CARDIAC-SPECIFIC OVEREXPRESSION OF PHOSPHOLAMBAN ALTERS CALCIUM KINETICS AND RESULTANT CARDIOMYOCYTE MECHANICS IN TRANSGENIC MICE

Phospholamban is the regulator of the cardiac sarcoplasmic reticulum ( SR) Ca2+-ATPase activity and an important modulator of basal contracti lity in the heart. To determine whether all the SR Ca2+-ATPase enzymes are subject to regulation by phospholamban in vivo, transgenic mice w ere generated which overexpressed phospholamban in the heart, driven b y the cardiac-specific alpha-myosin heavy chain promoter. Quantitative immunoblotting revealed a twofold increase in the phospholamban prote in levels in transgenic hearts compared to wild type littermate hearts . The transgenic mice showed no phenotypic alterations and no changes in heart/body weight, heart/lung weight, and cardiomyocyte size. Isola ted unloaded cardiac myocytes from transgenic mice exhibited diminishe d shortening fraction (63%) and decreased rates of shortening (64%) an d relengthening (55%) compared to wild type (100%) cardiomyocytes. The decreases in contractile parameters of transgenic cardiomyocytes refl ected decreases in the amplitude (83%) of the Ca2+ signal and prolonga tion (131%) in the time for decay of the Ca2+ signal, which was associ ated with a decrease in the apparent affinity of the SR Ca2+-ATPase fo r Ca2+ (56%), compared to wild type (100%) cardiomyocytes. In vivo ana lysis of left ventricular systolic function using M mode and pulsed-wa ve Doppler echocardiography revealed decreases in fractional shortenin g (79%) and the normalized mean velocity of circumferential shortening (67%) in transgenic mice compared to wild type (100%) mice. The diffe rences in contractile parameters and Ca2+ kinetics in transgenic cardi omyocytes and the depressed left ventricular systolic function in tran sgenic mice were abolished upon isoproterenol stimulation. These findi ngs indicate that a fraction of the Ca2+-ATPases in native SR is not u nder regulation by phospholamban. Expression of additional phospholamb an molecules results in: (a) inhibition of SR Ca2+ transport; (b) decr eases in systolic Ca2+ levels and contractile parameters in ventricula r myocytes; and (c) depression of basal left ventricular systolic func tion in vivo.