Blocking action of terodiline on calcium channels in single smooth muscle cells of the guinea-pig urinary bladder

The blocking action of terodiline, a nonspecific organic Ca++ antagonist, on smooth muscle Ca++ channels of the guinea pig urinary bladder was investigated. Inward Ca++ currents were recorded from smooth muscle cells isolated from the urinary bladder using the whole-cell patch-clamp technique. In the absence of terodiline, a use-dependent reduction in the amplitude of inward Ca++ current was observed at a stimulus frequency of 0.2 Hz. When terodiline (1-10 microM) was applied, the use-dependent reduction was accelerated markedly, depending on the stimulus frequency. The blocking action of terodiline was also dose-dependent; the Kd value as measured at the end of 20 times repetitive stimulation at 0.2 Hz was 1.7 microM. In addition to such a use-dependent block, terodiline produced a hyperpolarizing shift in the steady-state inactivation curve. The results suggest that terodiline preferentially binds to the Ca++ channel in the open state and also in the inactivated state.