The temperature dependence of intracellular pH in isolated frog skeletal muscle: Lessons concerning the Na+-H+ exchanger

We used 31P NMR to investigate the temperature-dependence of intracellular pH (pH(i)) in isolated frog skeletal muscles. We found that ln[H+(i)] is a linear function of 1/T(abs) paralleling those of neutral water (i.e., H+ = OH-) and of a solution containing the fixed pH buffers of frog muscle cytosol. This classical van't Hoff relationship was unaffected by inhibition of glycolysis and was not dependent upon the pH or [Na+] in the bathing solution. Insulin stimulation of Na+-H+ exchange shifted the intercept in the alkaline direction but had no effect on the slope. Acid loading followed by washout resulted in an amiloride-sensitive return to the (temperature dependent) basal pH(i). These results show that the temperature dependence of activation of Na+-H+ exchange is similar to that of the intracellular buffers, and suggest that constancy of [H+]/[OH-] with changing temperature is achieved in the short term by intracellular buffering and in the long term by the set-point of the Na+-H+ exchanger. Proton activation of the exchanger has an apparent standard enthalpy change (ΔH°) under both control and insulin-stimulated conditions that is similar to the ΔH°of the intracellular buffers and approximately half of the ΔH°for the dissociation of water. Thus, the temperature-dependent component of the standard free-energy change (ΔF°) is unaffected by insulin stimulation, suggesting that changes in Arrhenius activation energy (E(a)) may not be a part of the mechanism of hormone stimulation.