Oxytocin release and its relationship to dihydro-15-keto PGF(2 alpha) and arginine vasopressin release during parturition and to suckling in postpartum mares

Pituitary blood was collected from the intercavernal sinus in five mares be fore and during parturition, and in nine mares immediately after parturitio n to investigate oxytocin patterns during parturition and early lactation, and to determine the relationship between oxytocin, prostaglandin and argin ine vasopressin during parturition. In four mares in which sample collectio n began at least 6 h before rupture of the chorioallantois, a significant i ncrease (P < 0.05) in PGF(2 alpha) concentration was detected before a sign ificant increase in oxytocin concentration. Cross-correlation analysis of l og-transformed oxytocin and PGF(2 alpha) concentrations revealed a signific ant correlation (P < 0.05) at a 6 min lag period, indicating that in the 2h before delivery of the foal, an increase in prostaglandin was followed 6 m in later by an increase in oxytocin. A significant effect of suckling on ox ytocin release by the mare was detected in only two of nine mares, when oxy tocin concentrations were evaluated 0-3min after suckling. When foals were prevented from sucking for 1 h, by being either muzzled (n = 2) or separate d from the mare (n = 2), there was no significant association between resum ption of suckling and oxytocin release by the mare. The results of these st udies show that: (i) oxytocin secretion from the maternal posterior pituita ry gland begins before, or in association with, the onset of the second sta ge of labour, and that prostaglandin increases in the peripheral circulatio n before oxytocin release; and (ii) suckling is not significantly related t o oxytocin release in mares.