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The outward current that flows through the strong inward rectifier K+ (KIR) channel generates IK1, one of the major repolarizing currents of the cardiac action potential. The amplitude and the time dependence of the outward current that flows through KIR channels is determined by its blockage by cytoplasmic cations such as polyamines and Mg2+. Using the conventional whole-cell recording technique, we recently showed that the outward IK1 can show a time dependence during repolarization due to competition of cytoplasmic particles for blocking KIR channels. We used the amphotericin B perforated patch-clamp technique to measure the physiological amplitude and time dependence of IK1 during the membrane repolarization of guinea-pig cardiac ventricular myocytes. In 5.4 mM K+ Tyrode solution, the density of the current consisting mostly of the sustained component of the outward IK1 was about 3.1 A F-1 at around -60 mV. The outward IK1 showed an instantaneous increase followed by a time-dependent decay (outward IK1 transient) on repolarization to -60 to -20 mV subsequent to a 200 ms depolarizing pulse at +37 mV (a double-pulse protocol). The amplitudes of the transients were large when a hyperpolarizing pre-pulse was applied before the double-pulse protocol, whereas they were small when a depolarizing pre-pulse was applied. The peak amplitudes of the transients elicited using a hyperpolarizing pre-pulse were 0.36, 0.63 and 1.01 A F-1, and the decay time constants were 44, 14 and 6 ms, at -24, -35 and -45 mV, respectively. In the current-clamp experiments, a phase-plane analysis revealed that application of pre-pulses changed the current density at the repolarization phase to the extents expected from the changes of the IK1 transient. Our study provides the first evidence that an outward IK1 transient flows during cardiac action potentials.
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