J. &ton. Pharniac. (1988) zyxwvut 8, zyxwvut 3 zyxwvutsr 19-326 zyxwv Rilmenidine (S 3341) and the sympatho- adrenal system: adrenoreceptors, plasma and adrenal catecholamines in dogs zy P. Valet", M. A. Tran", C. Damase-Michel", G. de Saint-Blanquatt, L. Dang Tran", F. Angladet, E. Koenig-Berardt & J. L. Montastruc" *Laboratoire de Pharniacologie MPdicale et Clinique, INSERM U 31 7, FacultP zyx de MPdecine, 37 allkes Jules-Guesde, 31073 Toulouse CPdex, France, tlnserm U 8 7, Institut de Physiologie, CIniversitP Paul-Sabatier, 3 14000 Toulouse, France and SIR1 Servier, 32202 Neuilly sur Seine CPdex, France 1 The effects of rilmenidine, a new alpha,-adrenoreceptor agonist with antihy- pertensive 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-' 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-adrenorecep- tors observed in placebo-treated SAD dogs. There was no change in platelet alpha,-adrenoreceptors. 3 In anaesthetized normotensive dogs, rilmenidine (0.1 and 0.3 mg kg-l 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. Correspondence: Dr J. L. Montastruc.