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 pHirecovery-rate, multiplied byβi(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 pHo, in the apparent Michaelis constant for external Na+ions [KNao0.5] and a decrease in the maximum transport rate [VNaomax]). In addition, we confirmed that the stoichiometry of Naobinding is unaffected by the pHo(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+]ifrom the normal resting level (intracellular sodium activity, aiNa=7 to 13 mm) have little or no effect on acid efflux, suggesting that variation of aiNais not a physiologically important controller of NHE activity in heart.