Hypoxic pulmonary vasoconstriction (HPV) is the most significant factor causing pulmonary hypertension and right heart failure in chronic obstructive pulmonary disease. The pathogenesis of HPV is incompletely understood, but involves a derangement in intracellular Ca2+ homeostasis and defects of membrane channel function in pulmonary arterial smooth muscle cells. I propose to investigate the role of ionic channels in HPV by using patch clamp techniques and fluorescent imaging microscopy in order to identify the critical pathways leading to HPV. The overall hypothesis to be tested is that a) a rise in intracellular Ca2+ concentration, induced by membrane depolarization due to inhibition of voltage-gated K+ channels in pulmonary arterial smooth muscle cells, is a fundamental component in transducing hypoxia to contraction, and b) the pulmonary arterial endothelium modulates HPV by releasing nitric oxide which may affect pulmonary vascular tone through functional changes of ionic channels in pulmonary arterial smooth muscle cells. To test this hypothesis, I will determine (1) the ionic channel(s) responsible for controlling membrane potential and intracellular free Ca2+ concentration in the resting state in pulmonary arterial smooth muscle cells; (2) the effects of hypoxia, in addition to inhibiting voltage-gated K+ channels, on other K+ channels and voltage-activated Ca2+ channels i pulmonary arterial smooth muscle cells; (3) the cellular mechanisms leading to hypoxia-induced inhibition of voltage-gated K+ channels; and (4) whether endothelium-derived nitric oxide alters K+ or Ca2+ channel activity, membrane potential and intracellular free Ca2+ concentration in pulmonary arterial smooth muscle cells, and if it does, whether hypoxia influences these effects. All experiments will also be performed in mesenteric arterial smooth muscle cells which will serve as a reference: contrasting responses in pulmonary and mesenteric arterial smooth muscle cells may provide clues to the cellular mechanisms leading to HPV. These studies will provide insight into mechanisms responsible for the pathogenesis of pulmonary hypertension in chronic hypoxic pulmonary disease.