Site and mechanisms of action of kinins in rat ileal mucosa

Abstract

Kinin-induced secretion in the intestine is accompanied by marked increases in mucosal adenosine 3'',5''-cyclic monophosphate (cAMP) and prostanoids that undoubtedly contribute to the overall secretory response. The cellular site at which thee effects are initiated is unclear although a recent hypothesis has suggested the epithelial cell as the target for kinin action (6). We have investigated the effects of kallidin on the phospholipase-prostanoid-cAMP pathway in whole ileal mucosa and in epithelial cells isolated for the same tissue in the rat. Kallidin (1 .mu.M) stimulated a marked rise (24 to 30-fold) in PG (prostaglandin) E2 release from the serosal surface of stripped ileal mucosa within 1-2 min, which correlated closely with the rise in mucosal short-circuit current. Mucosal cAMP levels were also increased two to threefold by kallidin. However, kinins were unable to elicit effects under the same conditions in suspensions of viable epithelial cells. PGE2 release was unaffected by kallidin or bradykinin at concentrations up to 100 .mu.M, whereas cAMP levels could be stimulated by forskolin and PGE2 but not by kinin. Studies of intestinal phospholipase A2 (PLA2) activity also suggest a nonepithelial site for kinin action. This enzyme is responsible for liberating arachidonic acid from cellular phospholipids and has been shown to be activated by kinins in several tissues. In the intestine, PLA2 activity was found to be concentrated within the subepithelium with significantly lower levels in the epithelium itself. In addition, kallidin was unable to influence phospholipid labeling (an indirect measure of PLA2 activity) in cells incubated with [14C]arachidonic acid. These studies suggest that kinins initiate increases in intestinal prostaglandin and cAMP production within the subepithelium and not by a direct action on epithelial cells.