Contribution of the Na⁺/H⁺ Antiporter to the Regulation of Intracellular pH in a Crayfish Stretch Receptor Neurone

Regulation of intracellular pH (pHi) following acidosis induced by NH₄⁺/NH₃ exposures was re-investigated in a crayfish stretch receptor neurone using H⁺- and Na⁺-selective microelectrodes. All experiments were performed in nominally HCO₃⁻/CO₂ -free salines. From studies in Na⁺-free saline and from electrochemical calculations, we concluded that pHi regulation was dependent on extracellular Na⁺ concentration ([Na⁺]_o ). The half-maximal rate of pHi recovery had an apparent Michaelis–Menten constant of 21mmol l⁻ 1 [Na⁺]_o. The use of this experimental approach and an improved technique enabled us to observe pHi regulation even in Cl⁻ -free saline, in contrast to earlier findings. In addition, amiloride (2mmol l⁻ 1) inhibited the maximum rate of pHi recovery by about 80%, SITS (1mmol l⁻ 1) by about 20%.

The results strongly suggest the operation of two separate pHi-regulating mechanisms, a Na⁺-dependent HCO₃⁻ /Cl⁻ antiporter (probably the Na⁺/H⁺/HCO₃⁻ /Cl⁻ antiporter described earlier) and a Na⁺/H⁺ antiporter. Both mechanisms have been described in crayfish ganglion cells and muscle fibres, but the individual contribution to pHi regulation varies considerably in these preparations. Functional aspects of the pHi-regulating mechanisms in relation to ionic changes during the moulting cycle are discussed.