Intracellular pH regulation in the sensory neurone of the stretch receptor of the crayfish (Astacus fluviatilis).

The ionic mechanisms of intracellular pH (pHi) regulation were studied in the slowly adapting sensory cell of the crayfish stretch receptor by using pH-, Na+- and Cl(-)-sensitive liquid ion exchanger electrodes. Under control conditions a mean pHi of 7.23 +/- 0.12 (S.D.) at a mean membrane potential of 68.3 +/- 4.1 mV S.D. was found in sixteen cells. Thus pHi is about 1 pH unit more alkaline than predicted from passive distribution, implying the presence of an acid extrusion mechanism. In order to acidify the cytoplasm, the cell was either acid-loaded by NH4Cl or exposed to CO2 and CO2/HCO3- solutions. During CO2 exposures pHi was regulated only if calculated amounts of HCO3- were added to keep external pH (pHo) constant. The pHo per se was found to be an important determinant of pHi and its regulation. Substitution of external Na+ by choline inhibited pHi recovery almost completely. As soon as Na+ was readmitted H+ extrusion occurred immediately at a rate similar to that of the control. The internal Na+ activity (aiNa) ranged between 6 and 13 mM with a mean of approximately 9.1 +/- 2.5 mM (S.D.; n = 8). The effects of various solutions on aiNa and the temporal relationship between aiNa and pHi in NH4Cl acid-loaded cells were investigated. The amount of aiNa increased during cell internal acidification and recovered in parallel with pHi recovery in NH4Cl acid-loaded cells. Experiments with 10(-4) M-ouabain and K+-free conditions suggest that neither the Na+-K+ pump nor external K+ are directly involved in pHi regulation. The internal chloride activity (aiCl), which was lower than predicted from a passive distribution, fell during exposure to HCO3-/CO2. Regulation of pHi was inhibited if the cell was completely depleted of Cl- by prolonged exposures to Cl(-)-free solution (isethionate and/or gluconate substituted). The pHi-regulating system of the sensory cell requires Na+ and Cl- which probably operate in a combined mechanism such as Na+ -H+-Cl(-)-HCO3- or an equivalent.