Abstract 2696: Phospholemman and B-Adrenergic Stimulation in Intact Hearts
Phospholemman (PLM), the first cloned member of FXYD family of regulators of ion transport, is expressed abundantly in the heart and a major sarcolemmal substrate for phosphorylation. PLM associates with and regulates cardiac Na+-K+-ATPase and Na+/Ca2+ exchanger. Phosphorylation of PLM (at serine68) in response to β-adrenergic stimulation results in simultaneous inhibition of Na+/Ca2+ exchanger and relief of inhibition of Na+-K+-ATPase. The role of PLM in mediating β-adrenergic effects on in vivo and in vitro cardiac contractility was investigated using congenic PLM knockout (KO) mice. Resting cardiac output and ejection fraction were similar between wild-type (WT) and PLM-KO hearts. In vivo 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. The dose-response curves to Iso (0.5 to 25 ng) for WT and PLM-KO hearts were similar. Maximal +dP/dt attained 1–2 min. after Iso addition declined with time in WT but not PLM-KO hearts. In isolated myocytes paced at 2 Hz, both contraction and [Ca2+]i transient amplitudes reached maximum at 2 min. after Iso followed by decline in WT but not PLM-KO myocytes. Reducing pacing frequency to 0.5 Hz resulted in no decline in maximal contraction and [Ca2+]i transient amplitudes in Iso-stimulated WT and PLM-KO myocytes. Baseline Na+-K+-ATPase current (Ipump) was 41% higher in PLM-KO myocytes due to increased α1- but not α2-subunit activity. Iso increased Iα1 by 73% in WT compared to <10% in PLM-KO myocytes. Iso had no effect on Iα2 in both WT and PLM-KO myocytes. In WT hearts, PLM co-immunoprecipitated both α1- and α2-subunits of Na+-K+-ATPase. We conclude that under β-adrenergic stimulation, enhanced phosphorylation of PLM limited the increase in [Na+]i by relieving its inhibition of α1-subunit of Na+-K+-ATPase, at the expense of reduced inotropy in both intact hearts and isolated myocytes.
This research has received full or partial funding support from the American Heart Association, National Center.