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1. Intracellular pH (pHi) regulation in crayfish neurones was studied using pH-, Na+-, and Cl- sensitive micro-electrodes. Neuronal pH regulation has previously been studied only in molluscs. 2. The average resting pHi of crayfish neurones was 7.12 +/- 0.09, which is 1 pH unit more alkaline than that predicted were H+ ions distributed in equilibrium with the membrane potential. 3. When the cytoplasm was acidified (by NH4Cl loading, CO2 application, or HCl injection), pHi recovered towards its resting value. 4. Removal of Na+ from the external solution inhibited pHi recovery from an acid load by more than 90%. pHi recovery resumed immediately when external Na+ was reintroduced. 5. The resting intracellular Na+ concentration ([Na+]i) of crayfish neurones was 15-25 mM. During pHi recovery from an acid load, [Na+]i increased by 10-50 mM. 6. Reducing the external HCO3(-) concentration from 5 mM to 0 mM slowed pHi recovery by an average of about 45%. This slowing was appreciable even in cells in which Na+ removal almost totally blocked pHi recovery. 7. The resting intracellular Cl- concentration ([Cl-]i) was 30-40 mM, indicating that these cells actively accumulate Cl-. During pHi recovery from an acid load, [Cl-]i decreased by 3-5 mM. 8. In the presence of the anion exchange inhibitor SITS (4-acetamide-4'-isothiocyanostilbene-2,2'-disulphonic acid), pHi recovery was slowed to the rate which was normally seen in HCO3(-)-free Ringer solution. SITS abolished the dependence of pHi recovery on the external HCO3(-) concentration. 9. It is concluded that pHi regulation in crayfish neurones involves two separate mechanisms: a Na+-dependent, HCO3(-)-independent acid extrusion process, and a Cl---HCO3(-) exchange which is probably also Na+-dependent.

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