Integration of hypoxic dilation signaling pathways for skeletal muscle resistance arteries

Mediator contributions to hypoxic dilation of rat gracilis muscle resistance arteries were determined by measuring dilation, vascular smooth muscle hyperpolarization, and metabolite production after incremental hypoxia. Nitric oxide (NO) synthase inhibition abolished responses to mild hypoxia, whereas COX inhibition impaired responses to more severe hypoxia by 77%. Blocking 20-hydroxyeicosatetraenoic acid (20-HETE) impaired responses to moderate hypoxia. With only NO systems intact, responses were maintained with mild hypoxia (88% normal) mediated via K_Ca channels. When only COX pathways were intact, responses to moderate-severe hypoxia were largely retained (79% of normal) mediated via KATP channels. Vessel responses to moderate hypoxia were retained with only 20-HETE systems intact mediated via K_Ca channels. NO production increased 5.6-fold with mild hypoxia; greater hypoxia was without further effect. With increased hypoxia, 20-HETE levels fell to 40% of control values. 6-keto-PGF_1α levels were not altered with mild hypoxia, but increased 4.6-fold with severe hypoxia. These results suggest vascular reactivity to progressive hypoxia represents an integration of NO production (mild hypoxia), PGI₂ production (severe hypoxia), and reduced 20-HETE levels (moderate hypoxia).