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 K s ) and rapid ( I K r ) components of the native delayed rectifier K + channel in the absence and presence of the β‐adrenergic receptor agonist isoproterenol (isoprenaline; Iso) using the whole‐cell configuration of the patch‐clamp technique. Hypoxia reversibly inhibited basal I K s . 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 K s by Iso was significantly decreased from 18.3 to 1.9 n m . DTT mimicked the effect of hypoxia on the sensitivity of I K s to Iso while DTNB had no effect. Hypoxia increased the sensitivity of I K s to histamine and forskolin suggesting that the effect of hypoxia is not occurring at the β‐adrenergic receptor. The increase in sensitivity of I K s to Iso could be attenuated with addition of PKCβ peptide to the pipette solution. While hypoxia and DTT inhibited basal I K s they were without effect on I K r. In addition, Iso did not appear to alter the magnitude of I K r in the absence or presence of hypoxia. These data suggest that hypoxia regulates native I K s through two distinct mechanisms: direct inhibition of basal I K s and an increase in sensitivity to Iso that occurs downstream from the β‐adrenergic receptor. Both mechanisms appear to involve redox modification of thiol groups. In contrast native I K r does not appear to be regulated by Iso, hypoxia or redox state.