Patterns of oxytocin secretion during the oestrous cycle of the mare

From a group of 11 cyclic mares, blood samples were collected at 3-min intervals for 2 h and at 15-min intervals for an additional 6 h during four stages of the oestrous cycle. Mean plasma oxytocin concentrations (pg/ml, LSM +/- se) were greater on Day 15 after ovulation (169.9 +/- 17.6) than on Day 0 (82.6 +/- 17.6; P less than 0.01), Day 3 (97.2 +/- 20.4, P less than 0.01) and Day 7 after ovulation (104.0 +/- 25.0, P less than 0.05). Oxytocin was secreted in a pulsatile manner throughout the oestrous cycle, with short (0-29 min), medium (30-89 min) and long (greater than 90 min) duration rhythms. No differences in pulse frequencies were observed throughout the oestrous cycle. Pulse amplitudes for rhythms of short and medium duration were greater on Day 7 after ovulation than on Day 0 (P less than 0.01 for short and P less than 0.05 for medium rhythms), Day 3 (P less than 0.01) and Day 15 after ovulation (P less than 0.05 for short rhythms). Fluctuations in baseline values for oxytocin over the 8-h sampling period were indicative of a possible circadian rhythm. These results are consistent with previous observations for the mare and other species that oxytocin is secreted in a pulsatile manner and may be involved in the regulation of the oestrous cycle by promoting luteolysis via the synthesis and release of uterine PGF-2 alpha.