1. The intracellular pathways that modulate the opening of
oxygen-sensitive
ion channels during periods of
hypoxia are poorly understood. Different tissues appear to use either
NADPH oxidase or a
rotenone-sensitive mechanism as an
oxygen sensor. The aim of the present study was to identify the
oxygen-sensing pathway in the
oxygen-sensitive sheep adrenal medullary chromaffin cell (AMCC). 2. The whole-cell patch-clamp technique was used to measure K+ currents in dissociated adult ovine chromaffin cells as well as SK channel currents expressed in the H4IIE cell line. 3.
Diphenyliodonium, an inhibitor of
NADPH oxidase, had no effect on the
hypoxia-evoked closure of K+ channels in primary AMCC, whereas the mitochondrial inhibitor
rotenone abolished the
hypoxia-evoked response. Both these compounds significantly reduced K+ current amplitude under normoxic conditions. 4. One possible mechanism through which the
oxygen sensor may modulate K+ channel activity is by altering the redox state of the cell. In sheep AMCC, altering the redox state by the addition of H2O2 to the extracellular
solution increased K+ conductance. 5. The
oxygen-sensitive K+ (Ko2) channels in sheep chromaffin cells are from the SK family and the whole-cell conductance of cells expressing mouse SK2 or SK3, but not human SK1, was increased by H2O2 and decreased by the
reducing agent dithiothreitol. 6. These studies show that, in sheep AMCC, Ko2 channels are modulated via a
rotenone-sensitive mechanism and that alteration of the cellular redox state mimics the change produced by alterations in Po2. In a heterologous expression system, SK2 and SK3 channels, the channels that initiate
hypoxia-evoked changes in AMCC function, are modulated appropriately by changes in cellular redox state.