Alpha-Adrenoceptor Stimulation and Calcium Movements

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Abstract
A survey is given of the interaction between α-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 α2- and α1-adrenoceptor agonists, both in vivo and in vitro. In addition, experiments were performed with the calcium entry promoter Bay K 8644. α2-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 α2-adrenoceptor agonists and CEB, in vivo and in vitro, and in different animal species. The α1-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 α1-adrenoceptor agonist and on the experimental preparation used. It has been speculated that this ratio can be influenced by phenoxybenzamine.

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