The ionic mechanism of intracellular pH (pHi) regulation was investigated in isolated muscle fibres of the carpopodite adductor in the crayfish Astacus fluviatilis by electrophysiological means with pH, Na+ and Cl- -sensitive liquid ion exchanger micro-electrodes. In eighty-six cells a mean pHi of 7.14 +/- 0.12 (S.D.) at a membrane potential of--79.7 +/- 3.4 mV was found under control conditions which is about one pH unit more alkaline than predicted from passive distribution and indicates the presence of an acid-extrusion mechanism. In order to study pHi recovery the cells were acid loaded by exposure either to NH4 Cl or CO2. The effects of HCO3- and DIDS (an inhibitor of the anion exchange) on pHi recovery as well as the HCO3- -dependent decrease of intracellular Cl- during pHi recovery indicate that in pHi regulation a mechanism of acid extrusion is involved which exchanges extracellular HCO3- for intracellular Cl-. In CO2/HCO3- -free solution or in salines with DIDS, pHi recovery was retarded to the same degree, but the effects were not additive. Because of this the remaining pHi recovery must originate from an HCO3- -independent acid-extrusion mechanism. In Na+ -free solution any pHi recovery was blocked; if pHi recovery occurred it was accompanied by an increase of intracellular Na+ activity (aiNa). From these results it was concluded that all acid extrusion mechanisms which contributed to pHi recovery are coupled to an influx of Na+. A Na+/H+/HCO3-/Cl-, and a separate Na+/H+, exchange are proposed as a model of pHi regulation in the crayfish muscle fibre. Similar kinds of acid extrusion mechanisms are found in the neurone of the crayfish (Moody, 1981), with the difference that in the muscle fibre pHi regulation is achieved mainly by the former process. The rate of pHi recovery is considerably lower in the muscle fibre than in the neurone or in the sensory cell (Moser, 1985) of crayfish.
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