Rilmenidine in the hypertensive type-2 diabetic: a controlled pilot study versus captopril

BACKGROUND

Rilmenidine is an innovative antihypertensive agent that binds specifically to I1 imidazoline receptors. The antihypertensive efficacy of rilmenidine in treating type-2 diabetics has been demonstrated, and is associated with very good clinical and laboratory tolerance.

DESIGN

This was a 6-month, double-blind, randomized, controlled study comparing the effects of rilmenidine and captopril on the progression of microalbuminuria in a population of patients with mild-to-moderate hypertension [90 mmHg < diastolic blood pressure (DBP) < 110 mmHg], type-2 diabetes, and microalbuminuria (30 mg/24 h < urine albumin excretion < or = 300 mg/24 h).

RESULTS

Between month 0 and month 6, the mean supine blood pressure was reduced in a similar manner by rilmenidine (systolic blood pressure from 159 to 141 mmHg and diastolic blood pressure from 98 to 84 mmHg) and captopril (systolic blood pressure from 157 to 144 mmHg and diastolic blood pressure from 101 to 82 mmHg). The median value for microalbuminuria was reduced from 160 (90-260) to 56 (27-87) mg per 24 h by rilmenidine and from 144 (51-200) to 54 (41-123) mg per 24 h by captopril. Rate of clearance of creatinine was not significantly changed during the study by either treatment (with rilmenidine it varied from 95.2 to 95.6 ml/min; with captopril it varied from 86.2 to 90.4 ml/min). There was no statistical difference between the changes in levels of glycosylated hemoglobin for the groups treated with rilmenidine and captopril. Clinical and laboratory acceptabilities were good, and those for the two groups were comparable.

CONCLUSION

Rilmenidine exerts similar antihypertensive effects to those of captopril on the hypertensive with type-2 diabetes. Decreases in microalbuminuria elicited by the two treatments do not differ. That administration of rilmenidine decreases microalbuminuria suggests that it could exert nephroprotective effects.

[Imidazole receptor agonists--a new advance in the treatment of hypertension?]

Agonists of I1 imidazolin receptors are a new drug groups which was registered for the treatment of hypertension. Their antihypertensive action is comparable with current antihypertensives (hydrochlorothiazide, enalapril, atenolol, nifedipine retard) and causes a drop of the systolic BP by cca 15-20 mm Hg and a drop of the diastolic BP by 10-15 mm Hg with a probable normalization of the blood pressure in cca 60% patients with mild to moderate hypertension. Agonists of I1 imidazoline receptors are suitable in particular for the treatment of hypertension associated with metabolic syndrome. Their effect in patients with ischaemic heart disease or after a cerebrovascular attack is not known and despite very promising theoretical prerequisites they are not indicated in patients with chronic heart failure.

Imidazoline receptor agonist drugs for treatment of systemic hypertension and congestive heart failure

The imidazoline receptors recently have been discovered to be involved in the central nervous system control of sympathetic outflow. A new class of centrally acting antihypertensive agents, the imidazoline receptor agonists (rilmenidine and moxonidine), have been developed to control blood pressure effectively without the adverse effects of sedation and mental depression that usually are associated with centrally acting antihypertensive agents. This new generation of centrally acting antihypertensive agents is highly selective for the imidazoline receptor but has a low affinity for alpha(2)-adrenergic receptors. The usefulness of these agents in the treatment of congestive heart failure has not been demonstrated.

Drugs acting on imidazoline receptors: a review of their pharmacology, their use in blood pressure control and their potential interest in cardioprotection

Drugs acting within the autonomic nervous system are of particular interest when autonomic abnormalities are implicated in the development and maintenance of various cardiovascular pathologies. For example, it has been documented that in the early stages of hypertensive disease, i.e. hyperkinetic borderline hypertension, a sympathetic hyperactivity associated with a decreased parasympathetic activity results in increased cardiac output and heart rate. Several classes of drugs acting within the central, as well as the peripheral, autonomic nervous system are very efficient in treating hypertensive disease. One class - the second generation of a group of centrally acting drugs selective for imidazoline receptors - has proved beneficial in this respect, because drugs in this class are well tolerated and have interesting additional effects such as their antiarrhythmic action. Rilmenidine and moxonidine are the lead compounds of this class of drugs. Rilmenidine and moxonidine both proved more selective for cerebral imidazoline receptors than the reference drug, clonidine. It was suggested that this selectivity, attributable to their lower affinity for alpha2-adrenoceptors, explains the low incidence of adverse effects (including sedation) associated with these drugs. In addition, potentially beneficial actions on cardiac dysrythmias and congestive heart failure enlarge the therapeutic potential of the second generation of imidazoline-related drugs. This review focuses on the main pharmacological and clinical properties of rilmenidine and moxonidine, paying particular attention not only to their efficacy in hypertension but also to other potential cardiovascular indications.

Cardiovascular effects of clonidine-like drugs in pithed rabbits

Administration (3 to 100 microg/kg IV) of clonidine, rilmenidine, and an imidazoline derivative, 2-(2-chlorophenylamino)imidazoline, in pithed nonstimulated rabbits caused a dose-dependent increase in mean arterial pressure without affecting heart rate. Prazosin (0.1 mg/kg IV) almost abolished the pressor responses to 2-(2-chlorophenylamino)imidazoline, partially inhibited those induced by clonidine, but failed to affect those elicited by rilmenidine. In contrast, yohimbine (1 mg/kg IV) blunted the pressor responses of the 3 drugs. In sympathetically stimulated pithed rabbits, 2-(2-chlorophenylamino)imidazoline induced only pressor effects, whereas clonidine and rilmenidine caused a transient pressure increase followed by a dose-dependent depressor effect. Yohimbine abolished the depressor effect of both drugs, which may have involved presynaptic alpha(2)-adrenoceptors. In conclusion, peripheral effects of 2-(2-chlorophenylamino)imidazoline and clonidine involved at least alpha(1)- and alpha(2)-adrenoceptor activation, whereas pressor and depressor effects of rilmenidine were mediated by alpha(2)-adrenoceptors.

Effects of rilmenidine on stress-induced peak blood pressure and renal function

Rilmenidine is an imidazoline I1-receptor agonist that centrally acts by reducing the sympathetic tone. There is strong experimental evidence that natriuresis could be evoked by proximal tubular I1-receptors that have also been isolated in human kidneys. However, in humans, the natriuretic effects of proximal tubular I1-receptors have never been demonstrated. Because stress tests elicited a sympathetically mediated increase in blood pressure and in sodium reabsorption, this study examined whether a short-term infusion of rilmenidine (1 mg) may interfere with stress-induced cardiovascular response and renal sodium handling in normotensive men, in a double-blind, crossover, placebo-controlled study. The stress test used is an efficient and reproducible computerized version of the Stroop's stress test. During the experimental sessions, both basal and stress renal functional parameters were determined: glomerular filtration rate, renal plasma flow, filtration fraction, sodium excretion, and segmental sodium tubular reabsorption (lithium clearance). During the placebo phase, stress induced a significant increase in systolic blood pressure (SBP; 22.2+/-10.1 mm Hg) and diastolic blood pressure (DBP; 11.0+/-5.0 mm Hg). During stress, glomerular filtration rate and renal plasma flow tended to decrease, resulting in a nonsignificant increase in filtration fraction. Despite the increase in BP, stress induced a significant decrease in sodium excretion that was due mainly to a nonsignificant increase in sodium reabsorption in the proximal parts of the tubules. Rilmenidine significantly reduced rest and stress BP, but the cardiovascular reactivity to stress was not altered. The treatment slightly decreased basal glomerular filtration rate and increased renal plasma flow, so that the filtration fraction significantly decreased. The treatment-related decrease in BP was associated with a significant increase in basal sodium reabsorption. Stress-induced modifications in renal function and sodium handling were not altered by the treatment. In conclusion, rilmenidine reduced rest BP and preserved stress-induced reactivity in BP and heart rate. Renal effects of rilmenidine are characterized by a decrease in glomerular filtration rate and in filtration fraction and an increase in sodium reabsorption. The study failed to demonstrate any effect of rilmenidine on stress-induced increase in sodium reabsorption.

Identification of imidazoline-receptor binding sites in cortex and medulla of the bovine adrenal gland. Colocalization with MAO-A and MAO-B

The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) in bovine adrenal gland were determined using [3H]clonidine, [3H]2-(2-benzofuranyl)-2-imidazoline ([3H]2-BFI), and [3H]rilmenidine. In light of strong evidence that I-RBS and monoamine amine oxidase enzymes are linked, the selective radioligands [3H]RO41-1049 and [3H]RO19-6327 were used to label the distribution of MAO-A and -B enzymes, respectively. [3H]Clonidine (12 nM) labeled sites in two discrete regions of the bovine adrenal gland, the zona glomerulosa (39 +/- 7 fmol/mg tissue equivalent) and inner medulla (34 +/- 1 fmol/mg tissue). Binding was nonadrenergic (i.e., not inhibited by 100 nM methoxyidazoxan) and inhibited by 60-70% by 100 nM 2-BFI, the selective I2-RBS, suggesting binding predominantly to an I2-RBS. [3H]2-BFI (5 nM), the selective I2-RBS ligand, also labeled a high density of binding sites in the zona glomerulosa (57 +/- 9 fmol/mg) and chromaffin cells in the inner medulla (53 +/- 4 fmol/mg). These sites, however, were insensitive to clonidine (100 nM). By contrast, [3H]rilmenidine (40 nM) labeled I-RBS in all regions of the adrenal gland, that is, the zonae glomerulosa (59 +/- 10 fmol/mg), fasciculata (78 +/- 10 fmol/mg) and reticularis (63 +/- 7 fmol/mg), and outer and inner medullary chromaffin cells (42 +/- 1 and 55 +/- 2 fmol/mg, respectively). Binding to sites in the zona glomerulosa was partially inhibited (16%) by 100 nM 2-BFI. These results are consistent with previous studies indicating that [3H]rilmenidine labels an I2-RBS and additional I-RBS in rat brain and kidney. The distribution of [3H]RO19-6327 (5 nM) binding resembled that of [3H]2-BFI and [3H]clonidine binding with high densities of MAO-B enzyme located in the zona glomerulosa and chromaffin cells of the inner medulla (55 +/- 7 and 76 +/- 6 fmol/mg tissue, respectively), suggesting the colocalization of MAO-B enzyme with I2-RBS. [3H]RO41-1049 (20 nM) binding to MAO-A was highest in the zona reticularis (196 +/- 7 fmol/mg tissue) compared to the zonae glomerulosa and fasciculata (90 +/- 12 and 116 +/- 14 fmol/mg tissue) and inner medulla (149 +/- 38 fmol/mg tissue). Although the existence of I-RBS in bovine adrenal chromaffin cells is well established, this is the first description of I-RBS in the adrenal cortex. Further investigations are now required to determine whether imidazolines can affect adrenal function via actions at these sites.

Mechanism of the sympathoinhibition produced by the clonidine-like drugs rilmenidine and moxonidine

The mechanism of the sympathoinhibition produced by two new derivatives of clonidine, rilmenidine and moxonidine, was studied. One aim was to determine the receptor responsible for the central sympathoinhibition by these drugs. Rilmenidine and moxonidine were injected into the cisterna cerebellomedullaris. They decreased blood pressure and the plasma noradrenaline concentration. After rilmenidine and moxonidine, two selective alpha 2-adrenoceptor antagonists (devoid of affinity for I1 binding sites), yohimbine and SK&F86466, were given intracisternally. They completely counteracted the hypotensive effects of rilmenidine and moxonidine, indicating that alpha 2-adrenoceptors are involved in the central sympathoinhibition produced by these drugs. The other aim was to determine if peripheral presynaptic inhibition of noradrenaline release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone. Rilmenidine and moxonidine were injected i.v. in pithed rabbits with electrically stimulated sympathetic outflow. They dose-dependently lowered blood pressure and the plasma noradrenaline concentration and inhibited stimulation-evoked cardioacceleration. Moreover, the doses necessary for these peripheral effects were identical to the doses that reduce the sympathetic nerve firing rate and blood pressure in conscious rabbits. These observations indicate that peripheral presynaptic inhibition of noradrenaline release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by rilmenidine and moxonidine in intact animals.

Central imidazoline- and alpha 2-receptors involved in the cardiovascular actions of centrally acting antihypertensive agents

There has been a continuing and yet unresolved debate concerning the existence and contribution of imidazoline receptors to the antihypertensive actions of clonidine-like agents. Studies from our laboratory have examined the importance of imidazoline receptors and alpha 2-adrenoceptors in the mechanism of action of centrally acting antihypertensive drugs. We used conscious rabbits and imidazoline and specific alpha 2-adrenoceptor antagonists to show that second-generation agents rilmenidine and moxonidine act preferentially through imidazoline receptors but that alpha 2-adrenoceptors are important for the hypotension produced by clonidine and alpha-methyldopa. Using microinjections of the imidazoline antagonists into the rostral ventrolateral medulla (RVLM) of anesthetized rabbits we confirmed the generally held view that this is the major site of sympathoinhibitory actions of centrally acting antihypertensive agents. However, we also found that alpha 2-adrenoceptors are present in this nucleus and appear to be activated as a consequence of imidazoline receptor activation. In recent studies using a noradrenergic neurotoxin microinjected into the RVLM we found that this treatment selectively blocked the actions of moxonidine but did not affect the level of imidazole proteins, suggesting that I1-imidazoline receptors may be located presynaptic to the noradrenergic terminal. By contrast, clonidine acts directly on the alpha 2-adrenoceptors perhaps located on cell bodies in the nucleus. In conclusion, our studies suggest that imidazoline receptors and alpha 2-adrenoceptors within the RVLM are important for the antihypertensive actions of clonidine-like drugs.

Clinical pharmacology of moxonidine and rilmenidine

Moxonidine and rilmenidine are moderately selective I1-receptor stimulants. The imidazoline (I1) agonists cause peripheral sympathoinhibition, triggered at the level of central nervous imidazoline receptors. Imidazoline receptor stimulants are effective antihypertensive agents with a hemodynamic profile that is attractive from a pathophysiologic point of view. The antihypertensive activity of these agents is caused by vasodilatation and reduced peripheral vascular resistance. Left ventricular end-diastolic and end-systolic volume is reduced, whereas heart rate, stroke volume, cardiac output, and pulmonary artery pressures are largely unchanged. Long-term left ventricular hypertrophy is reduced. Both drugs, when applied in a once-daily dosage schedule, appear to control hypertension in most patients. Both drugs have been compared with representative agents from the major classes of antihypertensive drugs in controlled trials and found to be equally effective in blood pressure control. The incidence and severity of side effects are lower than those for clonidine, particularly with respect to sedation. A rebound (withdrawal) phenomenon has so far not been reported for moxonidine and rilmenidine. Therefore, I1-receptor stimulants offer the possibility of developing centrally acting agents with a better side-effect profile than do the classic alpha 2-adrenoceptor stimulants.

[New, modern, centrally active antihypertensive agents in the treatment of essential hypertension]

It was recently found that the rise of blood pressure leads to the excitation of a vasomotor centre in the brain stem and that the accompanying decrease in brain cortex excitability results in the reduced sensitivity to various adverse stimuli. Centrally acting antihypertensives, moxonidine and rilmenidine, do not impair circulatory reflexes and therefore do not deprive the patient of a chance to resist the pressure; thus the compliance of the patient might be increased. Both drugs activate I1-imidazoline receptors on the neurons of the rostral ventrolateral medulla oblongata. The reduction of neuronal firing rate results in the decrease of sympathetic activity and arterial pressure. Beside other advantages, centrally acting antihypertensives might be more promising than peripherally acting drugs due to their possible more favourable psychopharmacological profile; this component of their action might be underestimated at present.

Inhibition of water permeability in the rat collecting duct: effect of imidazoline and alpha-2 compounds

Arginine vasopressin (AVP) increases water permeability in the collecting duct of the nephron via activation of adenylyl cyclase. Alpha-2 (alpha2) agonists inhibit AVP-stimulated water permeability via binding to alpha2 adrenoceptors that have been divided into 3 subtypes- alpha2A, alpha2B, and alpha2C. Some biological effects mediated by alpha2 agonists result from nonadrenergic imidazoline receptors that exist in the rat kidney. Thus, alpha2-inhibition of AVP-stimulated water permeability in the rat collecting duct could be caused by imidazoline receptors. The purpose of this study was to test agonists and antagonists selective for alpha2 and imidazoline receptors on AVP-stimulated water permeability in the rat inner medullary collecting duct (IMCD). Some experiments were conducted where water permeability was stimulated by a nonhydrolyzable analog of adenosine 3', 5'-cyclic monophosphate (cAMP). Agonists included dexmedetomidine, clonidine, oxymetazoline, agmatine and rilmenidine. The latter two are selective imidazoline agonists. Antagonists included yohimbine, RX821002, atipamezole, prazosin, WB4101, idazoxan, and BU239. Prazosin and WB4101 demonstrate selectivity for the alpha2B and alpha2C subtypes, respectively, and oxymetazoline and RX821002 are selective for the alpha2A subtype. BU239 is selective for imidazoline receptors. Wistar rat terminal IMCDs were isolated and perfused to determine the osmotic water permeability coefficient (Pf). All agonists except agmatine inhibited AVP-stimulated Pf. Inhibition by rilmenidine indicated a different mechanism of action from other agonists. Dose-response data show dexmedetomidine to be the most potent inhibitor. Oxymetazoline and clonidine inhibited cAMP-stimulated Pf indicating that the mechanism involves postcAMP cellular events. It was reported previously that dexmedetomidine inhibits cAMP-stimulated Pf (1). All antagonists except prazosin and WB4101 reversed alpha2-inhibition of AVP-stimulated Pf. BU239 was effective at 1 microM but not at 100 nM. Results suggest that alpha2A adrenoceptors modulate water permeability in the IMCD. The involvement of imidazoline receptors is inconclusive.

Analysis of the receptor involved in the central hypotensive effect of rilmenidine and moxonidine

The aim of this study was to determine whether alpha2-adrenoceptors or imidazoline I1-receptors are responsible for the central sympathoinhibition produced by rilmenidine and moxonidine, two clonidine-like antihypertensive drugs. Rilmenidine and moxonidine were compared with the indirectly acting alpha2-adrenoceptor agonist alpha-methyldopa. Three antagonists were used. Yohimbine and SK&F86466 were used as selective alpha2-adrenoceptor antagonists. They were compared with efaroxan which is also an alpha2-adrenoceptor antagonist, but, in addition, possesses affinity for imidazoline I1-receptors. According to some but not all studies, the affinity of efaroxan for I1-receptors is much higher than its affinity for alpha2-adrenoceptors. Drugs were administered into the cisterna cerebellomedullaris of conscious rabbits by a catheter implanted previously under halothane anaesthesia. Rilmenidine (10 microg kg(-1)), moxonidine (0.3 microg kg(-1)) and alpha-methyldopa (0.4 mg kg(-1)) lowered blood pressure and the plasma noradrenaline concentration; the degree of sympathoinhibition produced by the three agonists was very similar. When injected after the agonists, efaroxan (0.1-14 microg kg(-1); cumulative doses), yohimbine (0.4-14 microg kg(-1)) and SK&F86466 (0.4-44 microg kg(-1)) counteracted the effects of the agonists on blood pressure and the plasma noradrenaline concentration. Efaroxan was about tenfold more potent than yohimbine and SK&F86466 at antagonizing the hypotensive effects of alpha-methyldopa. Similarly, efaroxan was two- to tenfold more potent than yohimbine and SK&F86466 against rilmenidine and moxonidine. Finally, efaroxan was about as potent against alpha-methyldopa as against rilmenidine and moxonidine. The results confirm previous observations that selective alpha2-adrenoceptor antagonists are capable of completely antagonizing effects of rilmenidine and moxonidine. The effects of the alpha2-adrenoceptor antagonist with an additional high affinity for imidazoline I1-receptors, efaroxan, can also be explained by blockade of alpha2-adrenoceptors. Efaroxan was more potent against rilmenidine and moxonidine than the selective alpha2-adrenoceptor antagonists. This was probably due to the fact that the affinity of efaroxan for alpha2-adrenoceptors is higher than the affinity of yohimbine and SK&F86466, since efaroxan was also the most potent of the three antagonists against the indirectly acting alpha2adrenoceptor agonist alpha-methyldopa. The observation that efaroxan was equally potent against rilmenidine and moxonidine and against alpha-methyldopa suggests that the same receptors were involved in the effects of the three agonists, alpha2-adrenoceptors; this observation is not compatible with the high I1/alpha2 selectivity of efaroxan and the hypothesis that rilmenidine and moxonidine activate I1-receptors, whereas alpha-methyldopa activates alpha2-adrenoceptors. Thus, the data do not indicate involvement of I1 imidazoline receptors in the central sympathoinhibition elicited by rilmenidine and moxonidine in rabbits. It is likely that rilmenidine and moxonidine produce sympathoinhibition by activating the same receptors which are activated by the indirectly acting catecholamine alpha-methyldopa, namely alpha2-adrenoceptors.

Cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor transgenic mice

1. We investigated the cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor mice. The in vitro pharmacology of these agonists was determined at recombinant (human) alpha2-adrenoceptors and at endogenous (dog) alpha2A-adrenoceptors. 2. In wild-type mice, rilmenidine, moxonidine (100, 300 and 1000 microg kg(-1), i.v.) and clonidine (30, 100 and 300 microg kg(-1), i.v.) dose-dependently decreased blood pressure and heart rate. 3. In D79N alpha2A-adrenoceptor mice, responses to rilmenidine and moxonidine did not differ from vehicle control. Clonidine-induced hypotension was absent, but dose-dependent hypertension and bradycardia were observed. 4. In wild-type mice, responses to moxonidine (1 mg kg(-1), i.v.) were antagonized by the non-selective, non-imidazoline alpha2-adrenoceptor antagonist, RS-79948-197 (1 mg kg(-1), i.v.). 5. Affinity estimates (pKi) at human alpha2A-, alpha2B- and alpha2C-adrenoceptors, respectively, were: rilmenidine (5.80, 5.76 and 5.33), moxonidine (5.37, <5 and <5) and clonidine (7.21, 7.16 and 6.87). In a [35S]-GTPgammaS incorporation assay, moxonidine and clonidine were alpha2A-adrenoceptor agonists (pEC50/intrinsic activity relative to noradrenaline): moxonidine (5.74/0.85) and clonidine (7.57/0.32). 6. In dog saphenous vein, concentration-dependent contractions were observed (pEC50/intrinsic activity relative to noradrenaline): rilmenidine (5.83/0.70), moxonidine (6.48/0.98) and clonidine (7.22/0.83). Agonist-independent affinities were obtained with RS-79948-197. 7. Thus, expression of alpha2A-adrenoceptors is a prerequisite for the cardiovascular effects of moxonidine and rilmenidine in conscious mice. There was no evidence of I1-imidazoline receptor-mediated effects. The ability of these compounds to act as alpha2A-adrenoceptor agonists in vitro supports this conclusion.

Ethanol counteraction of I1-imidazoline but not alpha-2 adrenergic receptor-mediated reduction in vascular resistance in conscious spontaneously hypertensive rats

Our recent findings have shown that ethanol selectively counteracts decreases in blood pressure (BP) evoked via activation of central I1-imidazoline receptors but not alpha-2 adrenoceptors in conscious spontaneously hypertensive rats (SHRs). This study investigated the role of sympathetic activity, cardiac output and total peripheral resistance (TPR) in the differential effect of ethanol on centrally mediated hypotension. Changes in plasma norepinephrine (NE), as index of sympathetic activity, BP, heart rate, cardiac index, stroke volume, and TPR elicited by rilmenidine or alpha-methylnorepinephrine (selective I1 and alpha-2 receptor agonists, respectively) and subsequent ethanol (0.5 or 1 g/kg) or saline, were evaluated in conscious SHRs. Intracisternal rilmenidine (25 microg) or alpha-methylnorepinephrine (alpha-MNE; 4 microg) elicited similar decreases in BP, TPR, and plasma NE, but cardiac index was not changed. Ethanol (0.5 g/kg i.v.) had no effect on hemodynamic responses to rilmenidine or alpha-MNE. The higher dose (1 g/kg i.v.) of ethanol counteracted the hypotensive response to rilmenidine and significantly (P <.05) elevated TPR and plasma NE. In contrast, ethanol (1 g/kg) had no effect on the hypotensive responses to alpha-MNE but significantly (P <.05) elevated plasma NE. However, this increase in NE was approximately one third of the increase evoked by ethanol when given after rilmenidine. These findings suggest that the selective counteraction by ethanol of the hypotension evoked via activation of central I1 but not alpha-2 receptors may relate, at least in part, to its greater ability to reverse the sympathoinhibition and the associated decrease in vascular resistance mediated by I1 receptors.

[Essential arterial hypertension treated with rilmenidin : what dose for what patient?]

OBJECTIVE

To study the therapeutic interest of dose adaptation from 1 mg to 2 mg daily when treating essential hypertension with rilmenidine in monotherapy, in terms of normalisation of blood pressure, of clinical tolerance and of laboratory parameters.

METHODS

In a non-comparative multicentre study conducted in France in patients with essential hypertension, rilmenidine was started at 1 mg daily. When the target blood pressure was not achieved (supine diastolic blood pressure (DBP) < or = 90 mm Hg) after 6 weeks, the dose was adapted to 2 mg daily. For persistent non-responders, a second antihypertensive was added from 3 months and a third from 6 months as necessary. Total follow up was for 12 months.

RESULTS

Eighteen thousand two hundred and thirty five (18,235) patients of either sex, average age 61.2 years and 90 mm Hg < supine DBP < 115 mm Hg were included. Sixteen thousand four hundred and ninety six (16,496) patients were followed for 12 months with complete data being available for 15,963 of them., 13,565 (84.9%) were treated with rilmenidine in monotherapy, 9,626 (60.3%) at 1 mg daily, and 3,939 (24.6%) at 2 mg daily. For patients who were treated with rilmenidine 1 mg daily throughout the study, mean reduction of blood pressure between D0 and M12 was -27.5/-18.9 mm Hg. Mean reduction was -30.0/-20.3 mm Hg for those who completed the study on 2 mg per day. 82.8% of the analysed population were normalised with rilmenidine in monotherapy, 59.1% with 1 mg daily. Dose adaptation to 2 mg normalised an additional 23.7% of patients. Rilmenidine monotherapy was associated with good clinical and laboratory acceptability.

CONCLUSION

The step-wise adaptation of rilmenidine dose in the treatment of essential hypertension is justified by the superior normalisation rate achieved by increasing the dose from 1 mg to 2 mg daily. This adaptation neither increases side effects nor changes laboratory parameters. These data support dose adaptation of rilmenidine monotherapy before association with another antihypertensive. However, neither ther blood pressure level nor associated risk factors predict the type of patients who will be normalised by 1 mg daily or those who require 2 mg. The practice of always starting rilmenidine at 1 mg daily in essential hypertension is therefore supported.

Influence of sino-aortic barodenervation on the cardiovascular effects of imidazoline-like drugs

Earlier findings have shown that hypotensive effects of centrally acting drugs, such as clonidine, are enhanced in animals after denervation of arterial baroreceptors. The purpose of this study was to investigate the dynamic of changes in arterial pressure, heart rate and hypotensive effects of clonidine, rilmenidine and moxonidine in Wistar rats after sino-aortic denervation (SAD) using radio-telemetry. SAD was followed by significant elevation of arterial pressure lability (the standard deviation of the mean arterial pressure), while the baseline mean arterial pressure (MAP) and heart rate in barodenervated rats (12 days after SAD) was similar to intact rats. The hypotension produced by clonidine, rilmenidine and moxonidine was much greater in SAD rats than in intact rats. The study suggests that baroreflex mechanisms are not only important for maintaining levels of blood pressure in the very short term, but also for buffering the effects of centrally acting antihypertensive drugs.

Comparison of the baroreceptor-heart rate reflex effects of moxonidine, rilmenidine and clonidine in conscious rabbits

In 10 conscious rabbits, the baroreceptor-heart rate (HR) reflex effects of centrally acting antihypertensive agents with high affinity for imidazoline receptors (IRs), moxonidine and rilmenidine, were compared with clonidine which acts predominantly via central alpha2-adrenoceptors. Dose regimens were chosen to give similar hypotension (-17+/-1 mm Hg) and bradycardia (-27+/-2 b/min) for all three agents given into the fourth ventricle. Baroreceptor-HR reflex curves were assessed by i.v. drug induced changes in blood pressure. With all treatments, the baroreflex curves with both vagal and sympathetic effectors intact were shifted to the left, corresponding to the hypotension, and the bradycardia plateau was reduced. Rilmenidine and moxonidine also reduced the upper plateau such that the curves were shifted parallel down the HR scale with no change in the HR range. By contrast, clonidine only decreased the lower plateau, and thus increased HR range (+19+/-6%). Moxonidine, but not rilmenidine, reduced the baroreflex gain by reducing the curvature. Clonidine also decreased curvature but this did not result in a reduction in gain as it was offset by the increase in HR range. The gain and range of the cardiac sympathetic component, as assessed after vagal blockade, was reduced by rilmenidine by 53 and 40% respectively, but was not affected by the other agents. The calculated vagal component of the curves showed that all agents produced a greater vagal bradycardia in response to a rise in pressure and that both rilmenidine and clonidine increased vagal HR range. The present study results show that many of the baroreflex effects of clonidine, such as facilitating cardiac vagal responses, are shared by the second generation agent rilmenidine, suggesting that they are primarily due to alpha2-adrenoceptor activation. In addition, the inhibition of the sympathetic component of the baroreflex, observed with rilmenidine, and not clonidine suggests that this effect may involve IRs. By contrast moxonidine, the most specific agent for I1 receptors, produces mainly a baroreflex independent inhibition of cardiac sympathetic activity with little effect on vagal activity.

Chronopharmacological dependence of antihypertensive effects of the imidazoline-like drugs in stroke-prone spontaneously hypertensive rats

In the present study we investigated the chronopharmacological dependence of dose-dependent hypotensive and cardiochronotropic effects of the imidazoline-like drugs (clonidine, rilmenidine and moxonidine) in stroke-prone spontaneously hypertensive rats (SHR-SP), using radio-telemetric system (Data Sciences, USA). The 24-h blood pressure, heart rate and locomotor activity profiles showed peak values during the rats' active phase during the night period. The degree of hypotensive and bradycardic effects of all drugs were most evident at this time and occurred in the absence of a change in locomotor activity. These studies show that clonidine, rilmenidine and moxonidine decrease blood pressure and heart rate in a time-dependent manner in SHR-SP. It was demonstrated that the degree and duration of hypotensive action of imidazoline-like drugs vary with the time of drug administration.

Effect of rilmenidine on the cadiovascular responses to stress in the conscious rabbit

Environmental stress can cause an increase in sympathetic nerve activity both in humans and animals. While centrally acting antihypertensive drugs such as rilmenidine are known to reduce sympathetic tone, it is not clear whether they also influence the cardiovascular responses to acute stress. In the present study we examined the effects of systemic treatment with rilmenidine on the sympathetic and haemodynamic responses to air jet or noise stress. Twelve conscious rabbits previously implanted with a renal nerve recording electrode were subjected to an 8 l/min stream of air directed at their face for 10 min or exposure to 10 min of white noise (approximately 85 dB). Both air jet and noise stress elicited increases in renal sympathetic nerve activity (RSNA) which were greatest in the first minute (+55+/-9% and +40+/-6%, respectively), but which quickly reached a stable level over the subsequent 9 min (+24+/-6% and +9+/-5%, respectively). This was accompanied by a small increase in heart rate (HR) and mean arterial pressure (MAP). Intravenous rilmenidine (273 microg/kg) reduced MAP from 85+/-3 mm Hg to 68+/-2 mm Hg and HR from 203+/-10 b/min to 188+/-10 b/min and lowered basal RSNA by 54%. Rilmenidine reduced the increase in RSNA seen during the first minute of air jet stress by 35% and reduced the average increase over the next 9 min by 68%. However, rilmenidine had little effect on either the initial or stable RSNA responses to noise stress. Saline treatment did not alter the RSNA responses to either air jet or noise stress. The results show that centrally-acting antihypertensive agents not only lower basal RSNA, but can differentially influence environmentally induced sympathetic responses. In addition, the differential effect of rilmenidine on noise and air jet stress suggests that they may involve quite different central processing.

Pharmacology and subcellular distribution of [3H]rilmenidine binding sites in rat brain

We have previously reported that in rat brain membranes, [3H]rilmenidine, in addition to labelling alpha2-adrenoceptors and the I2B-subtype of imidazoline receptor binding site (I2B-RBS), may label an additional I-RBS population, distinct from previously classified I1-RBS and I2-RBS. In this study, using crude or fractionated rat brain membranes we examined the possible association of [3H]rilmenidine-labelled I-RBS with the A- and B-isoforms of monoamine oxidase (MAO) by studying the inhibition of [3H]rilmenidine binding by a number of MAO inhibitors; and comparing the maximal binding density (Bmax) and subcellular distribution of [3H]rilmenidine binding sites with that of MAO-A and MAO-B catalytic sites labelled by [3H]RO41-1049 and [3H]RO19-6327 and 12-RBS labelled by [3H]2-BFI. Inhibition of [3H]rilmenidine binding by all MAO inhibitors tested produced very shallow curves (slope 0.29-0.56). Clorgyline and moclobemide (selective MAO-A inhibitors) displayed moderate affinities (60-140 nM), while pargyline (non-selective MAO-inhibitor), RO41-1049 (selective MAO-A inhibitor) and RO19-6327 (selective MAO-B inhibitor) exhibited very low affinities (> 2 microM) for 50-75% of [3H]rilmenidine-labelled I-RBS in crude brain membranes and even lower affinity for the remaining binding. Under identical buffer conditions, the Bmax of [3H]rilmenidine-labelled I-RBS (1.45+/-0.14 pmol/mg protein) was considerably lower than those of MAO-A (13.10+/-0.15 pmol/mg) and MAO-B (10.35+/-0.50 pmol/mg) sites. These results suggest that [3H]rilmenidine does not interact directly with the active catalytic site of either MAO enzyme and could at best only associate with a subpopulation of MAO molecules. Binding studies on five fractions of rat cortex homogenates-nuclear (N), heavy (M) and light (L) mitochondrial, microsomal non-mitochondrial (P), and soluble cytosolic (S) fractions-revealed that 45% of total [3H]rilmenidine binding was present in the P fraction cf. 20 and 23% in the M and L fractions, in contrast to [3H]RO19-6327 and [3H]2-BFI which bound 11-13% in the P fraction and 36-38% and 35-44% in the M and L fractions, respectively. Binding of all ligands in the N fraction was 6-15% of total. These studies reveal that [3H]rilmenidine-labelled I-RBS, unlike the I2-RBS, are not predominantly associated with mitochondrial fractions containing the MAO enzymes (and cytochrome oxidase activity), but appear to be distributed in both the mitochondrial and plasma membrane fractions in rat cerebral cortex.

Interaction of the dopamine D2 receptor agonist quinpirole with sympathetic vasomotor tone and the central action of rilmenidine in conscious rabbits

Previous studies in conscious rats have shown that systemic administration of the dopamine D2 receptor agonist quinpirole causes a centrally-mediated increase in blood pressure which is associated with increased plasma levels of noradrenaline and adrenaline. In addition, treatment with quinpirole caused a marked inhibition of the antihypertensive effect of centrally-acting sympatho-inhibitory drugs such as clonidine, rilmenidine and alpha-methyldopa, suggesting an interaction at the level of sympathetic vasomotor tone. The main aim of the present study was investigate in conscious rabbits the effect of quinpirole on renal sympathetic nerve activity. In addition, we studied the effect of pretreatment with quinpirole on responses to additional quinpirole injections or rilmenidine treatment. Quinpirole treatment caused a prolonged dose-dependent increase in blood pressure and heart rate. Additional injection of quinpirole, 30 min after the first treatment, caused a significantly smaller pressor response (7+/-2 vs. 17+/-2 mm Hg). Injection of rilmenidine caused a larger decrease in blood pressure in rabbits which had been pretreated with quinpirole than in controls (-28+/-3 vs. -14+/-3 mm Hg). Total renal sympathetic nerve activity was markedly increased by quinpirole treatment (3.5-fold), an effect which could be attributed to both increased amplitude and increased frequency of the renal nerve signal. After a second injection of quinpirole, 30 min after the first treatment, only total renal sympathetic nerve activity and amplitude were increased and the effects were reduced. These results show marked actions of quinpirole on renal sympathetic nerve activity in conscious rabbits. However, the previously described apparent desensitisation to the antihypertensive effect of rilmenidine could not be observed in rabbits, suggesting marked species differences in the mechanism and site of action of rilmenidine.

[3H]Rilmenidine-labelled imidazoline-receptor binding sites co-localize with [3H]2-(benzofuranyl)-2-imidazoline-labelled imidazoline-receptor binding sites and monoamine oxidase-B in rabbit, but not rat, kidney

The distribution and relative densities of imidazoline-receptor binding sites (I-RBS) and monoamine oxidase (MAO)-A and -B enzyme(s) in rat and rabbit kidney were compared autoradiographically using fixed nanomolar concentrations of [3H]rilmenidine and [3H]2-(benzofuranyl)-2-imidazoline ([3H]2-BFI) to label I-RBS, and [3H]RO41-1049 and [3H]RO19-6327 to label MAO-A and -B isoenzymes, respectively. In rat kidney, high densities of I-RBS labelled by [3H]rilmenidine were observed in the cortex and outer stripe (120-280 fmol/mg tissue), in contrast to low I-RBS densities labelled by [3H]2-BFI (<4 fmol/mg). A relatively high density of [3H]RO41-1049 binding to MAO-A enzyme was present in all regions of the rat kidney (160-210 fmol/mg) compared with a low density of [3H]RO19-6327 binding to MAO-B (< 25 fmol/mg). Comparison of MAO-A and -B distributions with that of [3H]rilmenidine-labelled I-RBS strongly suggests a lack of association in rat kidney. Similarly, the extremely low densities of [3H]2-BFI-labelled I2-RBS in rat kidney contrasts with the density of MAO-A, but is consistent with the low density of MAO-B. Rabbit kidney cortex and outer stripe contained high relative densities of [3H]rilmenidine-labelled I-RBS (200-215 fmol/mg) and [3H]2-BFI-labelled I2-RBS (45-60 fmol/mg) with lower densities in the inner stripe and inner medulla (< or = 100 and 30 fmol/mg respectively). A high density of MAO-A binding was observed in the inner stripe (515 fmol/mg) with lower levels in the cortex and outer stripe (100-240 fmol/mg), while high densities of MAO-B binding were observed in the cortex and outer stripe (290-450 fmol/mg) with lower levels in the inner stripe (65 fmol/mg). The correlation between the localization of [3H]rilmenidine-labelled I-RBS and [3H]RO19-6327-labelled MAO-B in rabbit kidney (r = 0.87, P = 0.057) suggest that [3H]rilmenidine may label a binding site co-existent with MAO-B, but not MAO-A (n.s.), in this tissue, but rilmenidine did not inhibit [3H]RO41-1049 or [3H]RO19-6327 binding. The distribution of [3H]2-BFI-labelled I2-RBS overlapped the combined distributions of both MAO-A and -B isoenzymes, suggesting that [3H]2-BFI may label sites on both enzymes in the rabbit, but [3H]2-BFI binding only correlated with [3H]RO19-6327 (r = 0.84, P = 0.07), not [3H]RO41-1049 binding (n.s.). Moreover, 2-BFI only inhibited [3H]RO19-6327, not [3H]RO41-1049 binding. These data are consistent with reports that I2-RBS are located on MAO-B and allosterically influence the catalytic site. The relationship of [3H]rilmenidine- and [3H]2-BFI-labelled I-RBS and the identity of non-MAO-associated [3H]rilmenidine-labelled I-RBS requires further investigation.

Rilmenidine elevates cytosolic free calcium concentration in suspended cerebral astrocytes

Rilmenidine, a ligand for imidazoline and alpha2-adrenergic receptors, is neuroprotective following focal cerebral ischemia. We investigated the effects of rilmenidine on cytosolic free Ca2+ concentration ([Ca2+]i) in rat astrocytes. Rilmenidine caused concentration-dependent elevation of [Ca2+]i, consisting of a transient increase (1-100 microM rilmenidine) or a transient increase followed by sustained elevation above basal levels (1-10 mM rilmenidine). A similar elevation in [Ca2+]i was induced by the imidazoline ligand cirazoline. The transient response to rilmenidine was observed in Ca2+-free medium, indicating that rilmenidine evokes release of Ca2+ from intracellular stores. However, the sustained elevation of Ca2+ was completely dependent on extracellular Ca2+, consistent with rilmenidine activating Ca2+ influx. Pretreatment with thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, abolished the response to rilmenidine, confirming the involvement of intracellular stores and suggesting that rilmenidine and thapsigargin activate a common Ca2+ influx pathway. The alpha2-adrenergic antagonist rauwolscine attenuated the increase in [Ca2+]i induced by clonidine (a selective alpha2 agonist), but not the response to rilmenidine. These results indicate that rilmenidine stimulates both Ca2+ release from intracellular stores and Ca2+ influx by a mechanism independent of alpha2-adrenergic receptors. In vivo, rilmenidine may enhance uptake of Ca2+ from the extracellular fluid by astrocytes, a process that may contribute to the neuroprotective effects of this agent.

[Rilmenidine in rosacea: a double-blind study versus placebo]

INTRODUCTION

The usual treatments of rosacea (cyclines and metronidazole) are mainly effective on reducing the number of papules and pustules. Clonidine only was employed in order to treat flushes observed in rosacea. Rilmenidine is a central hypotensive drug which acts more specifically than clonidine on imidazoline receptors and which has no sedative side effects. The purpose of this study was to evaluate the efficacy of rilmenidine 1 mg/d in the treatment of rosacea.

PATIENTS AND METHODS

A total of 41 patients suffering from typical rosacea were selected in this randomised double blind study rilmenidine versus placebo. The study comprised an 1-month observation period without treatment followed by 3 months of treatment. The major assessment criteria was the proportion of responders at the end of the treatment period. Responders were defined as patients showing a decrease of more than 50 p. 100 in the count of papules and pustules. Minor criterias were the variation of the number of flushes, self-evaluated by the patient and the variation of the redness of the face, noted by the investigator on a scale from 0 to 5.

RESULTS

Fifteen patients treated by rilmenidine (R) and 19 patients receiving the placebo (P) were evaluated. The proportion of responders was 69.2 p. 100 in group R and 57.1 p. 100 in group P (p = 0.69). The variations of the number of papules and pustules and of the redness of the face were not significantly different in the two groups. The decrease in the number of flushes was higher in group R (around -13) than in group P (around -5), but the difference was not really significant (p = 0.076). Arterial pressure decreased in 3 patients in group R and in 2 patients in group P. Minor side effects were noted in a similar proportion of patients in the two groups.

DISCUSSION

Rilmenidine is not efficient in reducing the number of papules and pustules but could decrease the number of flushes. Nevertheless, many patients were lost for follow-up and because of a major placebo effect, the conclusions of this study are not strong enough. Another study including more patients and using evaluation criteria based on the vascular components of rosacea could perhaps confirm this hypothesis.

Ethanol selectively counteracts hypotension evoked by central I(1)-imidazoline but not alpha2-adrenergic receptor activation in spontaneously hypertensive rats

Previous studies from our laboratory showed that ethanol counteracts hypotensive responses to clonidine in spontaneously hypertensive rats. This study investigated whether this effect of ethanol involves interaction with central alpha2-adrenoceptors or I(1)-imidazoline receptors or both. The effects of ethanol (0.5 or 1 g/kg, i.v.) or an equal volume of saline on hypotensive and bradycardic responses to clonidine (mixed alpha2-adrenoceptor/I(1)-imidazoline receptor agonist), rilmenidine (selective I(1)-imidazoline receptor agonist), or alpha-methylnorepinephrine (selective alpha2-adrenoceptor agonist) were studied in conscious spontaneously hypertensive rats. Intracisternal administration of clonidine (0.5 microg), rilmenidine (25 microg), or alpha-methylnorepinephrine (4 microg) elicited similar decreases in mean arterial pressure (MAP; 25-30 mm Hg) that lasted > or =60 min. Subsequent administration of ethanol (0.5 and 1 g/kg, i.v.) counteracted the hypotensive effect of clonidine in a dose-related manner. Ethanol (1 g/kg) increased the blood pressure to levels similar to baseline (preclonidine) levels, and blood pressure remained significantly (p < 0.05) higher compared with the corresponding values in saline-treated rats. Similarly, ethanol (0.5 and 1 g/kg, i.v.) dose-dependently counteracted the hypotensive effect of rilmenidine. The antagonizing effects of ethanol on hypotension evoked by clonidine and rilmenidine were comparable in terms of both magnitude and duration. In contrast, ethanol (0.5 or 1 g/kg) had no effect on hypotension evoked by alpha-methylnorepinephrine. Except for a brief increase in blood pressure by ethanol (1 g/kg) at 5 min, blood pressure values obtained in alpha-methylnorepinephrine-treated rats receiving any of the two doses of ethanol were similar to postsaline values. Ethanol had no effect on bradycardic responses to any of the three hypotensive agents. Blood ethanol concentrations were similar regardless of the antihypertensive drug used. We concluded that the adverse hemodynamic effect of ethanol on centrally mediated hypotensive responses depends on the types of receptors involved in the elicitation of this response. That ethanol counteracts decreases in blood pressure evoked by clonidine and rilmenidine but not by alpha-methylnorepinephrine suggests an interaction between ethanol and central pathways involved in I(1)-imidazoline receptor-mediated hypotension.

Rilmenidine reveals differences in the pharmacological characteristics of prejunctional alpha2-adrenoceptors in the guinea-pig, rat and pig

1. The alpha2A and alpha2D-adrenoceptor subtypes are thought to be species homologs most easily differentiated on the basis of the potency of antagonists. In the present study we have compared the effect of rilmenidine with two other selective alpha2-adrenoceptor agonists, UK-14304 (5-bromo-6- [2-imidazolin-2-ylamino]-quinoxaline) and clonidine, against electrically-evoked contractions in five isolated preparations from the rat, guinea-pig and pig, and, where possible, determined the receptor subtype involved. 2. UK-14034, clonidine and rilmenidine produced concentration-dependent inhibition of the electrically-evoked contractions of the rat isolated vas deferens and tail artery and the guinea-pig ileum. These inhibitory effects were reversed by the selective alpha2-adrenoceptor antagonist, RX-811058 (1 microM), except in the rat tail artery preparations where the remaining neurogenic response was inhibited; evidence for the involvement of 'innervated' alpha2-adrenoceptors. Both clonidine and UK-14304 produced concentration-dependent inhibition of responses in the porcine isolated tail artery and urinary bladder but clonidine was markedly less efficacious in these preparations. In contrast, rilmenidine failed to inhibit the neurogenic contractions in either preparation. 3. Although rilmenidine failed to elicit a detectable response in either the porcine isolated tail artery or urinary bladder, it (10 microM and 30 microM, respectively) competitively antagonised the inhibitory effects of UK-14304 with an estimated dissociation constant of (pK(B)) 5.82 and 5.93, respectively. 4. Prazosin (1 microM) failed to alter the effect of UK-14304 against neurogenic contractions in the porcine isolated urinary bladder, while rauwolscine (pK(B) 8.87) was 10 fold more potent than phentolamine (pK(B) 7.56). On the other hand, phentolamine (pK(B) 8.42) was only marginally more potent than rauwolscine (pK 8.05) against clonidine-induced inhibition of electrically-evoked contractions of the guinea-pig isolated ileum. This pharmacological evidence with antagonists supports the presence of alpha2D-adrenoceptors in the rat and guinea-pig and the alpha2A-adrenoceptors in the pig. 5. We have demonstrated that rilmenidine, unlike clonidine and UK-14304, is devoid of any agonist activity at prejunctional alpha2A-adrenoceptors in the pig, but is an efficacious agonist at alpha2D-adrenoceptors in the rat and guinea-pig.

Rilmenidine normalizes fructose-induced insulin resistance and hypertension in rats

OBJECTIVE

The aim of this study was to determine the effects of rilmenidine (an antihypertensive drug that lowers blood pressure by decreasing sympathetic outflow) in an animal model of hypertension associated with insulin resistance, i.e. rats fed on a high-fructose diet.

DESIGN

Wistar rats were fed for 4 weeks either on a standard diet (S group) or on a high-fructose diet (F group; 34.5% fructose). In half of the rats in the F group, rilmenidine (1 mg/kg per day) was added to the drinking water for the last 2 weeks of the diet (FR group).

RESULTS

Body weight gain was higher in the F than in the S rats (66+/-8g versus 45+/-8g, P< 0.05), but was prevented by rilmenidine treatment (32+/-2g). Arterial systolic blood pressure was increased in F rats (162+/-2 versus 155+/-2 mmHg, P< 0.05), rilmenidine reduced this value to normal (149+/-3 mmHg). Glucose tolerance, glucose turnover rate, and insulin secretion were not modified by the diet or by the drug. However, during a euglycemic hyperinsulinemic clamp, glucose utilization was lower (10+/-1 versus 14+/-1.5 mg/min per kg; P< 0.05) and hepatic glucose production higher (1+/-0.01 versus 0 mg/min per kg, P< 0.01) in F than in S rats. These changes in insulin action were totally abolished by rilmenidine.

CONCLUSIONS

These data demonstrate that rilmenidine can ameliorate the deleterious effects of a high-fructose diet, i.e. weight gain, hypertension, and resistance to the effects of insulin.

Sympatho-adrenal mechanisms regulating cardiovascular hypertrophy in primary hypertension: a role for rilmenidine?

OBJECTIVE

Overactivity of the sympatho-adrenal system has long been considered a major factor contributing to blood pressure elevation in primary hypertension in humans and experimental animals. Our aim has been to elucidate its role in the development of cardiovascular hypertrophy in hypertensives.

METHODS

Two studies have been performed in spontaneously hypertensive rats (SHR). One involved irreversible inhibition of the sympatho-adrenal system in newborn SHR using a sympathectomy procedure combined with prolonged alpha1-adrenoceptor blockade. The other involved reversible, long-term inhibition of the sympatho-adrenal system in young but mature SHR, by treatment with rilmenidine, a centrally active antihypertensive agent interacting with imidazoline receptors. Their effects on cardiovascular structure were examined.

RESULTS

Sympathectomy plus alpha1-adrenoceptor blockade prevented the development of cardiac and vascular hypertrophy in adolescent SHR and these effects were maintained later in life. Rilmenidine administered to older (9-week) SHR also attenuated cardiac hypertrophy, abolished perivascular fibrosis associated with the intramyocardial vessels, and normalized vessel structure in the richly sympathetically innervated mesenteric vasculature. These effects were only partially related to the level of blood pressure reduction.

CONCLUSIONS

Inhibition of the sympatho-adrenal system not only reduces blood pressure to normotensive levels in SHR but also has beneficial effects on cardiovascular structure, potentially reducing risk factors for cardiac and renal abnormalities frequently seen in long-term hypertensives. Therapeutically, these effects are likely to be achieved with rilmenidine.

Regression of left ventricular hypertrophy in hypertensive patients after 1 year of treatment with rilmenidine: a double-blind, randomized, controlled (versus nifedipine) study

OBJECTIVE

To assess the effect of 1-year treatment with rilmenidine, an oxazoline compound that exerts its antihypertensive effects through binding to imidazoline receptors in the brainstem, on left ventricular hypertrophy (LVH) secondary to essential, mild-to-moderate hypertension [supine diastolic blood pressure (DBP)95-115 mmHg].

METHODS

We performed a double-blind, randomized, controlled (versus slow-release nifedipine) trial. Adjustment of treatment took place every month (M) between inclusion (MO) and an evaluation after 6 months (M6), then during M9 and after 1 year (M12) to achieve supine DBP values < or = 90 mmHg. Patients were dropped from our study if they had DBP> 95mmHg during two consecutive visits or DBP>115 mmHg on one occasion. The daily dosage of rilmenidine was 1 mg, and could be increased to 2 mg/day. The daily dosage of slow-release nifedipine was started from the beginning at the maximum dosage of 40 mg/day, so that there was no true adjustment of treatment despite the allocation of patients to a different unit in the case of DBP> 95 mmHg. The primary criterion was the change in left ventricular mass index (LVMI, g/m2), assessed by echocardiography, between MO and M12 for patients who completed the trial.

RESULTS

After a 1-month placebo run-in period, 76 patients were selected and 73 were included (35 treated with rilmenidine and 38 treated with nifedipine). Fifteen patients withdrew from the study and two completed the study with a major deviation from protocol, leaving 56 patients (24 treated with rilmenidine and 32 treated with nifedipine) for a per-protocol analysis. Baseline demographic characteristics and history of arterial hypertension for the rilmenidine and nifedipine groups were similar, for included patients and for those taken into account for the per-protocol analysis. Between MO and M12, DBP in members of the per-protocol population was adequately controlled for those in the rilmenidine group (102.7+/-4.6 versus 88.5+/-7.1 mmHg, respectively) and for those in the nifedipine group (102.7+/-5.1 versus 85.6+/-79 mmHg, respectively). During MO, LVMI of patients in the rilmenidine group (176.9+/-41.3 g/m2) was slightly higher than that of patients in the nifedipine group (172.6+/-35.1 g/m2). During M12, LVMI was observed to have decreased both for patients in the rilmenidine group (to 154.8+/-40.2 g/m2, a decrease of 22.1+/-23.3 g/m2, P< 0.001) and for those in the nifedipine group (to 145.6+/-36.4 g/m2, a decrease of 26.9+/-29.5 g/m2, P< 0.001) but the difference between these two groups was not significant (P= 0.5).

CONCLUSION

One-year treatment with a daily dosage of 1 or 2 mg rilmenidine achieves a significant reduction of left ventricular mass, which is not statistically different than that occurring with a daily dosage of 40 mg of slow-release nifedipine.

Preventive effect of rilmenidine on the occurrence of neurogenic ventricular arrhythmias in rabbits

BACKGROUND

Centrally acting antihypertensive drugs bearing an imidazoline or a related chemical structure inhibit sympathetic nervous output to the heart and vascular beds, and enhance parasympathetic tone. Cardiac ischaemia and ventricular arrhythmias that can result from hypertension are likely to benefit from such effects.

OBJECTIVE

To investigate the effects of rilmenidine, an oxazoline with antihypertensive properties, in a model of neurogenically induced ischaemic ventricular arrhythmias.

METHODS AND RESULTS

Bicuculline, a alpha-aminobutyric acid (GABA(A)) receptor antagonist, was administered intracisternally in pentobarbitone anaesthetized rabbits; 10 microg/kg intracisternal bicuculline induced polymorphic ventricular ectopic beats and ventricular tachycardia, while blood pressure increased by about 50-60% and heart rate in sinus rhythm decreased by about 20%. Rilmenidine pretreatment (10 min), either administered intravenously (0.01, 0.1, 1 mg/kg) or intracisternally (3, 10, 30 microg/kg), dose-dependently prevented the occurrence of bicuculline-induced arrhythmias and, because of a lower baseline, the blood pressure values reached were less when compared with controls. Intracisternal idazoxan (15 microg/kg) had no significant antiarrhythmic effect but antagonized, in part, the haemodynamic and antiarrhythmic effects of rilmenidine (1 mg/kg intravenously; 30 microg/kg intracisternally).

CONCLUSION

The antiarrhythmic effects observed with rilmenidine are mainly mediated by blunting the bicuculline-induced increase in the sympathetic nervous output to the heart and the vascular beds. These effects of rilmenidine are likely to originate from action on the central as well as on the peripheral nervous systems. Direct coronary or cardiac effects might also play a role, in particular at low non-hypotensive intravenous doses.

Site and receptors involved in the sympathoinhibitory actions of rilmenidine

INTRODUCTION

It is well accepted that centrally acting antihypertensive agents such as rilmenidine reduce blood pressure through inhibition of the sympathetic nervous system. The central receptor involved, be it a central imidazoline receptor or an alpha2-adrenoceptor and its location, is less certain.

METHODS AND RESULTS

The present paper reviews studies from our laboratory examining these questions by using antagonists with differing affinity for imidazoline receptors and alpha2-adrenoceptors. In addition, we have used various routes of administration in conscious and anaesthetized normotensive rabbits. We found that rilmenidine was more potent in its hypotensive action when administered into the fourth ventricle than when given intravenously and considerably more potent when injected into the rostroventrolateral medulla (RVLM) compared to the nucleus of the solitary tract (NTS). By contrast, alpha-methylnoradrenaline, which acts solely through alpha2-adrenoceptors to produce hypotension, was similarly potent in both the NTS and RVLM. Injections of rilmenidine into the RVLM reduced renal sympathetic tone and produced a marked inhibition of renal sympathetic baroreflex responses. The pattern of renal sympathetic baroreflex effects of rilmenidine administered into the RVLM was similar to the effects of the fourth ventricular or intravenous administration. These studies together support the view that the RVLM is the major site of action. We have determined the relative importance of imidazoline receptors and alpha2-adrenoceptors in the inhibition of renal sympathetic nerve activity produced by rilmenidine administered into the RVLM, the fourth ventricle or intravenously. Initial studies in conscious rabbits showed that intravenous or fourth ventricular administration of rilmenidine induced renal sympatho-inhibition which was preferentially reversed by idazoxan or efaroxan (imidazoline/alpha2-adrenoceptor antagonist) compared to 2-methoxyidazoxan (alpha2-adrenoceptor antagonist). In anaesthetized rabbits, administration of idazoxan into the RVLM reversed the inhibition of the renal sympathetic activity induced by RVLM or intravenous administration of rilmenidine. In contrast, idazoxan had no effect on the responses produced by the alpha2-adrenoceptor agonist alpha-methylnoradrenaline.

CONCLUSION

Our studies suggest that rilmenidine given systemically acts primarily on imidazoline receptors in the RVLM to reduce sympathetic tone and to modulate sympathetic baroreflexes.

Complementary antihypertensive action of rilmenidine on the pressure-natriuresis relationship and sodium preference in spontaneously hypertensive rats

Previously, changes in position and slope of the pressure-natriuresis relationship have been used to characterize antihypertensive drugs in basic research. Rilmenidine may chronically reduce arterial pressure via central nervous system and renal imidazoline receptors. The present experiments were used to examine the shift in the pressure-natriuresis relationship during rilmenidine administration. We examined the effects of twice daily doses (1 and 3 mg/kg) for 6 days on the pressure-natriuresis relationship determined for control and treated spontaneously hypertensive rats (SHR) drinking tap water or 1% NaCl. The pressure-natriuresis relationship was shifted to the left for the 3 mg/kg dose and the slope was no different from the control. These experiments also indicated that rilmenidine might have an effect on sodium preference which was confirmed in a third series of experiments by permitting control and treated (3 mg/kg) SHR access to both tap water and 1% NaCl. This lack of change in slope indicates that, during rilmenidine treatment, the arterial pressure is relatively insensitive to sodium intake. The shift to the left indicates a restoration of the pressure-natriuresis relationship after chronic treatment with rilmenidine and a resetting of the long-term blood pressure control. Rilmenidine also reduces salt appetite in the SHR.

Relative importance of medullary brain nuclei for the sympatho-inhibitory actions of rilmenidine in the anaesthetized rabbit

OBJECTIVE

To determine the contribution of the rostral ventrolateral medulla and the nucleus of the solitary tract in mediating the attenuation of the renal sympathetic baroreflex produced by administration of rilmenidine to anaesthetized rabbits and to examine the relative contribution of alpha2-adrenoceptors and imidazoline receptors at these sites to the cardiovascular effects of rilmenidine.

METHODS AND RESULTS

Rilmenidine micro-injected into the rostral ventrolateral medulla produced hypotension and inhibition of renal sympathetic nerve activity with doses an order of magnitude lower than those required in the nucleus tractus solitarius. Alpha-methylnoradrenaline, however, was similarly potent at producing hypotension when it was injected into the rostral ventrolateral medulla or nucleus tractus solitarius but, unlike rilmenidine, did not lower renal sympathetic nerve activity when it was injected into the nucleus tractus solitarius. The alpha2-adrenoceptor antagonist 2-methoxyidazoxan partially reversed the hypotension and renal sympathetic nerve activity inhibition due to alpha-methylnoradrenaline when it was administered into the rostral ventrolateral medulla, whereas the mixed alpha2-adrenoceptor/imidazoline receptor antagonists, idazoxan and efaroxan, did not. 2-Methoxyidazoxan, but not idazoxan, also reversed the hypotension when alpha-methylnoradrenaline was administered into the nucleus tractus solitarius. The hypotension induced by rilmenidine in the rostral ventrolateral medulla was completely reversed both by 2-methoxyidazoxan and by idazoxan, as was the sympathetic inhibition. To assess any interaction between the nucleus tractus solitarius and the rostral ventrolateral medulla in mediating the baroreflex effects of rilmenidine, we injected rilmenidine into the rostral ventrolateral medulla, the nucleus tractus solitarius or both nuclei and determined renal baroreflex responses of sympathetic nerve activity using drug-induced changes in blood pressure. Injection of 0.5 nmol rilmenidine into the rostral ventrolateral medulla reduced mean arterial pressure and basal renal sympathetic nerve activity as well as renal sympathetic baroreflex range (by 27%) and gain (by 35%). In contrast, injection of rilmenidine into the nucleus tractus solitarius had no effect on basal renal sympathetic nerve activity and renal sympathetic baroreflex parameters. The effect of combined injection was similar to that of administration into the rostral ventrolateral medulla alone.

CONCLUSION

Our results show that the rostral ventrolateral medulla, rather than the nucleus tractus solitarius, is the major site involved in the hypotension and inhibition of the renal sympathetic baroreflex by rilmenidine. Comparison of the actions of alpha2-adrenoceptor and imidazoline receptor antagonists on the effects of rilmenidine and alpha-methylnoradrenaline suggests that these agents are acting at different receptors, presumably imidazoline and alpha2-adrenoceptors receptors, respectively, and that both are important in lowering sympathetic tone and blood pressure in the rostral ventrolateral medulla.

Antagonism of alpha1-adrenoceptor agonist-induced responses by rilmenidine in vascular smooth muscle

The effect of the centrally acting antihypertensive agent, rilmenidine, was examined on the contractile properties of isolated rat portal vein strips and on the free cytosolic [Ca2+] ([Ca2+]i) in isolated myocytes. Rilmenidine (1-30 microM) relaxed strips precontracted with noradrenaline. This effect was not inhibited by the alpha2-adrenoceptor antagonist, yohimbine, and was not mimicked by the alpha2-adrenoceptor agonist, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304). Rilmenidine dose dependently shifted to the right the concentration-response curves to noradrenaline and to phenylephrine but not that to carbachol. Rilmenidine alone (0.1-30 microM) caused a contraction which maximally corresponded to 18% of the maximal noradrenaline-induced contraction. This effect was not produced by UK 14,304, was not affected by yohimbine, but was inhibited by the alpha1-adrenoceptor antagonist, prazosin. In isolated myocytes, rilmenidine reduced the noradrenaline-induced [Ca2+]i increase but alone, it produced a rise in [Ca2+]i, the peak amplitude of which averaged 15% of the noradrenaline-induced transient [Ca2+]i rise. It is concluded that rilmenidine acts as a partial agonist of alpha1-adrenoceptors of vascular smooth muscle, causing relaxation of vessels precontracted by full agonists of alpha1-adrenoceptors.

Effect of N-dicyclopropylmethyl-amino-2-oxazoline (S-3341) on antioxidant status and nitric oxide in hypertensive patients

Defective endothelium-dependent vascular relaxation has been found in animal models of hypertension and in hypertensive patients. An imbalance due to reduced production of nitric oxide (NO) or increased production of free radicals, mainly the superoxide anion, may facilitate the development of an arterial functional spasm. Although it has been shown that many antihypertensive drugs can normalise both the antioxidant activity and NO, the antioxidant effect of N-dicyclopropylmethyl-amino-2-oxazoline (S-3341), an alpha-adrenoreceptor agonist, has not been investigated. In this study we investigated the antioxidant and NO status in hypertensive patients and whether there was any effect of S-3341 on these parameters. Eleven patients with mild hypertension (mean systolic blood pressure 159.5 +/- 2.5 mmHg) were administered S-3341 (1 mg/day) for 4 weeks. Plasma vitamin E, nitrite-nitrate and MDA levels, and catalase activity, were measured both before and after treatment with S-3341. There was significant reduction in both mean systolic and diastolic blood pressure during the treatment. We found an increase in catalase activity (p < 0.05), a decrease in malondialdehyde (MDA) levels (p < 0.01) and an insignificant increase in vitamin E levels in hypertensive patients following the S-3341 treatment. We propose that S-3341 may prevent oxidant stress in hypertensive patients by inhibiting free-radical formation.

[Comparative analysis of the effects of clonidine preparations depending on methods of arterial pressure recording]

Changes in hemodynamic parameters in intravenous infusion of drugs of the clonidine group were studied in Wistar rats by means of the radio-telemetry and catheter systems of arterial pressure (AP) recording. Clonidine (5 micrograms/kg), rilmenidine (300 micrograms/kg), and moxonidine (100 micrograms/kg) were found to cause significant decrease in AD and bradycardia whatever the method of AD measurement used. However, the degree of the hypotensive and negative cardiochronotropic effects of the central action drugs was higher in rats with radio-telemetry transmitters. The results provide evidence that the radio-telemetry system of monitoring of the hemodynamic parameters is more sensitive and accurate for evaluating the effect of the drugs under study.

The imidazoline receptor in control of blood pressure by clonidine and allied drugs

Clonidine, moxonidine, and rilmenidine are centrally acting antihypertensive agents that lower arterial pressure by inhibiting the tonic activity of sympathoexcitatory neurons in the rostral ventrolateral medulla. Competing hypotheses have been put forward by different investigators to explain the sympathoinhibition evoked by "imidazoline drugs": either via central actions at alpha 2-adrenergic receptors or novel I1-imidazoline receptors. These different perspectives are presented in the accompanying reviews.

Role of I1 imidazoline receptors in the sympathoinhibition produced by intracisternally administered rilmenidine and moxonidine

The role of alpha 2-adrenoceptors and I1 imidazoline receptors in the cardiovascular effects of rilmenidine (CAS 54187-04-1), moxonidine (CAS 75438-57-2) and guanabenz (CAS 23256-50-0) was studied in conscious rabbits. In radioligand binding studies, rilmenidine and moxonidine are selective for I1 imidazoline receptors (vs. alpha 2-adrenoceptors) and guanabenz is selective for alpha 2-adrenoceptors (vs. I1 imidazoline receptors). Efaroxan (CAS 89197-00-2; selective for I1 imidazoline receptors) and yohimbine (CAS 65-19-0; selective for alpha 2-adrenoceptors) were used as antagonists. All drugs were injected into the cisterna magna. Guanabenz (1, 3, 10 and 30 micrograms kg-1), rilmenidine (1, 3, 10 and 30 micrograms kg-1) and moxonidine (0.03, 0.1, 0.3 and 1 microgram kg-1) dose-dependently lowered blood pressure, heart rate and the plasma concentration of noradrenaline. In the antagonism experiments, guanabenz (10 micrograms kg-1), rilmenidine (10 micrograms kg-1) and moxonidine (0.3 micrograms kg-1) were administered first; they caused equal hypotension. Injection of the agonists was followed by injection of efaroxan (0.3-1 microgram kg-1) or yohimbine (0.1-0.3 micrograms kg-1). Efaroxan and yohimbine were equieffective at antagonizing the effects of guanabenz. In contrast, efaroxan was more effective than yohimbine at antagonizing the effects of rilmenidine and moxonidine. The results show that intracisternally administered guanabenz, rilmenidine and moxonidine cause sympathoinhibition. alpha 2-Adrenoceptors are responsible for the sympathoinhibition produced by guanabenz. In contrast, I1 imidazoline receptors are involved in the sympathoinhibition caused by rilmenidine and moxonidine.