Postsynaptic fall in intracellular pH and increase in surface pH caused by efflux of formate and acetate anions through GABA-gated channels in crayfish muscle fibres.

H(+)-selective microelectrodes and a two- or three-microelectrode voltage clamp were used to examine the influence of weak-acid, carboxylate anions on the actions of GABA on postsynaptic intracellular pH, surface pH and on membrane potential in fibres of the crayfish leg opener muscle. Substitution of 30 mM Cl- by formate or acetate promoted a GABA-induced decrease in intracellular pH, which was coupled to an increase in surface pH and to a depolarization. Such effects were not seen in the presence of an equivalent amount of lactate, methanesulphonate or glucuronate. Both the GABA-induced depolarization and the fall in internal pH promoted by formate and acetate were blocked by picrotoxin, and the fall in pH was reversibly inhibited by a K(+)-induced depolarization. The rate of the fall in intracellular pH produced by GABA (0.2 mM) was about 0.02 pH units/min in the presence of formate and 0.03 pH units/min in the presence of acetate. Under steady-state conditions, both 30 mM formate and acetate (but not lactate) induced a positive shift in the reversal potential of GABA-activated current, which was accounted for by a relative permeability vs Cl- of formate and acetate of 0.5 and 0.15, respectively. The conductance sequence of the anions was identical to the permeability sequence, i.e. Cl- greater than formate greater than acetate greater than lactate approximately equal to 0. This sequence is strictly correlated to the Stokes diameter of the anions. The relative permeabilities of the anions indicate that the effective diameter of the GABA-gated channel is about 0.5 nm. The fact that the GABA-induced acidosis was slower in the presence of formate than in the presence of acetate suggests that, in the former case, the rate-limiting step in the fall in internal pH is the entry of non-dissociated formic acid. All the above results are consistent with a scheme where GABA induces a channel-mediated efflux of permeant weak-acid anions, which gives rise to an inward (depolarizing) current and to an intracellular acidosis. A comparison of the permeability properties of crayfish and vertebrate GABA-gated channels suggests that effects similar to those seen in this work are likely to occur in mammalian and other vertebrate neurons in the presence of permeant weak-acid anions.