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 F2 alpha 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 microliters 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 F2 alpha 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 F2 alpha metabolite in this marsupial.
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