Comparison of luteolytic effectiveness of several prostaglandin analogs in heifers and relative binding affinity for bovine luteal prostaglandin binding sites

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The relative binding affinities for both the prostaglandin (PG)E 1 and PGF 2α specific bovine luteal binding sites were determined for five PGE and fourteen PGF derivatives and analogs. Relative binding affinity was determined in vitro using membranes prepared from bovine corpora luteal (CL) obtained from the slaughterhouse. The parent structure of the analog was a dominant feature in determining the affinity for the respective PG binding site. Luteolysis was determined in cattle following intramuscular injection of various doses of prostaglandin once between days 6 and 14 after estrus and measuring CL regression by ovarian palpation per rectum, interval between injection and return to estrus and duration of the subsequent estrous cycle. A dose which was luteolytic was established for each of eight PGF-type compounds, and a dose which was not luteolytic was also established. There appeared to be limited association between the relative affinity for the PGF 2α specific site in vitro and the estimated luteolytic dose range of these PGF analogs when tested in cattle. Differences in in vivo luteolytic potency for the compounds tested could not be explained by differences in binding affinity. Differences in metabolism and absorption may also be important in the determination of in vivo potency.