Tetracaine can inhibit contractions initiated by a voltage-sensitive release mechanism in guinea-pig ventricular myocytes

1. Effects of tetracaine on membrane currents and cell shortening were measured with high resistance electrodes, single-electrode voltage clamp (switch clamp) and a video edge detector at 37°C in cardiac ventricular myocytes. 2. Sequential voltage steps from -65 mV to -40 and 0 mV were used to activate two mechanisms of excitation-contraction (EC) coupling separately. The step to -40 mV activated the voltage-sensitive release mechanism (VSRM); the step to 0 mV activated Ca²⁺-induced Ca²⁺ release (CICR) coupled to inward Ca²⁺ current (I(L)). 3. Exposure to 100-300 μM tetracaine inhibited VSRM contractions but not CICR contractions. Inhibition of VSRM contractions was independent of I(Na) blockade. In contrast, 100 μM Cd²⁺ blocked I(L) and CICR contractions, but not VSRM contractions. Simultaneous application of both agents blocked both mechanisms of EC coupling. 4. Contraction-voltage relationships were sigmoidal when the VSRM was available. However, when the VSRM was inhibited with 100-300 μM tetracaine, contraction-voltage relationships became bell-shaped. The tetracaine-insensitive contractions were abolished by 0.1 μM ryanodine, indicating that they were dependent on release of SR Ca²⁺. 5. At a higher concentration (1 mM) tetracaine also inhibited I(L) and contractions triggered by I(L); however, the time course of effects on I(L) and associated contractions were different than for VSRM contractions. 6. With continuous application of tetracaine, the VSRM remained inhibited although SR Ca²⁺ stores increased 4-fold as assessed with caffeine. CICR contractions were not inhibited and maximum amplitude of contraction was not reduced. 7. Rapid application of tetracaine just before and during test steps also inhibited VSRM contractions, but without significantly affecting sarcoplasmic reticulum (SR) Ca²⁺ stores or CICR contractions. Maximum amplitude of contraction was reduced. 8. Rapid application of tetracaine (100-300 μM) allows preferential inhibition of the VSRM and provides a pharmacological method to assess the contribution of the VSRM to EC coupling.