Pharmacology of calcium entry blockers: interaction with vascular alpha-adrenoceptors.

The pharmacology of calcium entry blockers (CEB), as used in various types of cardiovascular disease, is reviewed. Their vasodilator potency is analyzed, which is the most common therapeutic characteristic of all CEB, apart from the antiarrhythmic activity of verapamil. All CEB relax vascular smooth muscle, particularly in arteriolar beds, thus reducing peripheral vascular resistance. Arteriolar relaxation appears to be associated with inhibition of transmembrane calcium influx. Recent studies have demonstrated that vasoconstriction induced by stimulation of vascular postsynaptic alpha 2-adrenoceptors, using selective agonists, is reduced by CEB through a noncompetitive mechanism. However, vasoconstriction evoked by selective excitation of vascular alpha 1-adrenoceptors remains virtually uninfluenced by CEB. The selective interference of CEB with alpha 2-adrenoceptor stimulation suggests that calcium movement is associated with alpha 2-adrenoceptor stimulation. This mechanism may further contribute to the vasodilator effect of CEB by inhibiting that part of vascular tone evoked by the stimulation of alpha 2-adrenoceptors by endogenous catecholamines.