Receptor subtypes involved in the action of calcium entry blockers

A survey is given of the interaction between alpha-adrenoceptor-triggered vasoconstriction and the influx of extracellular calcium ions. This problem was investigated by studying the influence of calcium entry blockers (CEB) on the vasoconstriction induced by several types of alpha 2- and alpha 1-adrenoceptor agonists both in vivo and in vitro. In addition, experiments were performed with the calcium entry promoter Bay K 8644. alpha 2-Adrenoceptor-triggered vasoconstriction is invariably accompanied by an influx of extracellular calcium ions which substantially contributes to the initiation of vascular smooth muscle contraction. This concept is a very general phenomenon, which holds for a variety of alpha 2-adrenoceptor agonists and CEB, in vivo and in vitro, and in different animal species. The alpha 1-adrenoceptor-induced vasoconstriction appears to be caused both by the release of intracellular calcium and by the transmembranous influx of intracellular calcium. The ratio between both processes is very different, depending upon the type of alpha 1-adrenoceptor agonist and on the experimental preparation used. It has been speculated that this ratio can be influenced by phenoxybenzamine.