Dependence of sodium-calcium exchange current through the membrane of salivary gland cells ofChironomus on pH of the extracellular solution

Abstract

The dependence of sodium-calcium exchange current (INa(Ca)) through the membrane of isolated secretory cells ofChironomus larva on pH of the extracellular solution was studied with the voltage-clamp technique with intracellular perfusion.INa(Ca) evoked by hyperpolarization of the membrane from −20 to −60 mV was recorded within physiological values of Na+ and Ca2+ gradients. It was established that acidification of extracellular solution from pH 7.2 to 4.0 gradually decreased the amplitude ofINa(Ca) with pK' — 3.72. In all cases at pH 3.0 an outward current of considerable amplitude emerged in response to membrane hyperpolarization. The reversal of the current occurred at pH around 3.25. A decrease inINa(Ca) was due to protonation of acid ionogenic groups (quite possibly, of the residues of aspartic or glutamic amino acids), which had been involved in binding of cations. Alkalization of extracellular solution from pH 7.2 to 10.0 produced a gradual increase in theINa(Ca) amplitude; pK' was in the pH range between 9 and 10. The increase inINa(Ca) in alkaline medium was probably due to the appearance of negatively charged cations at binding sites, which could be carried by deprotonated thiosulfate groups of cysteine residues. This was indicated by the possibility of initial decrease inINa(Ca) under the action of Hg2+ ions.