Plasma hormone concentrations after administration of dexamethasone during the middle of the luteal phase in cows.

The effect of glucocorticoids on gonadal steroid and gonadotropic hormone concentrations and subsequent follicular activity in cows undergoing normal estrous cycles was evaluated by administration of dexamethasone (DXM) during the middle of the luteal phase. Seven cows were given physiologic saline solution twice daily from day 13 to day 17 of the estrous cycle (control experiment). During the next estrous cycle, cows were administered DXM (2 mg, IM) twice daily on days 13 through 17. Plasma specimens obtained twice daily throughout the control and DXM-treatment cycles were assayed for progesterone and estradiol concentrations. The appearance of estrus after DXM treatment was delayed until days 23 to 25 in 3 cows and was not seen by day 35 in the other 4 cows, compared with mean (+/- SD) cycle length of 22.4 +/- 3.2 in cows during the control experiment. Progesterone concentration remained significantly (P less than 0.01) high on days 19 to 23, whereas estradiol values failed to increase (P less than 0.05) on days 19 and 20 after treatment with DXM. Blood samples were obtained at 15-minute intervals for 12 hours to compare (by analysis of covariance) the effect of DXM treatment on plasma hormone concentrations on day 15 of each cycle with those of day 10. Compared with values during the control experiment, a significant (P less than 0.05) decrease was observed in the size of the pulses of luteinizing hormone (LH) and estradiol, although the number of pulses of each hormone per 12 hours was not affected when cows were given DXM. Baseline concentrations of LH and estradiol were not altered by type of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)