Hypoxia activates ATP-dependent potassium channels in inspiratory neurones of neonatal mice

1. The respiratory centre of neonatal mice (4 to 12 days old) was isolated in 700 mu m thick brainstem slices. Whole-cell K+ currents and single ATP-dependent potassium (K-ATP) channels were analysed in inspiratory neurones. 2. In cell-attached patches, It,, channels had a conductance of 75 pS and showed inward rectification. Their gating was voltage dependent and channel activity decreased with membrane hyperpolarization. Using Ca2+-containing pipette solutions the measured conductance was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by extracellular Ca2+. 3. K-ATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3-4 min after oxygen removal from the bath. Hypoxic potentiation of open probability was due to an increase in channel open times and a decrease in channel closed times. 4. In inside-out patches and symmetrical Kf concentrations, channel currents reversed at about 0 mV. Channel activity was blocked bg ATP (300-600 mu M), glibenclamide (10-70 mu M) and tolbutamide (100-300 mu M). 5. In the presence of diazoxide (10-60 mu M), the activity of K-ATP channels was increased both in inside-out, outside-out and cell-attached patches. In outside-out patches, that remained within the slice after excision, the activity of K-ATP channels was enhanced by hypoxia, an effect that could be mediated by a release of endogenous neuromodulators. 6. The whole-cell K+ current (I-K) was inactivated at negative membrane potentials, which resembled the voltage dependence of K-ATP channel gating. After 3-4 min of hypoxia, K+ currents at both hyperpolarizing and depolarizing membrane potentials increased. I-K was partially blocked by tolbutamide (100-300 mu M) and in its presence, hypoxic potentiation of I, was abolished. 7. We conclude that K-ATP channels are involved in the hypoxic depression of medullary respiratory activity.