The relationship between alterations in alpha 1-adrenoceptor reserve by phenoxybenzamine and benextramine and the sensitivity of cirazoline-induced pressor responses to inhibition by nifedipine.

Nifedipine does not inhibit the alpha 1-adrenoceptor-mediated pressor response of cirazoline at a dose that significantly antagonizes the alpha 2-adrenoceptor-mediated pressor response of B-HT 933. After elimination of the alpha 1-adrenoceptor reserve with either phenoxybenzamine or benextramine, the response to cirazoline was rendered highly sensitive to antagonism by nifedipine. These results support the hypothesis that the resistance of alpha 1-adrenoceptor-mediated pressor responses to inhibition by calcium channel antagonists may be caused by a large alpha 1-adrenoceptor reserve.