Developmental Changes of Sarcoplasmic Reticular Calcium Ion Transport and Phospholamban in Rat Heart

This comparative study investigates the relationship between sarcoplasmic reticulum (SR) calcium (Ca2+) ATPase transport activity and phospholamban (PLB) phosphorylation in whole cardiac homogenates around birth and during postnatal development until weaning. At fetal day 20, the rate of homogenate oxalate-supported Ca2+ uptake was 23% of that at postnatal day 6 (0.34 ± 0.08 vs 1.46 ± 0.37 nmoles Ca2+/mg wet ventricular weight/min, respectively; P < 0.05). This was accompanied by a 2-fold gain in heart mass. Between postnatal days 6 and 21, there occurred a further 4-fold increase in heart mass which was accompanied by only a 1.3-fold increase in SR Ca2+ transport activity. Levels of phosphorylated PLB formed in vitro in the presence of radiolabelled ATP and catalytic subunit of protein kinase A increased approx. 4-fold between fetal day 20 and postnatal day 21. In this early developmental period, SR Ca2+-transport values were linearly related to the respective 32P-PLB levels suggesting coordinated developmental changes of SR Ca2+ ATPase and PLB. Developmental changes in SR Ca2+ transport and 32P-PLB levels were thyroid hormone(TH)-dependent as revealed from analysis of these parameters in 21-day-old rats with experimentally induced eu-, hypo- and hyperthyroid states. It appears that TH-dependent changes in the phosphorylation of PLB play a major role for the increase in SR Ca2+ transport activity between birth and weaning.