Phospholemman-mediated activation of Na/K-ATPase limits [Na]i and inotropic state during beta-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.


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.
Despa
Sanda
S
Department of Physiology, Loyola University Chicago, Maywood, Ill, USA.
Tucker
Amy L
AL
Bers
Donald M
DM
D000318:Adrenergic beta-Agonists / Q000633:toxicity*; D000818:Animals; D020296:Animals, Congenic; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000503:physiopathology*; D020013:Calcium Signaling; D002304:Cardiac Pacing, Artificial; D002316:Cardiotonic Agents / Q000633:toxicity*; D004789:Enzyme Activation / Q000187:drug effects; D006352:Heart Ventricles; D015640:Ion Channel Gating / Q000502:physiology; D007545:Isoproterenol / Q000633:toxicity; D008565:Membrane Proteins / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D032383:Myocytes, Cardiac / Q000187:drug effects* / Q000378:metabolism; D010750:Phosphoproteins / Q000502:physiology*; D010766:Phosphorylation; D011499:Protein Processing, Post-Translational; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
D000318:Adrenergic beta-Agonists; C506944:Atp2a1 protein, mouse; D002316:Cardiotonic Agents; D008565:Membrane Proteins; D010750:Phosphoproteins; D011943:Receptors, Adrenergic, beta; C069141:phospholemman; D012964:Sodium; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; C506947:Atp2a2 protein, mouse; D007545:Isoproterenol