Abstract
The effects of the calcium channel antagonist nifedipine and the protein kinase C (PKC) inhibitor, staurosporine were examined on the pressor actions of a full, cirazoline, and a partial, St587, alpha 1-adrenoceptor agonist as well as the alpha 2-adrenoceptor agonist B-HT 920 in the pithed rat preparation. Administration of nifedipine or staurosporine significantly reduced the diastolic blood pressure of pithed rats. Staurosporine displaced the dose-diastolic pressure response curves for B-HT 920 and St587 to the right in a dose-dependent manner. The ED50 value for the dose-response curves to B-HT 920 and St587 were found to be significantly increased after the administration of staurosporine. Staurosporine also caused a depression of the maximum response to B-HT 920 and St587. The presence of nifedipine resulted in an increase in the ED50 value of the dose-response curve to B-HT 920 and St587, and this was accompanied by significant reductions of the maximum response, whereas the administration of either staurosporine or nifedipine did not significantly affect the calculated ED50 value of the dose-response curve to cirazoline. In the presence of the calcium channel antagonist but not the PKC inhibitor the maximum response to cirazoline was significantly depressed. As judged from the slope function of the dose-response curves the nature of the inhibition produced by nifedipine compared to staurosporine also appeared to differ. Thus, with nifedipine as the antagonist, the slope function of the dose-response curve for alpha-agonists was significantly reduced. Moreover, in contrast to the actions of staurosporine, nifedipine reduced the maximum response to cirazoline in a dose-independent manner. This study supports the hypothesis that activation of alpha-adrenoceptors that have a substantial dependence on extracellular calcium for vasoconstriction are susceptible not only to the action of calcium channel antagonists but also to the actions of the PKC inhibitor staurosporine, thus suggesting that PKC may modulate directly/indirectly calcium channel activity in vascular smooth muscle.
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