POTENTIATION BY HYPOXIA OF CONTRACTIONS CAUSED BY ANGIOTENSIN-II IN DOG AND MONKEY CEREBRAL-ARTERIES

Background and Purpose: Hypoxia alters the responsiveness to endogenous substances of cerebral arteries, possibly resulting in the modulation of blood supply to ischemic brain regions. The present study was undertaken to analyze the mechanism of potentiation by hypoxia of angiotensin II-induced cerebroarterial contractions. Methods: Monkey and dog cerebral arterial strips with endothelium were suspended for isometric tension recording in Ringer-Locke solution aerated with 95% O2-5% CO2 (Partial O2 pressure, 570-600 mm Hg) or 95% N2-5% CO2 (approximately 10 mm Hg). Results: Contractions induced by angiotensin II and substance P were potentiated by exposure to hypoxia, whereas contractile responses to prostaglandin F2alpha were not influenced. Treatment with cyclooxygenase inhibitors abolished the peptide-induced contraction but did not alter the prostaglandin F2alpha-induced contraction. Relaxations induced by arachidonic acid were suppressed by indomethacin and hypoxia, whereas those caused by a prostaglandin I2 analogue were unaffected. Conclusions: The potentiation by hypoxia of cerebroarterial contractions caused by angiotensin II and substance P appears to be due to an interference with the synthesis of prostaglandin I2 from arachidonic acid and a resultant increase in the production of vasoconstrictor prostaglandins.