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

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.

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

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

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.

Pharmacology of moxonidine: an I1-imidazoline receptor agonist

The I1-imidazoline receptor is a novel neurotransmitter receptor found mainly in the brainstem, adrenal medulla and kidney. The actions of moxonidine are described at the level of individual biomolecules, cells, tissues, organs and finally with integrative functions. The receptor functions at the cellular level works through arachidonic acid and phospholipid signaling cascades in neuronal cells with the net result of inhibiting sympathetic premotor neurons.