Coordinate regulation of SR Ca(2+)-ATPase and phospholamban expression in developing murine heart

Phospholamban, the regulator of Ca(2+)-adenosinetriphosphatase (ATPase) activity in cardiac sarcoplasmic reticulum (SR), is an important determinant of basal myocardial performance. To determine whether phospholamban expression is developmentally regulated in the mouse and whether such regulation reflects alterations in Ca2+ pump activity, hearts from different stages of development were processed for molecular biological and biochemical studies. Both phospholamban and Ca(2+)-ATPase mRNAs were approximately 40% of adult (100%) levels at birth and gradually increased to approach adult levels by day 15 of development. These changes in transcript levels were indicative of changes at the protein level for both phospholamban and Ca(2+)-ATPase. Analysis of the initial rates of Ca2+ uptake demonstrated that over the course of development the upregulation of Ca(2+)-ATPase correlated with increases in the maximal rates of Ca2+ uptake and the constant apparent stoichiometric ratio of phospholamban to Ca(2+)-ATPase correlated with maintenance of a constant affinity of this enzyme for Ca2+ (0.25 +/- 0.03 microM Ca2+). Furthermore, targeted ablation of phospholamban in the mouse resulted in a much higher affinity of Ca2+ uptake for Ca2+ (0.10 +/- 0.02 microM Ca2+) than that observed in wild-type hearts, and this increased affinity was also maintained across different stages of postnatal development. These findings suggest that phospholamban is a major regulator of the affinity of Ca(2+)-ATPase for Ca2+, and coordinate regulation of the expression levels of these two SR proteins may be necessary for maintaining Ca2+ homeostasis in the developing mammalian heart.