A repolarization-induced transient increase in the outward current of the inward rectifier K+ channel in guinea-pig cardiac myocytes

1. Outward currents of the inwardly rectifying K+ current (I-Kir) in guinea-pig ventricular myocytes were studied in the presence of 1 mM intracellular free Mg2+ using the whole-cell patch-clamp technique. 2. During repolarizing voltage steps following a large depolarizing pulse (> 0 mV), outward I-Kir increased transiently at voltages positive to the K+ equilibrium potential (E-K, -84 mV for 5.4 mM extracellular [K+]). The rising phase was almost instantaneous, while the decay was exponential. The decay rate was faster at voltages closer to E-K (time constants, 33.9 +/- 9.8 and 4.8 +/- 1.4 ms at -30 and -50 mV, respectively). 3. The transient outward I-Kir was absent when the preceding depolarization was applied from -40 mV. Larger transient currents developed as the voltage before the depolarization was shifted to more hyperpolarized levels. 4. Shift of the depolarizing voltage from >0 mV to more negative ranges diminished the amplitudes of transient outward I-Kir and instantaneous inward I-Kir during the subsequent repolarizing steps positive and negative to E-K, respectively Since blockage of I-Kir by internal Mg2+ occurs upon large depolarization, and the block is instantaneously relieved at voltages negative to E-K, the rising phase of the transient outward I-Kir was attributed to the relief of Mg2+ block at voltages positive to E-K. Transient outward I-Kir was absent when intracellular [Mg2+] was reduced to 10 mu M or lower. 5. Prolongation of the repolarizing voltage step increased the amplitude of time-dependent inward I-Kir during the subsequent hyperpolarization, indicating the progress of a gating process (presumably the channel block by intracellular polyamine) during the decaying phase of outward I-Kir. 6. Progressive prolongation of the depolarizing pulse (> 0 mV) from 100 to 460 ms decreased the transient outward I-Kir amplitude during the subsequent re-polarizing step due to slow progress of the gating (polyamine block) at > 0 mV. 7. Current-voltage relations measured using repolarizing ramp pulses (-3.4 mV ms(-1)) showed an outward hump at around -50 mV, the magnitude of which increased as the voltage before the conditioning depolarization (10 mV) was shifted to more negative levels. With slower ramp speeds (-1.5 and -0.6 mV ms(-1)), the hump was depressed at voltages near E-K. 8. Our study suggests that the relief of Mg2+ block may increase outward I-Kir during repolarization of cardiac action potentials, and that the resting potential, the level/duration of action potential plateau and the speed of repolarization influence the outward I-Kir amplitude. 9. A kinetic model incorporating a competition between polyamine block and Mg2+ block was able to account for the time dependence of outward I-Kir.