Concentration-dependent effects of tetracaine on excitation-contraction coupling in frog skeletal muscle fibres

Summary

The effects of low (10–100 μm) concentrations of tetracaine on intramembrane charge movement and on the rate of calcium release (R_rel) from the sarcoplasmic reticulum (SR) were studied in cut skeletal muscle fibres of the frog using the voltage clamp technique. The fibres were mounted in a single or double vaseline gap chamber to study the events near the contraction threshold or in a wide membrane potential range. Although the ‘hump’ component of charge movement (Q_γ) was suppressed to some extent, the voltage dependence and the parameters of the Boltzmann distribution were not modified significantly at tetracaine concentrations below 50 μm. At 50 and 100 μm of tetracaine the midpoint voltage of the Boltzmann distribution was shifted to higher membrane potentials and the steepness was decreased. The total available charge remained the same at all concentrations tested. Using fura-2 to measure calcium transients at 100 μm tetracaine the threshold for calcium release was found to be significantly shifted to more positive membrane potentials. Tetracaine reversibly suppressed both the early inactivating peak and the steady-level of R_rel but the concentration dependence of these effects was markedly different. The inactivating component of calcium release was decreased with a Hill coefficient of approximately 1 and half effective concentration of 11.8 μm while the steady-level was decreased with a Hill coefficient of greater than 2 and a half effective concentration of 47.0 μm. These results favour two sites of action where tetracaine would suppress the calcium release from the SR.