Altered contractility and [Ca2+]i homeostasis in phospholemman-deficient murine myocytes: role of Na+/Ca2+ exchange.
Phospholemman (PLM) regulates contractility and Ca(2+) homeostasis in cardiac myocytes. We characterized excitation-contraction coupling in myocytes isolated from PLM-deficient mice backbred to a pure congenic C57BL/6 background. Cell length, cell width, and whole cell capacitance were not different between wild-type and PLM-null myocytes. Compared with wild-type myocytes, Western blots indicated total absence of PLM but no changes in Na(+)/Ca(2+) exchanger, sarcoplasmic reticulum (SR) Ca(2+)-ATPase, alpha(1)-subunit of Na(+)-K(+)-ATPase, and calsequestrin levels in PLM-null myocytes. At 5 mM extracellular Ca(2+) concentration ([Ca(2+)](o)), contraction and cytosolic [Ca(2+)] ([Ca(2+)](i)) transient amplitudes and SR Ca(2+) contents in PLM-null myocytes were significantly (P < 0.0004) higher than wild-type myocytes, whereas the converse was true at 0.6 mM [Ca(2+)](o). This pattern of contractile and [Ca(2+)](i) transient abnormalities in PLM-null myocytes mimics that observed in adult rat myocytes overexpressing the cardiac Na(+)/Ca(2+) exchanger. Indeed, we have previously reported that Na(+)/Ca(2+) exchange currents were higher in PLM-null myocytes. Activation of protein kinase A resulted in increased inotropy such that there were no longer any contractility differences between the stimulated wild-type and PLM-null myocytes. Protein kinase C stimulation resulted in decreased contractility in both wild-type and PLM-null myocytes. Resting membrane potential and action potential amplitudes were similar, but action potential duration was much prolonged (P < 0.04) in PLM-null myocytes. Whole cell Ca(2+) current densities were similar between wild-type and PLM-null myocytes, as were the fast- and slow-inactivation time constants. We conclude that a major function of PLM is regulation of cardiac contractility and Ca(2+) fluxes, likely by modulating Na(+)/Ca(2+) exchange activity.
Tucker
Amy L
AL
Dept. of Cellular and Molecular Physiology, Milton S. Hershey Medical Center, MC-H166, Hershey, PA 17003, USA.
Song
Jianliang
J
Zhang
Xue-Qian
XQ
Wang
Jufang
J
Ahlers
Belinda A
BA
Carl
Lois L
LL
Mounsey
J Paul
JP
Moorman
J Randall
JR
Rothblum
Lawrence I
LI
Cheung
Joseph Y
JY
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D018929:Cell Culture Techniques; D002478:Cells, Cultured; D003433:Crosses, Genetic; D006706:Homeostasis; D008565:Membrane Proteins / Q000172:deficiency* / Q000235:genetics; D051379:Mice; D020297:Mice, Congenic; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009200:Myocardial Contraction / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010750:Phosphoproteins / Q000172:deficiency* / Q000235:genetics; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D008565:Membrane Proteins; D010750:Phosphoproteins; D019831:Sodium-Calcium Exchanger; C069141:phospholemman; D002118:Calcium