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 21 mmol 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 (2 mmol l-1) inhibited the maximum rate of pHi recovery by about 80 %, SITS (1 mmol 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.

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