The Ca2+-transport ATPases in smooth muscle.

A calmodulin stimulated Ca2+-transport ATPase which has many of the characteristics of the erythrocyte type Ca2+-transport ATPase has been purified from smooth muscle. In particular, the effect of calmodulin on these transport enzymes is mimicked by partial proteolysis and antibodies against erythrocyte Ca2+-transport ATPase also bind to the smooth muscle (Ca2+ + Mg2+)ATPase. A correlation between the distribution of the calmodulin stimulated (Ca2+ + Mg2+)ATPase and (Na+ + K+)ATPase activities in smooth muscle membranes separated by density gradient centrifugation suggests a plasmalemmal distribution of this (Ca2+ + Mg2+)ATPase. A phosphoprotein intermediate in smooth muscle which strongly resembles the corresponding phosphoprotein in sarcoplasmic reticulum of skeletal muscle may indicate the presence in smooth muscle of a similar type of Ca2+-transport ATPase.