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The components and properties of the delayed rectifier K+ current (IK) in isolated guinea-pig sino-atrial (SA) node pacemaker cells were investigated using the whole-cell configuration of the patch-clamp technique. An envelope of tails test was conducted by applying depolarizing pulses from a holding potential of -50 mV to +30 mV for various durations ranging from 40 to 2000 ms. The ratio of the tail current amplitude elicited upon return to the holding potential to the magnitude of the time-dependent outward current activated during depolarizing steps was dependent on the pulse duration, while after exposure to the selective IKr inhibitor E-4031 (5 µM) this current ratio became practically constant irrespective of the pulse duration. These observations are consistent with the presence of the E-4031-sensitive, rapidly activating and E-4031-resistant, slowly activating components of IK (IKr and IKs, respectively) in guinea-pig SA node cells. The activation range for IKr, defined as the E-4031-sensitive current (half-maximal activation voltage (V1/2) of -26.2 mV) was much more negative than that for IKs, defined as the E-4031-resistant current (V1/2 of +17.2 mV). IKr exhibited a marked inward rectification at potentials positive to -50 mV, whereas IKs showed only a slight rectification. In the current-clamp experiments, bath application of E-4031 (0.5 and 5 µM) initially slowed the repolarization at potentials negative to approximately -30 mV and produced a significant depolarization of the maximum diastolic potential, followed by the arrest of electrical activity, thus indicating that the late phase of the repolarization leading to the maximum diastolic potential at around -60 mV in spontaneous action potentials is primarily produced by IKr in guinea-pig SA node cells. External application of the selective IKs inhibitor 293B (30 µM) also delayed the repolarization process at potentials negative to about -20 mV and induced moderate depolarization of the maximum diastolic potential leading to the arrest of the spontaneous activity. These results provide evidence to suggest that both IKr and IKs are present and play crucial roles in the spontaneous electrical activity of guinea-pig SA node pacemaker cells.
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