Hypoxic and posthypoxic responses in isolated coronary arteries and veins: role of endothelium.

: In isolated porcine and canine coronary artery and vein short-lasting exposure (8 min) to hypoxia (pO2 less than 20 mm Hg) evoked an increase of tone (6-34% of maximum contraction to potassium chloride). In arterial preparations of both species this response was significantly more pronounced in endothelium-intact vessels as compared to rubbed ones. Reintroduction of oxygen produced a posthypoxic dilation in coronary artery and a posthypoxic contraction in coronary vein. The latter was significantly higher in intact veins of the pig compared to the rubbed preparation but did not differ in canine veins. Repeated exposure to hypoxia with 1 h intervals of rest evoked mostly increased hypoxic and posthypoxic responses. The time of onset of hypoxic contraction was significantly shorter in intact canine coronary artery as compared to the rubbed preparation. The effects of indomethacin (inhibitor of cyclooxygenase), BM 13,505 (thromboxane A2 receptor antagonist) and D 600 (calcium entry blocker) differed in dependence on the response, the vessel and the species tested. Hypoxic contraction in canine coronary artery seems to be more sensitive to blockade of calcium entry whereas hypoxic contraction in porcine coronary artery and hypoxic contraction and posthypoxic contraction in coronary vein of both species are less sensitive to external calcium entry and probably mediated by products of the cyclooxygenase pathway of arachidonic acid. It is suggested that hypoxic and posthypoxic contraction might play a role in redistribution of blood flow from hypoxic to normoxic tissue of the heart during ischemia.