Comparison of the effects of phospholamban and jasmone on the calcium pump of cardiac sarcoplasmic reticulum. Evidence for modulation by phospholamban of both Ca²⁺ affinity and V(max(Ca)) of calcium transport

Regulation of the calcium pump of the cardiac sarcoplasmic reticulum by phosphorylation/dephosphorylation of phospholamban is central to the inotropic and lusitropic effects of β-adrenergic agonists on the heart. In order to study the mechanism of this regulation, we first obtained purified ruthenium red-insensitive microsomes enriched in sarcoplasmic reticulum membranes. The kinetics of microsomal Ca²⁺ uptake after phospholamban phosphorylation or trypsin treatment, which cleaves the inhibitory cytoplasmic domain of phospholamban, were then compared with those in the presence of jasmone, whose effects on the kinetics of fast skeletal muscle Ca²⁺-ATPase are largely known. All three treatments increased V(max(Ca)) at 25 °C and millimolar ATP; phosphorylation and trypsin decreased the K(m(Ca)), while jasmone increased it. Trypsin and jasmone increased the rate of E₂P decomposition 1.8- and 3.0-fold, respectively. The effects of phospholamban phosphorylation and jasmone on the Ca²⁺-ATPase activity paralleled their effects on Ca²⁺ uptake. Our data demonstrate that phospholamban regulates E₂P decomposition in addition to the known increase in the rate of a conformational 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 and, presumably, physiological conditions.