Influence of anaesthesia on the cardiovascular effects of rilmenidine and clonidine in spontaneously hypertensive rats

Blood pressure reduction induced by chronic intracerebroventricular or peroral clonidine administration in rats with salt-dependent or angiotensin II-dependent hypertension

The agonists of α2-adrenergic receptors such as clonidine, rilmenidine or monoxidine are known to lower blood pressure (BP) through a reduction of brain sympathetic outflow but their chronic antihypertensive effects in rats with low-renin or high-renin forms of experimental hypertension were not studied yet. Moreover, there is no comparison of mechanisms underlying BP reduction elicited by chronic peroral (po) or intracerebroventricular (icv) clonidine treatment. Male salt-sensitive Dahl rats fed 4% NaCl diet and Ren-2 transgenic rats were treated with clonidine administered either in the drinking fluid (0.5 mg/kg/day po) or as the infusion into lateral brain ventricle (0.1 mg/kg/day icv) for 4 weeks. Basal BP and the contributions of renin-angiotensin system (captopril 10 mg/kg iv) or sympathetic nervous system (pentolinium 5 mg/kg iv) to BP maintenance were determined in conscious cannulated rats at the end of the study. Both peroral and intracerebroventricular clonidine treatment lowered BP to the same extent in either rat model. However, in both models chronic clonidine treatment reduced sympathetic BP component only in rats treated intracerebroventricularly but not in perorally treated animals. In contrast, peroral clonidine treatment reduced angiotensin II-dependent vasoconstriction in Ren-2 transgenic rats, whereas it lowered residual blood pressure in Dahl rats. In conclusions, our results indicate different mechanisms of antihypertensive action of clonidine when administered centrally or systemically.

Blood pressure reduction induced by chronic intracerebroventricular or peroral clonidine administration in rats with salt-dependent or angiotensin II-dependent hypertension

The agonists of alpha(2)-adrenergic receptors such as clonidine, rilmenidine or monoxidine are known to lower blood pressure (BP) through a reduction of brain sympathetic outflow but their chronic antihypertensive effects in rats with low-renin or high-renin forms of experimental hypertension were not studied yet. Moreover, there is no comparison of mechanisms underlying BP reduction elicited by chronic peroral (po) or intracerebroventricular (icv) clonidine treatment. Male salt-sensitive Dahl rats fed 4% NaCl diet and Ren-2 transgenic rats were treated with clonidine administered either in the drinking fluid (0.5 mg/kg/day po) or as the infusion into lateral brain ventricle (0.1 mg/kg/day icv) for 4 weeks. Basal BP and the contributions of renin-angiotensin system (captopril 10 mg/kg iv) or sympathetic nervous system (pentolinium 5 mg/kg iv) to BP maintenance were determined in conscious cannulated rats at the end of the study. Both peroral and intracerebroventricular clonidine treatment lowered BP to the same extent in either rat model. However, in both models chronic clonidine treatment reduced sympathetic BP component only in rats treated intracerebroventricularly but not in perorally treated animals. In contrast, peroral clonidine treatment reduced angiotensin II-dependent vasoconstriction in Ren-2 transgenic rats, whereas it lowered residual blood pressure in Dahl rats. In conclusions, our results indicate different mechanisms of antihypertensive action of clonidine when administered centrally or systemically.

Responsiveness of α2-adrenoceptor/I1-imidazoline receptor in the rostral ventrolateral medulla to cardiovascular regulation is enhanced in conscious spontaneously hypertensive rat

Stimulation of α2-adrenoceptor/I1-imidazoline receptors in the rostral ventrolateral medulla decreases the blood pressure via sympathoinhibition. However, alteration of receptor responses in genetically hypertensive rats remains unclear. We examined cardiovascular responses of α2-adrenoceptor/I1-imidazoline receptor agonist and antagonists microinjected into the rostral ventrolateral medulla of conscious spontaneously hypertensive rats and normotensive Wistar Kyoto rats. Injection of 2-nmol clonidine-an α2-adrenoceptor/I1-imidazoline receptor agonist-unilaterally into the rostral ventrolateral medulla decreased the blood pressure, heart rate, and renal sympathetic nerve activity; the responses were significantly enhanced in spontaneously hypertensive rats than in Wistar Kyoto rats. Co-injection of 2-nmol 2-methoxyidazoxan (a selective α2-adrenoceptor antagonist) or 2-nmol efaroxan (an I1-receptor antagonist) with 2 nmol of clonidine attenuated the hypotensive and bradycardic effects of clonidine-only injection. Injection of 2-methoxyidazoxan alone increased the blood pressure and heart rate in spontaneously hypertensive rats, but not in Wistar Kyoto rats. These results suggest enhanced responsiveness of α2-adrenoceptor/I1-imidazoline receptors in the rostral ventrolateral medulla of spontaneously hypertensive rats.

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.

Brainstem adenosine A1 receptor signaling masks phosphorylated extracellular signal-regulated kinase 1/2-dependent hypotensive action of clonidine in conscious normotensive rats

Central adenosine A(1) and A(2A) receptors mediate pressor and depressor responses, respectively. The adenosine subtype A(2A) receptor (A(2A)R)-evoked enhancement of phosphorylated extracellular signal-regulated kinase (pERK) 1/2 production in the rostral ventrolateral medulla (RVLM), a major neuroanatomical target for clonidine, contributes to clonidine-evoked hypotension, which is evident in conscious aortic barodenervated (ABD) but not in conscious sham-operated (SO) normotensive rats. We conducted pharmacological and cellular studies to test the hypothesis that the adenosine A(2A)R-mediated (pERK1/2-dependent) hypotensive action of clonidine is not expressed in SO rats because it is counterbalanced by fully functional central adenosine subtype A(1) receptor (A(1)R) signaling. We first demonstrated an inverse relationship between A(1)R expression in RVLM and clonidine-evoked hypotension in ABD and SO rats. The functional (pharmacological) relevance of the reduced expression of RVLM A(1)R in ABD rats was verified by the smaller dose-dependent pressor responses elicited by the selective A(1)R agonist N(6)-cyclopentyladenosine in ABD versus SO rats. It is important that after selective blockade of central A(1)R with 8-cyclopentyl-1,3-dipropylxanthine in conscious SO rats, clonidine lowered blood pressure and significantly increased neuronal pERK1/2 in the RVLM. In contrast, central A(1)R blockade had no influence on the hypotensive response or the increase in RVLM pERK1/2 elicited by clonidine in ABD rats. These findings support the hypothesis that central adenosine A(1)R signaling opposes the adenosine A(2A)R-mediated (pERK1/2-dependent) hypotensive response and yield insight into a cellular mechanism that explains the absence of clonidine-evoked hypotension in conscious normotensive rats.

G-protein inwardly rectifying potassium channels are involved in the hypotensive effect of I1-imidazoline receptor selective ligands

OBJECTIVE

The present study examined the role of G-protein inwardly rectifying potassium (GIRK) channels in the depressor responses elicited by intracisternal injections of imidazoline-like drugs in anesthetized rabbits.

METHODS AND RESULTS

Intracisternal injections of the I1-imidazoline receptor (I1R) selective ligands LNP509 (30 microg/kg) and LNP640 (2 microg/kg) (subthreshold doses), and of the GIRK channel opener flupirtine (30 microg/kg) did not affect mean arterial blood pressure (MAP). LNP509 and LNP640, however, elicited substantial depressor responses in rabbits pretreated with flupirtine (-17 +/- 2 and -18 +/- 1 mmHg, respectively, P < 0.05). Injection of higher doses of LNP509 (200 microg/kg) or LNP640 (10 microg/kg) elicited substantial reductions in MAP (-45 +/- 3 and -39 +/- 2 mmHg, respectively, P < 0.05) in naive rabbits. The depressor responses elicited by the higher doses of LNP509 or LNP640 were markedly diminished by pretreatment with the GIRK channel blocker tertiapin-Q (10 microg/kg) (-23 +/- 3 and -26 +/- 2 mmHg, respectively, P < 0.05 compared with nonpretreated rabbits), whereas tertiapin-Q (10 microg/kg) did not affect MAP by itself. Maximal-specific binding (Bmax) of the I1R ligand [I]LNP911 to PC12 cell membranes (296 +/- 59 fmol/mg protein) was enhanced by flupirtine pretreatment whereas it was reduced by tertiapin-Q pretreatment (687 +/- 122 and 68 +/- 21 fmol/mg protein, respectively, P < 0.05 vs. control binding).

CONCLUSION

These findings demonstrate that the modulation of GIRK channels affects I1R's function and raise the possibility that GIRK channels, and I1Rs are parts of a single proteic complex.

Clonidine-evoked respiratory effects in anaesthetized rats

The respiratory effects of stimulation of alpha2-adrenergic receptors were studied in spontaneously breathing anaesthetized rats that were neurally intact, or bilaterally vagotomized, or subjected to bilateral combined midcervical vagotomy and section of the carotid sinus nerves. An intravenous clonidine bolus (15 microg kg(-1)) evoked a prolonged slowing of the respiratory rate in all the neural states explored. Vagotomy reduced the early clonidine-evoked decline, but not the augmentation of tidal volume that followed the decline. After section of the carotid sinus nerves, clonidine challenge continued to decrease the respiratory rate, but not the tidal volume. Blockade of alpha2-adrenergic receptors with intravenous doses of SKF 86466 (200 microg kg(-1)) abolished all respiratory effects of the clonidine challenge. In all the neural states studied, clonidine evoked a significant short-lived rise in mean arterial blood pressure followed by a decrease below the respective prechallenge value. The SKF 86466 pretreatment lowered mean arterial blood pressure control values and reduced the magnitude of postclonidine changes. These results indicate that: (i) clonidine-evoked activation of alpha2-adrenergic receptors affects the two components of the breathing pattern differently, and this occurs beyond the lung vagi; and (ii) changes in tidal volume result from excitation of the carotid bodies and are coupled with centrally mediated slowing of the respiratory rhythm.

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.

Effects of Long-Term Ovariectomy and Estrogen Replacement on Clonidine-Evoked Reductions in Blood Pressure and Hemodynamic Variability

The present study investigated the influence of 12-week ovariectomy (OVX) and estrogen supplementation (OVXE2) on the acute effects of the centrally acting antihypertensive agent clonidine on blood pressure, hemodynamic variability, and locomotor activity in Sprague-Dawley rats. The effects of estrogen depletion and repletion on the peripherally mediated hypotensive response to hydralazine were also evaluated to determine the selectivity of estrogen-clonidine interaction. The radiotelemetry technique was used for blood pressure and locomotor activity measurements. Three time-domain indices of hemodynamic variability were employed: (1). the standard deviation of mean arterial pressure (SDMAP) as a measure of blood pressure variability, and (2). the standard deviation of R-R intervals (SDRR) and the root mean square of successive differences in R-R intervals (rMSSD) as measures of heart rate variability. In control (sham-operated) rats, clonidine (30 microg/kg, i.p.) elicited significant decreases in MAP, blood pressure variability (SDMAP), and overall heart rate variability (SDRR). The reductions in MAP and its variability index (SDMAP) were significantly augmented in OVX rats and restored to sham-operated levels after estrogen replacement (17beta-estradiol subcutaneous pellet, 14.2 microg/d, 12 weeks). The locomotor activity was reduced by clonidine only in OVX rats. In contrast to clonidine, hydralazine (0.5 mg/kg, i.p.) hypotension was not altered by OVX or estrogen replacement. These findings suggest that estrogen negatively modulates centrally evoked hypotension versus no effect on hypotension of peripheral origin. Further, the results implicate the cardiovascular autonomic control in the enhanced hypotensive response to clonidine in OVX rats.

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Hypotensive and sedative effects of clonidine injected into the rostral ventrolateral medulla of conscious rats

We examined the effects of clonidine injected unilaterally into the rostral ventrolateral medulla (RVLM) of conscious, unrestrained rats. We also examined whether the local alpha(2)-adrenoceptor mechanism contributed to the action of clonidine injected into the RVLM. Injection of clonidine but not vehicle solution significantly decreased the mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) in conscious, unrestrained rats as well as in propofol-anesthetized rats. The frequency of natural behavior was significantly lower after clonidine injection than after vehicle injection. The depressor and sympathoinhibitory responses were significantly larger in the propofol-anesthetized rats than in the conscious rats. Coinjection of a selective alpha(2)-adrenoceptor antagonist, 2-methoxyidazoxan, with clonidine into the RVLM significantly attenuated the depressor, bradycardiac, sympathoinhibitory, and sedative effects of clonidine injected alone. In conclusion, clonidine injected into the RVLM decreased MAP, HR, and RSNA and caused sedation in conscious, unrestrained rats. The action of clonidine in the RVLM was at least partly mediated by alpha(2)-adrenoceptor mechanisms.

Relative importance of rostral ventrolateral medulla in sympathoinhibitory action of rilmenidine in conscious and anesthetized rabbits

The pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to compare the importance of the RVLM in mediating the inhibitory action of rilmenidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instrumented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressure in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast, the decrease in resting RSNA evoked by rilmenidine was similar in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmenidine microinjection into the RVLM was equally effective in inhibiting the RSNA baroreflex in both groups of animals. The upper plateau of the RSNA baroreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after rilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of rilmenidine on arterial pressure in conscious, chronically instrumented rabbits may relate to a lower level of sympathetic drive compared with anesthetized animals.

Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Hypertension associated with excessive liberation of circulating and tissue catecholamines is an independent risk factor for further cardiovascular complications and an important predictor of stroke. Moxonidine is a centrally acting anti-hypertensive drug with potent action on I1-imidazoline receptors. It inhibits catecholamine release and is therefore expected to exert an antiadrenergic effect at various levels in the regulation of the cardiovascular system. The aim of this study was to investigate the effect of moxonidine (0.025-0.1 mg/kg, i.v.) on the normal and unilateral carotid occlusion-induced impaired corticocerebral blood flow (cCBF) determined by hydrogen polarography, on mean arterial blood pressure (MABP) and heart rate (HR) in conscious rabbits. Moxonidine produced a reduction of MABP and HR. On the other hand, after administration of the drug, a significant increase in the normal and impaired cCBF was observed. Because the improvement in cCBF was conspicuous in both normal and ischemic conditions, moxonidine might be beneficial not only in the treatment of hypertension but also in the management of cerebral ischemia.

Renal vascular and biochemical responses to systemic renin inhibition in dogs at low renal perfusion pressure

Renin is produced by the kidney and secreted into the systemic circulation. However, its biochemical and physiological role of regulating renal blood flow with changing renal perfusion pressure (RPP) is not fully understood. In this study, the function of the intrarenal renin for production of angiotensin (Ang) I and maintenance of vascular tone was evaluated in dogs under normal conditions and when the kidney was perfused at low RPP. The dog left kidney was perfused first at normal (100 mm Hg) and then at low (30 mm Hg) RPP in the presence or absence of the renin inhibitor ciprokiren (3 mg/kg, i.v.). Both hemodynamic and biochemical parameters were measured. Lowering RPP markedly reduced left renal blood flow and elevated left renal vascular resistance. These effects were prevented by ciprokiren, which blocked the intrarenal production of Ang I. Lowering RPP increased the renal venous/ arterial ratio from 1.4+/-0.1 to 3.6+/-0.3 for plasma renin activity and from 2.4+/-0.2 to 9.8+/-1.1 for Ang I, but did not change the venous/arterial ratio for Ang II. The net renal venous conversion rate of Ang I to Ang II decreased from 0.22 to 0.09 after RPP was lowered, whereas the conversion rate in arterial blood was 1.35 and did not decrease significantly. Our results demonstrated the importance of intrarenal renin-angiotensin system for Ang I production and for the maintenance of the vascular tone, especially at low RPP. Our study also shows the limited capacity for Ang I conversion in the renal vasculature in vivo.

Central benzodiazepine involvement in clonidine cardiovascular actions

It is well known that the GABAergic and noradrenergic systems play an important role in blood pressure and heart rate regulation. Benzodiazepines and beta-carbolines, respectively, increase or decrease the probability of chloride-channel opening induced by GABA. The aim of this study was to determine, in conscious rats, the interaction existing between the central alpha2-adrenoceptor stimulation induced by clonidine and the facilitation or impairment of benzodiazepine receptor activity through the administration of either diazepam, a benzodiazepine receptor agonist, or methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), an inverse benzodiazepine agonist. Clonidine (5-10 µg, intracerebroventricularly) reduced heart rate and increased mean blood pressure by activation of central alpha2-adrenoceptors. Diazepam (2 mg/kg, intravenously (i.v.)) induced an increase in heart rate, while DMCM (0.3 mg/kg, i.v.) elicited a bradycardic effect. The bradycardic effects induced by both clonidine and DMCM were antagonized by the prior administration of methylatropine (1.5 mg/kg, i.v.). DMCM (0.3 mg/kg, i.v.) prevented the clonidine effects on heart rate and mean blood pressure, while diazepam (2 mg/kg, i.v.) failed to modify these effects. Our results suggest that the bradycardic effects of clonidine are mediated by a vagal stimulation and are related to the activation of a GABAergic pathway.Key words: blood pressure, clonidine, diazepam, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM), heart rate.

Adrenoceptor involvement in the cardiovascular responses to B-HT 920 in sinoaortic denervated rats

1. A study was made relating the involvement of alpha-adrenoceptors in the cardiovascular responses to intracerebroventricular (i.c.v.) injection of B-HT 920, a clonidine-type drug, in conscious sham-operated and sinoaortic-denervated rats. 2. Wistar rats were used, 7 days after the sham operation or sinoaortic denervation. For i.c.v. injection of drugs, a guide cannula had been previously implanted in the left lateral ventricle. 3. In sham-operated rats, cardiovascular responses to B-HT 920 (10-60 microg) were increased blood pressure and bradycardia; but, in sinoaortic-denervated rats, after the pressor response, a decrease in blood pressure also was seen. The responses to this agent were greater in sinoaortic-denervated rats than in sham-operated animals. Treatment with the alpha2-adrenoceptor antagonist yohimbine (30 microg), the imidazoline receptor antagonist idazoxan (15 microg) and the alpha1A-adrenoceptor antagonist 5-methylurapidil (15 microg) blocked the responses to B-HT 920 (30 microg). The alpha1-adrenoceptor antagonist prazosin (15 microg) and the alpha1B-adrenoceptor antagonist chloroethylclonidine (100 microg) did not modify the responses to agonist. 4. Sinoaortic denervation enhances the cardiovascular responses to B-HT 920. Moreover, the effects of i.c.v. administration of B-HT 920 could be mediated by several types of brain receptors: imidazoline receptors and alpha1A- and alpha2-adrenoceptors.

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.

Facilitation of reflex bradycardia does not contribute to the enhanced hypotensive effect of clonidine in aortic barodenervated rats

Our previous studies showed that the hypotensive effect of clonidine is enhanced in rats with surgically eliminated aortic baroafferents. In this study, we investigated whether this effect of clonidine is related to facilitation of baroreceptor control of reflex bradycardia. The effects of clonidine on blood pressure (BP), heart rate (HR), and baroreflex-mediated decreases in HR (baroreflex sensitivity, BRS) were studied in conscious aortic barodenervated (ABD) and sham-operated (SO) rats. The slope of the baroreflex curve relating increments in mean arterial pressure (MAP) induced by phenylephrine to corresponding baroreflex-mediated bradycardic responses was taken as an index of BRS. ABD but not the sham operation caused immediate and significant (p < 0.05) increases in BP and HR and an impairment of BRS. Two days after ABD, these parameters, except the BRS, subsided to near control levels. Starting from similar baseline values of BP and HR, clonidine (30 microg/kg, i.v.) elicited significantly (p < 0.05) greater decreases in MAP in conscious ABD rats compared with SO rats (-23 +/- 2 mm Hg vs. -7 +/- 2 mm Hg). The enhanced hypotensive effect of clonidine in ABD rats was associated with a significant reduction in baroreceptor-mediated reflex bradycardic responses to increments in BP evoked by phenylephrine. The slope of the baroreflex curves that represented the BRS showed approximately 40% reduction after treatment with clonidine (baseline BRS, -1.2 +/- 0.06 beats/min/mm Hg; clonidine, -0.73 +/- 0.07 beats/min/mm Hg). On the other hand, a threefold lower decrease in BP by clonidine in SO rats was not associated with any alteration in BRS. These findings support the hypothesis that aortic baroreceptors exert a tonically active restraining influence on centrally mediated hypotension. More important, the results do not favor a role for facilitation of baroreflexes in the enhanced hypotensive effect of clonidine in denervated rats.

Enhanced endothelial nitric oxide activity contributes to the reduced responsiveness of vascular alpha1-adrenoceptors following aortic barodenervation

We have recently shown that short-term aortic barodenervation diminishes constrictor responses to activation of alpha1-adrenoceptors in rat aortic smooth muscle. This study investigated the potential role of vascular endothelium and its derived vasoactive substances, nitric oxide and prostaglandins, in the reduced alpha1-adrenoceptor responsiveness after aortic barodenervation. Exposure of isolated aortic rings from aortic barodenervated and sham-operated rats 48 h after surgery to cumulative addition of phenylephrine (alpha1-adrenoceptor agonist, 3 x 10(-8) - 1 X 10(-4) M) resulted in concentration-related contractions that were significantly (P < 0.05) smaller in rings of denervated rats. Removal of the endothelium increased phenylephrine-mediated contractions in rings obtained from aortic barodenervated rats to near sham-operated levels as demonstrated by the similar contractile responses and slopes of the regression lines of the concentration-response curves. Pretreatment with indomethacin (cyclooxygenase inhibitor, 1 x 10[-5] M) had no effect on contractile responses to phenylephrine in rings from both groups of rats. In contrast, N(G)-nitro-L-arginine (nitric oxide synthase inhibitor, 3 x 10[-5] M) elevated basal vascular tone and significantly (P < 0.05) increased alpha1-adrenoceptor responsiveness, effects that were more evident in rings from denervated compared with sham-operated rats. N(G)-nitro-L-arginine produced significantly (P < 0.05) greater increases in the slopes of the regression lines (136.1 +/- 22% vs. 73.0 +/- 8.6% mg tension/mg tissue/log molar concentration) and maximum contractile response (Emax) to phenylephrine (161.2 +/- 8.2% vs. 76.7 +/- 6.1%) in rings from denervated compared with sham-operated rats suggesting an enhanced nitric oxide activity in aortas of denervated rats. This notion is further supported by the finding that cumulative i.v. administration of N(G)-nitro-L-arginine (1, 2, 4 and 8 mg/kg) elicited significantly (P < 0.05) greater pressor responses in conscious barodenervated compared with sham-operated rats. These results suggest that the endothelium plays a major role in the reduced constrictor responses to alpha1-adrenoceptor activation that occurs shortly after aortic barodenervation. This effect of the endothelium appears to involve, at least in part, enhancement of endothelial nitric oxide activity.

Effect of rilmenidine on arterial pressure and urinary output in the spontaneously hypertensive rat

Rilmenidine is an antihypertensive agent acting at the imidazoline receptor that may have both central effects in the ventral lateral medulla and direct effects on the kidney to alter Na+ excretion. The present experiments examined whether rilmenidine induces a leftward shift or change in the slope of the pressure-natriuresis curve in the spontaneously hypertensive rat (SHR). A single oral gavage dose indicated that 3 and 10 mg/kg rilmenidine significantly lowers arterial pressure at 4-12 h after administration by oral gavage. The effect of rilmenidine on pressure-natriuresis was studied using twice daily doses of 1 and 3 mg/kg for control and treated SHR drinking tap water or 1% NaCl for 3 days. Na+ excretion was measured over 24 h, and mean arterial pressure was measured 6-8 h after the morning dose of rilmenidine. The results indicate that 1 mg/kg had no effect, while the pressure-natriuresis relationship for the rats receiving the 3 mg/kg dose was shifted to the left and was not significantly different from the vertical slope of the untreated SHR. This experiment also suggested that rilmenidine may attenuate the salt preference of the rats. This was confirmed in an additional series of experiments in which the rats had access to both tap water and 1% NaCl. Thus, rilmenidine shifts the pressure-natriuresis relationship to the left and reduces salt preference in SHR.

Short-term aortic barodenervation diminishes alpha 1-adrenoceptor reactivity in rat aortic smooth muscle

Our previous studies have shown that aortic baroreceptor denervation elicits acute increases in blood pressure and significant elevations of sympathetic activity and peripheral vascular resistance. This study investigated the short-term (3 and 48 h) effect of aortic barodenervation and associated sympathetic hyperactivity on the functional activity of alpha 1-adrenoceptors in rat aortic smooth muscle. Compared with sham operation, aortic barodenervation caused acute rises in blood pressure and heart rate and reductions in baroreflex sensitivity. Blood pressure and heart rate remained elevated when measured in conscious aortic barodenervated rats 3 h after surgery but subsided to sham-operated levels at 48 h; the baroreflex sensitivity, however, remained attenuated. Hexamethonium (0.5-4 mg/kg, i.v.) elicited significantly (P < 0.05) greater depressor responses in conscious aortic barodenervated rats than in sham-operated rats at both 3 and 48 h, suggesting a higher sympathetic activity in denervated rats. Exposure of aortic rings from aortic barodenervated and sham-operated rats to cumulative addition of phenylephrine (alpha 1-adrenoceptor agonist, 3 x 10(-8)-1 x 10(-4) M) resulted in concentration related contractile responses that were similar in the two groups of rats at 3 h in contrast to significantly (P < 0.05) smaller contractions in rings from denervated rats at 48 h. The maximum contraction developed (Emax) at 48 h showed approximately 50% reduction in rings from aortic barodenervated compared with sham-operated rats (239 +/- 16 vs. 558 +/- 15 mg tension/mg tissue). The pA2 value for prazosin (alpha 1-adrenoceptor antagonist) was not altered by aortic barodenervation at 3 h but showed significant (P < 0.05) increases, compared with sham-operated values, at 48 h. It is concluded that short-term aortic barodenervation results in an elevation of sympathetic activity that coincides with reduced responsiveness of aortic smooth muscle to alpha 1-adrenoceptor activation. The aortic barodenervation-induced alpha 1-adrenoceptor desensitization is not a result of decreased receptor affinity but may involve an alteration of receptor density or in the post-receptor activation events.

Upregulation of imidazoline receptors in the medulla oblongata accounts for the enhanced hypotensive effect of clonidine in aortic barodenervated rats

The present study tested the hypothesis that an upregulation of the imidazoline receptor in the rostral ventrolateral medulla (RVLM) of aortic barodenervated (ABD) rats may account for the enhanced hypotensive effect of clonidine. In vitro autoradiographic radioligand binding studies were utilized to investigate the binding characteristics of imidazoline receptors in the RVLM and nucleus tractus solitarius (NTS), areas that play critical roles in cardiovascular regulation and elicitation of clonidine responses. ABD but not sham operation (SO) caused immediate and significant (P < 0.05) increases in mean arterial pressure (MAP) and heart rate (HR) and an impairment of the baroreflex-mediated HR response (baroreflex sensitivity, BRS). Two days after ABD, these parameters, except BRS, subsided to near-control (SO) levels. Intracisternal (i.c.) administration of clonidine (0.1 micrograms) elicited a 3-fold greater decrease in BP of conscious ABD compared with SO rats (-20.3 +/- 2.6 vs. -7.4 +/- 0.9 mmHg) thus demonstrating the ability of ABD to enhance centrally-mediated hypotensive responses. Autoradiographic visualization of brain sections obtained from separate groups of ABD and SO rats 48 h after surgery preincubated with [3H]idazoxan (2.5-3.5 nM) showed that [3H]idazoxan binding in RVLM, middle NTM (mNTS) and rostral NTS (rNTS) was saturable and of high affinity. Uneven distribution of imidazoline binding sites was evident since in control (SO) rats, Scatchard analysis of binding data revealed similar densities (Bmax) of [3H]idazoxan binding sites in the RVLM and mNTS versus significantly higher density in the rNTS. In ABD rats, the binding dissociation constant (Kd) was significantly decreased in both the RVLM (8.1 +/- 3.1 vs. 21.4 +/- 5.0 nM) and rNTS (12.3 +/- 1.3 vs. 18.6 +/- 3.1 nM) compared with SO rats while the Bmax was not affected. This finding suggests an increased receptor affinity in the RVLM and rNTS of barodenervated rats. The mNTS of ABD rats exhibited significant increases in the Bmax (861 +/- 96 vs. 570 +/- 87 fmol/mg protein) compared with values of SO rats but the receptor affinity was not affected. It is concluded that: (i) aortic baroreceptors exert a tonic inhibitory influence on central imidazoline receptor function; and (ii) the enhanced hypotensive effect of clonidine in conscious ABD rats may be accounted for by the increased affinity of the medullary imidazoline receptors particularly in the RVLM.

Involvement of peripheral presynaptic inhibition in the reduction of sympathetic tone by moxonidine, rilmenidine and UK 14304

We studied the possibility that presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by moxonidine, rilmenidine and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK 14304). In pithed rabbits without electric stimulation, moxonidine, rilmenidine and UK 14304 caused a long-lasting, > 10 min, increase in arterial pressure. Heart rate was not changed. In pithed rabbits in which sympathetic tone was created by electric stimulation through the pithing rod (2 Hz), the same doses of moxonidine, rilmenidine and UK 14304 caused only a brief, < 10 min, blood pressure rise. Heart rate was decreased, as were the plasma concentrations of noradrenaline and adrenaline. Dose-response curves for the effects on the plasma noradrenaline concentration (stimulated pithed rabbits) were compared with previously obtained dose-response curves for depression of renal sympathetic nerve activity (conscious rabbits). For each drug, the curve describing peripheral presynaptic inhibition and the curve describing central sympathoinhibition were very similar. Both the power and the dose dependence of the peripheral inhibitory effect support its contribution to the overall decrease in sympathetic tone produced by clonidine-like drugs in intact animals. The peripheral effect in all likelihood consists in activation of presynaptic alpha 2-autoreceptors. The agreement of the dose-response curves for the peripheral and for the central effect supports the view that the central effect, like the peripheral one, is mediated through alpha 2-adrenoceptors.

Clonidine and rilmenidine suppress hypotension-induced Fos expression in the lower brainstem of the conscious rabbit

Our current knowledge of the sites of action of the centrally-acting antihypertensive drug clonidine is based almost entirely on experiments in anesthetized animals. The aim of this study was to determine, in conscious rabbits, the sites of action in the brainstem of systemically administered clonidine, as well as its oxazoline analog rilmenidine. Three groups of experiments were carried out. In the first group, hypotension was produced by continuous intravenous infusion of sodium nitroprusside, at a rate sufficient to decrease arterial pressure by 20-30 mmHg, maintained for a period of 60 min. In the second and third groups of experiments, sustained hypotension was also produced by nitroprusside infusion as in the first group, but this was preceded by intravenous injection of clonidine (7-30 micrograms/kg i.v.) or rilmenidine (150-300 micrograms/kg i.v.), respectively. In confirmation of our previous study [Li Y.-W. and Dampney R. A. L. (1994) Neuroscience 61, 613-634], hypotension produced by nitroprusside alone induced a large increase (compared to sham control experiments) in the neuronal expression of Fos (a marker of neuronal activation) in the nucleus of the solitary tract, area postrema, the rostral, intermediate and caudal parts of the ventrolateral medulla, A5 area, locus coeruleus and subcoeruleus, and parabrachial nucleus. In comparison with this group, in rabbits pretreated with clonidine the numbers of Fos-positive cells were greatly reduced (by 76-94%) in the rostral, intermediate and caudal parts of the ventrolateral medulla, area postrema, A5 area, locus coeruleus and subcoeruleus. Clonidine pretreatment also caused a more moderate reduction (by 45%) in the number of Fos-positive cells in the nucleus of the solitary tract, but had no effect on Fos expression in the parabrachial nucleus. Double-labeling for tyrosine hydroxylase and Fos immunoreactivity showed that clonidine pretreatment greatly reduced the numbers of both catecholamine and non-catecholamine Fos-positive neurons. Rilmenidine pretreatment also greatly reduced Fos expression in the lower brainstem, with a very similar pattern to that observed after clonidine pretreatment. The results indicate that in conscious animals both clonidine and rilmenidine cause a widespread but selective inhibition of neurons in the pons and medulla that are normally activated by a hypotensive stimulus. In contrast to previous observations in anesthetized animals, the results suggest that (i) systemic administration of both drugs inhibits non-catecholamine as well as catecholamine neurons in the ventrolateral medulla, and (ii) the regional pattern of neuronal inhibition following administration of equipotent hypotensive doses of both drugs is very similar.

Involvement of imidazoline-preferring receptors in regulation of sympathetic tone

We examined the contribution of imidazoline-preferring receptors (IPR) and alpha 2-adrenoceptors at different levels of the central nervous system in the antihypertensive and sympathoinhibitory actions of rilmenidine in 2 conscious animal models, the spontaneously hypertensive rat (SHR) and the normotensive rabbit. In conscious SHRs, we compared the potency of rilmenidine and clonidine administered intravenously into the lateral cerebral ventricle, the cisterna magna, and into the subarachnoidal space of the thoracolumbar spinal cord. In SHRs, we found that rilmenidine was more potent and more effective by the intrathecal than the intracisternal route. By contrast, clonidine was most effective after administration into the cisterna magna. Intravenous administration of rilmenidine or clonidine induced dose-dependent and prolonged decreases in blood pressure and heart rate. Neither rilmenidine nor clonidine altered mean arterial pressure or heart rate when given into the lateral cerebral ventricle. These data suggest that in SHRs the spinal cord may be an important site for the antihypertensive action of rilmenidine. We therefore characterized the receptor type involved. We observed in conscious SHRs that intrathecal post-treatment with idazoxan, a mixed alpha 2-adrenoceptor and IPR antagonist, abolished the antihypertensive effect of rilmenidine, whereas 2-methoxyidazoxan, a selective alpha 2-adrenoceptor antagonist, caused only a partial reversal of the blood pressure effects of rilmenidine. These results suggest that rilmenidine acts mainly through IPR rather than alpha 2-adrenoceptors in the spinal cord. In view of these findings, we compared the hypotensive actions of rilmenidine and clonidine, administered into the lateral cerebral ventricle, the cisterna magna, and the subarachnoid space of the thoracolumbar spinal cord in conscious normotensive rabbits. Both drugs were less potent and effective when administered intrathecally than intracisternally. These experiments suggest that the hypotensive action of rilmenidine and clonidine in the rabbit is mediated through receptors mainly located in the brainstem. Further, we found that idazoxan reversed the hypotensive action of rilmenidine more readily than 2-methoxyidazoxan. Surprisingly, both idazoxan and 2-methoxyidazoxan completely reversed the depressor effects of clonidine. Therefore, in the rabbit, rilmenidine acts through IPR located in the brainstem and clonidine acts predominantly through alpha 2-adrenoceptors. In conclusion, our studies demonstrate that IPR are involved in the vasodepressor action of rilmenidine in both conscious SHRs and rabbits. However, although the main site of action of rilmenidine in SHRs may be located in the thoracolumbar spinal cord, in the rabbit it appears to be in the brainstem.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol-clonidine hemodynamic interaction in normotensive rats is modified by anesthesia

Our previous findings have shown that ethanol attenuates the decreases in plasma norepinephrine (NE) levels, and blood pressure elicited by centrally acting antihypertensive drugs in spontaneously hypertensive rats (SHRs). The present study investigated whether this interaction can be influenced by baseline blood pressure (BP) and sought direct evidence to support the involvement of the SNS by recording the sympathetic neural activity (SNA) of the splanchnic nerve. The conscious aortic barodenvervated (ABD) rat model was utilized because it exhibits greater hypotensive responses to clonidine compared with sham-operated (SO) rats. Although ABD and SO rats (332 + 33 vs. 227 + 18 pg/ml). Clonidine (30 micrograms/kg, i.v.) elicited significantly greater decreases in mean arterial pressure (MAP; -20.0 + 2.1 vs. -10.4 + 0.8 mm Hg) and plasma NE (-194 + 26 vs. -50 + 11 pg/ml) in conscious ABD vs. SO rats. Ethanol (1 g/kg, i.v.) reversed clonidine-evoked decreases in BP and plasma NE levels, but the interaction was more prominent in ABD rats. To support the hypothesis that the interaction occurs within the CNS, the effect of ethanol was studied on clonidine-evoked decreases in preganglionic SNA and BP in anesthetized rats. In contrast to its effects in conscious rats, ethanol augmented both the hypotensive and sympathoinhibitory responses to clonidine in anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine reverses the slowly developing hypertension produced by low doses of angiotensin II

We used the centrally acting sympatholytic drug clonidine to evaluate neurogenic pressor activity in rats made hypertensive by chronic intravenous infusion of a low dose (4.0 ng.min-1) of angiotensin II (Ang II). Male Sprague-Dawley rats were instrumented with arterial and venous catheters and then housed in metabolic cages for the duration of the experiment. After 3 days of recovery from surgery, daily measurements were begun of mean arterial pressure, heart rate, water balance, and urinary electrolyte excretion. After 3 days of control measurements rats received either Ang II (4.0 ng.min-1 IV, n = 5) or saline vehicle (n = 4) continuously over a 15-day period. After the Ang II and vehicle infusions were ended, measurements were made during 3 days of recovery. On days 2, 7, and 12 of the experimental infusion period, clonidine hydrochloride was administered as a bolus (10.0 micrograms.kg-1 IA). The resulting changes in mean arterial pressure and heart rate were assessed over a 6-hour period. Fluid measurements were evaluated on a daily basis. In rats receiving only vehicle, clonidine produced no significant changes in any variable at any time. In rats given Ang II, mean arterial pressure increased from a control value (mean +/- SEM) of 106 +/- 1 mm Hg to 135 +/- 6, 139 +/- 6, and 148 +/- 4 mm Hg on days 2, 7, and 12, respectively. The antihypertensive response to clonidine after 6 hours on days 2, 7, and 12 of the Ang II infusion in these rats was -18 +/- 8, -16 +/- 5, and -23 +/- 9 mm Hg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathoinhibition by rilmenidine in conscious rabbits: involvement of alpha 2-adrenoceptors

Cardiovascular and sympathetic nervous system effects of the mixed alpha 2-adrenoceptor and imidazoline receptor agonist rilmenidine were studied in conscious rabbits chronically instrumented for the recording of the firing rate of renal sympathetic fibers. Separate experiments were carried out on pithed rabbits with electrically stimulated (2 Hz) sympathetic outflow. Drugs were administered intravenously in a cumulative manner. In conscious rabbits, rilmenidine 0.1, 0.3 and 1.0 mg kg-1 dose-dependently lowered blood pressure, renal sympathetic nerve activity, heart rate and the plasma concentration of noradrenaline and adrenaline. The effect on blood pressure and plasma catecholamines was maximal after 0.3 mg kg-1 whereas heart rate and renal sympathetic nerve activity decreased further after rilmenidine 1.0 mg kg-1. Yohimbine 0.1 and 0.5 mg kg-1, when injected subsequently, attenuated and at the higher dose abolished all effects of rilmenidine. The effects of rilmenidine were also antagonized by the alpha 2-adrenoceptor antagonist 2-(2,3-dihydro-2-methoxy-1,4-benzodioxin-2-yl)-4,5-dihydro-1H-imid azole HCl (RX821002; 0.1 and 0.5 mg kg-1). Yohimbine 0.1 and 0.5 mg kg-1 did not attenuate or attenuated only slightly the decrease of heart rate and renal sympathetic nerve activity produced by infusion of vasopressin. In pithed rabbits with electrically-stimulated sympathetic outflow, yohimbine 0.1 submaximally and yohimbine 0.5 mg kg-1 maximally increased the plasma noradrenaline concentration. The experiments show by direct measurement of sympathetic nerve firing and plasma catecholamines that rilmenidine causes sympathoinhibition in conscious rabbits, presumably through central sites of action.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathoinhibitory effects of rilmenidine may be mediated by sites located below the brainstem

1. To determine the site of action of rilmenidine, we examined its effets on arterial blood pressure (BP), heart rate (HR) and postganglionic renal sympathetic nerve activity (RSNA) after intracerebroventricular (i.c.v.) administration (300 micrograms kg-1), in groups (all n = 6) of conscious and freely moving, pentobarbitone-anaesthetized and pentobarbitone-anaesthetized and spinally transected, fifteen week-old male spontaneously hypertensive rats (SHRs). 2. In conscious SHRs, which exhibited a low sympathetic nerve activity (RSNA: 3.4 +/- 0.9 muV), rilmenidine was inactive on systolic BP (SBP), diastolic BP (DBP), HR and RSNA. 3. In intact pentobarbitone-anaesthetized SHRs, which exhibited an elevated sympathetic nerve activity (RSNA: 10.6 +/- 0.9 muV), rilmenidine exerted potent antihypertensive (delta SBP: -37 +/- 4%; delta DBP: -43 +/- 6%), bradycardic (delta HR: -32 +/- 3%) and sympathoinhibitory (delta RSNA: -68 +/- 9%) activities. 4. In pentobarbitone-anaesthetized SHRs with cervical spinal cord transection, BP was markedly decreased and sympathetic nerve activity (RSNA: 10.3 +/- 3.1 muV) returned to the level observed in conscious SHRs (RSNA: 3.6 +/- 0.5 muV). In these conditions, rilmenidine remained sympathoinhibitory (delta RSNA: -74 +/- 5%). 5. In conclusion, we have shown that pentobarbitone-anaesthesia enhances the peripheral sympathetic tone by a central action, as the spinal cord transection allows RSNA to return to normal levels and that, spinal or ganglionic structures could be a major site of the sympathoinhibitory action of rilmenidine.