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

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Abstract
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 16 cells. pHi is .apprx. 1 pH unit more alkaline than predicted from passive distribution, implying the presence of an acid extrusion mechanism. 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 .**GRAPHIC**. ranged between 6 and 13 mM with a mean of .apprx. 9.1 .+-. 2.5 mM (S.D.; n = 8). The effects of various solutions on .**GRAPHIC**. and the temporal relationship between .**GRAPHIC**. and pHi in NH4Cl acid-loaded cells were investigated. The amount of .**GRAPHIC**. 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 Cl activity .**GRAPHIC**. 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.