The Role of the Enzyme Cyclooxygenase and Bile in the Damage and Repair of Intestinal Epithelium

Abstract

Colic describes poorly localized abdominal pain in horses and accounts for the majority of emergency calls received by equine veterinarians. The main drugs used to treat colic are the nonsteroidal anti-inflammatory drugs (NSAIDs), which inhibit the enzyme cyclooxygenase (COX), and prevent endotoxin-induced elaboration of prostaglandins. There are 2 isoforms of cyclooxygenase: COX-1, which constitutively produces prostaglandins and COX-2, which is induced by inflammation. Study 1: We hypothesized that the non-specific cyclooxygenase inhibitor flunixin would retard repair of ischemic intestinal injury by preventing production of reparative prostaglandins whereas the selective COX-2 inhibitor, etodolac, would permit repair as a result of continued COX-1 prostaglandin production. Equine jejunum was subjected to ischemia for 1 hour, and recovered for 4 hours in Ussing chambers. In ischemic tissue treated with flunixin, production of prostaglandins was inhibited, and there was no evidence of recovery based on measurements of transepithelial resistance (TER). Conversely, untreated ischemic tissues or tissues treated with etodolac had significant elevations in prostaglandins, and significant recovery of TER. These studies suggest that specific COX-2 inhibitors may provide an advantageous alternative to non-specific cyclooxygenase inhibitors in horses with colic. Study 2: A potential adverse effect of NSAIDS in horses is colitis. It was hypothesized that the non-selective COX inhibitor flunixin would retard repair of bile-injured colon by preventing production of reparative prostaglandins, whereas the selective COX-2 inhibitor, etodolac would not inhibit repair as a result of continued COX-1 activity. Equine colon was exposed to 1.5mM deoxycholate for 30-minutes, after which they were recovered for 4 hours in Ussing chambers. Contrary to the proposed hypothesis, recovery of bile-injured colonic mucosa was not affected by flunixin or etodolac, despite significantly depressed prostanoid production. However, treatment of control tissue with flunixin led to increases in mucosal permeability, whereas treatment with etodolac had no significant effect. Therefore, although recovery from bile-induced colonic injury maybe independent of COX-elaborated prostanoids, treatment of control tissues with non-selective COX inhibitors may lead to marked increases in permeability. Alternatively, selective inhibition of COX-2 may reduce the incidence of adverse effects in horses requiring NSAID therapy. Study 3: It has been shown that rapid in vitro recovery of barrier function in porcine ischemic-injured ileal mucosa, is attributable principally to reductions in paracellular permeability. However, these experiments did not take into account the effects of normal luminal contents, such as bile salts, which, according to our preliminary studies, reach concentrations as high as 10⁻⁵M in the porcine ileum. The objective of this study was to evaluate the role of deoxycholic acid in recovery of mucosal barrier function. Porcine ileum was subjected to 45-minutes of ischemia, after which mucosa was mounted in Ussing chambers, and exposed to varying concentrations of deoxycholic acid. The ischemic episode resulted in significant reductions in TER, which recovered control levels of TER within 2-hours, associated with significant reductions in mucosal-to-serosal ³H-mannitol flux. However, treatment of ischemic-injured tissues with 10⁻⁵M deoxycholic acid fully inhibited recovery of TER with significant increases in mucosal-to-serosal 3H-mannitol flux, whereas 10⁻⁶M deoxycholic acid had no effect. Histologic evaluation at 2 hours revealed complete restitution regardless of treatment, indicating the breakdown in barrier function was due to changes in paracellular permeability. Similar effects were noted with application of 10⁻⁵M taurodeoxycholic acid, and the effects of deoxycholic acid were reversed with application of the Ca²⁺ mobilizing agent thapsigargin. Deoxycholic acid at physiologic concentrations significantly impairs recovery of epithelial barrier function by an effect on paracellular pathways, and these effects appear to be Ca²⁺-dependent.