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 sarcolemma of muscle fibres and with the very distal branchings of the excitatory motoneurons. The transmitter of these synapses is GABA (gamma-aminobutyric 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 increase in the Cl- permeability but also to a considerable HCO3- conductance that causes an intracellular acidification. To investigate possible physiological implications, we measured the intracellular pH of various muscle fibre types of crayfish and crab using pH-sensitive microelectrodes. Independent of the presence or absence of inhibitory innervation, bath application of 10(-5) mol l(-1) GABA led to acidification in all fibre types (pH change: 0.14 +/- 0.08, n = 11). In no preparation was a change in intracellular pH observed upon stimulation of specific or common inhibitory motoneurons with 1040 pulses s(-1) for 2-5 min. The results suggest that HCO3- conductance cannot be activated through synaptic GABA receptors. However, all crustacean muscle fibre types seem to possess extrasynaptic GABA-sensitive channels that exhibit a considerable HCO3- conductance. The physiological importance of these channels remains to be elucidated.