Oxytocin-induced secretion of 13,14-dihydro-15-keto-prostaglandin F 2α in mares with prolonged corpus luteum function

Oxytocin treatment to prolong corpus luteum (CL) function for estrus suppression in mares is replacing use of intrauterine glass ball. Most common oxytocin protocol involves 60 IU of oxytocin treatment given intramuscularly once daily from days 7 - 14 postovulation (day 0). About 70% of treated mares had prolonged CL function lasting for 60 - 90 days. However, a longer duration of CL function would be more favorable. Serial treatment of human chorionic gonadotropin (hCG) during oxytocin-induced prolonged CL function would extend duration of CL function by having a luteotropic effect and/or by inducing ovulation of diestrous follicle(s) resulting in new CL(s) formation that would remain functional for additional 60 - 90 days (i.e. 120 - 180 days of CL function). 1 Although diestrous ovulations were documented during prolonged CL function period, there was no difference between control and hCG-treated mares in duration of CL function. Notably, in some mares, progesterone concentrations decreased abruptly after a diestrous ovulation, suggesting return of endogenous luteolytic mechanism responsible for terminating CL function. Objective was to evaluate the ability of endometrium to secrete prostaglandin F 2α (PGF2α ) during oxytocin-induced prolonged CL function. Oxytocin treatment was used to prolong CL function in mares that were subsequently assigned to 1 of 3 groups: 50 - 59 days (n = 4), 60 - 69 days (n = 4), or 70 - 79 days (n = 3); groups designated as 50s, 60s, and 70s, respectively). Day14 mares served as control group (n = 3). Endometrial secretion of PGF 2α was evaluated by measuring systemic concentrations of 13,14-dihydro-15-keto-prostaglandin F 2α (PGFM) following a single 10 IU intravenous oxytocin bolus (time 0) treatment to mares in all groups. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 25, 30, 45, 60, 75, 90, 105, and 120 minutes. Area under PGFM curve was higher in 70s group compared to 50s (p < 0.001) and 60s groups (p < 0.02), with no difference between 70s and control groups. CL function was maintained after oxytocin bolus in all 4 50s mares, 3 of 4 60s mares, and 0 of 3 70s mares. These results provided clear evidence for return of uterine luteolytic mechanism between days 50 - 70 in mares with prolonged CL function.

Keywords: Equine, mare, estrus suppression, oxytocin, corpus luteum, PGFM

Reference

1.Vanderwall DK, Kinney DL, Mason H, et al: Effect of human chorionic gonadotropin (hCG) treatment on the duration of oxytocin-induced prolonged corpus luteum (CL) function in mares. Clinical Theriogenology 2017;9:445.

This manuscript was originally published in the journal Clinical Theriogenology Vol 12(3) Sept 2020.  Clinical Theriogenology is the official journal of the Society for Theriogenology (SFT) and the American College of Theriogenologists (ACT).  This content has been reproduced on the IVIS website with the explicit permission of the SFT/ACT.