Biological activity of metabolites of PGD2 on canine proximal colon
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The responses of the canine colonic epithelium to the metabolites of prostaglandin D2 (PGD2) were compared with those elicited by the parent prostanoid. Dose-response relations to PGD2 showed three distinct patterns: 1) a dose-dependent decrease in short-circuit current (Isc) at lower concentrations followed by a dose-dependent increase at higher concentrations; 2) dose-dependent decreases, with no increase even at the highest concentrations tested; and 3) dose-dependent increases in Isc, with no decreases at any concentration. The colon responded differently to the two enzymatically derived metabolites 13,14-dihydro-15-keto-PGD2 (DK) and 11 beta-PGF2 alpha. The former consistently produced only dose-dependent decreases in Isc, while the latter elicited only dose-dependent increases. Pretreatment of tissues with 11 beta-PGF2 alpha altered the responses to PGD2 such that only decreases were noted. Conversely, pretreatment with DK caused PGD2 to elicit only increases in Isc. The nonenzymatically derived PGJ2 elicited responses comparable to those seen with PGD2. Pretreatment of tissues with indomethacin abolished responses to 11 beta-PGF2 alpha as well as its isomer, PGF2 alpha, suggesting the involvement of a cyclooxygenase product. Responses to PGE2 were, however, amplified. Cross-desensitization was noted between the two isomers. Tissues desensitized to either 11 beta-PGF2 alpha or PGF2 alpha were responsive to DK as well as PGE2; however, tissues desensitized to PGE2 were unresponsive to 11 beta-PGF2 alpha. Thus the canine colonic epithelium responds not only to PGD2 but also to its derived metabolites. Variability in the response to PGD2 between animals could stem from differences at the receptor level and/or differential production of these metabolites.
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