Contractile activation by voltage clamp depolarization of cut skeletal muscle fibres.

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Abstract
Single frog skeletal muscle fibers bathed in a relaxing solution were cut close to the tendon and mounted across a single vaseline gap so that a short segment of intact terminated fiber extended beyond 1 side of the gap. A compensating circuit, set with a microelectrode in the terminated fiber segment, was used to correct total current for external current crossing the gap and to correct pool voltage for the voltage drop across the fiber segment in the gap. The microelectrode was then removed and the fiber voltage-clamped using the compensating circuit. This allowed movement without damage under controlled voltage. Strength-duration curves for contraction thresholds of cut fibers exposed externally to TTX [tetrodotoxin] Ringer solution and internally to a predominantly K glutamate solution were similar to strength-duration curves reported for intact fibers. The change from TTX Ringer to a predominantly (TEA [tetraethylammonium])2SO4 external solution shifted the strength-duration curve for cut fiber contraction thresholds in the negative direction as reported for intact fibers. When studied at 3-4.degree. C, fibers from warm-adapted frogs apparently had higher contraction thresholds than fibers from cold-adapted frogs. Delayed rectifier currents recorded from cut fibers were similar to those reported for intact fibers.