Adenine nucleotides via activation of ATP-sensitive K+ channels modulate hypoxic response in rat pulmonary artery

We examined the role of ATP-sensitive K+ channels in hypoxic pulmonary vasoconstriction, using isolated rat pulmonary arterial rings. Isolated rat pulmonary arterial rings displayed a rapid contraction followed by relaxation under hypoxic conditions. The ATP-sensitive K+ channel blocker glibenclamide (concentration >1 mu M) or a hyperglycemic buffer (15 mM glucose) attenuated the hypoxic relaxation in a dose-dependent manner but did not affect the hypoxia-induced contraction. To examine the relationship between hypoxia, energy, and redox state, intracellular levels of adenine nucleotides and pyridine coenzymes were determined by high-performance liquid chromatography in freeze-dried isolated rat pulmonary arteries at three time points (0, 4, and 10 min) before and during hypoxia. Hypoxia time dependently decreased the ATP content and the ATP-to-ADP ratio and increased the ADP and the AMP content in association with a rapid increase in the NADH and the NADH-to-NAD(+) ratio. Hyperglycemic buffer (15 mM glucose) suppressed the hypoxia-induced changes of the adenine nucleotides (the decrease of the ATP content and the ATP-to-ADP ratio) but did not affect the hypoxia-induced changes of the NADH and the NADH-to-NAD(+) ratio. Hypoxia did not affect the NADP(+) or the NADPH content of pulmonary arteries. These findings indicate that an ATP-sensitive K+ channel regulates the tone of rat pulmonary arteries. Furthermore, an imbalance of the energy state may be involved in ATP-sensitive K+ channel activation during hypoxic vasorelaxation.