Plasma prolactin concentrations in mares and their neonates after oxytocin induction of parturition

Studies were undertaken to investigate the effects of oxytocin induction on prolactin release in term (Group II) and preterm (Group III) mares and to compare these effects to spontaneously foaling mares (Group I). Since physiological concentrations of prolactin in blood have not been measured in the neonatal foal, experiments were designed to monitor prolactin in the cord artery and jugular blood of the foals from all groups of mares. Although prolactin levels varied in term mares (Group I and II) during the last 11 days of pregnancy, an increase was observed between Day -6 and Day 0 (2.7 and 11.9 ng/ml respectively; P less than 0.1). The average concentration of prolactin over the last 4 days (Days -3 to 0) had increased by 40% when compared to the average concentration on Days -6, -5, and -4. These findings indicate a rising trend which appears to occur concomitantly with changes in concentrations of 2 mammary components tested, sodium and potassium. Prolactin concentrations did not significantly increase in term mares after oxytocin treatment or in spontaneously foaling mares. However, the preterm induced mares had higher prolactin concentrations during the first stage of labor (19.3 +/- 7.2 ng/ml) than prior to treatment with oxytocin (4.7 +/- 2.0 ng/ml; P less than 0.01). Levels of prolactin in all groups significantly declined by 20-min post-placental expulsion. For the first 30 min after birth, prolactin concentrations in foals from oxytocin-induced mares appeared to be 2-fold higher than those from spontaneously foaling mares. Thereafter, prolactin values declined to baseline values by 48 hrs. When comparing cord arterial plasma with cord venous plasma in each group, prolactin concentrations were similar. However, the average prolactin levels in both the cord artery and vein appeared higher (ave: 1.1 ng/ml) in Group II and III than in Group I (less than 0.5 ng/ml). From these results, the authors suggest that 1) prolactin may have a role in regulating mammary secretory products in mares just prior to parturition; 2) oxytocin may increase prolactin secretion in preterm induced mares; 3) oxytocin induction may have a short term effect to increase circulatory prolactin concentrations in neonates in utero regardless whether their dams were treated preterm or term.