The (Ca2+ -Mg2+)-ATPases of the Plasma Membrane and of the Endoplasmic Reticulum in Smooth Muscle Cells and Their Regulation

Abstract

Smooth muscle cells contain two distinct Ca²⁺ -transport ATPases with a different subcellular localization. The plasmalemmal Ca²⁺- pump has a relative molecular weight (M_r) of 140k and its phospho-intermediate level is increased by La³⁺. Its resemblance to the erythrocyte Ca²⁺ pump is further confirmed by its calmodulin-binding capacity and its antigenic properties. A 100k Ca²⁺ -transport ATPase is localized in the endoplasmic reticulum. Its phospho-intermediate level is de-creased by La³⁺, and it is antigenically related to the cardiac sarcoplasmic reticulum Ca²⁺ -transport ATPase. 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. The endoplasmic-reticulum Ca²⁺ -transport ATPase is regulated via phospholamban. Phosphorylation of this regulatory protein by cAMP-dependent as well as by cGMP-dependent protein kinase stimulates the endoplasmic-reticulum Ca²⁺ pump. The activity of the plasmalemmal Ca²⁺-transport ATPase can be modulated by calmodulin, negatively charged phospholipids, and by receptor-binding agonists. cGMP-dependent protein kinase also exerts a stimulatory effect on the plasmalemmal Ca²⁺ pump, but this effect is not mediated via a direct phosphorylation of the Ca²⁺ pump.