Blocking effect of 1389-S on the sodium current in isolated guinea-pig ventricular myocytes

Under whole cell patch conditions, 1389-S blocked the I Na in guinea-pig ventricular myocytes under steady state conditions (K d rest = 30 μ M, K d i = 2.4 μ M) with a shift of the inactivation curve to the hyperpolarizing direction. Both brief and long conditioning pulses could produce a use-dependent block of 1389-S. These results suggest that 1389-S had a higher affinity to the inactivated than to the rested state under steady state conditions and had a higher affinity to the activated state during train pulses as well as to the inactivated state, making channels unavailable for conduction upon activation.