Localization of ionic conductances in crayfish muscle fibers

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 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.

Sites of action of D2O in intact and skinned crayfish muscle fibers

The effect on tension development of replacing 90% of the H2O of the bathing saline with D2O was studied on intact single fibers, and on skinned fibers before and after the latter were treated so as to eliminate Ca-accumulation by the sarcoplasmic reticulum (SR). Excitation-contraction coupling (ECC) of intact fibers is not abolished, but is depressed by D2O so that higher depolarizations are required to elicit a given tension. The reduction in tension at a given level of depolarization is not due to inhibition of the contractile system. The latter showed an enhanced Ca sensitivity; that is, skinned fibers respond to Ca concentrations that are 1-2 orders of magnitude smaller in D2O than in H2O saline. When bathed in D2O saline, intact fibers or skinned fibers with functional SR can still accumulate and release Ca in sufficient quantities to allow repeated induction of maximum tensions. Relaxation is slowed in all three types of preparation, perhaps because of an increased affinity of troponin to Ca in D2O salines.