Phospholemman-mediated activation of Na/K-ATPase limits [Na]i and inotropic state during β-adrenergic stimulation in mouse ventricular myocytes

Background - Cardiac Na/K-ATPase ( NKA) regulates intracellular Na ([Na](i)), which in turn affects intracellular Ca and thus contractility via Na/Ca exchange. Recent evidence shows that phosphorylation of the NKA-associated small transmembrane protein phospholemman ( PLM) mediates beta-adrenergic - induced NKA stimulation. Methods and Results - Here, we tested whether PLM phosphorylation during beta-adrenergic activation limits the rise in [ Na](i), Ca transient amplitude, and triggered arrhythmias in mouse ventricular myocytes. In myocytes from wild-type (WT) mice, [ Na](i) increased on field stimulation at 2 Hz from 11.1 +/- 1.8 mmol/ L to a plateau of 15.2 +/- 1.5 mmol/ L. Isoproterenol induced a decrease in [ Na](i) to 12.0 +/- 1.2 mmol/ L. In PLM knockout ( PLM-KO) mice in which beta-adrenergic stimulation does not activate NKA, [ Na](i) also increased at 2 Hz ( from 10.4 +/- 1.2 to 17.0 +/- 1.5 mmol/ L) but was unaltered by isoproterenol. The PLM-mediated decrease in [ Na](i) in WT mice could limit the isoproterenol-induced inotropic state. Indeed, the isoproterenol-induced increase in the amplitude of Ca transients was significantly smaller in the WT mice ( 5.2 +/- 0.4- versus 7.1 +/- 0.5- fold in PLM-KO mice). This also was the case for the sarcoplasmic reticulum Ca content, which increased by 1.27 +/- 0.09- fold in WT mice versus 1.53 +/- 0.09- fold in PLM-KO mice. The higher sarcoplasmic reticulum Ca content in PLM-KO versus WT mice was associated with an increased propensity for spontaneous Ca transients and contractions in PLM-KO mice. Conclusions - These data suggest that PLM phosphorylation and NKA stimulation are an integral part of the sympathetic fight-or-flight response, tempering the rise in [Na](i) and cellular Ca loading and perhaps limiting Ca overload - induced arrhythmias.