Cyclo-oxygenase-1 or -2-mediated metabolism of arachidonic acid in endothelium-dependent contraction of mouse arteries.

This study aimed to determine whether the cyclo-oxygenase (COX) substrate arachidonic acid (AA) evokes endothelium-dependent contraction and, if so, the specific COX isoform(s) involved and whether prostacyclin (prostaglandin I2; PGI2), a mediator of endothelium-derived vasoconstrictor activity, can be generated in medial smooth muscle from the intermediate COX product prostaglandin H2 (PGH2) that might diffuse from the endothelium. Aortae and/or carotid arteries were isolated from C57BL/6 mice or those lacking one of the two COX isoforms (COX-1(-/-) or COX-2(-/-)) for functional and/or biochemical analyses. Results showed that in vessels from C57BL/6 mice, exogenous AA evoked not only endothelium-dependent production of the PGI2 metabolite 6-keto-PGF1α, but also contractions reduced by thromboxane-prostanoid receptor antagonism or endothelial denudation. The minimal concentration for AA to evoke contraction was 0.3 μm, a level thought to activate only COX-2. However, neither the contraction nor 6-keto-PGF1α production was altered in vessels from COX-2(-/-) mice, while both were reduced in COX-1(-/-) counterparts. In vessels from COX-1(-/-) mice, AA also caused minor contractions that were sensitive to non-selective COX inhibition. Real-time PCR showed that like COX-1, COX-2 mainly existed in the endothelium, but it was unaltered in COX-1(-/-) mice. Also, we noted that in endothelium-denuded aortae, PGH2 generated PGI2 as in intact vessels. These results demonstrate a predominant role for COX-1 and suggest that in the given mouse arteries, metabolites from either COX isoform cause contraction. Moreover, our results imply that some of the PGI2 involved in vasoconstrictor activity of endothelial COX-mediated metabolism could possibly be generated from PGH2 in medial smooth muscle.