Altered Ca fluxes and contractile state during pH changes in cultured heart cells

We studied mechanisms underlying changes in myocardial contractile state produced by intracellular (pHi) or extracellular (pHo) changes in pH using cultured chick embryo ventricular cells. A change in pHo of HEPES-buffered medium from 7.4 to 6.0 or to 8.8 changed the amplitude of cell motion by -85 or +60%, and 45Ca uptake at 10 s by -29 or +22%, respectively. The pHo-induced change in Ca uptake was not sensitive to nifedipine (10 microM), but was Na gradient dependent. Changes in pHi produced by NH4Cl or preincubation in media at pH values ranging from 6.0 to 8.8 failed to alter significantly 45Ca uptake or efflux. However, larger changes in pHi were associated with altered Ca uptake. Changes in pHo from 7.4 to 6.0 or to 8.8 were associated with initial changes in 45Ca efflux by +17 or -18%, respectively, and these effects were not Na dependent. Exposure of cells to 20 mM NH4Cl produced intracellular alkalinization and a positive inotropic effect, whereas subsequent removal of NH4Cl caused intracellular acidification and a negative inotropic effect. There was, however, a lack of close temporal relationship between pHi and contractile state. These results indicate that pHo-induced changes in contractile state in cultured heart cells are closely correlated with altered transsarcolemmal Ca movements and presumably are due (at least in part) to these Ca flux changes. In contrast, pHi-induced changes in contractile state appear principally to involve altered Ca handling within the cell and/or altered Ca sensitivity of myofibrils.