Hypoxic pulmonary vasoconstriction in chronically hypoxic rats.

Reactivity of lung vessels to acute hypoxia was found increased or decreased in chronically hypoxic (CH) rats by different authors. We examined severity and duration of hypoxia and age as possible explanations. Isolated blood-perfused lungs of CH rats ventilated with low-O2 mixtures were compared with control (C) rats. Juvenile CH rats (10 or 12% O2, 3 weeks) showed increased reactivity; reactivity of mature CH rats (10% O2, 3 weeks) was not significantly different from controls. Reactivity declined with age in CH but not C rats. Juvenile and mature rats exposed to 10% O2 for 48 h showed reduced reactivity; after 3 weeks but not 48 h hypoxia, pulmonary arterioles are narrowed by new muscle. The pressure/flow relationship differed in CH from C rats in that resistance and critical closing pressure (CCP) were greater during normoxia. Hypoxia caused increased resistance and CCP in C rats but mainly an increase in CCP in CH rats. The difference may be attributable to muscularisation of collapsible alveolar vessels in chronic hypoxia.