"Sleepy" inward rectifier channels in guinea-pig cardiomyocytes are activated only during strong hyperpolarization.
K(+) channels of isolated guinea-pig cardiomyocytes were studied using the patch-clamp technique. At transmembrane potentials between -120 and -220 mV we observed inward currents through an apparently novel channel. The novel channel was strongly rectifying, no outward currents could be recorded. Between -200 and -160 mV it had a slope conductance of 42.8 +/- 3.0 pS (S.D.; n = 96). The open probability (P(o)) showed a sigmoid voltage dependence and reached a maximum of 0.93 at -200 mV, half-maximal activation was approximately -150 mV. The voltage dependence of P(o) was not affected by application of 50 microM isoproterenol. The open-time distribution could be described by a single exponential function, the mean open time ranged between 73.5 ms at -220 mV and 1.4 ms at -160 mV. At least two exponential components were required to fit the closed time distribution. Experiments with different external Na(+), K(+) and Cl(-) concentrations suggested that the novel channel is K(+) selective. Extracellular Ba(2+) ions gave rise to a voltage-dependent reduction in P(o) by inducing long closed states; Cs(+) markedly reduced mean open time at -200 mV. In cell-attached recordings the novel channel frequently converted to a classical inward rectifier channel, and vice versa. This conversion was not voltage dependent. After excision of the patch, the novel channel always converted to a classical inward rectifier channel within 0-3 min. This conversion was not affected by intracellular Mg(2+), phosphatidylinositol (4,5)-bisphosphate or spermine. Taken together, our findings suggest that the novel K(+) channel represents a different "mode" of the classical inward rectifier channel in which opening occurs only at very negative potentials.
Liu
Gong Xin
GX
Institut für Normale und Pathologische Physiologie, Universität Marburg, Deutschhausstrasse 2, D-35037 Marburg, Germany.
Daut
Jürgen
J
D000818:Animals; D001464:Barium / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D004594:Electrophysiology; D006168:Guinea Pigs; D007545:Isoproterenol / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D024661:Potassium Channels, Inwardly Rectifying / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000502:physiology*
D024661:Potassium Channels, Inwardly Rectifying; D002586:Cesium; D001464:Barium; D007545:Isoproterenol