The differential effects of twitch potentiators on charge movements in frog skeletal muscle.

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Abstract
The effects of twitch potentiators at physiologically effective concentrations on intramembrane charge movements were examined in voltage‐clamped frog skeletal muscle in the presence and absence of tetracaine. Caffeine, even at high concentrations (2.5 mM) did not alter the potential dependence of either tetracaine‐sensitive or tetracaine‐resistant portions of non‐linear charge through a voltage range that included the mechanical threshold. Perchlorate (8 mM) altered the form and shifted the potential dependence of the non‐linear charge by about ‐25 mV in the hyperpolarizing direction, but did not alter the total available charge. Comparing charge movements in the presence and absence of 4 mM‐tetracaine demonstrated that perchlorate shifted the threshold and voltage dependence of the delayed ('q gamma') transients but had no action on the steady‐state tetracaine‐resistant charge. Thiocyanate (10 mM) shifted the voltage dependence of charge movements by about 20 mV in the hyperpolarizing direction. Experiments employing 4 mM‐tetracaine demonstrated that this was the result of effects on both tetracaine‐sensitive and tetracaine‐resistant charge movements. On the basis of their effects on contractile activation reported on earlier occasions, these differential effects of twitch potentiators are explicable in terms of tetracaine‐sensitive and tetracaine‐resistant charge components being causally separate and not requiring a direct participation of tetracaine‐resistant charge in contractile activation.