Abstract 2375: Regulation of Cardiac Myocyte Contractility by Phospholemman
Phospholemman (PLM) regulates cardiac contractility by modulating Na+/Ca2+ exchanger (NCX1) and/or Na+-K+-ATPase activities. PLM, when phosphorylated at serine68, disinhibits Na+-K+-ATPase but inhibits NCX1. In this study, we first demonstrated that adult mouse cardiac myocytes cultured for 48 hours had normal surface membrane area and t-tubule appearance, and exhibited near normal contractility. Contractile differences between wild-type (WT) and PLM knockout (KO) myocytes were preserved after 48h of culture. Infection with adenovirus overexpressing GFP did not affect contractility at 48h. When WT PLM was overexpressed in PLM-KO myocytes, part of PLM was phosphorylated, both Na+-K+-ATPase current (Ipump) and Na+/Ca2+ exchange current (INaCa) were depressed, and contractility and [Ca2+]i transients reverted back to those observed in cultured WT myocytes. Overexpressing PLMS68E mutant (phosphomimetic) in PLM-KO myocytes resulted in suppression of INaCa but no effect on Ipump. Contractility and [Ca2+]i transient amplitudes in PLM-KO myocytes overexpressing PLMS68E mutant were depressed when compared to PLM-KO myocytes overexpressing GFP. Overexpressing PLMS68A mutant (mimicking unphosphorylated PLM) in PLM-KO myocytes had no effects on INaCa but decreased Ipump. Contractility and [Ca2+]i transient amplitudes in PLM-KO myocytes overexpressing S68A mutant were similar to PLM-KO myocytes overexpressing GFP. Neither WT PLM nor its mutants had any effect on SR Ca2+ uptake in KO myocytes. We conclude that at the single myocyte level, PLM affects cardiac contractility and [Ca2+]i homeostasis primarily by its direct inhibitory effects on Na+/Ca2+ exchange.