Abstract

Prostanoids have been implicated in the regulation of lung vascular tone both under physiological and inflammatory conditions. The conversion of arachidonic acid (AA) to prostaglandin H₂ is catalyzed at least by two isoforms of cyclooxygenase, named Cox-1 and Cox-2. Cox-1 is thought to be ubiquitously expressed, enrolled in physiological processes, whereas Cox-2 is mostly assumed to be dynamically regulated, responding to inflammatory conditions. We have recently shown by immunohistochemistry that Cox-2 is constitutively expressed in control rat lungs, with a predominant localization in smooth muscle cells of partially muscular vessels. We now asked whether Cox-2 is basically involved in the physiological regulation of pulmonary vascular tone. Isolated perfused rat lungs were challenged with intravascular bolus application of free AA to elicit thromboxane-related vasoconstrictor responses and to investigate the effects of three different selective Cox-2 inhibitors (NS-398, DUP697, SC-236). AA induced the liberation of prostaglandin I₂ and thromboxane A₂ into the intravascular space, and it provoked marked pulmonary artery pressure responses and concomitant lung edema formation. All events were dose-dependently inhibited by 1 to 50 μmol/liter NS-398, whereas control vasoconstrictor responses to angiotensin II and the stable thromboxane analogue U46619 were not affected by this agent. Similarly, marked inhibition of the AA elicited pressor response was achieved by 25 μmol/l DUP697 and by 10 μmol/l SC-236. These data suggest a physiological role of Cox-2 rather than Cox-1 in the regulation of vascular tone in rat lungs.