Phospholemman And Beta-Adrenergic Stimulation In The Heart

Wang J, Gao E, Song J, Zhang XQ, Li J, Koch WJ, Tucker AL, Philipson KD, Chan TO, Feldman AM, Cheung JY. Phospholemman and beta-adrenergic stimulation in the heart. Am J Physiol Heart Circ Physiol 298: H807-H815, 2010. First published December 11, 2009; doi:10.1152/ajpheart.00877.2009.-Phosphorylation at serine 68 of phospholemman (PLM) in response to beta-adrenergic stimulation results in simultaneous inhibition of cardiac Na+/Ca2+ exchanger NCX1 and relief of inhibition of Na+-K+-ATPase. The role of PLM in mediating beta-adrenergic effects on in vivo cardiac function was investigated with congenic PLM-knockout (KO) mice. Echocardiography showed similar ejection fraction between wild-type (WT) and PLM-KO hearts. Cardiac catheterization demonstrated higher baseline contractility (-dP/dt) but similar relaxation (-dP/dt) in PLM-KO mice. In response to isoproterenol (Iso), maximal -dP/dt was similar but maximal -dP/dt was reduced in PLM-KO mice. Dose-response curves to Iso (0.5-25 ng) for WT and PLM-KO hearts were super-imposable. Maximal -dP/dt was reached 1-2 min after Iso addition and declined with time in WT but not PLM-KO hearts. In isolated myocytes paced at 2 Hz. contraction and intracellular Ca2+ concentration ([Ca2+](i)) transient amplitudes and [Na+](i) reached maximum 2-4 min after Iso addition, followed by decline in WT but not PLM-KO myocytes. Reducing pacing frequency to 0.5 Hz resulted in much smaller increases in [Na+](i) and no decline in contraction and [Ca2+](i) transient amplitudes with time in Iso-stimulated WT and PLM-KO myocytes. Although baseline Na+-K+-ATPase current was 41% higher in PLM-KO myocytes because of increased alpha(1)- but not alpha(2)-subunit activity, resting [Na+](i) was similar between quiescent WT and PLM-KO myocytes. Iso increased alpha(1)-subunit current (I-alpha 1) by 73% in WT but had no effect in PLM-KO myocytes. Iso did not affect alpha(2)-subunit current (I-alpha 2) in WT and PLM-KO myocytes. In both WT and NCX1-KO hearts, PLM coimmunoprecipitated with Na+-K+-ATPase alpha(1)- and alpha(2)-subunits, indicating that association of PLM with Na+-K+-ATPase did not require NCX1. We conclude that under stressful conditions in which [Na+](i) was high, beta-adrenergic agonist-mediated phosphorylation of PLM resulted in time-dependent reduction in inotropy due to relief of inhibition of Na+-K+-ATPase.