ENHANCED MYOCARDIAL-CONTRACTILITY AND INCREASED CA2-TWITCH SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM CA2+-ATPASE( TRANSPORT FUNCTION IN TRANSGENIC HEARTS EXPRESSING THE EAST)

In this study, we investigated whether the fast-twitch skeletal muscle sarco(endo)plasmic reticulum Ca2+ transport pump (SERCA1a) can functi onally substitute the cardiac SERCA2a isoform and how its overexpressi on affects cardiac contractility. For this purpose, we generated trans genic (TG) mice that specifically overexpress SERCA1a in the heart, us ing the cardiac-specific cy-myosin heavy chain promoter. Ectopic expre ssion of SERCA1a resulted in a 2.5-fold increase in the amount of tota l SERCA protein. At the same time, the level of the endogenous SERCA2a protein was decreased by 50%, whereas the level of other muscle prote ins, including calsequestrin, phospholamban, actin, and tropomyosin, r emained unchanged. The steady-state level of SERCA phosphoenzyme inter mediate was increased 2.5-fold, and the maximal velocity of Ca2+ uptak e was increased 1.7-fold in TG hearts, demonstrating that the overexpr essed protein is functional. Although the basal cytosolic calcium sign al was decreased by 38% in TG cardiomyocytes, the amplitude of cytosol ic calcium signal was increased by 71.8%. The rate of calcium reseques tration was also increased in TG myocytes, which was reflected by a 51 .6% decrease in the normalized time to 80% decay of calcium signal. Th is resulted in considerably increased peak rates of myocyte shortening and relengthening (50.0% and 66.6%, respectively). Cardiac functional analysis using isolated work-performing heart preparations revealed s ignificantly faster rates of contraction and relaxation in TG hearts ( 41.9% and 39.5%, respectively). The time to peak pressure and the time to half-relaxation were shorter (29.1% and 32.7%, respectively). In c onclusion, our study demonstrates that the SERCA1a pump can functional ly substitute endogenous SERCA2a, and its overexpression significantly enhances Ca2+ transport and contractile function of the myocardium. T hese results also demonstrate that the SERCA pump level is a critical determinant of cardiac contractility.