NO RELATION OF THE SUPPRESSIVE EFFECT ON THE SYMPATHETIC NERVOUS-SYSTEM TO THE ACUTE HYPOTENSION CAUSED BY IMIDAPRIL AND ENALAPRIL

To investigate the involvement of the sympathoinhibitory effect of imi dapril and enalapril in their antihypertensive effect at a clinically reasonable dose, we studied whether some responses induced by the stim ulation of the sympathetic nervous system (SNS) were affected by intra venous administration of imidaprilat and enalaprilat in curarized pith ed spontaneously hypertensive rats. Imidaprilat and enalaprilat (both at 100 mu g/kg, i.v.), which are active metabolites of imidapril and e nalapril, respectively, Suppressed the presser responses to electrical stimulation (ES) of the spinal cord (T-1-L(7)) and exogenous noradren aline (NA). The presser responses to NA were significantly suppressed after either alpha(1)- or alpha(2)-adrenoceptors were blocked. Further more, imidaprilat (100 mu g/kg, i.v.) suppressed these reduced respons es. When the reduced basal blood pressure was restored by vasopressin infusion, imidaprilat and enalaprilat (both at 100 mu g/kg, i.v.) did not suppress the responses to ES and exogenous alpha-adrenoceptor agon ists. They affected neither basal plasma concentrations of NA and adre naline nor ES-induced increase of these catecholamines. These results suggest that the suppressive effects of imidaprilat and enalaprilat on the presser responses to ES and alpha-adrenoceptors agonists are appa rently observed in pithed SHR because of a reduction of vascular tone and that imidapril and enalapril do not lower the blood pressure throu gh suppressing SNS.