Contractile responses to hypoxia of isolated rings from the left branch of rabbit pulmonary artery.

Contractile responses of left extrapulmonary artery segments with either intact or damaged endothelium were examined with changes in PO2 at constant pH. Hypoxia consistently reduced noradrenaline-induced contractions. This hypoxia-induced relaxation was sometimes followed by a contraction and a second relaxation. Hypoxia-induced relaxations were also obtained if precontraction was elicited with KCl (40 mM), but no triphasic response was observed. Relaxations faded away with time and only contractions were then observed. Relaxations were more considerable in rings from young animals and, although always present, decreased with the age of the animal. Endothelium damage reduced hypoxia-induced relaxations. Indomethacin, a potent blocker of cyclooxygenase, increased hypoxia-induced contraction and reduced relaxation in segments with intact endothelium. Without endothelium the indomethacin effect was less significant. It is concluded that the response to hypoxia of the pulmonary artery is similar to that of systemic vessels. Endothelium seems to play a modulatory role in the hypoxia-induced response in extrapulmonary artery. Prostaglandin metabolism seems to play a minor role in this modulation.