Measurement of GABA-Evoked Conductance Changes of Lobster Muscle Fibres by a Three-Microelectrode Voltage Clamp Technique

The effective membrane conductance and capacity of lobster muscle fibres was measured by a three-intracellular-microelectrode voltage clamp technique. Conductance values agreed well with those determined under current clamp, by means of the `short' cable equations. Reversible increases in conductance evoked by γ -aminobutyric acid (GABA) were reflected by differences (Δ V) in electrotonic potential amplitude recorded at the centre, and midway between the centre and fibre end respectively. GABA dose--conductance curves derived from cable theory or from Δ V measurements were virtually identical. The effective capacity (c<sub>eff</sub>), determined from the area beneath the `on' Δ V capacity transient, yielded values of the membrane time constant consistently lower than those obtained by the graphical method of E. Stefani & A.B. Steinbach (J. Physiol., London. 203, 383--401 (1969)); one possible explanation for this discrepancy is discussed. In the presence of GABA, the effective capacity was reduced in a dose-related manner. The results were interpreted in terms of an equivalent circuit in which surface membrane was arranged in parallel with cleft--tubular membrane of finite conductance, charged through an access resistance. GABA was thought to be decreasing c_eff by selectively increasing the conductance of the cleft--tubular membranes.