The effects of tetracaine on charge movements and on antipyrylazo III signals monitoring intracellular Δ[Ca2+] were compared in cut frog semitendinosus muscle fibers in a single vaseline gap-voltage clamp. Low tetracaine concentrations (25-40 µM) markedly reduced Δ[Ca2+] signals and shifted the rheobase. However, they neither influenced charge movement nor that peak Δ[Ca2+] value associated with the contractile threshold. Higher tetracaine concentrations (100-200 µM) partly inhibited charge movements in cut fibers. They separated a steeply voltage-sensitive charge, some of whose features resembled ‘qr' reported in intact fibers, and whose movement preceded Δ[Ca2+] signals at threshold. These findings: (a) directly confirm an earlier suggestion that tetracaine acts on steps in excitation-contraction coupling rather than my ofilament activation; (b) show that tetracaine at low concentrations can directly interfere with sarcoplasmic reticular calcium release without modifying charge movement; (c) show that the tetracainesensitive charge, first found in intact fibers, also exists in cut fibers; and (d) make it unlikely that tetracaine-sensitive charge transfer is a consequence of Ca2+ release as suggested on earlier occasions.
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