Effects of COX-2 inhibitors on aortic prostacyclin production in cholesterol-fed rabbits

Prostacyclin (PGI2) is a potent vasodilator and inhibitor of platelet aggregation that is produced by prostacyclin synthase via the cyclooxygenase (COX) pathway of arachidonic acid metabolism. We investigated the potential role of COX-2 in the production of vasoactive prostanoids by aortic tissue in a rabbit model of dietary cholesterol-induced atherosclerosis. COX-1 was detected as the major isoform by immunoblot analysis in extracts from aortas of normal and 8 week cholesterol-fed animals with COX-2 being induced in atherosclerotic plaques from cholesterol-fed animals. Aortic tissue from cholesterol-fed animals showed decreased levels of basal 6-keto-PGF1α and PGE2 production as compared to the normal controls but showed no difference with respect to their ability to synthesize these prostanoids in response to exogenous arachidonic acid. The highly selective COX-2 inhibitors rofecoxib and the furanone DFP at concentrations of up to 10 μmol/l had no effect on the arachidonic acid-dependent production of 6-keto-PGF1α, in contrast to indomethacin, which caused a complete inhibition at 0.5 μmol/l. Celecoxib caused a significant inhibition of 6-keto-PGF1α at 10 μmol/l but had little effect when the dose was lowered to 1 μmol/l. Similar effects of these inhibitors were observed with respect to the production of PGE2 and no major difference was observed between aortic tissues from normal and cholesterol-fed animals with regard to inhibitor sensitivity. These results indicate that in a rabbit model of early stage cardiovascular disease, the basal production of 6-keto-PGF1α and PGE2 by aortic tissue is decreased. Furthermore, COX-2 expression is induced in atherosclerotic plaques and may play a role in altering localized synthesis of prostanoids in these lesions but does not appear to significantly impact the arachidonic acid-dependent prostacylin production of aortic tissues, which is largely mediated by COX-1.