Regulatory role of phospholamban in the efficiency of cardiac sarcoplasmicreticulum Ca2+ transport

Phospholamban is an inhibitor of the sarcoplasmic reticulum Ca2+ transport apparent affinity for Ca2+ in cardiac muscle. This inhibitory effect of pho spholamban can be relieved through its phosphorylation or ablation. To bett er characterize the regulatory mechanism of phospholamban, we examined the initial rates of Ca2+-uptake and Ca2+-ATPase activity under identical condi tions, using sarcoplasmic reticulum-enriched preparations from phospholamba n-deficient and wild-type hearts. The apparent coupling ratio, calculated b y dividing the initial rates of Ca2+ transport by ATP hydrolysis, appeared to increase with increasing [Ca2+] in wild-type hearts. However, in the pho spholamban-deficient hearts, this ratio was constant, and it was similar to the value obtained at high [Ca2+] in wild-type hearts. Phosphorylation of phospholamban by the catalytic subunit of protein kinase A in wild-type sar coplasmic reticulum also resulted in a constant value of the apparent ratio of Ca2+ transported per ATP hydrolyzed, which was similar to that present in phospholamban-deficient hearts. Thus, the inhibitory effects of dephosph orylated phospholamban involve decreases in the apparent affinity of sarcop lasmic reticulum Ca2+ transport for Ca2+ and the efficiency of this transpo rt system at low [Ca2+], both leading to prolonged relaxation in myocytes.