Abstract

We recently reported that cyclooxygenase (COX)-2 expression was up-regulated in the rat small intestine after administration of indomethacin, and this may be a key to nonsteroidal anti-inflammatory drug (NSAID)-induced intestinal damage. In the present study, we investigated the effect of inhibiting COX-1 or COX-2 on various intestinal events occurring in association with NSAID-induced intestinal damage. Rats without fasting were treated with indomethacin, SC-560 (a selective COX-1 inhibitor), rofecoxib (a selective COX-2 inhibitor), or SC-560 plus rofecoxib, and the following parameters were examined in the small intestine: the lesion score, the enterobacterial number, myeloperoxidase (MPO) and inducible nitric-oxide synthase (iNOS) activity, and intestinal motility. Indomethacin decreased mucosal prostaglandin (PG)E₂ content and caused damage in the intestine within 24 h, accompanied by an increase in intestinal contractility, bacterial numbers, and MPO as well as iNOS activity, together with the up-regulation of COX-2 and iNOS mRNA expression. Neither SC-560 nor rofecoxib alone caused intestinal damage, but their combined administration produced lesions. SC-560, but not rofecoxib, caused intestinal hypermotility, bacterial invasion, and COX-2 as well as iNOS mRNA expression, yet the iNOS and MPO activity was increased only when rofecoxib was also administered. Although SC-560 inhibited the PG production, the level of PGE₂ was restored 6 h later, in a rofecoxib-dependent manner. We conclude that inhibition of COX-1, despite causing intestinal hypermotility, bacterial invasion, and iNOS expression, up-regulates the expression of COX-2, and the PGE₂ produced by COX-2 counteracts deleterious events, and maintains the mucosal integrity. This sequence of events explains why intestinal damage occurs only when both COX-1 and COX-2 are inhibited.