Rilmenidine (S 3341) and the sympathoadrenal system: adrenoreceptors,plasma and adrenal catecholamines in dogs

1 The effects of rilmenidine, a new alpha₂-adrenoreceptor agonist with antihypertensive properties, were investigated on plasma catecholamines, blood cell adrenoreceptors and adrenal medullary function. 2 In conscious sino-aortic denervated (SAD) dogs, rilmenidine (1 mg kg^?1 orally for 2 weeks) significantly reduced both blood pressure and heart rate when compared with placebo treatment. The drug decreased plasma noradrenaline and adrenaline levels and corrected the decrease in leucocyte beta-adrenoreceptors observed in placebo-treated SAD dogs. There was no change in platelet alpha₂-adrenoreceptors. 3 In anaesthetized normotensive dogs, rilmenidine (0.1 and 0.3 mg kg^?1 i.v.) induced a dose-dependent decrease in both cardiovascular parameters (blood pressure and heart rate) and catecholamine release from the adrenal medulla. 4 The present study shows that rilmenidine decreases sympathetic tone mainly by an action on the adrenal medulla. In addition, its ability to lower blood pressure in SAD dogs, i.e. a model of hypertension in which high sympathetic tone is present, indicates that rilmenidine may also depress other parts of the sympathetic nervous system.