Plasma membrane and sarcoplasmic reticulum Ca-ATPase and smooth muscle.

Calcium ions (Ca2+) are involved in the regulation of many cellular activities. The Ca-ATPase(s) of the plasma membrane and of the endoplasmic reticulum play an important role in controlling the intracellular Ca2+ concentration. Therefore, it is not unexpected that these enzymes are modulated by different factors. The activity of the plasma membrane Ca-ATPase is modified by the amount of negatively charged phospholipids surrounding the enzyme. The regulation of the endoplasmic reticulum Ca-ATPase depends on the phosphorylation of phospholamban by cAMP- and cGMP-dependent protein kinase. These two different Ca2+ transport ATPases are present in both visceral and vascular smooth muscle, but tissue- and species-dependent differences in their relative amount have been observed. In this article we will review the characteristics of Ca-ATPases of the smooth muscle.