Effect of potassium channel openers on hypoxic pulmonary vasoconstriction

The ATP-sensitive potassium channel (K+ATP) has been suggested as an important mechanism for the reactivity of vascular smooth muscle. We investigated the effects of K+ channel openers (lemakalim, pinacidil) on hypoxic pulmonary vasoconstriction (HPV) and angiotensin II (Ag II) induced vasoconstriction in isolated rat lungs (Sprague-Dawley rats: 300-450 g). Ventilation with hypoxic gas (2% O2, 5% CO2) was performed for 6 min after the injection of Ag II (0.1 microgram). Isolated lungs were perfused under constant flow (0.04 ml/g/min) using 20 ml of blood from donor rat. The perfusion pressure was used as the pulmonary artery pressure. Lemakalim or pinacidil was pre-administered through the reservoir. Pretreatment with pinacidil (10(-4) M) or lemakalim (10(-5) M) inhibited the pressor response to hypoxia, but did not inhibit the response to angiotensin II. Although the effect of lemakalim on HPV was reversed by administration of glibenclamide (10(-5) M) or tolbutamide (10(-3) M), the effect of pinacidil on HPV was not influenced by either drug. These results suggest that 1) K+ channel openers (lemakalim and pinacidil) inhibit the pressor response to hypoxia, and 2) lemakalim seems to act through K+ATP, whereas pinacidil may have other mechanisms of inhibition of vascular smooth muscle contraction. K+ATP may play an important role in the regulation of pulmonary vascular reactivity to hypoxia.