Release of an oxytocic peptide at parturition in the marsupial, Macropus eugenii

Abstract

The oxytocic peptide mesotocin was measured in plasma samples collected throughout pregnancy in the conscious tammar wallaby, Macropus eugenii. Plasma mesotocin and the prostaglandin metabolite 13,14-dihydro-15-oxo-prostaglandin F(2α) were also assessed immediately prepartum and during parturition. A radioimmunoassay for mesotocin was validated in the tammar and this assay allowed direct measurement in 50 μl unextracted plasma with a sensitivity of 12.5 pmol l-1. Plasma concentrations of mesotocin remained basal (approximately 15 pmol l-1) at all stages of pregnancy, including prepartum. A significant (P < 0.05) increase in plasma mesotocin was observed only immediately after delivery of the neonate and this increase was maintained for at least 15 min postpartum. Mesotocin concentrations returned to basal values 2 h after birth. Peak concentrations of mesotocin of 516.7 ± 108.1 pmol l-1 were measured within 2 min of birth. This peak coincided with a short-lived peak in concentration of prostaglandin F(2α) metabolite immediately after birth (2.1 ± 0.4 nmol l-1) which decreased to less than 0.3 nmol l-1 within 2 h postpartum. These data demonstrate that mesotocin is released during, or immediately after, delivery and appears to parallel the profile of circulating prostaglandin F(2α) metabolite in this marsupial.