The ionic mechanism of intracellular pH regulation in crayfish neurones.

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Abstract
Intracellular pH (pHi) regulation in crayfish [Procambaras clarki] neurons was studied using pH-, Na+-, and Cl--sensitive micro-electrodes. Neuronal pH regulation has previously been studied only in mollusk. The average resting pHi of crayfish neurons was 7.12 .+-. 0.09, which is 1 pH unit more alkaline than that predicted were H+ distributed in equilibrium with the membrane potential. When the cytoplasm was acidified (by NH4Cl loading, CO2 application, or HCl injection), pHi recovered towards its resting value. Removal of Na+ from the external solution inhibited pHi recovery from an acid load by > 90%. pHi recovery resumed immediately when external Na+ was reintroduced. The resting intracellular Na+ concentration ([Na+]i) of crayfish neurons was 15-25 mM. During pHi recovery from an acid load, [Na+]i increased by 10-50 mM. Reducing the external HCO3- concentration from 5 mM to 0 mM showed pHi recovery by an average of about 45%. This slowing was appreciable even in cells in which Na+ removal almost totally blocked pHi recovery. 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. In the presence of the anion exchange inhibitor SITS (4-acetamide-4''-isothiocyanostilbene-2,2''-disulfonic 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. pHi regulation in crayfish neurons involves 2 separate mechanisms: a Na+-dependent, HCO3--independent acid extrusion process, and a Cl--HCO3- exchange which is probably also Na+-dependent.