An open-label study to assess the feasibility and tolerability of rilmenidine for the treatment of Huntington's disease

Preclinical data have shown that rilmenidine can regulate autophagy in models of Huntington's disease (HD), providing a potential route to alter the disease course in patients. Consequently, a 2-year open-label study examining the tolerability and feasibility of rilmenidine in mild-moderate HD was undertaken. 18 non-demented patients with mild to moderate HD took daily doses of 1 mg Rilmenidine for 6 months and 2 mg for a further 18 months followed by a 3-month washout period. The primary outcome was the number of withdrawals and serious adverse events. Secondary outcomes included safety parameters and changes in disease-specific variables, such as motor, cognitive and functional performance, structural MRI and serum metabolomic analysis. 12 patients completed the study; reasons for withdrawal included problems tolerating study procedures (MRI, and venepuncture), depression requiring hospital admission and logistical reasons. Three serious adverse events were recorded, including hospitalisation for depression, but none were thought to be drug-related. Changes in secondary outcomes were analysed as the annual rate of change in the study group. The overall change was comparable to changes seen in recent large observational studies in HD patients, though direct statistical comparisons to these studies were not made. Chronic oral administration of rilmenidine is feasible and well-tolerated and future, larger, placebo-controlled, studies in HD are warranted.

TRIAL REGISTRATION

EudraCT number 2009-018119-14.

MDMA-induced neurotoxicity of serotonin neurons involves autophagy and rilmenidine is protective against its pathobiology

Toxicity of 3,4-methylenedioxymethamphetamine (MDMA) towards biogenic amine neurons is well documented and in primate brain predominantly affects serotonin (5-HT) neurons. MDMA induces damage of 5-HT axons and nerve fibres and intracytoplasmic inclusions. Whilst its pathobiology involves mitochondrially-mediated oxidative stress, we hypothesised MDMA possessed the capacity to activate autophagy, a proteostatic mechanism for degradation of cellular debris. We established a culture of ventral pons from embryonic murine brain enriched in 5-HT neurons to explore mechanisms of MDMA neurotoxicity and recruitment of autophagy, and evaluated possible neuroprotective actions of the clinically approved agent rilmenidine. MDMA (100 μM-1 mM) reduced cell viability, like rapamycin (RM) and hydrogen peroxide (H₂O₂), in a concentration- and time-dependent manner. Immunocytochemistry revealed dieback of 5-HT arbour: MDMA-induced injury was slower than for RM and H₂O₂, neuritic blebbing occurred at 48 and 72 h and Hoechst labelling revealed nuclear fragmentation with 100 μM MDMA. MDMA effected concentration-dependent inhibition of [³H]5-HT uptake with 500 μM MDMA totally blocking transport. Western immunoblotting for microtubule associated protein light chain 3 (LC3) revealed autophagosome formation after treatment with MDMA. Confocal analyses and immunocytochemistry for 5-HT, Hoechst and LC3 confirmed MDMA induced autophagy with abundant LC3-positive puncta within 5-HT neurons. Rilmenidine (1 μM) protected against MDMA-induced injury and image analysis showed full preservation of 5-HT arbours. MDMA had no effect on GABA neurons, indicating specificity of action at 5-HT neurons. MDMA-induced neurotoxicity involves autophagy induction in 5-HT neurons, and rilmenidine via beneficial actions against toxic intracellular events represents a potential treatment for its pathobiology in sustained usage.

Improvement of obesity by activation of I1-imidazoline receptors in high fat diet-fed mice

Imidazoline I1-receptor (I1R) is known to regulate the blood pressure, and rilmenidine, as the agonist, is used to treat hypertension in clinics. However, the role of I1R in obesity is still unclear. In the present study, we investigated the changes of obesity by activation of I1R in high fat diet (HFD)-fed mice. Chronic administration of rilmenidine into HFD-fed mice for 8 weeks significantly reduced body weight, which was reversed by efaroxan at the dose sufficient to block I1R. Also, rilmenidine significantly decreased the energy intake of HFD-fed mice. This reduction of energy intake was abolished by efaroxan at the same dosing for blockade of I1R. However, hypothalamic I1R protein expression in HFD-fed mice was markedly lower than that in normal chow-fed mice. In addition, epididymal white adipose tissue (eWAT) cell size in HFD-fed mice was decreased by rilmenidine via the activation of I1R. Moreover, effect of rilmenidine on appetite disappeared in db/db mice. Taken together, we suggest that rilmenidine can improve obesity in HFD-fed mice through an activation of I1R to ameliorate energy intake and eWAT accumulation.

Improvement of hyperphagia by activation of cerebral I(1)-imidazoline receptors in streptozotocin-induced diabetic mice

Imidazoline I1-receptors (I1R) are known to regulate blood pressure and rilmenidine, an agonist, is widely used as antihypertensive agent in clinic. However, the role of I1R in feeding behavior is still unclear. In the present study, we used the agonist of I1R to investigate the effect on hyperphagia in streptozotocin (STZ)-induced diabetic mice. Rilmenidine decreased the food intake of STZ-diabetic mice in a dose-dependent manner. The reduction of food intake was abolished by pretreatment with efaroxan at the dose sufficient to block I1R. Intracerebroventricular (icv) administration of rilmenidine into STZ-diabetic mice also significantly reduced hyperphagia, which was reversed by icv administration of efaroxan. In addition, similar results were observed in STZ-diabetic mice, which received chronic treatment with rilmenidine 3 times daily (t.i.d.) for 7 days. Moreover, the hypothalamic neuropeptide Y (NPY) level was reduced by rilmenidine that was also reversed by pretreatment with efaroxan. In conclusion, the obtained results suggest that rilmenidine can decrease food intake in STZ-diabetic mice through an activation of I1R to lower hypothalamic NPY level.

Investigation of the effects of the chronic administration of some antihypertensive drugs on enzymatic and non-enzymatic oxidant/antioxidant parameters in rat ovarian tissue

OBJECTIVE

In this study, effects of chronic antihypertensive drug (clonidine, methyldopa, amlodipine, ramipril, and rilmenidine) treatment on antioxidant-oxidant parameters were investigated in rat ovarian tissue.

STUDY DESIGN

Chronic drug administration for 30 days and at the end, biochemical examinations (total glutathione (tGSH), glutathione peroxidase (GPO), glutathione reductase (GR), glutathione s-transferase (GST), superoxide dismutase (SOD), nitric oxide (NO), catalase (CAT), malondialdehyde (MDA), and myeloperoxidase (MPO) analyses) were performed.

RESULTS

The levels of glutathione (GSH) and NO, and the activities of GPO, GR, GST, SOD, and CAT were measured the lowest in ramiprile group. Also in ramiprile group, the level of MDA and the activity of MPO was the highest.

CONCLUSION

We divided the drugs into four groups according to their biochemical side effect potentials in ovarian tissue: (I) Drugs which have no clear negative effect on ovarian tissue: clonidine, rilmenidine; (II) Drugs which have mild negative effect on ovarian tissue: methyldopa; (III) Drugs which have moderate negative effect on ovarian tissue: amlodipine; (IV) Drugs which have severe negative effect on ovarian tissue: ramipril. These data might be useful in the selection of the least toxic antihypertensive drug in pregnant and/or normal females.

A novel mechanism for decreasing plasma lipid level from imidazoline I-1 receptor activation in high fat diet-fed mice

The imidazoline I-1 receptor (I-1 R) agonists are widely used to lower blood pressure, but their effects on hyperlipidemia are still obscure. The present study is aimed to evaluate the possible mechanism(s) of I-1 R in the regulation of lipid homeostasis. Farnesoid X receptor (FXR) plays an important role in blood lipid homeostasis; however, the role of FXR in rilmenidine-induced blood lipid lowering action is still unknown. Thus, we administered rilmenidine, a selective agonist of I-1 R, into high fat diet-fed (HFD) mice showing hypertriglyceridemia and hypercholesterolemia. Rilmenidine significantly ameliorated hyperlipidemia in HFD mice after 7 days of administration. Pretreatment with efaroxan, at a dose sufficient to inhibit I-1 R activation, blocked the effects of rilmenidine. Also, in cultured HepG2 cells, rilmenidine dose-dependently induced the expression of farnesoid X receptor (FXR). The rilmenidine-induced FXR expression and FXR-related genes were blocked by efaroxan. However, rilmenidine treatment did not affect the expression of enzymes related to β-oxidation. In conclusion, activation of I-1 R may activate FXR to lower plasma lipids, suggesting I-1 R as a new target for the treatment of hyperlipidemia.

Development of a fast LC-MS/MS method for quantification of rilmenidine in human serum: elucidation of fragmentation pathways by HRMS

Rilmenidine is an alpha 2 adrenoreceptor agonist used in the treatment of mild and moderate hypertension. In this study, a fast and accurate liquid chromatographic method with tandem mass spectrometric detection has been validated in order to assure quantification of rilmenidine in human serum. The fragmentation pathway of protonated rilmenidine was studied using high-resolution mass spectrometry (HRMS). This study compared selectivity, linearity, accuracy, precision, extraction efficiency, matrix effect and sensitivity using common liquid-liquid extraction (LLE) and solid-phase extraction (SPE) procedures. The limit of quantitation for both extraction techniques was 0.1 ng/ml. Several differences between the LLE and SPE have been observed in terms of linearity, accuracy, precision and matrix effect. Additionally, the advantages of SPE included less manual work load and increased recovery of rilmenidine in human serum to approximately 80% (LLE, 57%). The developed method involving SPE was found to be accurate (relative error (RE) < 5%), reproducible (relative standard deviation, RSD < 7%), robust and suitable for quantitative analysis of rilmenidine in serum samples obtained from patients under antihypertensive treatment.

Rilmenidine attenuates toxicity of polyglutamine expansions in a mouse model of Huntington's disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a polyglutamine expansion in huntingtin. There are no treatments that are known to slow the neurodegeneration caused by this mutation. Mutant huntingtin causes disease via a toxic gain-of-function mechanism and has the propensity to aggregate and form intraneuronal inclusions. One therapeutic approach for HD is to enhance the degradation of the mutant protein. We have shown that this can be achieved by upregulating autophagy, using the drug rapamycin. In order to find safer ways of inducing autophagy for clinical purposes, we previously screened United States Food and Drug Administration-approved drugs for their autophagy-stimulating potential. This screen suggested that rilmenidine, a well tolerated, safe, centrally acting anti-hypertensive drug, could induce autophagy in cell culture via a pathway that was independent of the mammalian target of rapamycin. Here we have shown that rilmenidine induces autophagy in mice and in primary neuronal culture. Rilmenidine administration attenuated the signs of disease in a HD mouse model and reduced levels of the mutant huntingtin fragment. As rilmenidine has a long safety record and is designed for chronic use, our data suggests that it should be considered for the treatment of HD and related conditions.

Rilmenidine – its antihypertensive efficacy, safety and impact on quality of life in perimenopausal women with mild to moderate essential hypertension

OBJECTIVES

A prospective, open-labeled study to assess the antihypertensive effect of monotherapy with rilmenidine and its impact on quality of life (QoL), as well as on biochemical parameters in perimenopausal women with essential hypertension.

DESIGN AND METHODS

Fifty-five perimenopausal women with mild to moderate essential hypertension (mean age 51.4+/-2.4 years) were enrolled. At baseline and after 12-week monotherapy with rilmenidine, we assessed: systolic (SBP) and diastolic (DBP) blood pressure (BP), heart rate (HR), fasting glucose, serum creatinine and total cholesterol levels. QoL was also assessed at baseline and at 12 weeks by two standardized questionnaires: the Short Form - 36 (SF-36) and the Subjective Symptoms Assessment Profile (SSAP). Statistical analysis was performed using Student's t-test to compare changes in BP, QoL and biochemical parameters during therapy with rilmenidine.

RESULTS

After 12 weeks of therapy, there was a significant decrease in BP and HR. Normalization of BP was observed in 46 (84%) women. Rilmenidine did not influence serum creatinine, fasting glucose and lipid profile. Treatment was very well tolerated by the patients and no side-effects were noted. Both the SF-36 and the SSAP demonstrated improvement in general QoL. We observed a significant improvement in all SF-36 subscales. In the SSAP, a similar significant improvement was found, except dizziness subscale. Improvement in QoL was not related to reduction of BP.

CONCLUSIONS

Monotherapy with rilmenidine is safe and effective in BP treatment and significantly improves QoL in perimenopausal women with essential hypertension.

Central Actions of Agmatine in Conscious Spontaneously Hypertensive Rats

Agmatine (decarboxylated arginine) is an endogenous ligand at alpha-2 adrenergic and imidazoline nonadrenergic receptors. In conscious spontaneously hypertensive rats (SHRs), we have studied its central effects on cardiovascular function and its interaction with the second generation centrally acting antihypertensive agent, rilmenidine, and the reference imidazoline, clonidine, which are mixed alpha-2 adrenoceptor/imidazoline receptor agonists. Agmatine, when administered in low doses (30-100 microg/kg) into the fourth ventricle had no effect on blood pressure and caused an increase in heart rate. A higher dose of 1,000 microg/kg produced an adverse reaction in conscious SHRs and a marked and long-lasting increase in blood pressure. The effects of fourth ventricular rilmenidine (300 microg/kg) and clonidine (10 microg/kg) were equihypotensive and equibradycardic. The antihypertensive and bradycardic effects of rilmenidine were not reversed by cumulative intracisternal doses (30-100-300 microg/kg) of agmatine. The bradycardia obtained 20 min after intracisternal administration of clonidine in the fourth ventricle was reversed by 30 microg/kg agmatine. Only the highest dose of agmatine (1,000 microg/kg) did reverse the antihypertensive effects of rilmenidine and clonidine. Agmatine neither did mimic nor block the antihypertensive response to rilmenidine and clonidine at well-tolerated doses. Yet agmatine produced a small tachycardia at relatively low doses and was able to reverse the bradycardia induced by clonidine. Therefore, its affinity for alpha-2 adrenoceptors in vitro might partially explain its cardiovascular effects in vivo.

The synergistic interaction between rilmenidine and paracetamol in the writhing test in mice

The aim of the study was to ascertain antinociceptive effects of rilmenidine, a second-generation imidazoline-alpha-2-adrenoreceptor agonist, and to see whether rilmenidine was able to increase the analgesic effects of paracetamol in the writhing test in mice. An acetic acid (0.7%) solution was injected into the peritoneal cavity and the number of writhes was counted. The influence on locomotor performance was tested using the rotarod test. Rilmenidine, paracetamol, and rilmenidine-paracetamol fixed-ratio combinations produced dose-dependent antinociceptive effects. ED(50) values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical additive ED(50) value for the rilmenidine-paracetamol combination was 109.23 +/- 35.05 mg/kg. This value was significantly greater than the observed ED(50) value which was 56.35 +/- 20.86 mg/kg, indicating a synergistic interaction. Rilmenidine did not impair motor coordination, as measured by the rotarod test, at antinociceptive and higher doses.

Bioequivalence evaluation of rilmenidine in healthy volunteers

The bioavailability of two rilmenidine tablet formulations was compared in healthy male (17) and female (8) subjects, aged 18 to 36 years, during a laboratory-blind, randomized, two-treatment, two-period, cross-over study under fasting conditions. In each treatment phase subjects received a single dose of 1.544 mg rilmenidine dihydrogen phosphate (CAS 85409-38-7), equivalent to 1 mg rilmenidine (CAS 54187-04-1). Consecutive dosing was separated by a drug-free wash-out period of 7 d. Following each dosing, serial venous blood samples were collected over a period of 48 h for the determination of plasma rilmenidine concentrations by means of a validated LCMS/MS method. The most frequently reported drug-related adverse events were dizziness and headache ranging from mild to moderate in intensity. The geometric mean C(max) of rilmenidine for the reference and test products was 3.73 and 3.97 ng/ml, respectively. The corresponding geometric mean AUC(0-infinity)) was 34.0 and 35.1 ng x h/ml. T(max) for both products under investigation appeared at 1.33 h. The test product was shown to be bioequivalent to the reference product with respect to all pharmacokinetic variables investigated.

Synergistic interaction between rilmenidine and ibuprofen in the writhing test in mice

OBJECTIVES

The aim of the study was to ascertain whether rilmenidine, a second generation imidazoline-alpha-2-adrenoreceptor agonist, is able to increase analgesic effects of ibuprofen in the writhing test in mice. Experimental studies combining these agents have not yet been published.

METHODS

An acetic acid (0.7%) solution was injected into the peritoneal cavity and the number of writhes was counted. The influence on locomotor performance was tested using the rotarod test.

RESULTS

Rilmenidine, ibuprofen, and rilmenidine-ibuprofen fixed-ratio combinations produced dose-dependent antinociceptive effects. ED50 values were estimated for the individual drugs and an isobologram was constructed. The derived theoretical additive ED50 value for the rilmenidine-ibuprofen combination was 34.00 +/- 9.39 mg/kg. This value was significantly greater than the observed ED50 value which was 18.07 +/- 5.41 mg/kg, indicating a synergistic interaction. Rilmenidine did not impair motor coordination, as measured by the rotarod test, at antinociceptive and higher doses.

CONCLUSIONS

The present results suggest that rilmenidine enhances the analgesic activity of ibuprofen. If rilmenidine produces antinociception in humans, then the synergistic antinociception of rilmenidine with ibuprofen could offer therapeutic advantage for clinical treatment of pain.

[Analysis of central mechanisms involved in gastric mucosal integrity]

Beta-endorphin, deltorphin II, [D-Ala2, Phe4, Gly5-ol-enkephalin (DAGO) as well as endomorphin-1 and endomorphin-2 injected intracerebroventricularly (i.c.v.) induced gastroprotective action. It has been raised that endogenous opioids may have a central role in maintaining gastric mucosal integrity. Therefore we aimed to study the role of endogenous opioid system in the gastroprotective action induced by activation of alpha 2-adrenoceptors, nociceptin- and cannabinoid-receptors. Our results suggest that the non-selective opioid receptor antagonist naloxone (27 nmol i.c.v.) and the delta-opioid receptor antagonist naltrindole (5 nmol i.c.v.) abolished the mucosal protective effect of alpha 2-adrenoceptor agonists clonidine (470 pmol i.c.v.) and rilmenidine (45 pmol i.c.v.), nociceptin (1 nmol i.c.v.) and the cannabinoid receptor agonist anandamide (110 nmol i.c.v.). Based on our findings it can be raised that opioid system besides its well known regulatory functions might be involved in maintenance of gastric mucosal integrity.

Centrally acting antihypertensive agents: an update

Centrally acting agents stimulate alpha(2) receptors and/or imadozoline receptors on adrenergic neurons situated within the rostral ventrolateral medulla and, in so doing, sympathetic outflow is reduced. Centrally acting agents also stimulate peripheral alpha(2) receptors, which, for the most part, is of marginal clinical significance. Central a agonists have had a lengthy history of use, starting with alpha-methyldopa, which has had a dramatic decline in use, in part, because of bothersome side effects. Patients who require multidrug therapy with otherwise resistant hypertension, such as diabetic and/or renal failure patients, are typically responsive to these drugs, as are patients with sympathetically driven forms of hypertension. Perioperative forms of hypertension respond well to clonidine, a circumstance where the additional anesthesia- and analgesia-sparing effects of this drug may offer additional clinical benefits. Clonidine can be used adjunctively with other more traditional therapies in heart failure, particularly when hypertension is present. Sustained-release moxonidine, however, is associated with early mortality and morbidity when used in patients with heart failure. Escalating doses of drugs in this class often give rise to salt and water retention, in which case diuretic therapy becomes a valuable adjunctive therapy.

Effects of rilmenidine on 24-h rhythmicity of blood pressure and spontaneous baroreflex sensitivity in essential hypertensive subjects

OBJECTIVE

To study the effects of the centrally acting imidazoline-like compound rilmenidine on the circadian and short-term cardiovascular rhythms derived from continuous blood pressure (BP) recordings in patients with mild essential hypertension.

METHODS

This was a single-center, open study. Recordings were obtained from eight subjects, using a Portapres during two 24-h hospitalizations: the first after the inclusion visit and the second 4 weeks after starting rilmenidine treatment (1 or 2 mg/day). For circadian analysis of cardiovascular variables, 10 min were selected every hour to obtain 24 periods per subject for each session. Spontaneous baroreflex sensitivity (BRS) was estimated using the sequence technique and the cross-spectral analysis between systolic BP and interbeat intervals.

RESULTS

Rilmenidine significantly reduced the overall systolic and diastolic BP and heart rate (P < 0.001). The effects of rilmenidine on BP and heart rhythm were marked during the daytime. Rilmenidine reduced the low-frequency (LF) component of systolic BP variability throughout the 24 h. The highest values of spontaneous BRS were observed at night. Rilmenidine increased the BRS obtained by the slope of the sequence method throughout the 24-h period (P < 0.001). The LF gain was significantly increased with rilmenidine during the day and the night.

CONCLUSIONS

Rilmenidine may differentially affect the baroreflex-dependent (phasic or reflex) and the baroreflex-independent (tonic) autonomic outflow. The 24-h approach reinforced this concept, since indexes of BRS were increased throughout the 24-h period while BP was reduced during the daytime.

Direct evidence for imidazoline (I1) receptor modulation of ethanol action on norepinephrine-containing neurons in the rostral ventrolateral medulla in conscious spontaneously hypertensive rats

BACKGROUND

Enhancement of the rostral ventrolateral medulla (RVLM) presympathetic (norepinephrine, NE) neuronal activity represents a neurochemical mechanism for the pressor effect of ethanol. In this study, we tested the hypothesis that ethanol action on RVLM presympathetic neurons is selectively influenced by the signaling of the local imidazoline (I1) receptor. To support a neuroanatomical and an I1-signaling selectivity of ethanol, and to circumvent the confounding effects of anesthesia, the dose-related neurochemical and blood pressure effects of ethanol were investigated in the presence of selective pharmacological interventions that cause reduction in the activity of RVLM or nucleus tractus solitarius (NTS) NE neurons via local activation of the I1 or the alpha2-adrenergic receptor in conscious spontaneously hypertensive rats.

RESULTS

Local activation of the I1 receptor by rilmenidine (40 nmol) or by the I1/alpha2 receptor mixed agonist clonidine (1 nmol), and local activation of the alpha2-adrenergic receptor (alpha2AR) by the pure alpha2AR agonist alpha-methylnorepinephrine (alpha-MNE, 10 nmol) caused reductions in RVLM NE, and blood pressure. Intra-RVLM ethanol (1, 5, or 10 microg), microinjected at the nadir of the neurochemical and hypotensive responses, elicited dose-dependent increments in RVLM NE and blood pressure in the presence of local I1--but not alpha2-receptor activation. Only intra-NTS alpha-MNE, but not rilmenidine or clonidine, elicited reductions in local NE and blood pressure; ethanol failed to elicit any neurochemical or blood pressure responses in the presence of local activation of the alpha2AR within the NTS.

CONCLUSION

The findings support the neuroanatomical selectivity of ethanol, and support the hypothesis that the neurochemical (RVLM NE), and the subsequent cardiovascular, effects of ethanol are selectively modulated by I1 receptor signaling in the RVLM.

Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine

OBJECTIVES

To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM).

METHODS

Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).

RESULTS

Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well.

CONCLUSIONS

The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.

Involvement of phosphatidylcholine-selective phospholipase C in activation of mitogen-activated protein kinase pathways in imidazoline receptor antisera-selected protein

Imidazoline receptor antisera-selected protein (IRAS) is considered as a candidate for the I1-imidazoline receptor (I1R), but the signaling pathway mediated by IRAS remains unknown. In our study, the signal transduction pathways of IRAS were investigated in CHO cells stably expressing IRAS (CHO-IRAS), and compared to the native I1R signaling pathways. Rilmenidine or moxonidine (10 nM-100 microM), I1R agonists, failed to stimulate [35S]-GTPgammaS binding in CHO-IRAS cell membrane preparations, suggesting that G protein may not be involved in IRAS signaling pathway. However, incubation of CHO-IRAS with rilmenidine or moxonidine for 5 min could induce an upregulation of phosphatidylcholine-selective phospholipase C (PC-PLC) activity, and an increase in the accumulation of diacylglycerol (DAG), the hydrolysate of PC-PLC, in a concentration-dependent manner. The elevated activation of PC-PLC by rilmenidine or moxonidine (100 nM) could be blocked by efaroxan, a selective I1R antagonist. Cells treated with rilmenidine or moxonidine showed an increased level of extracellular signal-regulated kinase (ERK) phosphorylation in a concentration-dependent manner, which could be reversed by efaroxan or D609, a selective PC-PLC inhibitor. These results suggest that the signaling pathway of IRAS in response to I1R agonists coupled with the activation of PC-PLC and its downstream signal transduction molecule, ERK. These findings are similar to those in the signaling pathways of native I1R, providing some new evidence for the relationship between I1R and IRAS.

How can we block sympathetic overactivity? Effects of rilmenidine and atenolol in overweight hypertensive patients

The aim of the present study was to evaluate effects of long-term treatment with rilmenidine compared with atenolol on lipid and glucose metabolism and cardiovascular remodelling in hypertension. In total, 37 patients with hypertension were randomised to rilmenidine 1-2 mg/day or atenolol 50-100 mg/day for 26 weeks. Standard oral glucose tolerance test with a parallel measurement of insulin and glucose levels was performed. The 'areas under the curve' (AUC) for insulin and glucose were calculated. Plasma lipids, left ventricular mass index (LVMI), and intima-media thickness (IMT) were measured. Brachial artery diameter during reactive hyperaemia was used to test endothelium-dependent vasodilatation (EDVD). Blood pressure reduction was equally achieved in both treatment arms. The fasting glucose level increased in the atenolol group from 4.8+/-0.6 to 5.2+/-0.7 mmol/l (P<0.01). The AUC of glucose in rilmenidine group decreased from 860+/-93 to 737+/-66 mmol/min/l (P<0.05), and in the atenolol group it increased from 937+/-86 to 989+/-88 mmol/min/l (P<0.05). Rilmenidine showed a positive effect on lipid levels, whereas in the atenolol group a significant decrease of high-density lipoprotein cholesterol was observed. Left ventricular mass index decreased with rilmenidine by 9.6% and by 6.9% with atenolol (P<0.05). Intima-media thickness significantly decreased in the rilmenidine group. Endothelium-dependent vasodilatation slightly increased on in the rilmenidine group, while on in the atenolol group it remained unchanged. Our data suggest that in hypertensive patients central inhibition of sympathetic drive can produce favourable effects on glucose and lipid metabolism compared with standard beta-blockade with a similar antihypertensive efficacy. Rilmenidine also provides beneficial effects on cardiovascular remodelling and altered endothelial function in hypertension.

Possible role of NMDA receptors in antinociception induced by rilmenidine in mice in the formalin test

OBJECTIVES

The aim of the study was to investigate the possible role of MK-801, an NMDA antagonist, in analgesia induced by rilmenidine, an imidazoline (I(1)) agonist, in mice in the formalin test.

METHODS

25 microl of formalin 2.5% was injected into the dorsal surface of the right hind paw of the mouse. Pain response was scored after formalin injection for a period of 50 min. A weighted average of nociceptive score, ranging from 0 to 3, was calculated. The mean +/-SEM of scores between 0-5 and 15-40 min after formalin injection was presented.

RESULTS

The study showed that rilmenidine (1.25, 2.5 and 5 mg/kg, i.p.) produced analgesia dose-dependently (p<0.001) in formalin test. In addition, the results demonstrated that efaroxan (0.1 and 1 mg/kg, i.p.) could reduce the antinociceptive effect of rilmenidine (2.5 mg/kg, i.p.) (p<0.01) in animals, however, yohimbine (0.1 and 0.2 mg/kg, i.p.) could not block the analgesia induced by rilmenidine (2.5 mg/kg, i.p.) (p>0.05). On the other hand, MK-801 (0.05 mg/kg, i.p.) reduced the pain related behaviors in mice (p>0.05). Moreover, our findings demonstrated that MK-801 (0.01 mg/kg, i.p.) could potentiate the analgesic effect of rilmenidine (1.25 mg/kg, i.p.) significantly (p<0.01).

CONCLUSIONS

The present study suggests that imidazoline (I(1)) receptors play an important role in mediating the antinociception induced by rilmenidine in formalin test. Furthermore, it may be concluded that there is an interaction between NMDA receptors and imidazoline (I(1)) binding sites.

Centrally Acting Imidazolines Stimulate Vascular Alpha 1A-Adrenergic Receptors in Rat-Tail Artery

: 1. Centrally acting imidazoline antihypertensive agents clonidine and moxonidine also act peripherally to contract blood vessels. While these agents act at both I(1)-imidazoline and alpha 2 adrenergic receptors centrally, the receptor types by which they mediate contraction require further definition. We therefore characterized the receptor subtype by which these agents mediate contraction of proximal rat-tail artery. 2. Dose-response curves were determined for phenylephrine and for several imidazoline ligands, using endothelium denuded, isolated ring segments, of tail arteries from adult male Sprague-Dawley rats. Ring segments were mounted on a force transducer with platinum wires and immersed in a tissue bath containing Krebs solution, to which drugs could be added. Signals were digitized and recorded by a computer. 3. Tail artery contractions expressed as a percent of contraction to 106 mM potassium were phenylephrine (96%), moxonidine (88%), clonidine (52%), and UK14304 (30%). Neither rilmenidine nor harmane caused contraction. Contraction of tail artery to moxonidine or clonidine could be blocked by alpha 1 antagonist urapidil or prazosin, and also by alpha 1A subtype selective antagonist WB4101. Schild plots were generated and a calculated pA2 value of 9.2 for prazosin in the presence of clonidine confirms clonidine as an agonist at alpha 1A receptors in proximal segments of rat-tail artery. 4. Our work suggests that clonidine and moxonidine are promiscuous compounds at micromolar concentrations and that harmane and rilmenidine are more selective compounds for in vivo imidazoline research.

Opposite to α2-adrenergic agonists, an imidazoline I1 selective compound does not influence reflex bradycardia in rabbits

This work aimed to study the respective effects of central alpha2-adrenergic receptors (alpha2-ARS) and I1 imidazoline receptors (I1Rs) in the facilitatory effects of imidazoline-like drugs on the reflex bradycardia (RB). Experiments were performed in anaesthetized rabbits. The reflex bradycardic response was induced by phenylephrine injected i.v. LNP 509, rilmenidine and dexmedetomidine were administered intracisternally (i.c.). LNP509 (1 mg/kg, i.c.), a ligand highly selective for I1Rs, induced hypotension (54+/-3 vs. 93+/-2 mm Hg) and bradycardia (260+/-13 vs. 322+/-13 beats/min) (p<0.05, n=5) but did not affect RB. Rilmenidine (1 microg/kg, i.c.), a hybrid ligand which binds to both I1 and alpha2-ARS, also decreased arterial pressure (61+/-2 vs. 101+/-2 mm Hg) and heart rate (260+/-4 vs. 308+/-8) (p<0.01, n=5); it potentiated the RB (maximum R-R interval: 284+/-17 vs. 196+/-6 ms) (p<0.05, n=5). Dexmedetomidine (1 microg/kg, i.c.), a ligand selective for alpha2-ARs, reduced blood pressure (53+/-3 vs. 104+/-2 mm Hg) and heart rate (246+/-4 vs. 312+/-8 beats/min) (p<0.05, n=5) and potentiated the RB (maximum R-R interval: 518+/-38 vs. 194+/-4 ms) (p<0.05, n=5). The potentiation of RB was much greater than that observed with rilmenidine and was significantly prevented by L-NNA injected centrally. This study shows that: (i) an exclusive action on I1Rs which decreases arterial pressure, does not potentiate the RB ii) activation of alpha2-ARs potentiates the RB (iii) the R-R prolongation caused by alpha2-ARs stimulation is prevented by central NOS inhibition.

Efficacy and tolerability of rilmenidine compared with isradipine in hypertensive patients with features of metabolic syndrome

OBJECTIVES

A high prevalence of associated metabolic cardiovascular risk factors is often observed among hypertensive subjects. The aim of the present study was to assess the effects of 1-2 mg/day of rilmenidine, a centrally acting antihypertensive agent with selectivity for I(1) imidazoline receptors, vs. 2.5-5 mg/twice daily of isradipine, a dihydropyridine calcium channel blocker, in hypertensive patients with features of the metabolic syndrome.

RESEARCH DESIGN AND METHODS

In this 6-month multicentre, comparative, double-blind, parallel group study, the primary objective was to assess the effects of the treatments on blood pressure (BP); the secondary endpoints were to assess glucose and lipid metabolism, in addition to clinical and biological tolerability. In non-responder patients, dose adjustment was possible from the first month and adding a diuretic from the third month.

RESULTS

Of an intention-to-treat population of 93 patients, 84 per protocol patients completed the study: 42 in the rilmenidine group and 42 in the isradipine group. BP decreased significantly (p < 0.001) and similarly in both groups (systolic blood pressure, SBP: -16.0 +/- 17.2 mmHg and -15.0 +/- 13.0 mmHg, and diastolic blood pressure, DBP: -9.0 +/- 9.4 mmHg and -9.0 +/- 8.7 mmHg with rilmenidine and isradipine, respectively). Normalisation (DBP < 90 mmHg and SBP < 140 mmHg) and response (normalisation or decrease in SBP >or= 20 mmHg or decrease in DBP >or= 10 mmHg) rates were respectively 57% and 72% with rilmenidine and 64% and 79% with isradipine (NS between groups). The effects of the treatments on both glucose and lipid metabolism were comparable: no significant difference from baseline was observed on the main parameters including insulin sensitivity indexes. The two treatments appeared to be well tolerated throughout the study, with no serious adverse reaction reported in the rilmenidine group and one serious adverse event in the isradipine group (a perimalleolar oedema), leading to withdrawal from the study for the affected patient.

CONCLUSION

This study suggests that in hypertensive patients with metabolic disorders, rilmenidine is an effective antihypertensive treatment, comparable to isradipine, with metabolic neutrality and a good tolerance profile.

Nischarin as a functional imidazoline (I1) receptor

Gene matching shows that Nischarin is a mouse homologue of human imidazoline receptor antisera-selective (IRAS) protein, a viable candidate of the imidazoline (I1) receptor. Nischarin and IRAS share the functions of enhancing cell survival, growth and migration. Bioinformatics modeling indicates that the IRAS and Nischarin may be transmembrane proteins and the convergence information raises the interesting possibility that Nischarin might serve as the I1-receptor. To test this hypothesis, we developed antibodies against the Nischarin protein, and conducted signal transduction (functional) studies with the I1-receptor agonist rilmenidine in the presence and absence of Nischarin antisense oligodeoxynucleotides (ODNs). NIH3T3 cells transfected with the Nischarin cDNA and incubated with the newly synthesized antibody expressed a 190 kD band. The antibody identified endogenous Nischarin in differentiated PC12 cells around 210 kD, which is consistent with reported findings in other cells of neuronal origin. The immunoflourescence findings showed the targeted protein to be associated with the cell membrane in PC12 cells. Nischarin ODNs abolished the expression of Nischarin in PC12 cells. Equally important, the Nischarin ODNs eliminated the production of MAPK(p42/44), a recognized signal transduction product generated by I1-receptor activation in differentiated PC12 cells. Together, the present findings suggest that Nischarin may serve as the functional I1-receptor or at least share a common signaling pathway in the differentiated PC12 cells.

Reduced Hemodynamic Responses to Physical and Mental Stress Under Low-Dose Rilmenidine in Healthy Subjects

Activation of the sympathetic nervous system plays a major role in the pathogenesis and prognosis of cardiovascular diseases. Rilmenidine is an I(1)-imidazoline receptor agonist that reduces blood pressure by modulation of central sympathetic activity, but the effects of low-dose rilmenidine on the hemodynamic responses to physiological maneuvers that increase adrenergic drive is not known. To assess the effects of low-dose rilmenidine on the hemodynamic responses to stress, 32 healthy subjects (20-56 years old) underwent acute physical exercise (n = 15, individualized ramp protocol on treadmill) and mental stress (n = 17, word color Stroop and mental arithmetics tests) two hours after the oral administration of 0.5 mg of rilmenidine (RIL) or placebo (PLA) following a randomized, double-blind, placebo controlled crossover study. No subject complained of any side effect. Rilmenidine reduced peak exercise heart rate (PLA: 187 +/- 7; RIL: 181 +/- 9 bpm; P = 0.003), but did not modify peak aerobic power (VO(2max) - PLA: 41.7 +/- 6.2; RIL: 42.3 +/- 6.7 ml/kg/min; P = 0.26). During mental stress, rilmenidine inhibited the peak systolic (PLA: 123 +/- 10; RIL: 114 +/- 8 mmHg; P = 0.02) and diastolic (PLA: 86 +/- 7; RIL: 81 +/- 7 mmHg; P <0.05) blood pressure responses. In conclusion, rilmenidine reduced the hemodynamic response to physical and mental stress stimuli without limiting exercise capacity. These results support the concept that rilmenidine, at a dose lower than the ones recommended to treat hypertension, reduced the myocardial oxygen demand to stress and may carry potential clinical impact.

Lipid-lowering actions of imidazoline antihypertensive agents in metabolic syndrome X

Agonists active at I1-imidazoline receptors (I1R) not only lower blood pressure but also ameliorate glucose intolerance, insulin resistance, and hyperlipidemia with long-term treatment. We sought to determine the possible mechanism for the lipid-lowering actions of imidazolines in a model of metabolic Syndrome X, the spontaneously-hypertensive obese (SHROB) rat. The acute actions of moxonidine and rilmenidine, selective I1R agonists, were compared to a specific alpha2-adrenergic receptor agonist, guanabenz, with and without selective receptor blockers. Moxonidine and rilmenidine rapidly reduced plasma triglyceride (20+/-4% and 21+/-5%, respectively) and cholesterol (29+/-9% and 27+/-9%). In contrast, the specific alpha2-adrenergic receptor agonist guanabenz failed to reduce plasma lipids. Blocking experiments showed that moxonidine's actions were mediated by I1R and not alpha2-adrenergic receptors. To evaluate a hepatic site of action, radioligand binding studies with liver plasma membranes confirmed the presence of I1R. Intraportal moxonidine reduced plasma triglycerides by 23+/-3% within 10 min. Moxonidine inhibited hepatic triglyceride secretion by 75% compared to vehicle treatment. Tracer studies with 2H2O suggested that moxonidine inhibits de novo fatty acid synthesis. Thus, activation of I1R lowers plasma lipids, with the main site of action probably within the liver to reduce synthesis and secretion of triglycerides. More selective I1R agonists might provide monotherapy for hyperlipidemic hypertension.

Cause and Consequences of Sympathetic Hyperactivity in Chronic Kidney Disease

Patients with chronic kidney disease and patients undergoing hemodialysis treatment show a sustained overactivity of the sympathetic nervous system, which originates from signals arising in the failing kidneys and traveling via afferent renal nerves to cardiovascular centers in the brainstem. Additional important factors are increased levels of angiotensin II and asymmetrical dimethylarginine. The sympathetic overactivity contributes to hypertension and cardiovascular morbidity and mortality in that patient population. Sympathetic overactivity can be reduced by adrenergic receptor blockers, centrally acting sympathicolytic drugs such as moxonidine and rilmenidine, angiotensin-converting enzyme inhibition, and angiotensin II type 1 receptor antagonists. Daily short hemodialysis and long nocturnal hemodialysis may reduce the elevated sympathetic activity, possibly because of an increased clearance of asymmetrical dimethylarginine, an endogenous nitric oxide synthase inhibitor. Prospective trials examining the potential impact of both beta-blockers and centrally acting sympatholytic drugs on cardiovascular mortality in chronic kidney disease and hemodialysis patients are very much needed.

Blockade of sympathetic nervous system activity by rilmenidine increases plasma adiponectin concentration in patients with essential hypertension

BACKGROUND

Adiponectin is an adipose tissue-specific protein with antiatherogenic and insulin-sensitizing properties. In patients with essential hypertension plasma adiponectin levels are lower than in healthy subjects. It is hypothetized that low plasma adiponectin concentration may be involved in the pathogenesis of vascular complications in hypertension. The aim of the study was to evaluate the effect of 6 months of antihypertensive therapy with rilmenidine on plasma adiponectin concentration in patients with essential hypertension.

METHODS

In 20 patients with essential hypertension (mean age 39 +/- 14 years; 11 F, 9 M; mean duration of hypertension 8 +/- 9 years) plasma adiponectin concentration (ELISA; B-Bridge International, San Jose, CA), insulin sensitivity (metabolic clearance of glucose-M value and M/plasma insulin ratio) by the euglycemic-hyperinsulinemic glucose clamp technique, and body fat content (DEXA) were estimated twice, before and after 6 months of antihypertensive monotherapy with rilmenidine (daily dose 1 to 2 mg).

RESULTS

The 6 months of treatment with rilmenidine resulted in a significant decrease of systolic (P = .007), diastolic (P = .002), and mean arterial blood pressure (P = .002), no significant change of body mass index (27.7 +/- 4.7 and 27.6 +/- 4.4 kg/m(2)), total body fat content (25.4 +/- 7.3 and 24.9 +/- 6.8 kg), and insulin sensitivity parameters (M value 6.0 +/- 2.5 and 5.8 +/- 4.3 mg/kg/min and M/plasma insulin ratio 7.4 +/- 5.0 and 6.9 +/- 4.3 mg/kg/min/mU/L). However after 6 months of treatment with rilmenidine a significant increase in plasma adiponectin concentration (from 12.5 +/- 6.1 to 16.9 +/- 11.1; P = .0002) was observed.

CONCLUSIONS

Antihypertensive therapy with rilmenidine correlates with an increase of plasma adiponectin concentration without any significant changes of insulin sensitivity and body fat content in patients with essential hypertension.

Role of imidazoline receptors in the anti-aversive properties of clonidine during opiate withdrawal in rats

Clonidine is used as a treatment for heroin addiction. Previous studies have reported that clonidine attenuated conditioned place aversion (CPA) to naloxone-precipitated opiate withdrawal by acting on alpha2 adrenoceptors (alpha2R). However, clonidine acts as a partial agonist both at alpha2R and at imidazoline-1 receptors (I1Rs). The current study was designed to determine the role of I1R in the induction of naloxone-induced CPA in morphine-dependent rats. Morphine dependence was induced by subcutaneous implantation of morphine pellets. Morphine-dependent rats were tested in a three-chamber place-aversion apparatus. A range of agonists were chosen on the basis of their differential selectivity for alpha2R and I1R. As expected, pretreatment with clonidine prevented naloxone-induced CPA. By contrast, pretreatment with a selective alpha2R agonist (UK14304) failed to prevent the CPA. We then tested whether the high affinity of clonidine for I1R was responsible for the difference between these two alpha2R agonists. Rilmenidine (a mixed alpha2R/I1R agonist) attenuated aversion to opiate withdrawal in a dose-dependent manner. The action of clonidine on I1R was studied by co-administering clonidine with RX821002, a specific alpha2R antagonist. Co-treatment with RX821002 and clonidine blocked naloxone-induced CPA. These results indicate that the pharmacologically protective effects of clonidine on naloxone-induced CPA are related to actions on I1RS as well as alpha2Rs.

Neuronal Norepinephrine Responses of the Rostral Ventrolateral Medulla and Nucleus Tractus Solitarius Neurons Distinguish the I1- from the α2-Receptor-Mediated Hypotension in Conscious SHRs

We tested the hypothesis that the I1 receptor mediates the reduction in rostral ventrolateral medulla (RVLM) neuronal norepinephrine (NE; index of sympathetic activity) that leads to hypotension independent of other brainstem areas or the alpha2-adrenergic receptor. To this end, we developed a model that permitted measurement of real-time changes in neuronal NE in the RVLM or nucleus tractus solitarius (NTS) along with blood pressure and heart rate in the conscious SHR in response to localized microinjections of selective I1 (rilmenidine) or alpha2-adrenergic (alpha-methylnorepinephrine; alpha-MNE) agonist versus the mixed I1/alpha2 agonist clonidine. To further support the hypothesis, we investigated the effects of localized selective alpha2- (SK&F86466) or I1 (efaroxan) blockade on the reductions in neuronal NE and blood pressure elicited by intra-RVLM rilmenidine. In the latter experiment, changes in RVLM neuronal c-Fos (another marker of sympathetic neural activity) were also investigated. Intra-RVLM rilmenidine (40 nmol) or clonidine (1 nmol) similarly reduced RVLM NE and blood pressure; these responses were approximately 2-fold greater than those elicited by the pure alpha2-adrenergic agonist alpha-MNE (10 nmol). By contrast, intra-NTS rilmenidine or clonidine had no effect on NTS NE or blood pressure versus significant reductions in both parameters by alpha-MNE. Intra-RVLM rilmenidine decreased c-Fos expression, and these responses were abolished by efaroxan but not by SK&F 86466. These findings suggest: (1) in the RVLM, I1-receptor signaling suppresses cardiovascular neuron activity, which leads to lowering of blood pressure; (2) although the alpha2-adrenergic receptor in the RVLM serves a similar role, it does not exert a tonic neuronal inhibitory effect and is not essential, as a downstream signaling entity, for the I1-evoked neurobiological effects in the brainstem. The potential confounding effects of anesthetics on the I1 and/or alpha2 receptor-mediated neuronal and cardiovascular responses were circumvented in the present study.

Mitogen-activated protein kinase phosphorylation in the rostral ventrolateral medulla plays a key role in imidazoline (i1)-receptor-mediated hypotension

Our previous study showed that rilmenidine, a selective I(1)-imidazoline receptor agonist, enhanced the phosphorylation of mitogen-activated protein kinase (MAPK)(p42/44), via the phosphatidylcholine-specific phospholipase C pathway in the pheochromocytoma cell line (PC12). In the present study, we tested the hypothesis that enhancement of MAPK phosphorylation in the rostral ventrolateral medulla (RVLM) contributes to the hypotensive response elicited by I(1)-receptor activation in vivo. Systemic rilmenidine (600 microg/kg i.v.) elicited hypotension and bradycardia along with significant elevation in MAPK(p42/44), detected by immunohistochemistry, in RVLM neurons. To obtain conclusive evidence that the latter response was I(1)-receptor-mediated, similar hypotensive responses were elicited by intracisternal (i.c.) rilmenidine (25 microg/rat) or the highly selective alpha(2)-agonist alpha-methylnorepinephrine (4 microg/rat). An increase in RVLM MAPK(p42/44) occurred only after rilmenidine. Furthermore, pretreatment with efaroxan (0.15 microg/rat i.c.), a selective I(1)-imidazoline receptor antagonist, or with PD98059 (2'-amino-3'-methoxyflavone) (5 microg/rat i.c.), a selective extracellular signal-regulated kinase 1/2 inhibitor, significantly attenuated the hypotensive response and the elevation in RVLM MAPK(p42/44) elicited by i.c. rilmenidine. The findings suggest that MAPK phosphorylation in the RVLM contributes to the hypotensive response induced by I(1)-receptor activation and presents in vivo evidence that distinguishes the neuronal responses triggered by the I(1)-receptor from those triggered by the alpha(2)-adrenergic receptor.

Site-dependent inhibition of neuronal c-jun in the brainstem elicited by imidazoline I1 receptor activation: Role in rilmenidine-evoked hypotension

Clonidine (a mixed alpha2-adrenoceptor and imidazoline I1 receptor agonist)-evoked hypotension was associated with dissimilar reductions in c-jun gene expression in the rostral ventrolateral medulla (RVLM) and the nucleus tractus solitarius (NTS) in normotensive rats. In the present study, we investigated the relative contribution of the alpha2-adrenoceptor vs. the imidazoline I1 receptor to the reduction in c-jun gene expression in these two brainstem areas. In conscious spontaneously hypertensive rats (SHRs), equihypotensive doses of three centrally acting hypotensive drugs with different selectivity for the two receptors were administered intracisternally (4 microl) to limit their actions to the brain. As a control, a similar hypotensive response was elicited by i.v. hydralazine. Clonidine (0.5 microg), or alpha-methylnorepinephrine (alpha-MNE, 4 microg), a highly selective alpha2-adrenoceptor agonist, similarly reduced c-jun mRNA expression in the NTS and rostral ventrolateral medulla. In contrast, a similar hypotensive response (-37+/-3.5 mm Hg) caused by the selective imidazoline I1 receptor agonist rilmenidine (25 microg) was associated with reduction in c-jun mRNA expression in the rostral ventrolateral medulla, but not in the NTS. Further, intra-rostral ventrolateral medulla rilmenidine (40 nmol) reduced c-Jun protein expression in rostral ventrolateral medulla and blood pressure and both responses were antagonized by selective imidazoline I1 receptor (efaroxan, 4 nmol), but not alpha2-adrenoceptor (SK&F 86466, 10 nmol) blockade. These results suggest: (1) the c-jun containing neurons in the brainstem are involved in the centrally mediated hypotension elicited by centrally acting antihypertensive agents, and (2) the alpha2-adrenoceptor modulates c-jun gene expression in the NTS and rostral ventrolateral medulla implicated in centrally mediated hypotension, and (3) the imidazoline I1 receptor mediated inhibition of c-jun gene expression in the rostral ventrolateral medulla, but not in the NTS, contributes to the centrally mediated hypotension by the second generation drugs.

Rilmenidine sympatholytic activity preserves mental stress, orthostatic sympathetic responses and adrenaline secretion

BACKGROUND

Heightened central sympathetic nervous outflow is common in essential hypertension, contributing to hypertension development and possibly also to complications. Acute sympathetic nervous activation is a proven trigger for adverse cardiovascular events. Accordingly, antihypertensive drugs inhibiting sympathetic outflow represent a theoretically attractive therapeutic option.

OBJECTIVES

To study the sympatholytic and blood pressure-lowering activity of the imidazoline binding agent rilmenidine at rest and during reflex sympathetic activation.

DESIGN AND METHODS

We used a randomized, double-blind, 6-week cross-over study, with a 1-week placebo run-in period, two 2-week active treatment intervals (rilmenidine 1 mg twice daily or placebo) and intervening 1-week placebo washout. In 15 hypertensive patients, noradrenaline and adrenaline plasma kinetics and intra-arterial blood pressure measurements were performed at rest, after mental stress (difficult mental arithmetic) and during head-up tilting, at the end of the 2-week dosing periods.

RESULTS

The noradrenaline spillover rate, indicative of whole body sympathetic activity, was reduced 35% by rilmenidine at rest (P < 0.01) and remained significantly lower during mental stress and tilting, although the increases in noradrenaline spillover with both stimuli were preserved. The effects on intra-arterial blood pressure ran in parallel, a fall in supine resting pressure, but no reduction in blood pressure rise during mental stress and a lack of fall in blood pressure with tilting. On placebo, adrenaline secretion was 0.88 +/- 0.15 nmol/min (mean +/- SE) at rest, increased by 0.42 +/- 0.23 nmol/min with mental stress (P = 0.019) and was unchanged with tilting. Rilmenidine left adrenaline secretion untouched under all conditions.

CONCLUSIONS

The present study confirms a sympatholytic effect of rilmenidine during supine rest but preservation of sympathetic responses during mental stress and tilting, with the latter underlying a freedom from postural hypotension on the drug. The absence of suppression of reflexive sympathetic responses contrasts with the described effects of rilmenidine in experimental animals, and emphasizes the previously demonstrated unique importance in humans of suprabulbar noradrenergic neuronal projections from the brainstem in regulating tonic sympathetic activity, with these being inhibited by imidazoline binding agents. Sympathetic nervous inhibition with rilmenidine contrasted with an absence of suppression of adrenaline secretion, affirming that sympathetic nervous and adrenal medullary function can be disconnected.

Comparison of rilmenidine and lisinopril on ambulatory blood pressure and plasma lipid and glucose levels in hypertensive women with metabolic syndrome

OBJECTIVE

In previous studies, the I1 imidazoline specific agonist rilmenidine effectively lowered office blood pressure (BP) in patients with metabolic syndrome, improved glucose metabolism and did not demonstrate unfavourable effects on plasma lipids. The aim of the present study was to investigate the effects of 12weeks therapy with rilmenidine compared with the ACE inhibitor lisinopril on ambulatory BP, plasma lipid and fasting glucose levels in women with metabolic syndrome.

RESEARCH DESIGN

Prospective randomised open-label, blinded end-points study.

METHODS

Female patients (n = 51) with hypertension and other components of metabolic syndrome were treated with 1 mg rilmenidine (n = 24) or 10 mg lisinopril (n = 27), once- or twice-daily. Anthropometric measurements, office BP and heart rate (HR) measurements, ambulatory BP monitoring, lipid and fasting glucose assessment were performed before and after 12weeks of treatment

MAIN OUTCOME MEASURES

Changes in ambulatory BP and HR, including 24-h, daytime and night-time values, and in lipids and glucose levels. All changes were adjusted for baseline values using the analysis of covariance method.

RESULTS

Ambulatory 24-h systolic BP and diastolic BP were decreased significantly in the rilmenidine group (-11.9 +/- 1.9 and -7.7 +/- 0.8 mm Hg, p < 0.001) respectively and the lisinopril group (-11.0 +/- 1.8 and -6.7 +/- 0.7 mm Hg respectively, p < 0.001). There were no significant differences between the two groups. Rilmenidine reduced 24-h ambulatory HR (-3.6 +/- 0.8 bpm versus 0.3 +/- 0.8 bpm with lisinopril; p = 0.002). The reductions of day-time and night-time BP were also significant for both treatment groups, but the rilmenidine group demonstrated a greater decrease in night-time diastolic BP (p = 0.046). Rilmenidine significantly increased HDL cholesterol and decreased fasting glucose levels (p = 0.009 and p = 0.012, respectively). HDL cholesterol tended to increase and fasting glucose tended to decrease in the lisinopril group. However, differences between groups were not significant.

CONCLUSION

Rilmenidine has similar effects on ambulatory BP patterns in hypertensive women with metabolic syndrome as lisinopril. Rilmenidine compared with lisinopril significantly reduces ambulatory HR. In this study, rilmenidine and lisinopril demonstrate similar effects on plasma lipid and fasting glucose levels.

Imidazoline binding sites and their ligands: an overview of the different chemical structures

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

Rilmenidine prevents blood pressure increase in rats with compromised nitric oxide production

AIM

To search tools of high blood pressure in the model of nitric oxide (NO)-defective hypertension, and the study focused on the effect of rilmenidine, agonist of imidazoline receptors, which was suggested to modulate central sympathetic outflow.

METHODS

Three experimental groups, each consisting of 7 rats, were used: (I) rats with inhibition of NO synthase (NOS) by N(G)-nitro-L-arginine methyl ester (L-NAME) 40 mg.kg(-1).d(-1) for 4 weeks in drinking water, (II) rats with inhibited NOS as in group I , plus agonist of imidazoline receptors rilmenidine 3 mg.kg(-1).d(-1) for 4 weeks by gavage, and (III) control rats. Systolic blood pressure was measured weekly noninvasively. At the end of experiment aortic ring isometric tension was followed, NOS expression (aorta, left ventricle), and NOS activity (left ventricle and brain) were determined.

RESULTS

In the group I systolic blood pressure increased significantly, aortic ring relaxation to acetylcholine was significantly attenuated. Rilmenidine administered simultaneously with L-NAME (group II) prevented the increase of blood pressure which did not differ significantly from control values; aortic ring relaxation to acetylcholine did not differ from control. No change in NOS expression (aorta and left ventricle) was found in groups I and II. Significant decline in NOS activity (left ventricle and brain) was found in groups I and II.

CONCLUSION

Rilmenidine has a remarkable role in NO-defective hypertension, possibly by inhibiting central sympathetic outflow and by affecting receptors in vascular smooth muscle also. The prime cause of hypertension in this experimental model--the compromised production of NO due to inhibition of NOS--was not affected by rilmenidine.

Chronic ethanol administration attenuates imidazoline I1 receptor- or alpha 2-adrenoceptor-mediated reductions in blood pressure and hemodynamic variability in hypertensive rats

Our previous studies have demonstrated that acute ethanol administration counteracts imidazoline I(1) receptor but not alpha(2)-adrenoceptor-mediated hypotension in spontaneously hypertensive rats (SHR). In the present study, we investigated the effect of chronic ethanol administration on hypotensive responses elicited by acute administration of selective imidazoline I(1) receptor (rilmenidine) or alpha(2)-adrenoceptor (alpha-methyldopa) agonist along with ethanol effects on: (i) locomotor activity and (ii) time-domain indices of variability in blood pressure (standard deviation of mean arterial pressure) and heart rate (standard deviation of beat-to-beat intervals and root mean square of successive differences in R-R intervals). Hemodynamic and locomotor responses elicited by rilmenidine or alpha-methyldopa were assessed in radiotelemetered ethanol-fed (2.5% or 5% w/v, 12 week) and control SHR. In control SHR, i.p. rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg) significantly reduced blood pressure. Rilmenidine had no effect on heart rate whereas alpha-methyldopa elicited a biphasic response (tachycardia followed by bradycardia). Blood pressure and heart rate oscillations were also reduced by both drugs, which may conform to sympathoinhibition. The hypotensive effect of rilmenidine or alpha-methyldopa was significantly attenuated by ethanol feeding (2.5% or 5%) in a concentration-dependent manner. In addition, ethanol attenuated alpha-methyldopa-evoked reduction in heart rate, but not blood pressure, variability in marked contrast to attenuating rilmenidine-evoked reductions in blood pressure, but not heart rate, variability. These findings demonstrate that, unlike its acute effects, chronic ethanol attenuates both imidazoline I(1) receptor and alpha(2)-adrenoceptor-mediated hypotension whereas its effect on hemodynamic variability depended on the nature of the hypotensive stimulus.

Effects of centrally acting antihypertensive drugs on the microcirculation of spontaneously hypertensive rats

We investigated the acute effects of centrally acting antihypertensive drugs on the microcirculation of pentobarbital-anesthetized spontaneously hypertensive rats (SHR). The effects of the sympatho-inhibitory agents clonidine and rilmenidine, known to activate both alpha2-adrenoceptors and nonadrenergic I1-imidazoline binding sites (I1BS) in the central nervous system, were compared to those of dicyclopropylmethyl-(4,5-dimethyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride (LNP 509), which selectively binds to the I1BS. Terminal mesenteric arterioles were observed by intravital microscopy. Activation of the central sympathetic system with L-glutamate (125 microg, ic) induced marked vasoconstriction of the mesenteric microcirculation (27 +/- 3%; N = 6, P < 0.05). In contrast, the marked hypotensive and bradycardic effects elicited by intracisternal injection of clonidine (1 microg), rilmenidine (7 microg) and LNP 509 (60 microg) were accompanied by significant increases in arteriolar diameter (12 +/- 1, 25 +/- 10 and 21 +/- 4%, respectively; N = 6, P < 0.05). The vasodilating effects of rilmenidine and LNP 509 were two-fold higher than those of clonidine, although they induced an identical hypotensive effect. Central sympathetic inhibition elicited by baclofen (1 microg, ic), a GABA B receptor agonist, also resulted in vasodilation of the SHR microvessels. The acute administration of clonidine, rilmenidine and LNP 509 also induced a significant decrease of cardiac output, whereas a decrease in systemic vascular resistance was observed only after rilmenidine and LNP 509. We conclude that the normalization of blood pressure in SHR induced by centrally acting antihypertensive agents is paralleled by important vasodilation of the mesenteric microcirculation. This effect is more pronounced with substances acting preferentially (rilmenidine) or exclusively (LNP 509) upon I1BS than with those presenting important alpha2-adrenergic activity (clonidine).

[The effects of rilmenidine on cardiac autonomic function in healthy volunteers]

INTRODUCTION AND OBJECTIVES

Rilmenidine is an antihypertensive drug whose antihypertensive effect occurs via a sympatholytic action on the central nervous system. However, the effects of rilmenidine on autonomic cardiovascular function are not clear. The aim of this study was to evaluate the acute effect of rilmenidine on autonomic cardiac function by measuring heart rate variability.

SUBJECTS AND METHOD

A total of 20 healthy men (mean age, 263 years) were included in the study; 1 mg of rilmenidine or placebo was given to participants on different days in a double-blind crossover randomized study protocol. After drug administration, time domain and frequency domain parameters of heart rate variability were determined before and after 2 h with the patient in supine decubitus and during the handgrip exercise with 5-min electrocardiographic recordings.

RESULTS

Rilmenidine caused an increase in mean RR values after administration when compared to pre-drug administration recordings with the patient in supine decubitus (929 ms vs 860 ms, P<.05), but this effect was not found in the placebo group. However, there were no differences in other time domain parameters or in any of the frequency domain parameters (normalized low frequency unit, normalized high frequency unit and low frequency/high frequency ratio) with the participant in supine position in either group. In addition, neither rilmenidine nor placebo modified heart rate variability parameters during the handgrip exercise.

CONCLUSION

Administration of a single dose of rilmenidine increased vagal tone without affecting vagal modulation in the supine position. The absence of vagal tone increase during the handgrip exercise suggests that this effect of rilmenidine is minimal.

[Rilmenidine sympatholytic activity preserves mental and orthostatic sympathetic response and epinephrine secretion]

BACKGROUND

Heightened central sympathetic nervous outflow is common in essential hypertension, contributing to hypertension development and perhaps also to complications. Acute sympathetic nervous activation is a proven trigger for adverse cardiovascular events. Accordingly, antihypertensive drugs inhibiting sympathetic outflow represent a theoretically attractive therapeutic option.

OBJECTIVES

To study the sympatholytic and blood pressure lowering activity of the imidazoline binding agent rilmenidine at rest and during reflex sympathetic activation.

DESIGN AND METHODS

The HERA study (Hyperium Effect on the sympathetic Reflex activation and Adrenaline) is a randomised, double-blind, 6-week cross-over trial, with a 1-week placebo run-in period, two 2-week active treatment intervals (rilmenidine 1 mg bid, placebo) and intervening one week placebo wash-out. In 15 hypertensive patients, noradrenaline and adrenaline plasma kinetics and intra-arterial blood pressure measurements were performed at rest, after mental stress (difficult mental arithmetic) and during head-up tilting, at the end of the 2-week dosing periods.

RESULTS

The noradrenaline spillover rate, indicative of whole body sympathetic activity, was reduced 35% by rilmenidine at rest (p<0.01) and remained significantly lower during mental stress and tilting, although the increases in noradrenaline spillover with both stimuli were preserved. The effects on intraarterial blood pressure ran in parallel, a fall in supine resting pressure, but no reduction in BP rise during mental stress and a lack of fall in BP with tilting. On placebo, adrenaline secretion was 162 +/- 27 ng/min (mean, SE) at rest, increased by 77 +/- 42 ng/min with mental stress (p=0.019) and was unchanged with tilting. Rilmenidine left adrenaline secretion untouched under all conditions.

CONCLUSIONS

This study confirms a sympatholytic effect of rilmenidine during supine rest but demonstrates that sympathetic responses during mental stress and tilting are preserved, the latter underlying a perhaps surprising absence of postural hypotension on the drug. The absence of suppression of reflexive sympathetic responses contrasts with the effects of rilmenidine in experimental animals, and emphasises the previously demonstrated unique importance in humans of suprabulbar noradrenergic neuronal projections from the brainstem, which are inhibited by imidazoline binding agents, in regulating tonic sympathetic activity in essential hypertension. Sympathetic nervous inhibition with rilmenidine contrasted with an absence of suppression of the secretion of adrenaline affirming that here, as elsewhere, sympathetic nervous and adrenal medullary function can be disconnected.

Chronic treatment with rilmenidine in spontaneously hypertensive rats: differences between two schedules of administration

Rilmenidine is one of the lead compounds of the second generation of centrally acting antihypertensive drugs. In the first part of this study, 2 routes of administration of chronic treatment (1 month) with rilmenidine were compared. In conscious and pentobarbital-anesthetized spontaneously hypertensive rats (SHR), rilmenidine was delivered intraperitoneally either 250 microg/kg b.i.d. or 500 microg/kg/d infusion by means of minipumps. The possibility of rilmenidine-induced desensitization of central (brain cortex) and/or peripheral (kidney) alpha2-adrenoreceptors was studied in saturation experiments with the classic alpha2-adrenergic antagonist [H]rauwolscine. In the second part of this study, the cardiovascular and cardiac antihypertrophic effects of the most efficient procedure were investigated. The discontinuous administration of the drug was more effective than infusion. In rats treated with rilmenidine b.i.d., mean blood pressure was reduced by nearly 15% when no reduction occurred in SHRs treated with minipumps. With the first schedule of administration, plasma concentration of the drug reached a maximum of approximately 30 ng/ml when it was only 12 ng/ml with the continuous infusion of the same dose. Anesthesia with pentobarbital potentiated the antihypertensive effect of rilmenidine in rats treated discontinuously and unmasked an antihypertensive action in rats receiving the drug with minipumps. In saturation binding experiments, no significant changes in adrenergic receptors were observed in kidney membrane preparations. In contrast, in brain cortical membranes a reduction by about 50% of the Bmax of [H]rauwolscine value was observed in rats treated discontinuously with rilmenidine. In contrast, a 400% increase of the Bmax was observed in the brain of rats treated with minipumps. Over the one-month period of the second study, the discontinuous treatment with the 500 microg/kg/d dose of rilmenidine was still able to reduce blood pressure, at least at the peak concentration time, but did not induce any significant reduction of the ventricular mass. In conclusion, rilmenidine has only weak antihypertensive effects in conscious SHRs, even at doses higher than those that are active in rabbits and humans. As a consequence, it lacks significant cardiac antihypertrophic effects in this species. Pharmacokinetic data show that the rapid plasma withdrawal of this drug may explain this particular feature in rats.

Pharmacokinetic/pharmacodynamic assessment of tolerance to central nervous system effects of a 3 mg sustained release tablet of rilmenidine in hypertensive patients

Previous single-dose studies have shown clear blood pressure-lowering effects of a potential sustained release (SR) profile of rilmenidine, with concentration-dependent effects on the central nervous system. The aim of this study was to evaluate potential changes in concentration-effect-relationships for these central nervous system effects during a 4-week treatment period with an experimental SR formulation of rilmenidine 3 mg once daily in 15 mild-to-moderate hypertensive patients. The central nervous system effects of the treatment were evaluated using saccadic eye movements for sedative effects and visual analogue scales for subjective effects on alertness, mood and calmness. Measurements for pharmacokinetic and pharmacodynamic evaluations were performed on the first day of the treatment period and repeated after 1 week and 4 weeks of treatment. Drug concentrations increased during the study, whereas treatment related reductions in saccadic peak velocity (SPV) remained similar on all three study days. The slopes of the concentration-effect-curves for SPV remained unchanged throughout the study, while the intercepts tended to increase as a result of increased pre-dose values. Similar effects were observed for visual analogue scales for alertness: pre-dose values increased significantly during the study, while the size of the treatment responses (slopes) remained unaltered. The reasons for these adaptations cannot be determined but may include drug tolerance and habituations to study procedures. Blood pressure control remained stable and adequate throughout the study.

Differential modulation by estrogen of alpha2-adrenergic and I1-imidazoline receptor-mediated hypotension in female rats

We have recently shown that estrogen negatively modulates the hypotensive effect of clonidine (mixed alpha2-/I1-receptor agonist) in female rats and implicates the cardiovascular autonomic control in this interaction. The present study investigated whether this effect of estrogen involves interaction with alpha2- and/or I1-receptors. Changes evoked by a single intraperitoneal injection of rilmenidine (600 microg/kg) or alpha-methyldopa (100 mg/kg), selective I1- and alpha2-receptor agonists, respectively, in blood pressure, hemodynamic variability, and locomotor activity were assessed in radiotelemetered sham-operated and ovariectomized (Ovx) Sprague-Dawley female rats with or without 12-wk estrogen replacement. Three time domain indexes of hemodynamic variability were employed: the standard deviation of mean arterial pressure as a measure of blood pressure variability and the standard deviation of beat-to-beat intervals (SDRR) and the root mean square of successive differences in R-wave-to-R-wave intervals as measures of heart rate variability. In sham-operated rats, rilmenidine or alpha-methyldopa elicited similar hypotension that lasted at least 5 h and was associated with reductions in standard deviation of mean arterial pressure. SDRR was reduced only by alpha-methyldopa. Ovx significantly enhanced the hypotensive response to alpha-methyldopa, in contrast to no effect on rilmenidine hypotension. The enhanced alpha-methyldopa hypotension in Ovx rats was paralleled with further reduction in SDRR and a reduced locomotor activity. Estrogen replacement (17beta-estradiol subcutaneous pellet, 14.2 microg/day, 12 wk) of Ovx rats restored the hemodynamic and locomotor effects of alpha-methyldopa to sham-operated levels. These findings suggest that estrogen downregulates alpha2- but not I1-receptor-mediated hypotension and highlight a role for the cardiac autonomic control in alpha-methyldopa-estrogen interaction.

Effect of ethanol on reductions in norepinephrine electrochemical signal in the rostral ventrolateral medulla and hypotension elicited by I1-receptor activation in spontaneously hypertensive rats

BACKGROUND

The mechanism of the antagonistic hemodynamic interaction between ethanol and centrally acting sympatholytics is not known. In this study, we tested the hypothesis that the imidazoline (I1)-receptor modulation of norepinephrine (NE) release within the rostral ventrolateral medulla (RVLM) plays a pivotal role in this clinically relevant hemodynamic interaction. METHOD In anesthetized spontaneously hypertensive rats, the effects of centrally acting sympatholytics on RVLM NE electrochemical signal were investigated by in vivo electrochemistry along with cardiovascular responses in the absence and presence of ethanol. In vivo microdialysis in conscious spontaneously hypertensive rats was used to confirm the electrochemical findings. RESULTS Clonidine (30 microg/kg, intravenously) or rilmenidine (400, 600, or 800 microg/kg) significantly reduced RVLM NE electrochemical signal (index of neuronal activity) and mean arterial pressure; rilmenidine effects were dose-related, and ethanol (1 g/kg) counteracted these responses. Ethanol (1 g/kg) pretreatment increased both RVLM NE electrochemical signal and blood pressure but did not influence the reductions in both variables elicited by subsequently administered clonidine. The alpha2-adrenergic antagonist 2-methoxyidazoxan (30 microg/kg) counteracted rilmenidine (800 microg/kg)-evoked responses. In vivo microdialysis in conscious spontaneously hypertensive rats confirmed the electrochemical findings since clonidine- (30 microg/kg, intravenously) evoked reductions in RVLM NE and the associated hypotension were counteracted by ethanol (1 g/kg). CONCLUSIONS (1) Ethanol counteracts centrally mediated hypotension, at least in part, by increasing RVLM NE; (2) the interaction involves the I1 receptor modulation of RVLM neuronal activity; (3) the alpha2-adrenergic receptor contributes to the electrochemical and cardiovascular effects of high doses of rilmenidine, and (4) the RVLM is a neuroanatomical target for systemically administered ethanol.

Moxonidine and rilmenidine injected into the medial septal area reduces the salivation induced by pilocarpine

We determined the effects of moxonidine and rilmenidine 20 nmol (alpha(2)-adrenergic and imidazoline receptor agonists) injected into the medial septal area (MSA) on the pilocarpine-induced salivation, when injected intraperitoneally (i.p.), of male Holtzman rats weighing 250-300 g, with stainless-steel cannula implanted into the MSA. The rats were anesthetized with zoletil 50 mg kg(-1) b.wt. (tiletamine chloridrate 125.0 mg and zolazepan chloridrate 125.0 mg) into quadriceps muscle intramuscularly (IM), saliva was collected using pre-weighed small cotton balls inserted in the animal's mouth. The pre-treatment with moxonidine injected into the MSA reduced the salivation induced by pilocarpine (1 mg kg(-1)) injected i.p. (12+/-3 mg min(-1)) vs. control (99+/-9 mg min(-1)). The pre-treatment with rilmenidine 40 nmol also reduced the salivation induce by pilocarpine injected i.p. (20+/-5 mg min(-1)) vs. control (94+/-7 mg min(-1)). Idazoxan 40 nmol (imidazoline receptor antagonist) injected into the MSA previous to moxonidine and rilmenidine partially blocked the effect of moxonidine and totally blocked the rilmenidine effect in pilocarpine-induced salivation injected i.p. (60+/-8 and 95+/-10 mg min(-1), respectively). Yohimbine 40 nmol (alpha(2)-adrenergic receptor antagonist) injected into the MSA previously to moxonidine and rilmenidine partially blocked the moxonidine effect but produced no change on the rilmenidine effect on i.p. pilocarpine-induced salivation (70+/-6 and 24+/-6 mg min(-1), respectively). Injection of these alpha(2)-adrenergic and imidazoline agonists and antagonists agents i.p. produced no change on i.p. pilocarpine-induced salivation. These results show that central, but not peripheral, injection of alpha(2)-adrenergic and imidazoline agonists' agents inhibit pilocarpine-induced salivation. Idazoxan, an imidazoline receptor antagonist, totally inhibits the rilmenidine effect and partially inhibits the moxonidine effect on pilocarpine-induced salivation. Yohimbine produced no change on rilmenidine effect but partially inhibited the moxonidine effect. Both of these antagonists when injected into the MSA previous to pilocarpine i.p. potentiated the sialogogue effect of pilocarpine. The results suggest that alpha(2)-adrenergic/imidazoline receptor of the MSA when stimulated blocked pilocarpine-induced salivation in rats when injected intraperitonially. These receptors of the medial septal area have an inhibitory mechanism on salivary secretion.

EFFECTS OF RILMENIDINE ON PROXIMAL TUBULAR FLUID ABSORPTION IN RATS

The antihypertensive agent rilmenidine has threefold higher affinity for I(1) imidazoline receptors compared with alpha(2)-adrenoceptors and acts on the central nervous system by reducing sympathetic activity and in the kidney by inhibiting Na(+)/H(+) exchange activity. In the present study, we examined: (i) the effects of luminal and peritubular administration of rilmenidine on fluid absorption in superficial proximal tubules; and (ii) the nature of the receptors involved in mediating the action of this drug in the presence of specific antagonists (efaroxan, idazoxan and 2-methoxy-idazoxan). Studies were performed in anaesthetized Sprague-Dawley rats using shrinking split-drop micropuncture. Luminal administration of rilmenidine (10(-5) and 10(-13) mol/L) inhibited proximal tubular fluid absorption. Peritubular rilmenidine at 10(-12) and 10(-13) mol/L also inhibited fluid uptake, whereas rilmenidine at 10(-11) mol/L had a significant stimulatory action. In the presence of the I(2) > I(1)/alpha(2)-adrenoceptor antagonist idazoxan (10(-5) mol/L), luminal rilmenidine (10(-5) mol/L) stimulated fluid absorption. Stimulation of fluid uptake was also observed when rilmenidine (10(-5) mol/L) and the I(1) imidazoline receptor antagonist efaroxan (10(-5) mol/L) were added together in the luminal fluid. Luminal administration of the selective alpha(2)-adrenoceptor antagonist 2-methoxy-idazoxan (10(-5) mol/L) resulted in significant attenuation of the inhibitory action of luminal rilmenidine (10(-5) mol/L). This indicates that both I(1) imidazoline receptors and alpha(2)-adrenoceptors are involved in the luminal actions of rilmenidine. The effects of luminal and peritubular administration of alpha-methylnoradrenaline (an alpha(2)-adrenoceptor agonist) were compared with those of rilmenidine. Luminal alpha-methylnoradrenaline, at higher concentrations (10(-7) and 10(-5) mol/L), inhibited fluid absorption, as was seen with peritubular rilmenidine, but, in contrast with rilmenidine, no stimulatory action was observed. Peritubular alpha-methylnoradrenaline inhibited fluid uptake at higher concentrations (10(-5) and 10(-7) mol/L), whereas rilmenidine at these concentrations had no effect. The differences in the concentration-dependent responses for rilmenidine and alpha-methylnoradrenaline indicate that both imidazoline receptors and alpha(2)-adrenoceptors are involved in the actions of these compounds on proximal fluid uptake.

Apparent absence of direct renal effect of imidazoline receptor agonists

Imidazoline receptor agonists such as moxonidine and rilmenidine increase sodium excretion whether administered within the central nervous system, intravenously, or directly into the renal artery. To determine if this natriuresis was mediated by a direct renal effect and was independent of the renal sympathetic nerves, we used two different preparations in the pentobarbital-anesthetized rat. In the first series of studies, rats were unilaterally nephrectomized 7 to 10 days before the experiment. On the day of the experiment, the remaining kidney was denervated (surgical and 10% phenol/ 95% ethyl alcohol) or sham treated. The effect of an intravenous infusion of rilmenidine was determined. Rilmenidine (10 nmol/kg/minute) decreased blood pressure and increased urine flow rate and sodium excretion in the sham- but not the denervation-treated rats. The response to furosemide (5.05 nmol/kg/minute) remained intact following denervation. We then used a two-kidney rat model that allowed for separate urine collection from each ureter. We used low infusion rates of moxonidine directly into the left renal artery. An increase in urine flow rate from the left but not the right kidney would suggest a direct renal action. Low infusion rates of moxonidine (10, 30 nmol/kg/minute) increased urine flow rate similarly from both ureters. A low infusion rate of furosemide (9.1 nmol/kg/minute) into the left renal artery increased urine flow rate only from the left ureter. The failure of moxonidine to increase urine flow rate selectively only in the left kidney indicated the agonist acts at an extrarenal site to increase urine flow rate from both kidneys equally. The complete attenuation of the response to rilmenidine indicates the importance of the renal nerves and suggests that the extrarenal site is most probably the central nervous system. Collectively, these studies do not support a direct renal action of imidazoline agonists in producing natriuresis.

Alpha(2A)-adrenergic versus imidazoline receptor controversy in rilmenidine's action: alpha(2A)-antagonism in humans versus alpha(2A)-agonism in rabbits

At the alpha(2A)-autoreceptors on the sympathetic nerve terminals of the human atrial appendages and rabbit pulmonary artery, rilmenidine and oxymetazoline exhibit different properties (antagonism and agonism, respectively). These opposite pharmacodynamic properties of alpha(2)-adrenoceptor ligands seem to be due to substantial differences in the nucleotide and amino acid sequences between human and rabbit alpha(2A)-adrenoceptors. Hence, the rabbit alpha(2A)-adrenoceptor is not reliably predictive for the action of ligands at the human alpha(2A)-adrenoceptor.

Are centrally acting imidazoline agents appropriate therapy for renovascular hypertension?

An increased role of the sympathetic nervous system has been suggested to be a major contributor to the chronic elevation of BP in renovascular hypertension. We assessed the effects of rilmenidine, a centrally acting antihypertensive imidazoline agent, on BP and renal sympathetic nerve activity (RSNA) in 2K1C renovascular hypertensive conscious rabbits. Rabbits were made hypertensive with a renal clip or were sham-operated and were studied 3 and 6 weeks later. Acute treatment with rilmenidine reduced BP to a greater extent in the hypertensive rabbits. Although rilmenidine reduced the heart rate by the same extent in all groups, rilmenidine produced much less inhibition of RSNA in the hypertensive animals. These studies suggest that the contribution of the sympathetic nervous system is greater in 2K1C hypertension and that imidazoline agents may be beneficial in treating renovascular hypertension.