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–/CO2free 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 l21 [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 l21) inhibited the maximum rate of pHi recovery by about 80%, SITS (1mmol l21) 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 pHiregulating mechanisms in relation to ionic changes during the moulting cycle are discussed.