Ca2+-flux modulation by calciductin phosphorylation in cardiac sarcolemma

The epinephrine-induced inotropic effect of the myocardium can be attributed to phosphorylation of the sarcolemmal protein calciductin, as this event is accompanied by a 3.5-fold increase in ATP-independent, voltage-dependent CA2+ uptake by isolated sarcolemmal vesicles. This can be considered as the in vitro equivalent of the Ca2+ slow channel. Ca2+ uptake under these conditions is linear, with the degree of calciductin phosphorylation and inhibitor studies indicate the properties of the unphosphorylated channels are similar to those of the fully activated state. Calciductin has been purified and shows great similarities to phospholamban, a protein modulator of the sarcoplasmic reticulum Ca2+ pump. This raises the interesting possibility that calciductin and phospholamban are identical, although they serve different purposes in the sarcolemma and sarcoplasmic reticulum membranes.