The transgenic expression of highly inhibitory monomeric forms of phospholamban in mouse heart impairs cardiac contractility

Transgenic mice were generated with cardiac-specific overexpression of the monomeric, dominant-acting, superinhibitory L37A and I40A mutant forms of p hospholamban (PLN), and their phenotypes were compared with wild-type (wt) mice or a-fold overexpressors of wt PLN (wtOE). The level of PLN monomer in cardiac microsomes was increased 11-13-fold, and the apparent affinity of the sarco(endo)plasmic reticulum Ca2+ ATPase for Ca2+ was decreased from pC a 6.22 in wt or 6.12 in wtOE to 5.81 in L37A and 5.72 in I40A. Basal physio logical parameters, measured in isolated myocytes, indicated a significant reduction in the rates of shortening (+dL/dt) and relengthening (-dL/dt), H emodynamic measurements indicated that peak systolic pressure was unaffecte d but that pressure changes (+dP/dt and -dP/dt) were lowered significantly in both mutant lines, and relaxation time (tau) was also lengthened signifi cantly. Echocardiography for both mutants showed depressed systolic functio n and an increase in left ventricular mass of over 1.4-fold. Significant de creases in left ventricular shortening fraction and velocity of circumferen tial shortening and increases in ejection time were corrected by isoprotere nol. The use of antibodies specific against Ser(16)- and Thr(17)-PLN peptid es showed. that phosphorylation of both pentameric and monomeric PLN were i ncreased between 1.2- and 2.4-fold in both the L37A and I40A lines but not in the wtOE line. These observations show that overexpression of superinhib itory mutant forms of PW causes depression of contractile parameters with i nduction of cardiac hypertrophy, as assessed with echocardiography.