Dual actions of tetracaine on intramembrane charge in amphibian striated muscle

1 The effects of graded concentrations of tetracaine on the steady‐state and kinetic properties of intramembrane charge were examined in intact voltage‐clamped amphibian muscle fibres. 2 The micromolar tetracaine concentrations that were hitherto reported to abolish Ca 2+ transients in skeletal muscle failed to affect significantly the steady‐state charge. Maximal reductions of such intramembrane charge required relatively high, 1–2 mm, concentrations of tetracaine. 3 The plots of maximum charge against tetracaine concentration suggested a saturable 1:1 drug binding that spared a fixed amount of tetracaine‐resistant ( q β ) charge but inhibited a discrete fraction of susceptible ( q γ ) charge with a K D between 0.1 and 0.2 m m . 4 The q β charge thus isolated by 2 m m tetracaine was conserved through a wide range of applied test voltages and pulse durations and regardless of whether the imposed transition from the holding potential (−90 mV) to the test potential took place in one or more steps. 5 Similarly, ‘on’ and ‘off’ q β currents that were elicited by voltage steps from fixed conditioning to varying test levels mapped onto non‐linear phase‐plane trajectories that nevertheless depended uniquely upon voltage. In contrast, the currents that followed voltage steps made from varying prepulse levels to fixed −90 or −20 mV test potentials mapped onto identical q β phase‐plane trajectories that were independent of the prepulse history. 6 The charge movements that followed strong depolarizing voltage clamp steps to test potentials in the range −50 to 0 mV approximated simple monotonic decays that could empirically be described by a single time constant. Nevertheless, a complete inhibition of a tetracaine‐sensitive ( q γ ) charge movement by 2 m m tetracaine that left only q γ charge, sharply altered both the magnitude and the voltage dependence of these time constants. This establishes a distinct contribution of the q γ species to overall charge kinetics even at such test voltages. 7 Under such a criterion for the voltage dependence of charging kinetics, even the micromolar (0.05‐0.2m m ) tetracaine concentrations that failed to markedly alter the steady‐state charge consistently increased the charging time constants yet did not influence their voltage sensitivity. 8 These findings demonstrate the existence of separate kinetic and steady‐state effects of tetracaine on intramembrane charge movements, at micromolar and millimolar anaesthetic concentrations, respectively. These parallel earlier effects of tetracaine that have been reported upon the transient and sustained components of sarcoplasmic reticular Ca 2+ release. They also establish that maximally effective concentrations of tetracaine isolate a single distinct species of conserved ( q β ) intramembrane charge.