NO HYPERPOLARIZES PULMONARY-ARTERY SMOOTH-MUSCLE CELLS AND DECREASES THE INTRACELLULAR CA2-GATED K+ CHANNELS( CONCENTRATION BY ACTIVATING VOLTAGE)

NO causes pulmonary vasodilation in patients with pulmonary hypertensi on, In pulmonary arterial smooth muscle cells, the activity of voltage -gated K+ (K-v) channels controls resting membrane potential, In turn, membrane potential is an important regulator of the intracellular fre e calcium concentration ([Ca2+](i)) and pulmonary vascular tone. We us ed patch clamp methods to determine whether the NO-induced pulmonary v asodilation is mediated by activation of K-v channels. Quantitative fl uorescence microscopy was employed to test the effect of NO on the dep olarization-induced rise in [Ca2+](i). Blockade of K-v channels by 4-a minopyridine (5 mM) depolarized pulmonary artery myocytes to threshold for initiation of Ca2+ action potentials, and thereby increased [Ca2](i), NO (approximate to 3 mu M) and the NO-generating compound sodium nitroprusside (5-10 mu M) opened K-v channels in rat pulmonary artery smooth muscle cells, The enhanced K+ currents then hyperpolarized the cells, and blocked Ca2+-dependent action potentials, thereby preventi ng the evoked increases in [Ca2+](i). Nitroprusside also increased the probability of K-v channel opening in excised, outside-out membrane p atches. This raises the possibility that NO may act either directly on the channel protein or on a closely associated molecule rather than v ia soluble guanylate cyclase, In isolated pulmonary arteries, 4-aminop yridine significantly inhibited NO-induced relaxation. We conclude tha t NO promotes the opening of K-v channels in pulmonary arterial smooth muscle cells. The resulting membrane hyperpolarization, which lowers [Ca2+](i), is apparently one of the mechanisms by which NO induces pul monary vasodilation.