COMPARISON OF THE EFFECTS OF PHOSPHOLAMBAN AND JASMONE ON THE CALCIUM-PUMP OF CARDIAC SARCOPLASMIC-RETICULUM - EVIDENCE FOR MODULATION BY PHOSPHOLAMBAN OF BOTH CA2-MAXCA OF CALCIUM-TRANSPORT( AFFINITY AND V)

Regulation of the calcium pump of the cardiac sarcoplasmic reticulum b y phosphorylation/dephosphorylation of phospholamban is central to the inotropic and lusitropic effects of beta-adrenergic agonists on the h eart. In order to study the mechanism of this regulation, we first obt ained purified ruthenium red-insensitive microsomes enriched in sarcop lasmic reticulum membranes. The kinetics of microsomal Ca2+ uptake aft er phospholamban phosphorylation or trypsin treatment, which cleaves t he inhibitory cytoplasmic domain of phospholamban, were then compared with those in the presence of jasmone, whose effects on the kinetics o f fast skeletal muscle Ca2+-ATPase are largely known. All three treatm ents increased V-max (Ca) at 25 degrees C and millimolar ATP; phosphor ylation and trypsin decreased the K-m (Ca), while jasmone increased it . Trypsin and jasmone increased the rate of E(2)P decomposition 1.8- a nd 3.0-fold, respectively. The effects of phospholamban phosphorylatio n and jasmone on the Ca2+-ATPase activity paralleled their effects on Ca2+ uptake. Our data demonstrate that phospholamban regulates E(2)P d ecomposition in addition to the known increase in the rate of a confor mational change in the Ca2+-ATPase upon binding the first of two Ca2+. These steps in the catalytic cycle of the Ca2+-ATPase may contribute to or account for phospholamban's effects on both V-max (Ca) and K-m ( Ca), whose relative magnitude may vary under different experimental an d, presumably, physiological conditions.