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

1. The acute cardiovascular effects of two alpha 2-adrenoceptor agonists, rilmenidine and clonidine, were studied in 15-week-old male spontaneously hypertensive rats (SHRs). The effects of these drugs were compared with intravenous (i.v.) and intracerebroventricular (i.c.v.) administration in conscious and pentobarbitone-anaesthetized SHRs, in which aortic blood pressure (BP) was continuously recorded. 2. In conscious SHRs, i.v. doses of either rilmenidine (30, 100, 300 micrograms kg-1) or clonidine (3, 10, 30 micrograms kg-1) induced dose-dependent short-lasting increases in BP followed by moderate decreases associated with bradycardia, while the same three doses of both drugs given i.c.v. were devoid of BP and heart rate (HR) effects. 3. Pentobarbitone-anaesthesia increased the sympathetic control of BP and suppressed the cardiac baroreflex sensitivity. 4. In anaesthetized SHRs, i.v. injections of the same 3 doses of rilmenidine and clonidine induced a slight increase in BP, rapidly followed by profound and long-lasting BP and HR decreases. Surprisingly, when given i.c.v., these 3 doses lowered BP and HR to the same extent but in a more progressive manner. 5. The lack of efficacy of both drugs in conscious SHRs after the i.c.v. administration of i.v. active doses and the lack of more marked and rapid effects in anaesthetized SHRs, after i.c.v. than after i.v. injections, question the involvement of a major central site of action for these antihypertensive alpha 2-adrenoceptor agonists. Moreover, these results show that the cardiovascular effects of these drugs are profoundly influenced by baseline sympathetic nervous system activity which is enhanced by pentobarbitone-anaesthesia.

Preliminary clinical pharmacological studies of S3341, a new hypotensive agent, and comparison with clonidine in normal males

S3341, a new hypotensive agent which binds to alpha2-receptors in animal brain preparations, was studied in normal healthy male volunteers. A dose ranging study with 15 and 25 micrograms/kg of S3341 was performed in a double blind, placebo controlled randomised and balanced manner with 3 subjects. A decrease or BP without noticeable sedation (assessed by visual analogue scales) was seen. One, 2 mg of S3341, 0.1 and 0.2 mg of clonidine were then compared in a double blind, placebo controlled, randomised and balanced manner in 10 subjects. BP, heart rate, systolic time intervals (STI), critical flicker frequency, choice reaction time, pursuit rotor, stimulated salivary volume, and dryness of mouth and sedation with visual analogue scales, were measured at 0, 1.5, 3.0, 4.5 and 6.0 h after drug administration. The relationship between decreases in BP and sedation was assessed by linear regression analysis, with the former as the independent (predictor) response and the latter as the dependent (response) variate. Both drugs produced a similar decrease of BP which was significantly different from placebo. Changes in psychomotor function tests were not significant. Both drugs produced dryness of mouth and sedation which were significantly different from placebo but changes were less with S3341. Clonidine showed a significantly steeper slope than S3341 in the relationship between decreases in BP and sedation. This must be interpreted with caution as there was wide variation in the correlation between decreases in BP and sedation, but it may be possible to achieve lesser sedation with S3341.

Importance of imidazoline receptors in the cardiovascular actions of centrally acting antihypertensive agents

Increasing evidence indicates that the hypotensive effect of centrally acting antihypertensive drugs is not due to stimulation of alpha 2-adrenoceptors but to action on imidazoline receptors (IR). This has led to the development and recent clinical use of second generation agents such as rilmenidine and moxonidine that possess a much greater selectivity toward these nonadrenergic receptors. However, relatively few studies have examined the role of these receptors in conscious animals or have adequately accounted for the alpha 2-adrenoceptor antagonist properties of IR antagonists such as idazoxan. We have taken the approach of initially calibrating the alpha 2-adrenoceptor antagonist potency of intracisternally (ic) administered idazoxan and the IR-1 receptor antagonist efaroxan against 2-methoxyidazoxan, a highly selective alpha 2-adrenoceptor antagonist with little or no imidazoline antagonist effect. This was done using alpha-methyldopa, a hypotensive agent affecting only alpha 2-adrenoceptors. Thus, we chose doses of the antagonists with equal alpha 2-adrenoceptor blocking action such that differences in the ability of idazoxan or efaroxan compared to 2-methoxy-idazoxan to reverse the hypotension produced by rilmenidine, moxonidine, or clonidine indicate an interaction with IR. By this method we found that the hypotensive effects of rilmenidine and moxonidine at moderate intracisternal doses were more readily reversed by the imidazoline antagonists than by 2-methoxy-idazoxan, indicating that IR were largely responsible for their hypotensive actions. By contrast, clonidine's effects were equally reversed by all antagonists, suggesting interaction mainly with alpha 2-adrenoceptors. In conscious rabbits with chronic renal sympathetic nerve electrodes we examined the effect of rilmenidine and alpha-methyldopa on the renal sympathetic baroreflex. Both drugs reduced renal sympathetic nerve activity and sympathetic baroreflex responses, but only the effect of rilmenidine was preferentially reversed by idazoxan. Thus, both IR and central alpha 2-adrenoceptor receptors can influence the renal baroreflex, but the former are relatively more important for the actions of rilmenidine. We recently examined the possible sites of action of rilmenidine in anesthetized rabbits and showed that sixfold lower doses were required to reduce blood pressure when the drug was injected into the rostral ventrolateral medulla compared to intracisternal administration. At this site rilmenidine also reduced renal sympathetic tone and inhibited renal sympathetic baroreflex responses. By contrast, rilmenidine was relatively ineffective when injected into the nucleus of the solitary tract. These experiments support the view that rilmenidine acts primarily at IR in the rostral ventrolateral medulla to reduce sympathetic tone and modulate sympathetic baroreflexes.

Efficacy and acceptability of rilmenidine for mild to moderate systemic hypertension

A double-blind multicenter trial compared rilmenidine with placebo in the treatment of 126 patients with mild to moderate hypertension after a 4-week placebo run-in period. Patients with mild hypertension (study 1) with mean supine diastolic blood pressure (BP) between 95 and 104 mm Hg received either rilmenidine 1 mg/day (n = 31) or placebo (n = 35) for 4 weeks. In study 2, patients with moderate hypertension (mean supine diastolic BP between 105 and 115 mm Hg) received either rilmenidine 1 mg twice a day (n = 30) or placebo twice a day (n = 30) for 4 weeks. All 61 patients taking rilmenidine completed the study; 8 of the 65 patients taking placebo were withdrawn because of an increase in BP. Rilmenidine significantly reduced mean systolic and diastolic BP compared with placebo in both studies. BP was normalized (systolic less than 160 mm Hg and diastolic less than or equal to 90 mm Hg in 61% of the patients taking rilmenidine as opposed to 23% of those taking placebo (p less than 0.001). There was no significant difference in the incidence of either dry mouth or daytime drowsiness between rilmenidine, 1 mg/day, and placebo. Dry mouth was significantly more frequent with rilmenidine, 2 mg/day, than with placebo, but this difference was transient and no longer significant at the end of the study. No unexpected adverse effects occurred. Rilmenidine as single therapy appears to be effective and well accepted in the management of mild to moderate hypertension, in particular at the 1-mg/day dose, which normalized 84% of mild hypertensive patients and did not induce any significant adverse effects compared with placebo.

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

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

Rilmenidine-induced hypotension in conscious rabbits involves imidazoline-preferring receptors

The relative contributions of imidazoline-preferring receptors (IPR) and alpha 2-adrenoceptors to hypotensive and bradycardic effects of intracisternal (i.c.) rilmenidine were investigated in conscious rabbits. We compared the antagonist potencies of two alpha 2-adrenoceptor antagonists, 2-methoxy-idazoxan (0.001-10 micrograms/kg i.c.), which has very low affinity for IPR, and idazoxan (0.003-30 micrograms/kg i.c.), which has high affinity for blocking IPR. We also compared the i.c. effects of the antagonists on responses to alpha-methyldopa (alpha-MD), a drug with centrally acting alpha 2-adrenoceptor agonist metabolites that have no affinity for IPR. Rilmenidine (22 micrograms/kg i.c.) and alpha-MD (400 micrograms/kg i.c.) produced similar decreases in mean arterial pressure (MAP) (delta MAP = -23 +/- 2 and -24 +/- 2%, respectively) and in heart rate (HR) (delta HR = -11 +/- 1 and -9 +/- 2%, respectively, n = 30). The hypotension and bradycardia produced by alpha-MD and rilmenidine were completely reversed by 2-methoxy-idazoxan, but 2-methoxy-idazoxan was 16 and 9 times more potent at restoring MAP and HR, respectively, after alpha-MD than after rilmenidine. In contrast, idazoxan was more potent in reversing the hypotension elicited by i.c. injections of rilmenidine than that elicited by alpha-MD. Idazoxan, however, had no effect on rilmenidine-induced bradycardia, but did dose-dependently reverse the decrease in HR produced by alpha-MD. In separate experiments, we observed that the doses of each antagonist drug in themselves did not modify MAP nor HR significantly, but a 10-fold higher dose of idazoxan (300 micrograms/kg) caused immediate although brief hypertension and tachycardia.(ABSTRACT TRUNCATED AT 250 WORDS)

New concepts on the central regulation of blood pressure. Alpha 2-adrenoceptors and "imidazoline receptors"

The most usual hypothesis to explain the central hypotensive effect of clonidine-like substances was to admit that these drugs stimulated alpha 2-adrenoceptors within the brainstem. Now it has been demonstrated that neither the endogenous ligand to the alpha-adrenoceptors, noradrenaline, nor any other catecholamine or phenylethylamine was hypotensive in the medullary nucleus reticularis lateralis, where all imidazolines proved to be such. Recently, a membrane receptor population sensitive to clonidine and insensitive to catecholamines was described within the nucleus reticularis lateralis; this subgroup of receptors represented 20 to 30 percent of the [3H]clonidine binding sites in the bovine nucleus reticularis lateralis and 100 percent within the human nucleus reticularis lateralis region. Thus, the existence of such imidazoline specific receptors was clearly established and the endogenous ligand for those receptors, which is neither a catecholamine nor likely a peptide, is under processing for purification. Therefore, it appeared that the hypotensive effect of substances with an imidazoline or imidazoline-like structure might be due to their action within medullary receptors specific for this endogenous ligand temporarily named "clonidine displacing substance." Rilmenidine, structurally close to imidazolines, also interfered with these receptors. The central component of its hypotensive effect was recently confirmed in rabbits, where its central cardiovascular effects were antagonized by "the clonidine displacing substance." Although exhibiting a lower affinity than the reference substance for these receptors, rilmenidine might have a higher selectivity, thus explaining its restricted side effects. A structure-activity study with this molecule would bring a confirmation to these first observations.

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.

Imidazoline I(1) receptor-induced activation of phosphatidylcholine-specific phospholipase C elicits mitogen-activated protein kinase phosphorylation in PC12 cells

In the present study, we tested the hypothesis that the activation of imidazoline I(1)-receptor, which is coupled to phosphatidylcholine-specific phospholipase C, results in downstream activation of mitogen-activated protein kinase (p42(mapk) and p44(mapk) isoforms) in PC12 cells. PC12 cells pretreated with nerve growth factor (50 ng/ml, 48 h) to initiate neuronal differentiation were incubated with [methyl-3H]choline and [3H]myristate. Activation of imidazoline I(1) receptor by rilmenidine (10 microM) caused time-dependent increases in diacylglycerol accumulation and phosphocholine release. The Western blotting analysis showed that rilmenidine (10 microM) produced a time-dependent activation of p42(mapk) and p44(mapk) that reached its maximum at 15 min and returned to control levels after 30 min. This finding was confirmed by immunofluorescence labeling of activated mitogen-activated protein kinase in the same model system. Efaroxan (imidazoline I(1)-receptor antagonist) or tricyclodecan-9-yl-xanthogenate (D609, phosphatidylcholine-specific phospholipase C inhibitor) attenuated the phosphorylation of p42(mapk) and p44(mapk) induced by rilmenidine. Nerve growth factor-induced phosphorylation of both mitogen-activated protein kinase isoforms was not affected by D609. These results support the hypothesis that the activation of the imidazoline I(1) receptor coupled phosphatidylcholine-specific phospholipase C results in the downstream activation of mitogen-activated protein kinase.

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

BACKGROUND

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

OBJECTIVES

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

DESIGN AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Sympathetic Responses to Stress and Rilmenidine in 2K1C Rabbits

We determined whether the sympathetic excitatory responses to environmental stressors and the sympathoinhibitory responses to rilmenidine are altered by renovascular hypertension. Rabbits were made hypertensive with a clip on the right renal artery, and a left renal nerve recording electrode was implanted. After 3 or 6 weeks, the animals were given air-jet stress and loud noise stress before and after intravenous rilmenidine. Three and 6 weeks after renal clipping, mean arterial pressure was 28% and 36% greater than preclip values. Air-jet stress elicited a marked increase in renal sympathetic nerve activity, mean arterial pressure, and heart rate. Renal sympathetic nerve activity responses were much greater in hypertensive rabbits, but the pressor responses were similar to those observed in normotensive animals. Acute administration of rilmenidine decreased blood pressure more in hypertensive animals but with a much lesser inhibition of sympathetic activity. Rilmenidine markedly reduced increased sympathetic activity during air-jet stress in 3-week clipped rabbits but to a lesser extent in the other groups. These studies show that while sympathetic responses to stress were markedly enhanced in renal clip hypertensive rabbits, they did not result in greater pressor responses, thus suggesting that vascular neuroeffector mechanisms were not altered. By contrast, the increased effects of rilmenidine suggest a much greater contribution to the hypertension by the sympathetic nervous system, but one that is caused by an enhanced "nonvascular" neuroeffector mechanism. As such, sympathoinhibitory agents such as rilmenidine are very suitable and very effective agents for the treatment of renovascular hypertension.

Imidazoline receptor agonist drugs: a new approach to the treatment of systemic hypertension

The imidazoline receptors have recently been discovered to be involved in central nervous system control of blood pressure (I-1 receptor) and in neuroprotection for cerebral ischemia (I-2 receptor). A new class of central-acting antihypertensive agents has been developed, the imidazoline receptor agonists (rilmenidine and moxonidine), which control blood pressure effectively without the adverse effects of sedation and mental depression that are usually associated with central-acting antihypertensives. This new generation of central-acting antihypertensive agents are highly selective for the imidazoline receptor, while having a low affinity for alpha 2-adrenergic receptors.

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

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

Nischarin as a functional imidazoline (I1) receptor

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

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)

Cause and Consequences of Sympathetic Hyperactivity in Chronic Kidney Disease

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

Central imidazoline receptors and centrally acting anti-hypertensive agents

We have examined the location and contribution of imidazoline receptors (IR) in mediating the hypotensive and sympatholytic actions of first and second generation anti-hypertensive agents in rabbits. We found that the hypotension produced by rilmenidine and moxonidine given intravenously (i.v.) or into the fourth ventricle (4V) was preferentially reversed by the IR antagonists idazoxan and efaroxan (compared to a selective alpha(2)-adrenoceptor antagonist 2-methoxy-idazoxan), suggesting that IR are important in the sympatho-inhibition produced by these agents. Clonidine was not preferentially reversed by the IR antagonists suggesting an action via alpha(2)-adrenoceptors. In anaesthetised rabbits, the rostral ventrolateral medulla (RVLM) was the most potent site for rilmenidine to produce the sympatho-inhibition and modulation of sympathetic baroreflexes. alpha-Methylnoradrenaline was also sympatholytic suggesting alpha(2)-adrenoceptors are also present in this site. Microinjection of the IR and alpha(2)-adrenoceptor antagonists showed that rilmenidine activates IR in the RVLM but that alpha(2)-adrenoceptors are also activated as a consequence. These studies suggest that rilmenidine acts primarily via IR in the RVLM to reduce sympathetic tone but also imply an important association of alpha(2)-adrenoceptors and IR in the region.

Efficacy and safety of rilmenidine for arterial hypertension

To assess the long-term acceptability and efficacy of rilmenidine (S 3341), patients with placebo-resistant hypertension (diastolic blood pressure [BP] greater than or equal to 95 mm Hg and less than 115 mm Hg) were included in an open 1-year treatment study. Eight examinations allowed treatment adaptation if diastolic BP remained greater than or equal to 90 mm Hg (monotherapy with rilmenidine, 1 or 2 mg/day, followed by the addition of a diuretic, then tritherapy). Three hundred seventeen patients, aged 58.0 +/- 0.7 years, were included. Two hundred sixty-nine were followed for 1 year and 48 withdrew from the trial without any symptom suggesting a withdrawal syndrome: 4 because of adverse effects; 6, lack of efficacy despite triple therapy; 9, intercurrent diseases; 10, noncompliance independent of adverse effects; 18, personal reasons not associated with treatment; and 1, lost to follow-up. On the 12th month, the decrease in supine systolic and diastolic BP reached 25 and 17 mm Hg with monotherapy (n = 150), 26 and 17 mm Hg with double therapy (n = 90) and 20 and 15 mm Hg with triple therapy (n = 29). BP was normalized (diastolic BP less than or equal to 90 mm Hg) on months 6 and 12 in 80 and 84% of the patients, respectively. Monotherapy was maintained in 66 and 60% of these patients, respectively, two-thirds being treated with 1 mg once daily. Adverse effects with monotherapy were mainly observed at the beginning of treatment in 3 to 8%: dry mouth, asthenia, gastralgia, palpitations, drowsiness, insomnia; other adverse effects were rare (1 to 2%).(ABSTRACT TRUNCATED AT 250 WORDS)

Haemodynamic and metabolic effects of rilmenidine in hypertensive patients with metabolic syndrome X. A double-blind parallel study versus amlodipine

OBJECTIVE

To compare the effects of rilmenidine with those of amlodipine on blood pressure, glucose metabolism, plasma lipid concentration and fibrinolysis parameters.

DESIGN

A four-month randomized double-blind, parallel group study.

PATIENTS AND METHODS

Obese hypertensive patients with hypertriglyceridaemia (> or = 2.3 mmol/l) and impaired glucose tolerance (OMS-ADA) were included (n = 52). A placebo run-in period of 2 weeks was followed by 4 months of double-blind treatment with either rilmenidine or amlodipine. Blood pressure was recorded using a mercury sphygmomanometer. Glucose metabolism was evaluated by an oral glucose tolerance test

RESULTS

Of the 52 patients recruited, 47 (21 rilmenidine and 26 amlodipine) completed the 4-month treatment period. The intention-to-treat analysis showed a comparable reduction in systolic and diastolic blood pressure (SBP, DBP) with the two anti-hypertensive treatments (rilmenidine -13.9/-13.5 mmHg; amlodipine - 17.6/-15.0 mmHg). Insulin concentrations under basal conditions and 2 h after a standard oral glucose load did not change significantly after treatment in both groups. Plasma glucose under basal conditions and 2 h after a standard oral glucose load as well as the area under the plasma glucose concentration curve tended to decrease in the rilmenidine group and to increase in the amlodipine group so that the changes in these parameters were significantly different between the two study groups (P= 0.041, P = 0.042 and P = 0.015, respectively). Plasminogen activator inhibitor type 1 (PAI-1) antigen and PAI-1 activity were only decreased in the rilmenidine group (not statistically significant).

CONCLUSION

Our results demonstrate that rilmenidine and amlodipine have a comparable anti-hypertensive effect but only rilmenidine is able to improve glucose metabolism.

L 4 - IMPORTANCE OF IMIDAZOLINE RECEPTORS IN THE CARDIOVASCULAR RESPONSES TO CLONIDINE AND RILMENIDINE IN CONSCIOUS RABBITS

The present paper summarizes our studies concerning the involvement of imidazoline and alpha 2-adrenoceptors in the cardiovascular actions of centrally acting drugs rilmenidine, clonidine and methyldopa. We have found that they produce very similar cardiovascular autonomic effects which relate directly to the function of central monoamine neurotransmitters. They mimic certain elements of the noradrenergic neuron system in the central nervous system, in particular the brainstem actions which involve hypotension, bradycardia and resetting of the baroreceptor heart rate reflex. By contrast they turn off serotonergic pathways that are pressor, produce tachycardia and inhibit the baroreceptor heart rate reflex. Recent studies using specific receptors antagonist drugs idazoxan and 2-methoxy-idazoxan indicate that in conscious rabbits the imidazoline receptor actions of rilmenidine is of primary importance at doses which would be considered clinically relevant. We further conclude that the alpha 2-adrenoceptors and the imidazoline receptors are likely to be located in series i.e. along the same cardiovascular autonomic pathways in the brainstem but presumably at different sites.

The hypotensive action of rilmenidine is dependent on functional N-methyl-D-aspartate receptor in the rostral ventrolateral medulla of conscious spontaneously hypertensive rats

Rilmenidine is a second-generation centrally acting antihypertensive drug that acts mainly through the activation of the imidazoline (I(1)) receptor in the rostral ventrolateral medulla (RVLM). To investigate the contribution of the N-methyl-D-aspartate receptor (NMDAR) to the hypotensive action of rilmenidine, experiments were undertaken in conscious male spontaneously hypertensive rats (SHRs). Microinjection of cumulative doses of rilmenidine (10, 20, and 40 nmol) at 10- to 15-min intervals, into the RVLM elicited dose-dependent hypotensive and bradycardic response. Pretreatment with intra-RVLM 2-amino-5-phosphonopentanoic acid (AP5) (2 nmol), a selective NMDAR antagonist, not only abolished the hypotensive response elicited by intra-RVLM rilmenidine (40 nmol) but also converted it to a pressor response (-24 +/- 1 versus 17 +/- 7 mm Hg; P < 0.05) and significantly attenuated the bradycardic response (-72 +/- 18 versus -24 +/- 20 bpm; P < 0.05). The blood pressure response to intra-RVLM N-methyl-D-aspartate (NMDA) depended on the dose applied. Whereas intra-RVLM NMDA (>20 pmol) produced the expected pressor response, a lower dose (10 pmol) reduced mean arterial pressure (MAP) (-14 +/- 3 mm Hg) and heart rate (-21 +/- 12 bpm). The divergent MAP responses were attenuated by intra-RVLM AP5 (2 nmol), which implicates the NMDAR in the pressor as well as the depressor response. The present findings suggest that the NMDAR in the RVLM of the SHR 1) exerts dual effects on blood pressure, with the response type depending on the level of NMDAR activation, and 2) plays a pivotal role in the hypotension mediated by I(1) receptor activation in the RVLM.

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.

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.

I1 receptors, cardiovascular function, and metabolism

When injected into the medullary site of the hypotensive action of clonidine, imidazolines and related compounds decrease blood pressure (BP), whereas no phenylethylamine compounds were capable of producing such an effect at the same site. There is much biochemical and pharmacologic evidence to support the involvement of imidazoline receptors in the regulation of vasomotor tone as well as in the mechanism of action of some centrally acting antihypertensive drugs. Imidazoline-specific binding sites, which do not recognize catecholamines, have been described in various tissues. Functional studies using selective antagonists have confirmed that the hypotensive effects of clonidine-like drugs are mediated, at least in part, by nonadrenergic imidazoline-specific receptors, whereas their sedative action clearly involves alpha2-adrenergic receptors located in the locus coeruleus. Compared with clonidine, newer centrally acting antihypertensive drugs such as rilmenidine are more selective for imidazoline receptors than for alpha2-adrenergic receptors. This selectivity may explain the reduced incidence of side effects of these drugs at therapeutic doses. Very recently, imidazoline-like compounds with no affinity and no activity at alpha2-adrenergic receptors have become available. Some of these compounds lowered the BP when injected centrally, indicating that an action on imidazoline I1 receptors alone is sufficient to cause hypotension. Nevertheless, imidazoline receptors and alpha2-adrenoceptors cooperate in the control of the vasomotor tone and in the hypotensive action of centrally acting hybrid drugs (ie, drugs that bind to both types of receptor). Additional noncardiovascular effects of imidazoline-like drugs have also been described, such as insulin secretion stimulation and renal sodium reabsorption inhibition. These effects may account for the long-term benefits of imidazoline selective drugs, such as rilmenidine.

Rilmenidine: a clinical overview

Rilmenidine is an antihypertensive agent with selectivity for I1 imidazoline receptors that acts both centrally by reducing sympathetic overactivity and in the kidney by inhibiting the Na+/H+ antiport. Rilmenidine provides antihypertensive efficacy comparable with that of diuretics, beta-blockers, calcium channel blockers, and angiotensin-converting enzyme (ACE) inhibitors. Experience from trials and clinical practice highlights rilmenidine's clinical and metabolic acceptability in hypertensive populations, including those at special risk because of old age, renal impairment, diabetes mellitus, or dyslipidemia. In the at-risk hypertensive, rilmenidine reduces left ventricular hypertrophy to a similar degree to other reference agents. New studies show a significant improvement in glucose metabolism in metabolic syndrome patients treated with rilmenidine, and a significant reduction in microalbuminuria during rilmenidine treatment of hypertensive type 2 diabetics. Thus the efficacy/tolerance ratio of rilmenidine supports its role as a first-line antihypertensive option for all groups of hypertensive patient, with specific advantages in some at-risk populations.