Differential regulation of the slow and rapid components of guinea-pig cardiac delayed rectifier K+ channels by hypoxia.

The aim of this study was to examine the effects of acute hypoxia on the slow (I(Ks)) and rapid (I(Kr)) components of the native delayed rectifier K+ channel in the absence and presence of the beta-adrenergic receptor agonist isoproterenol (isoprenaline; Iso) using the whole-cell configuration of the patch-clamp technique. Hypoxia reversibly inhibited basal I(Ks). The effect could be mimicked by exposing the cells to the thiol-specific reducing agent dithiothreitol (DTT) and attenuated upon exposure of cells to the membrane-impermeant thiol-specific oxidizing compound 5,5'-dithio-bis[2-nitrobenzoic acid] (DTNB). In the presence of hypoxia, the K(0.5) for activation of I(Ks) by Iso was significantly decreased from 18.3 to 1.9 nm. DTT mimicked the effect of hypoxia on the sensitivity of I(Ks) to Iso while DTNB had no effect. Hypoxia increased the sensitivity of I(Ks) to histamine and forskolin suggesting that the effect of hypoxia is not occurring at the beta-adrenergic receptor. The increase in sensitivity of I(Ks) to Iso could be attenuated with addition of PKCbeta peptide to the pipette solution. While hypoxia and DTT inhibited basal I(Ks) they were without effect on I(Kr.) In addition, Iso did not appear to alter the magnitude of I(Kr) in the absence or presence of hypoxia. These data suggest that hypoxia regulates native I(Ks) through two distinct mechanisms: direct inhibition of basal I(Ks) and an increase in sensitivity to Iso that occurs downstream from the beta-adrenergic receptor. Both mechanisms appear to involve redox modification of thiol groups. In contrast native I(Kr) does not appear to be regulated by Iso, hypoxia or redox state.