Expression and phosphorylation of the Na-pump regulatory subunit phospholemman in heart failure

Intracellular [Na] is approximate to 3 mmol/L higher in heart failure (HF; in our arrhythmogenic rabbit model;(3)), and this can profoundly affect cardiac Ca and contractile function via Na/Ca exchange and Na/H exchange. Na/K- ATPase is the primary mechanism of Na extrusion. We examine here in HF rabbits (and human hearts) expression of Na/K-ATPase isoforms and phospholemman (PLM), a putative Na/K-ATPase regulatory subunit that inhibits pump function and is a major cardiac phosphorylation target. Na/K- ATPase alpha 1- and alpha 2- isoforms were reduced in HF in rabbit ventricular homogenates (by 24%) and isolated myocytes (by 30% and 17%), whereas alpha 3 was increased (50%) in homogenates and decreased (52%) in myocytes (P < 0.05). Homogenate Na/K-ATPase activity in left ventricle was also decreased in HF. However, we showed previously that Na/K-ATPase characteristics in intact ventricular myocytes were unaltered in HF. To reconcile these findings, we assessed PLM expression, phosphorylation, and association with Na/K- ATPase. PLM coimmunoprecipitated with Na/K-ATPase alpha 1 and alpha 2 in control and HF rabbit myocytes. PLM expression was reduced in HF by 42% in isolated rabbit left ventricular (LV) myocytes, by 48% in rabbit LV homogenates, and by 24% in human LV homogenate. The fraction of PLM phosphorylated at Ser-68 was increased dramatically in HF. Our results are consistent with a role for PLM analogous to that of phospholamban for SR Ca-ATPase (SERCA): inhibition of Na/ K-ATPase function that is relieved on PLM phosphorylation. So reduced Na/ K-ATPase expression in HF may be functionally offset by lower inhibition by PLM (because of reduced PLM expression and higher PLM phosphorylation).