THE EFFECT OF γ‐AMINOBUTYRIC ACID ON THE INPUT CONDUCTANCE AND MEMBRANE POTENTIAL OF ASCARIS MUSCLE

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Abstract
Twin intracellular recordings were made from the bag region of Ascaris muscle in order to make conductance measurements. The preparation was bathed in a cool (22°C) Ringer solution to abolish the large spontaneous depolarizing potentials and to improve stability for recording. The resting membrane potential was − 31 ± 1 mV, mean ± s.e. mean (n = 17). The current‐voltage plots were linear in the hyperpolarizing direction but showed evidence of delayed rectification during the application of depolarizing currents. The input conductance of the bag was measured from the slope of these plots during the application of hyperpolarizing current. The resting conductance of the bags was 2.4 ± 0.2 μS, mean ± s.e. mean (n = 12). When the preparation was perfused with γ‐aminobutyric acid (GABA) in concentrations greater than 3 μm, a dose‐dependent increase in conductance associated with a hyperpolarizing potential was recorded. The log dose‐response relationship obtained from 6 preparations was sigmoidal and had an ED50 of 3 μm. When Cl− in the Ringer was replaced by SO2‐4, the GABA‐induced conductance changes decreased and were associated with depolarizing potentials. Voltage responses were recorded in the bag region during the iontophoretic application of GABA to different regions of the muscle cell. The largest responses were recorded when GABA was applied to the bag region. Smaller responses were recorded when GABA was applied to the arms and syncytial regions. The responses of the bags were dose‐dependent and were antagonized by bath‐applied picrotoxin. An extrapolation method using 10−5 m GABA suggested that the GABA reversal potential was about 30 mV more negative than the resting membrane potential. This was confirmed by means of a two microelectrode voltage clamp technique. The reversal potential was thus estimated as − 61.2 + 2.2 mV mean ± s.e. mean (n = 27).