IN-VITRO RESPONSES OF LUNG ARTERIES TO ACUTE-HYPOXIA AFTER NO SYNTHASE BLOCKADE OR CHRONIC HYPOXIA

Responses to hypoxia of lung arteries (200-350 mu m) from control (C) and chronically hypoxic (CH) rats were compared in a myograph before a nd after blockade of NO synthase with N-G-nitro-L-arginine methyl este r (L-NAME). After precontraction with prostaglandin F-2 alpha (PGF(2 a lpha)), hypoxia caused a four-phase tension change: brief dilation, tr ansient contraction, prolonged dilation, and slow contraction (we stud ied the first three phases). In CH rats, the first dilation and first contraction were significantly reduced. After L-NAME, the first dilati on was reduced in C rats and abolished in CH rats; thus the first phas e is attributable to NO release and is affected by chronic hypoxia. Th e first contractile phase was significantly reduced by L-NAME in C but not in CH rats, where it was small. Thus NO synthase inhibition inhib its hypoxic constriction in isolated vessels, whereas it enhances hypo xic constriction in perfused lungs. The third dilator phase was unaffe cted by chronic hypoxia; it was increased after L-NAME in CH rats. Thu s, in vitro, responses to hypoxia are complex; there is a balance betw een two dilator and two constrictor processes.