Effects on the macroscopic sodium current: GE 70 (1, 3 and 10 |J.mol/L) was added to the bathing solution in all experiments. The drug concentration dependently decreased sodium current (peak sodium current and mean current per record) with a 50% inhibitory concentration of 2.3±0.2 |j,mol/L. The percentage reduction of the ensemble-averaged current (n=5) was almost the same at any measured holding potential (-140, -120, -110, -100, -90 and -80 mV). The membrane potential required for half-maximal steady state inactivation of the cardiac sodium channels (peak sodium current, 2 Hz) was not shifted by GE 70 (3 |j,mol/L: n=5) (Figure 2).Effects on the current-voltage relationship were measured by applying test pulses between -70 and +60 mV from a holding potential of -120 mV at a rate of 2 Hz. The peak of the current-voltage relation curve at -30 mV was not shifted by GE 70 (3 |j,mol/L: n=4), and the reversal potential remained unchanged (Figure 3).
Figure 2 Effect of GE 70 on the steady state inactivation curve. In cell-attached patches, 20 ms depolarizing pulses to a test potential (Et) of-30 mV were applied from different holding potentials (Eh) between -140 and -70 mV at a constant stimulation rate of 2 Hz. The inactivation curve for the peak of the ensemble-averaged current is shown in control conditions (O) and with 3 fimol/L GE 70 (l). Each symbol represents the arithmetic mean of five experiments ± SEM. Inset The ensemble-averaged currents (202 sweeps each) in control conditions and in the presence of 3 fimol/L GE 70 are shown (cell-attachedpatch, Eh=-100 mV, Et=-30 mV; vertical calibration bar: 2 pA, horizontal bar: 2 ms). h^ Fraction of sodium channels in the rested state at equilibrium
Figure 3 Effect of GE 70 on the current-voltage relationship. In cell-attached patches 20 ms depolarizing pulses ranging from -70 to +60 mV were applied from a holding potential of-120 mV at a rate of 2 Hz. The relative decrease of the sodium current (iNa) by 3 fimol/L GE 70 was independent of the test potential (Et). Each symbol represents the arithmetic mean of four experiments ± SEM. m Control; • GE 70
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