Role of cyclooxygenase system in cerebrovascular responsiveness: different effects of indomethacin on CO2 responsiveness and dilatation by Ca(2+)-channel blocker.

To investigate roles of prostaglandins in the regulation of cerebral blood flow, we compared effects of indomethacin, a cyclooxygenase inhibitor, on the cerebrovascular CO2 responsiveness with those on the cerebrovascular dilatory action of diltiazem, a Ca(2+)-channel blocker. Fifteen adult cats were used. The cerebral tissue oxygen tension, carbon dioxide tension, pH and blood pressure were measured continuously. Indomethacin (1 mg/kg) was infused into the carotid artery. In 8 cats, 3 min inhalation of 5% CO2 in air was performed before and after the indomethacin infusion. In 7 cats, diltiazem (100 micrograms/kg) was infused into the carotid artery for 3 min before and after the indomethacin infusion. The cerebrovascular CO2 responsiveness was significantly decreased (p < 0.05) after the administration of indomethacin. On the other hand, the cerebrovascular dilatation induced by the Ca(2+)-channel blocker was significantly increased (p < 0.05) after the administration of indomethacin. It is concluded that the products of cyclooxygenase system are involved in the cerebrovascular responsiveness both to CO2 and to Ca(2+)-channel blocker, but action mechanisms of prostaglandins may be different, that is, prostaglandians may enhanced cerebrovascular responsiveness to CO2 but diminish it to Ca(2+)-channel blocker.