Concentrations of oxytocin in the intercavernous sinus of mares during luteolysis: temporal relationship with concentrations of 13,14-dihydro-15-keto-prostaglandin F2 

Abstract

The reproductive tracts of nine thoroughbred mares were examined by ultrasound to determine the day of ovulation (day 0). Mares were fitted with intercavernous sinus cannulae on the day before the start of sample collection of pituitary venous effluent rich in oxytocin. Intercavernous sinus blood samples were collected for at least 36 h at 5 min intervals beginning at noon on day 13 (n = 2), day 15 (n = 5) or day 16 (n = 2) after ovulation. Concentrations of oxytocin and 13,14-dihydro-15-keto prostaglandin F2 alpha (PGFM) in plasma were determined by radioimmunoassay. Three high-magnitude surges of PGFM (> 1 ng ml-1) were found in these samples. Three high magnitude pulses of oxytocin (> 200 pg ml-1) were also observed, one associated with each of the PGFM surges. In each of these cases, the oxytocin pulse appeared to follow or coincide with the onset of the PGFM surge. Lower magnitude pulses of both hormones were detected throughout the bleeding period in every mare. The average interval between these pulses was 122.3 min for oxytocin and 121.0 min for PGFM. The interval between pulses for individual mares varied from 90 to 199 min for oxytocin, and from 87 to 213 min for PGFM. However, there was no correlation between PGFM and oxytocin pulse intervals among mares. Within each mare, there was no discernable association between low magnitude pulses of oxytocin and PGFM. From these data, it was concluded that high-magnitude surges of PGF2 alpha are associated with similar surges of oxytocin from the posterior pituitary gland, and that PGF2 alpha may induce their secretion. The posterior pituitary gland also appears to secrete oxytocin in a pulsatile manner at a frequency of approximately 1 pulse every 2 h but these pulses do not appear to be associated with the low magnitude pulses of PGF2 alpha secreted from the uterus.

Oxytocin – a peptide serving manifold roles during the reproductive cycle in mares

“…Daily application of OT to mares at days 4, 5, 6, 7 and 8 of the oestrous cycle had no effect on PGFM concentrations and duration of dioestrus (Neely et al 1979), supporting the hypothesis of downregulated OTreceptors up to day 10 of the cycle. In contrast, Vanderwall et al (1998) could not find synchronous OT and PGFM patterns in blood samples, which were drawn from the cavernous sinus during luteolysis, although they support the positive feedback role of OT on PGF 2α as well.…”

Oxytocin induction of pulses of a prostaglandin metabolite and luteolysis in mares

“…Oxytocin is synthesized in the hypothalamus, stored in the posterior pituitary, and secreted in pulses [11,12]. The CL also synthesizes and secretes OT during the estrous cycle in domestic ruminants [13,14], but the CL apparently does not secrete OT in mares [15].…”

Effect of Administration of Oxytocin During Diestrus on Corpus Luteum Function and Endometrial Oxytocin Receptor Concentration in Cycling Mares

“…We hypothesized that when oxytocin treatment is initiated before day 10 postovulation, it prevents luteolysis by inhibiting the increase in endometrial oxytocin receptor concentration that would otherwise permit endogenous oxytocin-induced PGF2a secretion at the time of luteolysis; however, there was no difference in endometrial oxytocin-binding capacity between salinetreated control mares and oxytocin-treated mares in this study, which does not support that hypothesis. It is possible that oxytocin treatment prevents luteolysis through a different mechanism or that our experimental methods were not sensitive enough to detect a difference in oxytocin receptor concentrations between groups; for example, cervical manipulation induces an acute release of oxytocin from the posterior pituitary gland in nonpregnant mares [13], and because the endometrial tissue samples in this study were obtained transcervically, it is plausible that release of endogenous oxytocin could have led to the saturation of endometrial oxytocin receptors, which prevented us from detecting a difference between groups. Therefore, further work will be needed to determine the mechanism through which oxytocin treatment disrupts luteolysis, leading to prolonged CL function in mares.…”

“…Endogenous oxytocin secretion is involved in regulating prostaglandin F2a (PGF2a) secretion from the endometrium during spontaneous luteolysis in the mare [13,14], and although administration of exogenous oxytocin to mares around the time of luteolysis (i.e., days 11-15 postovulation) stimulates an acute onset of PGF2a secretion [15][16][17], when oxytocin is administered in the midluteal phase before the expected time of luteolysis (i.e., before day 10 postovulation), it does not induce PGF2a secretion and often disrupts spontaneous luteolysis causing prolonged CL function [16,18]. We previously reported the results of a "proof of principle" study [7] that showed IM administration of 60 U of oxytocin twice daily on days 7-14 postovulation was an efficacious method of disrupting luteolysis because it caused prolonged CL function through day 30 postovulation in all six treated mares, whereas all six saline-treated control mares underwent spontaneous luteolysis by day 16 postovulation.…”