Intestinal effects of isoprostanes: evidence for the involvement of prostanoid EP and TP receptors.
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The isoprostanes, which differ from prostaglandins by the cis orientation of their side chains, are believed to exert their biological effects on either a prostanoid TP receptor or a "unique" isoprostane receptor. Preliminary experiments suggested that canine colonic epithelium possessed no prostanoid TP receptor activity, in contrast to the muscularis mucosae, which responds well to the selective prostanoid TP receptor agonist U46619. To define the receptors involved, the in vitro responses of the epithelium and muscularis mucosae from the canine proximal colon to both 8-iso-PGE2 and 8-iso-PGF2alpha were compared. The epithelium responded to 8-iso-PGE2 but not to 8-iso-PGF2alpha. Under basal conditions, 8-iso-PGE2 produced concentration-dependent increases in short circuit current (pEC50 = 6.4 +/- 0.1) that were not antagonized by the selective prostanoid TP receptor antagonist SQ29548 (10(-6) M). Cross-desensitization experiments suggested that the stimulant effects involved a prostanoid EP receptor. Desensitization of the epithelium to PGE2 resulted in unexpected decreases in short circuit current in response to 8-iso-PGE2 (10(-6) M). This effect was mimicked by the selective prostanoid TP receptor agonist U46619 (10(-5) M), and antagonized by three structurally different prostanoid TP receptor antagonists: L670596 (10(-6) M), SQ29548 (10(-6) M) and GR32191 (10(-6) M). 8-Iso-PGE2, 8-iso-PGF2alpha and U46619 caused concentration-dependent increases in the force of contraction of the muscularis mucosae strips. These responses were antagonized by selective prostanoid TP receptor antagonists, arguing for the involvement of prostanoid TP receptors. Thus, the effects of isoprostanes on the canine colon involve both prostanoid TP and EP receptors.
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