DUAL ACTIONS OF TETRACAINE ON INTRAMEMBRANE CHARGE IN AMPHIBIAN STRIATED-MUSCLE

1. The effects of graded concentrations of tetracaine on the steady-st ate and kinetic properties of intramembrane charge were examined in in tact voltage-clamped amphibian muscle fibres. 2. The micromolar tetrac aine concentrations that were hitherto reported to abolish Ca2+ transi ents in skeletal muscle failed to affect significantly the steady-stat e charge. Maximal reductions of such intramembrane charge required rel atively high, 1-2 mM, concentrations of tetracaine. 3. The plots of ma ximum charge against tetracaine concentration suggested a saturable 1 : 1 drug binding that spared a fixed amount of tetracaine-resistant (q (beta)) charge but inhibited a discrete fraction of susceptible (q gam ma) charge with a K-D between 0.1 and 0.2 mM. 4. The q(beta) charge th us isolated by 2 mM tetracaine was conserved through a wide range of a pplied test voltages and pulse durations and regardless of whether the imposed transition from the holding potential(-90 mV) to the test pot ential took place in one or more steps. 5. Similarly, 'on' and 'off' q (beta) currents that were elicited by voltage steps from fixed conditi oning to varying test levels mapped onto non-linear phase-plane trajec tories that nevertheless depended uniquely upon voltage. In contrast, the currents that followed voltage steps made from varying prepulse le vels to fixed -90 or -20 mV test potentials mapped onto identical q(be ta) phase-plane trajectories that were independent of the prepulse his tory. 6. The charge movements that followed strong depolarizing voltag e clamp steps to test potentials in the range -50 to 0 mV approximated simple monotonic decays that could empirically be described by a sing le time constant. Nevertheless, a complete inhibition of a tetracaine- sensitive (q(gamma) ) charge movement by 2 mM tetracaine that left onl y q(beta) charge, sharply altered both the magnitude and the voltage d ependence of these time constants. This establishes a distinct contrib ution of the q(gamma) 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 (0.05-0.2 mM) tetracaine conce ntrations that failed to markedly alter the steady-state charge consis tently increased the charging time constants yet did not influence the ir voltage sensitivity. 8. These findings demonstrate the existence of separate kinetic and steady-state effects of tetracaine on intramembr ane charge movements, at micromolar and millimolar anaesthetic concent rations, respectively. These parallel earlier effects of tetracaine th at have been reported upon the transient and sustained components of s arcoplasmic reticular Ca2+ release. They also establish that maximally effective concentrations of tetracaine isolate a single distinct spec ies of conserved (q(beta)) intramembrane charge.