Lack of differential inhibition by nifedipine of pressor responses induced by alpha 1- and alpha 2-adrenoceptor agonists and by angiotensin II in anaesthetized cats.

The effect of nifedipine on pressor dose-response curves to phenylephrine, alpha-methylnoradrenaline and angiotensin II was determined in anaesthetized cats pretreated with propranolol and atropine. The selectivity of phenylephrine and alpha-methylnoradrenaline for postjunctional alpha 1- and alpha 2-adrenoceptors respectively was demonstrated by using the selective alpha 1-adrenoceptor antagonist doxazosin and the relatively selective alpha 2-adrenoceptor antagonist rauwolscine. Nifedipine infused intravenously produced a degree of inhibition of rises in diastolic blood pressure similar to that induced by all three agonists. It is concluded that alpha 1- and alpha 2-adrenoceptor activation induced by phenylethanolamine derivatives is equally dependent on extracellular calcium for vascular smooth muscle contraction. Antagonism of sympathetically mediated or angiotensin-induced vasoconstriction could contribute to the vasodilator effects of nifedipine.