title: Measurement of GABA-evoked conductance changes of lobster muscle fibres by a three-microelectrode voltage clamp technique.
abstract: 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 gamma-aminobutyric acid (GABA) were reflected by differences (delta 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 delta V measurements were virtually identical. The effective capacity (ceff), determined from the area beneath the 'on' delta 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 though to be decreasing ceff by selectively increasing the conductance of the cleft-tubular membranes.
mesh_terms: D000818:Animals; D004553:Electric Conductivity; D004594:Electrophysiology / Q000379:methods; D007473:Ion Channels / Q000502:physiology; D008564:Membrane Potentials; D008839:Microelectrodes; D009132:Muscles / Q000502:physiology*; D008121:Nephropidae; D011956:Receptors, Cell Surface / Q000502:physiology*; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D011956:Receptors, Cell Surface; D011963:Receptors, GABA-A; D005680:gamma-Aminobutyric Acid