Stimulation of the plasma membrane Na+/H+ exchanger NHE1 by sustained intracellular acidosis -: Evidence for a novel mechanism mediated by the ERK pathway

Activity of the Na+/H+ exchanger ( NHE) isoform 1 (NHE1) is increased by intracellular acidosis through the interaction of intracellular H+ with an allosteric modifier site in the transport domain. Additional regulation is achieved via kinase-mediated modulation of the NHE1 regulatory domain. To determine if intracellular acidosis stimulates NHE1 activity solely by the allosteric mechanism, we subjected cultured neonatal rat ventricular myocytes (NRVM) with native NHE1 expression to intracellular acidosis ( pH(i) similar to 6.6) for up to 6 min by transient exposure to NH4Cl and its washout in the presence of NHE inhibition (by zero [Na+](o) or the NHE1 inhibitor cariporide) in HCO3--free medium. After the desired duration of acidosis, NHE was reactivated (by reintroduction of [Na+](o) or removal of cariporide), and the rate of recovery of pH(i) (dpH(i)/dt) was measured as the index of NHE activity. Regardless of the method used when intracellular acidosis was sustained for greater than or equal to 3 min, subsequent NHE activity was significantly increased (>4-fold). Similar NHE stimulatory effects of sustained acidosis were observed in adult rat ventricular myocytes and COS-7 cells. Sustained ( 3 min) intracellular acidosis activated several NHE1 kinases in NRVM, in an in-gel kinase assay using as substrate a glutathione S-transferase fusion protein of the NHE1 regulatory domain. Detailed investigation of ERK and its downstream effector p90(RSK), two putative NHE1 kinases, revealed time-dependent activation of both by intracellular acidosis in NRVM. Furthermore, inhibition of MEK1/2 by pretreatment of NRVM with two structurally distinct inhibitors, PD98059 ( 30 muM) or UO126 ( 3 muM), inhibited the activation of ERK and p90(RSK) and abolished the stimulation of NHE activity by sustained ( 3 min) intracellular acidosis. Our data show that not only the extent but also the duration of intracellular acidosis regulates NHE1 activity and suggest that the stimulatory effect of sustained intracellular acidosis occurs through a novel mechanism mediated by activation of the ERK pathway.