Interaction between R 56865 and alpha-adrenoceptors in the pithed rat

In pithed normotensive rats, the benzothiazolamine derivative R 56865 in high doses exhibits a competitive antagonism of alpha 1-adrenoceptor-mediated vasoconstrictions. The absence of a depression of the maximum of the dose-response curve of ST 587 and the very moderate attenuation of the maximal B-HT 920-induced increase in diastolic blood pressure (BP) confirms the lack of major calcium entry blocking properties of R 56865 for alpha-adrenoceptor-activated calcium channels in vitro. In doses up to 10(-5) mol/kg, the interaction of R 56865 with the sympathetic neurotransmission can solely be explained by alpha 1-adrenoceptor blockade. This was confirmed by the comparable antagonism of the selective alpha 1-adrenoceptor antagonist prazosin in a concentration of 6 x 10(-8) mol/kg. In contrast to the isolated rat aorta, where R 56865 showed an allosteric interaction with the NA binding site on the alpha 1-adrenoceptor, R 56865 acts like an alpha 1-adrenoceptor antagonist of the competitive type in vivo.