Calcium dependency of vasoconstriction mediated by alpha 1- and alpha 2-adrenoceptors.

A survey is given of the calcium dependency of vasoconstriction mediated by alpha 1- and alpha 2-adrenoceptors. In all species studied and with all drugs investigated, it has been confirmed that alpha 2-adrenoceptor-mediated vasoconstriction is sensitive to calcium entry blockade with calcium antagonists, both in vivo and, under suitable experimental circumstances, also in vitro. The calcium entry promotor (calcium agonist) Bay k8644 appears to behave as a mirror image of the calcium-entry blockers. alpha 1-Adrenoceptor-agonist-induced vasoconstriction shows a differential picture with respect to the dependency of extracellular calcium. Accordingly, the vasoconstriction induced by the alpha 1-agonists cirazoline, (-)-phenylephrine, (+/-)-erythro-methoxamine, and (-)-amidephrine is not primarily dependent upon an influx of calcium, whereas other compounds like St 587 and Sgd 101/75 cause vasoconstriction which is very sensitive to drug-induced calcium entry blockade. It may be possible that different recognition sites of the alpha1-adrenoceptors are involved. In this connection, the calcium-entry promotor Bay k8644 does not simply behave as the mirror image of calcium entry blockers.