MECHANISM OF ACTION OF GABA ON INTRACELLULAR PH AND ON SURFACE PH IN CRAYFISH MUSCLE-FIBERS

1. The mode of action of gamma‐aminobutyric acid (GABA) on intracellular pH (pHi) and surface pH (pHs) was studied in crayfish muscle fibres using H(+)‐selective microelectrodes. The extracellular HCO3‐ concentration was varied (0‐30 mM) at constant pH (7.4). 2. GABA (5 x 10(‐6)‐10(‐3) M) produced a reversible fall in pHi which showed a dependence on the concentrations of both GABA and HCO3‐. The fall in pHi was associated with a transient increase in pHs and it was inhibited by a K(+)‐induced depolarization. 3. In the presence of 30 mM‐HCO3‐, a near‐saturating concentration of GABA (0.5 mM) produced a mean fall in pHi of 0.43 units. This change in pHi accounted for about two‐thirds of the GABA‐induced decrease (from ‐66 to ‐29 mV) in the sarcolemmal H+ driving force, while the rest was due to the simultaneous depolarization. 4. The apparent net efflux of HCO3‐ (JHCO3e) produced by a given concentration of GABA was estimated on the basis of the instantaneous rate of change of pHi. In the presence of 30 mM‐HCO3‐, JHCO3e following exposure to 0.5 mM‐GABA had a mean value of 8.0 mmol l‐1 min‐1. Under steady‐state conditions (at plateau acidosis), the intracellular acid load produced by 0.5 mM‐GABA was about 25% of that seen at the onset of the application. 5. The GABA‐induced HCO3‐ permeability, calculated on the basis of the flux data, showed a concentration dependence similar to that of the GABA‐activated conductance described in previous work. 6. The GABA‐induced increase in pHs was immediately blocked by both a membrane‐permeant inhibitor of carbonic anhydrase (acetazolamide, 10(‐6) M) and by a poorly permeant inhibitor (benzolamide, 10(‐6) M). 7. Application of acetazolamide (10(‐4) M) for 5 min or more produced a decrease of up to 60% in the maximum rate of fall of pHi at GABA concentrations higher than 20 microM. 8. The recovery of the GABA‐induced acidosis was associated with a fall in pHs. The recovery was completely blocked in solutions devoid of Na+ or of Cl‐, as well as by DIDS (4,4'‐diisothiocyanostilbene‐2,2'‐disulphonic acid, 10(‐5) M). This indicates that the maintenance of a non‐equilibrium H+ gradient at plateau acidosis and the recovery of pHi are attributable to Na(+)‐dependent Cl(‐)‐HCO3‐ exchange. 9. We conclude that the effects of GABA on pHi and pHs are due to electrodiffusion of HCO3‐ across postsynaptic anion channels.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society