Calcium and alpha-adrenoceptors in activation of vascular smooth muscle.

Calcium entry blockers have differing effects on various vascular smooth muscle preparations during alpha-adrenoceptor activation. One possible explanation for these differences is that norepinephrine activates different subtypes of postjunctional alpha-adrenoceptors, which are linked preferentially to either the entry of extracellular or the mobilization of intracellular calcium. Therefore, differences in the sensitivity to calcium entry blockers during vasoconstrictor responses to exogenous or circulating catecholamines or to sympathetic nerve stimulation may reflect the heterogeneous pharmacological properties of the postjunctional alpha-adrenoceptors involved. However, depending on the blood vessel or the species studied, equally convincing evidence favoring the interpretation that either alpha 1- or alpha 2-adrenoceptors modulate the entry of extracellular calcium in vascular smooth muscle can be obtained. Therefore, it seems reasonable to conclude that the varying degrees of dependency of alpha-adrenoceptor activation on calcium entry are determined by the functional differentiation of the vascular smooth muscle cells rather than by the apparent pharmacological characteristics of their alpha-adrenoceptors.