γ-Aminobutyric acid-stimulated chloride permeability in crayfish muscle

Abstract

γ-Aminobutyric acid selectively increased Cl − permeability in isolated strips of crayfish abdominal muscle. Muscle fibers incubated in VAn Harreveld's solution at room temperature took up 36Cl − to the extent of 700 ml/kg wet weight with a halftime of 2.5 min. During 15- s incubations, the control 36Cl − uptake space was 131 ± 4 ml/kg ( n = 60) and this was significantly increased by γ-aminobutyric acid at 200 μM or higher concentrations to 177 ± 4 ml/kg ( n = 48, P < 0.05). This effect was specific for chloride since γ-aminobutyric acid did not increase the uptake by crayfish muscle of radioactive sucrose, inositol, or propionate. γ-Aminobutyric acid stimulation of 36Cl − uptake is mediated by receptor-ionophore function since the process shows pharmacological properties virtually identical to those observed by electrophysiological techniques. The γ-aminobutyric acid stimulation of Cl − permeability is dose dependent with 50% of the maximal effect at 40 μM γ-aminobutyric acid and the dose vs. response curve is somewhat sigmoid. The γ-aminobutyric acid agonist muscimol causes the same maximal effect on Cl − uptake as γ-aminobutyric acid, but acts at 5-fold lower concentrations, i.e. is more potent. However, the partial agonist γ-amino, β-hydroxybutyric acid produced little or no stimulation of 36Cl − flux. The response to γ-aminobutyric acid was blocked by 2 mM β-guanidinopropionate or γ-guanidinobutyrate, 0.5 mM bicuculline, and 10 μM picrotoxinin. Picrotoxinin inhibition was dose dependent with 50% inhibition occurring at 4 μM. Antagonists did not affect control 36Cl − uptake. These results confirm electrophysiological observations that the postsynaptic response to the inhibitory neurotransmitter γ-aminobutyric acid involves a rapid increase in membrane permeability to Cl −