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

Sildenafil citrate long-term treatment effects on cardiovascular reactivity in a SHR experimental model of metabolic syndrome

Much evidence indicates that metabolic syndrome is strongly correlated with a decrease in nitric oxide and an increase in oxidative stress leading to cardiovascular alterations. In recent years, gut microbiota has emerged as a new contributor to the metabolic syndrome establishment and associated cardiovascular diseases, but the underlying mechanisms remain unclear. We hypothesized that a positive modulation of cyclic guanosine monophosphate (cGMP) pathway, through phosphodiesterase type 5 (PDE5) inhibition could prevent cardiovascular alterations and gut dysbiosis that may be associated to metabolic syndrome. Spontaneously hypertensive rats (SHR) were randomly divided into 4 groups: control, cafeteria diet (CD) and sildenafil citrate treated groups (5mg/kg per os) were given either a CD or a standard chow diet for 10 weeks. Body weight, arterial blood pressure and glucose tolerance test were monitored. At the 10^th week, cardiac inotropy and coronary perfusion pressure were evaluated on isolated heart according to Langendorff method. Cumulative concentration response curves to phenylephrine and acetylcholine were determined on thoracic aorta rings for vascular reactivity evaluation. Faecal samples were collected for the gut microbiota analysis. Compared to the control group, CD-fed rats showed a significant increase in body weight gain, arterial blood pressure and were glucose intolerant. This group showed also a decrease in β-adrenoceptor-induced cardiac inotropy and coronary vasodilation. Gut microbiota analysis revealed a significant reduction in the abundance of Lactobocillus spp in cafeteria diet-fed rats when compared to the control ones. Sildenafil citrate long-term treatment decreased weight gain and arterial blood pressure, improved coronary vasodilation and reduced α_1-adrenoceptor-induced vasoconstriction in CD group. However, it did not reverse gut dysbiosis induced by chronic CD feeding. These results suggest that cGMP pathway targeting may be a potential therapeutic strategy for the management of the metabolic syndrome and associated cardiovascular disorders.

Baroreflex control of renal sympathetic nerve activity and spontaneous rhythms at Mayer wave's frequency in rats

The effects of sedation with pentobarbital sodium (15 mg/kg followed by 15.9+/-1.2 mg/kg/h, i.v.) on arterial pressure (AP) Mayer waves and accompanying oscillations of renal sympathetic nerve activity (RSNA) were examined in rats (n=8). As compared with values observed in the conscious state, pentobarbital significantly (P<0.05) decreased AP (from 119+/-2 to 93+/-3 mm Hg), heart rate (HR; from 427+/-11 to 355+/-11 beats/min) and RSNA (from 1.20+/-0.27 to 0.62+/-0.13 microV). The baroreflex control of RSNA was analyzed by fitting a sigmoid logistic function to changes in RSNA and AP observed during nitroprusside and phenylephrine administrations. During pentobarbital infusion, the RSNA-AP relationship was reset towards lower AP values, but neither its maximum gain nor its gain at resting AP were significantly altered (from 6.3+/-1.0 to 5.8+/-1.4 and from 3.2+/-0.5 to 3.8+/-1.3 normalized units (n.u.)/mm Hg, respectively). Spectral power in the frequency band containing Mayer waves (0.29-0.73 Hz) was significantly decreased by pentobarbital for both AP (from 4.65+/-0.90 to 0.95+/-0.25 mm Hg2) and RSNA (from 1437+/-245 to 488+/-79 n.u.2). This effect was mainly secondary to the attenuation of strongly coherent oscillations of both variables at approximately 0.4 Hz. Although previous experimental evidence points to a major involvement of the sympathetic limb of the arterial baroreceptor reflex in the genesis of Mayer waves, the present study indicates that the amplitude of these oscillations cannot be used as a quantitative index of sympathetic baroreflex sensitivity.

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.

Involvement of imidazoline receptors in the baroreflex effects of rilmenidine in conscious rabbits

OBJECTIVE

It has been suggested that imidazoline receptors rather than alpha2-adrenoceptors are involved in the sympathoinhibitory action of centrally acting antihypertensive drugs such as rilmenidine. In the present study, we examined the relative importance of alpha2-adrenoceptors and imidazoline receptors in modulating the renal sympathetic and heart rate (HR) baroreflex in response to central administration of rilmenidine in conscious normotensive rabbits.

METHODS

In seven conscious rabbits, chronically instrumented with a fourth ventricular (4V) catheter, aortic and vena caval cuff occluders and a renal nerve electrode, we continuously recorded renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and HR and assessed baroreflex MAP-RSNA and MAP-HR relationships with balloon-induced ramp rises and falls in MAP. Rabbits were treated with 4V rilmenidine (22 microg/kg) followed by 4V idazoxan (30 microg/kg; a mixed alpha2-adrenoceptor and imidazoline receptor antagonist) or 4V 2-methoxy-idazoxan (1 microg/kg; an alpha2-adrenoceptor antagonist with little affinity for imidazoline receptors).

RESULTS

Rilmenidine lowered blood pressure by 24% and reduced both upper and lower plateaus of the renal sympathetic baroreflex curve, such that the RSNA range (difference between plateaus) was reduced by 40% (-32 +/- 10 normalized units). Curves were shifted to the left with the fall in MAP. Idazoxan restored MAP, maximum RSNA and the RSNA baroreflex range. By contrast the alpha2-adrenoceptor antagonist 2-methoxy-idazoxan caused only a partial recovery of MAP and RSNA baroreflex upper plateau and range (-9 +/- 2 mmHg, 29 and 33% lower than control). Both antagonists partially restored the HR baroreflex.

CONCLUSION

These findings suggest that in conscious rabbits, both imidazoline receptors and alpha2-adrenoceptors are involved in the central antihypertensive and baroreflex actions of rilmenidine, but that activation of imidazoline receptors is more important for its renal sympathoinhibitory action.

Acute cardiovascular effects of the alpha2-adrenoceptor antagonist, idazoxan, in rats: influence of the basal sympathetic tone

Intravenous administration of the alpha2-adrenoceptor antagonist, idazoxan, elicits variable cardiovascular effects, depending on experimental conditions. In this study, the effects of idazoxan were investigated in rats with high, low, or no basal sympathetic tone. In a group of conscious Sprague-Dawley rats (n = 9), mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nervous activity (RSNA) were recorded. Idazoxan (250 microg/kg, i.v.) induced a transient decrease in MAP (-12+/-3 mm Hg) that was accompanied by increases in HR (49+/-14 beats/min) and RSNA (53+/-14%). In six of nine rats, a light pentobarbitone anesthesia was given. Basal RSNA was decreased (6.0+/-1.3 microV from 12.8+/-4.1 microV; p<0.05), and the depressor effect of idazoxan was reversed to a pressor effect (21+/-6 mm Hg) associated with bradycardia (-16+/-8 beats/min) and sympathoinhibition (-56+/-15%). In eight conscious intact rats, idazoxan (250 microg/kg, i.v.) attenuated by approximately 40% the pressor response to the selective alpha1-adrenoceptor agonist, cirazoline (0.5 microg/kg, i.v.). In three groups of six to seven ganglion-blocked (chlorisondamine, 2.5 mg/kg, i.v.) conscious rats, idazoxan dose-dependently increased mean arterial pressure (MAP: 39+/-2, 55+/-3, and 69+/-4 mm Hg at 125, 250, and 500 microg/kg, i.v., respectively) with minimal changes in HR. In contrast, the noradrenaline-releasing agent, tyramine (62.5, 125, and 250 microg/kg, i.v.), dose-dependently increased both MAP and HR. The alpha1-adrenoceptor antagonist, prazosin (1 mg/kg, i.v.; n = 8) blunted by approximately 70% (p<0.01) the pressor effect of 250 microg/kg idazoxan. It is concluded that in rats with high sympathetic tone, idazoxan has depressor effects, most likely related to its peripheral alpha-adrenoceptor antagonist properties. In rats with low or no sympathetic tone, idazoxan induced pressor responses mainly secondary to its partial agonist activity at vascular postjunctional alpha1-adrenoceptors.

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

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

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

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

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.

Renal denervation altered the hemodynamic and renal effects following intracerebroventricular administration of the I1-imidazoline receptor agonist, rilmenidine, in pentobarbital anaesthetized rats

Previous studies have reported on the effects of intracerebroventricular (icv) administration of the I1-imidazoline receptor agonist moxonidine. In the present study, the relationship between increasing doses of the I1-agonist rilmenidine (administered icv) with blood pressure and renal function has been determined. Moreover, the importance of the renal nerves in this response have also been assessed. In pentobarbitone anesthetized rats, icv rilmenidine (30, 100, 300 nmol in 5 microliters) produced a dose related decrease in blood pressure and heart rate. Urine flow was not altered at the lower doses although at the highest dose (300 nmol) the increase approached significance (p = 0.06). Sodium excretion and osmolar clearance were not altered. Free water clearance was increased at 100 and 300 nmol rilmenidine (p < 0.05). Consistent with the above dose response studies, in sham denervated rats icv rilmenidine (300 nmol) decreased blood pressure and increased free water clearance. In rats having undergone renal denervation, baseline levels of urine flow rate, sodium excretion and osmolar clearance were increased. In these denervated rats, icv rilmenidine (300 nmol) failed to decrease blood pressure. Urine flow rate was increased with a decrease in sodium excretion and osmolar clearance. Free water clearance was increased. These results indicate the importance of the renal nerves in mediating the acute decrease in blood pressure following icv administration of the I1-imidazoline receptor agonist rilmenidine. The increase in free water clearance seen following icv rilmenidine appears to be mediated independent of the renal nerves. The changes associated with sodium excretion on the contrary are dependent on intact renal nerves.

Rilmenidine and reflex renal sympathetic nerve activation in Wistar and hypertensive rats

1. This study sets out to examine the effect of rilmenidine administered systemically on basal and reflexly activated renal nerve activity in Wistar and stroke prone spontaneously hypertensive rats (SHRSP). 2. Animals were anaesthetized with chloralose/urethane, stimulating electrodes were placed on the brachial plexi and the renal nerves were isolated and put on recording electrodes. Both brachial nerves were stimulated electrically at 0.8, 1.6 and 3.2 Hz (15 V, 0.2 ms) in the absence and in the presence of rilmenidine given at 100 and 200 micrograms kg-1 i.v. in a cumulative manner. 3. Stimulation of the brachial nerves caused graded increases in blood pressure, heart rate and integrated renal nerve activity (P < 0.05) in both Wistar and SHRSP. Fast Fourier transformation of the renal nerve activity signal to generate a power spectrum demonstrated that both total power and percentage power at heart rate was higher in the SHRSP than Wistar (P < 0.05). Total power was raised during brachial nerve stimulation in both Wistar and SHRSP by some 200-300% (P < 0.05) but the percentage power at heart rate was decreased by some 60% (P < 0.01) in the Wistar but was raised by some 40-50% (P < 0.05) in the SHRSP. 4. Administration of rilmenidine caused dose-related decreases in blood pressure and heart rate and integrated renal nerve activity in both Wistar and SHRSP (all P < 0.05). Both doses of rilmenidine decreased (P < 0.05) the total power in the signal in both strains of rat by about one-half but the power occurring at heart rate only fell at the higher dose of compound in the Wistar, whereas in the SHRSP it was decreased by both doses by approximately 60-70%. In the presence of rilmenidine, coherence of the renal nerve signal was reduced in the Wistar and SHRSP and although the drug had no effect on phase difference in the Wistar, this parameter was decreased in the SHRSP by the low and high doses of rilmenidine (P < 0.05). 5. In the presence of 100 micrograms kg-1 rilmenidine, stimulation of the brachial nerves caused increases in total power in the Wistar and SHRSP (two to three fold, P < 0.05), together with a decrease (P < 0.05) in the percentage power occurring at heart rate in the Wistar, of some 60%, and an increase (P < 0.01) in the SHRSP, of some two to three times, which were very similar in magnitude and pattern to those obtained in the absence of the drug. Following the 200 micrograms kg-1 dose of rilmenidine, brachial nerve stimulation increased total power in the Wistar and SHRSP groups (P < 0.05) and whereas in the Wistar the percentage power at heart rate did not change in the SHRSP it was again increased in response to the electrical stimulation of the brachial plexus (P < 0.001) by between two to three fold. 6. These results showed that in both the Wistar and SHRSP rilmenidine depressed blood pressure, heart rate and integrated renal nerve activity. Moreover, rilmenidine did not affect the reflex activation of renal nerve activity via the somatosensory system although the characteristics within the power spectra underwent certain changes which might have a functional impact at the level to the kidney.

Mechanism of sympathoinhibition by imidazolines

The aim of the present study is to characterize the cardiovascular effects of rilmenidine and moxonidine, two recently developed centrally acting antihypertensive drugs. Rilmenidine and moxonidine are alpha 2-adrenoceptor agonists and, in addition, possess affinity for imidazoline (I1)-receptors. To determine if alpha 2- or I1-receptors are involved in sympathoinhibition, rilmenidine and moxonidine were compared with UK 14304, an alpha 2-agonist devoid of affinity for I1-receptors, and antagonism by the "pure" alpha 2-adrenoceptor antagonists yohimbine, SK&F86466, and RX821002 was studied. When injected intravenously into conscious rabbits, rilmenidine and moxonidine, on the one hand, and UK 14304, on the other, elicited a similar pattern of effects. Thus, transient hypertension was followed by long-lasting hypotension accompanied by a decrease in heart rate, renal sympathetic nerve firing rate, and plasma norepinephrine concentration. The effects of rilmenidine, moxonidine, and UK 14304 were antagonized by intravenously administered yohimbine, SK&F86466, and RX821002. The effects of moxonidine and UK 14304 were also prevented by yohimbine injected into the cisterna magna. Altogether, the degree of antagonism of the effects of rilmenidine and moxonidine did not differ from the degree of antagonism of the effects of UK 14304. Rilmenidine, moxonidine, and UK 14304 were also given to pithed rabbits in which a constant sympathetic tone was maintained by electrical stimulation of the sympathetic nerves. At doses that caused sympathoinhibition in conscious rabbits, they lowered the plasma norepinephrine concentration markedly. Our experiments show by direct measurement of sympathetic nerve activity and plasma norepepinephrine concentration that rilmenidine, moxonidine, and UK 14304 cause sympathoinhibition. As a consequence, blood pressure and heart rate decrease. The simplest interpretation of the blockade of central sympathoinhibition by the selective alpha 2-adrenoceptor antagonists is that rilmenidine, moxonidine, and UK 14304 primarily activated alpha 2-adrenoceptors. The decrease in plasma norepinephrine in pithed rabbits indicates peripheral presynaptic alpha 2-adrenoceptor-mediated inhibition of norepinephrine release per action potential from postganglionic sympathetic axons and suggests a contribution of this mechanism to the overall reduction in sympathetic tone.

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.

Why imidazoline receptor modulator in the treatment of hypertension?

The influence of the sympathetic nervous system on blood pressure control was impressively demonstrated in 1940 by bilateral excision of sympathetic nerve fibers. Thereafter, the first generation of drugs lowering blood pressure by central modulation of the sympathetic outflow through alpha 2-adrenoceptor for stimulation, such as alpha-methyldopa, guanabenz, clonidine, and guanfacine, were marketed. However, these compounds were often tolerated poorly, because they caused orthostatic hypotension, sedation, tachycardia or bradycardia, dry mouth, and reduced cardiac output. The mode of action of the second generation centrally acting antihypertensive drugs moxonidine and rilmenidine is different from that of the first generation compounds (e.g., clonidine). Contrary to clonidine, the newer drugs bind more selectively to I1-imidazoline receptors rather than to alpha 2-adrenoceptors where first-generation drugs act. The high affinity and selectivity of these two drugs for this recently discovered new receptor class make it possible to discriminate between I1-imidazoline receptor-mediated blood pressure lowering, on the one hand, and alpha 2-adrenoceptor-mediated side effects, on the other. Discrimination of the two effects was substantiated either by studies using moxonidine alone or in interaction experiments with I1-imidazoline receptor or alpha 2-adrenoceptor antagonists. The high selectivity of moxonidine at the I1-imidazoline receptor allows discrimination between alpha 2-adrenoceptors and I1-imidazoline receptors and is reflected in man by the relatively low incidence of adverse drug events during moxonidine treatment. Concentration of endazoline, a specific mediator of I1-imidazoline receptors, is elevated in some patients with essential hypertension. Modulation of I1-imidazoline receptors by moxonidine could be interpreted as antagonism with regard to the endogenous agonistic effect of the endogenous "transmitter" endazoline. On the other hand, moxonidine acted directly as an agonist at the putative I1-imidazoline receptor. Therefore, to clear the ground, characterization as well as physiological function of the mediator for imidazoline receptors seems essential. The therapeutic relevance of using drugs selective for I1-imidazoline receptors for blood pressure reduction in hypertensive patients is substantiated by the finding that in human rostral ventrolateral medulla (RVLM), which is essential in central blood pressure regulation, the relation between alpha 2-adrenoceptors and I1-imidazoline receptors is about one to ten (1:10). Reduction of a long-lasting sympathetic overdrive may avoid the deteriorating effects on the heart and peripheral circulation. These recent findings give a rational explanation for the very low incidence of sedation and the absence of respiratory depression, orthostatic hypotension, and rebound hypertension that banned the former central acting antihypertensive drugs from first-line treatment despite the advantages of central mediated blood pressure control.

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)

Natriuretic effect of rilmenidine in anesthetized rats

Rilmenidine binds to alpha 2-adrenoceptors and imidazoline receptors in the central nervous system and the kidney. To test the hypothesis that rilmenidine would increase sodium excretion, renal function was studied in rats with innervated and denervated kidneys to distinguish between indirect (via renal sympathetic nerves) and direct effects of rilmenidine on the kidney. Standard clearance techniques were used in Wistar rats anesthetized with thiobutabarbital to measure renal function during 80 minutes of infusion of 0.9% NaCl or rilmenidine (20 or 50 micrograms.kg-1.min-1 intravenously). Snares on abdominal arteries were used to offset hypotension induced by rilmenidine. Heart rate decreased by 80-120 beats/min with either dose of rilmenidine. At 20 micrograms.kg-1.min-1, rilmenidine increased total and fractional excretion of sodium and clearance of osmoles while decreasing free water clearance from innervated kidneys. There were no changes in these variables in chronically denervated kidneys. Direct recording of renal sympathetic nerve activity showed a progressive, marked decrease in nerve activity during the low-dose infusion of rilmenidine. At 50 micrograms.kg-1.min-1, rilmenidine produced a differential effect on the clearance of osmoles by innervated and denervated kidneys but both kidneys had an increase in free water clearance. The data indicate that rilmenidine increases sodium excretion indirectly in anesthetized rats by decreasing renal sympathetic nerve activity. At doses and infusion periods used in these studies, there was no evidence for a direct effect of rilmenidine on sodium excretion. The increase in free water clearance seen with the high dose of rilmenidine suggests that the inhibitory effect of alpha 2-adrenoceptor activation on vasopressin is involved at this dose.(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)