HYPOXIA ACTIVATES ATP-DEPENDENT POTASSIUM CHANNELS IN INSPIRATORY NEURONS OF NEONATAL MICE

1. The respiratory centre of neonatal mice (4 to 12 days old) was isol ated in 700 mu m thick brainstem slices. Whole-cell K+ currents and si ngle ATP-dependent potassium (K-ATP) channels were analysed in inspira tory neurones. 2. In cell-attached patches, It,, channels had a conduc tance of 75 pS and showed inward rectification. Their gating was volta ge dependent and channel activity decreased with membrane hyperpolariz ation. Using Ca2+-containing pipette solutions the measured conductanc e was lower (50 pS at 1.5 mM Ca2+), indicating tonic inhibition by ext racellular Ca2+. 3. K-ATP channel activity was reversibly potentiated during hypoxia. Maximal effects were attained 3-4 min after oxygen rem oval from the bath. Hypoxic potentiation of open probability was due t o an increase in channel open times and a decrease in channel closed t imes. 4. In inside-out patches and symmetrical Kf concentrations, chan nel currents reversed at about 0 mV. Channel activity was blocked bg A TP (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 sli ce after excision, the activity of K-ATP channels was enhanced by hypo xia, an effect that could be mediated by a release of endogenous neuro modulators. 6. The whole-cell K+ current (I-K) was inactivated at nega tive membrane potentials, which resembled the voltage dependence of K- ATP channel gating. After 3-4 min of hypoxia, K+ currents at both hype rpolarizing and depolarizing membrane potentials increased. I-K was pa rtially blocked by tolbutamide (100-300 mu M) and in its presence, hyp oxic potentiation of I, was abolished. 7. We conclude that K-ATP chann els are involved in the hypoxic depression of medullary respiratory ac tivity.