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

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.