EFFECTS OF DEXAMETHASONE ADMINISTRATION TO DIESTRUS COWS ON SYSTEMIC PROGESTERONE, ESTROGEN AND UTERINE CYCLOOXYGENASE PRODUCTION

Parenteral administration of dexamethasone to diestrus cattle can exte nd the length of the natural estrous cycle. In mice, dexamethasone has been shown to inhibit production of the second isozyme of the cycloox ygenase (COX) enzyme (a rate limiting enzyme in prostaglandin formatio n). Therefore, the purpose of this study was to determine the effect o f dexamethasone on estrous cycle length and COX-1 and -2 production by the uterine endometrium of cyclic cattle. Nine crossbred beef cows th at exhibited two previous normal estrous cycles were randomly assigned to two treatments; a control group administered intramuscular injecti ons of vehicle, and a dexamethasone group administered 8 mg of dexamet hasone (Azium(R), Schering Corp., Kenilworth, NJ). Both groups receive d twice daily injections on day 13-22 of the treatment cycle. Uterine endometrial biopsies were collected on days 16, 19 and 22 of the treat ment cycle. Blood samples were collected daily on day 13-22 of the tre atment cycle for plasma progesterone and estradiol concentrations. The mean treatment cycle length was extended(P < 0.05) in the dexamethaso ne group (31 d) compared with the control group (24 d). However, no di fference was noted in the time to progesterone decline between treatme nts. In contrast, estradiol levels were lower in the dexamethasone tre ated animals compared with the control group on day 19 to 22 of treatm ent. A western blot analysis revealed no COX-2 in the uterine samples of either treatment. The COX-I isoform was found on all days examined, but no treatment effect was detected. These results suggest that dexa methasone extends the cycle length by inhibiting follicle growth, and that COX-2 may not be involved in prostaglandin formation by the uteru s during luteolysis. (C) 1997 Elsevier Science B.V.