Contraction followed by relaxation in response to hypoxia in the sheep isolated middle cerebral artery.

In rings of sheep middle cerebral artery, precontracted with 5-hydroxytryptamine (5-HT) 10 microM, hypoxia (pO2 = 10 mm Hg) caused contraction and re-oxygenation caused a transient further relaxation. The hypoxic contraction was abolished and the re-oxygenation relaxation was reduced by removal of the endothelium. BW755C 20 microM reduced the hypoxic contraction and re-oxygenation relaxation. Haemolysate 1 microliter/ml and methylene blue 10 microM reduced the hypoxic contraction and re-oxygenation relaxation and enhanced the hypoxic relaxation phase. It is concluded that the endothelium of isolated rings of sheep middle cerebral artery releases vasoconstrictor substances during hypoxia and releases vasodilator substances during re-oxygenation.