Vascular smooth muscle calcium channels.

Vascular smooth muscle Ca channels function in excitation-contraction coupling. A survey of recent literature reveals several types of excitable Ca channels. There are at least two plasmalemmal Ca channels that are primarily activated by depolarization. In addition, there also exists evidence for the presence of Ca channels in conduit arteries that are primarily activated by agonists. Under circumstances of compromised sarcoplasmic reticulum (SR) Ca accumulation, Ca that enters through the nonregulated Ca leak also contributes to tension development. The Ca release from the SR appears to be mediated by large Ca channels that are activated by free Ca, inositol-1,4,5-trisphosphate, and free ATP. The differential sensitivity to procaine suggests the presence of two separate excitable Ca channels in vascular smooth muscle SR in addition to a basal Ca leak. This presentation concludes with a theoretical model describing how vascular smooth muscle Ca metabolism may be altered in hypertension and how a Ca antagonist may lead to reduction of blood pressure.