Oxytocin-induced secretion of prostaglandin F2α in postpartum beef cows: Effects of progesterone and estradiol-17β treatment1

The purpose of the present study was to determine the effect of progesterone or progesterone + estradiol-17β on oxytocin-induced prostaglandin F 2α (PGF 2α ) secretion in postpartum beef cows. Thirty-four anestrous postpartum beef cows were ovariectomized (d 32 [Groups 1 to 3] or d 23 [Groups 4 to 6] postpartum [d 0 = parturition]) and allotted to six treatments (Group 1; negative control) to simulate short (Groups 2 through 5) or normal (Group 6) length estrous cycles. Steroid treatments for the respective groups were as follows: Group 1) no estradiol-17β or progesterone treatment (n = 8; negative control); Group 2) progesterone (d 34 to 40; n = 6); Group 3) estradiol-17β (d 32 to 33) and progesterone (d 34 to 40; n = 6); Group 4) progesterone (d 23 to 29), no estradiol-17β (d 32 to 33), and progesterone (d 34 to 40; n = 5); Group 5) progesterone (d 23 to 29), estradiol-17β (d 32 to 33), and progesterone (d 34 to 40; n = 5); and Group 6) progesterone (d 23 to 29), estradiol-17β (d 32 to 33), and progesterone (d 34 to 50; n = 4; positive control). Oxytocin (100 IU) was injected (i.v.) at the end of each treatment to test the ability of the postpartum uterus to secrete PGF 2α as measured by a stable metabolite of PGF 2α , 15keto-13,14 dihydro-PGF 2α (PGFM). Peak concentrations of PGFM (P ≤ 0.08) and total PGFM secreted (area under the curve; P < 0.05) were increased on d 6 following first (Group 2) or second (Group 4) exposure to progesterone and were similar to peak concentrations and total PGFM secreted 16 d following a simulated normal estrous cycle (Group 6). Administration of estradiol-17β before first progesterone exposure (Group 3) did not reduce peak concentrations of PGFM or total PGFM secreted relative to the preceding groups. Peak concentrations of PGFM (P ≤ 0.08) and total PGFM secreted (P < 0.05) were reduced following a second progesterone exposure, provided that cows were pretreated with estradiol-17β (Group 5). In , oxytocin-induced release of PGFM was inhibited on d 6 following second exposure to progesterone only when cows were pretreated with estradiol-17β. Therefore, estradiol-17β and progesterone were both associated with the timing of PGF 2α secretion in postpartum cows.