Sustained intracellular acidosis activates the myocardial Na+/H+ exchanger independent of amino acid Ser703 and p90rsk

The mammalian Na+/H+ exchanger isoform 1 (NHE1) is a ubiquitously expressed pH-regulatory membrane protein that functions in the myocardium and other tissues. It is an important mediator of the myocardial damage that occurs after ischemia–reperfusion injury and is implicated in heart hypertrophy. Regulation of NHE1 has been proposed as a therapeutic target for cardioprotection. We therefore examined mechanisms of control of NHE1 in the myocardium. Several different amino acids have been implicated as a being critical to NHE1 regulation in a number of tissues including Ser703, Ser770, and Ser771. In the myocardium, NHE1 is activated in response to a variety of stimuli including activation by an ERK-dependent sustained intracellular acidosis. In this study, we determined whether Ser703 and p90rsk activity are critical in activation of NHE1 by sustained intracellular acidosis. In vitro phosphorylation of NHE1 C-terminal fusion proteins determined that ERK-dependent phosphorylation of the cytoplasmic region was not dependent on Ser703; however, phosphorylation by p90rsk required Ser703. A Ser703Ala mutation decreased basal NHE1 activity in CHO cells but not in cardiomyocytes. NHE1 with a Ser703Ala mutation was activated in response to sustained intracellular acidosis in CHO cells. In addition, sustained intracellular acidosis also activated the Ser703Ala mutant protein in isolated cardiomyocytes and phosphorylation levels were also increased by acidosis. The presence of a dominant-negative p90rsk kinase also did not prevent activation and phosphorylation of NHE1 by sustained intracellular acidosis in isolated cardiomyocytes. We conclude that Ser703 and p90rsk are not required for activation by sustained intracellular acidosis and that p90rsk phosphorylation of Ser703 is independent of this type of activation.