Intracellular pH of crab and crayfish muscle fibre types during GABA application and inhibitory nerve stimulation

The inhibitory motoneurons of crustaceans form synapses both with the sarco lemma of muscle fibres and with the very distal branchings of the excitator y motoneurons. The transmitter of these synapses is GABA (gamma -aminobutyr ic acid) which is known to open Cl- channels. Studies on the dactyl opener muscle of crayfish suggest that application of GABA not only leads to an in crease in the Cl- permeability but also to a considerable HCO3- conductance that causes an intracellular acidification. To investigate possible physio logical implications, we measured the intracellular pH of various muscle fi bre types of crayfish and crab using pi-I-sensitive microelectrodes. Indepe ndent of the presence or absence of inhibitory innervation, bath applicatio n of 10(-5) mol l(-1) GABA led to acidification in all fibre types (pH chan ge: 0.14 +/- 0.08, n = ii). In no preparation was a change in intracellular pH observed upon stimulation of specific or common inhibitory motoneurons with 10-40 pulses s(-1) for 2-5 min. The results suggest that HCO3- conduct ance cannot be activated through synaptic GABA receptors. However, all crus tacean muscle fibre types seem to possess extrasynaptic GABA-sensitive chan nels that exhibit a considerable HCO3- conductance. The physiological impor tance of these channels remains to be elucidated.