Interaction between Na+ and H+ ions on Na-H exchange in sheep cardiac Purkinje fibers

The interaction between Na+ and H+ ions upon Na-H exchange (NHE) was examined in sheep cardiac Purkinje fibers. Acid equivalent fluxes through NHE were examined using recordings of intracellular pH and Na+ in isolated preparations measured with ion selective microelectrodes. The extent of acid-extrusion by NHE was estimated from pH, recovery-rate, multiplied by beta(i) (intracellular buffering power) in response to an internal acid load induced by 20 mm NH4Cl removal (nominally HCO3- free media). A mixed inhibitory effect was found of extracellular H+ on external Na+-activation of NHE (i.e. an increase, at low pH(o), in the apparent Michaelis constant for external Na+ ions [KN0.5Nao] and a decrease in the maximum transport rate [VNmaxNao]). In addition, we confirmed that the stoichiometry of Na-o binding is unaffected by the pH(o) (between 7.5 and 6.5), showing a Hill coefficient close to one. The interaction between Na+ and H+ ions at the internal face of the cardiac NHE was also studied. Our evidence suggests that an increase in the intracellular Na+ ion concentration ([Na+](i)) inhibits acid efflux and that this inhibition can be approximated by the decrease in thermodynamic driving force caused by reducing the transmembrane Na+ gradient. It appears, however, that small variations in [Na+](i) from the normal resting level (intracellular sodium activity, a(Na)(i)=7 to 13 mm) have little or no effect on acid efflux, suggesting that variation of a(Na)(i) is not a physiologically important controller of NHE activity in heart. (C) 1997 Academic Press Limited.