Levels of alpha-melanotrophin in the plasma and pituitary glands of fetal and juvenile rats, and in the plasma of adult female rats during pregnancy and lactation.

alpha-Melanotrophin was detected by radioimmunoassay in the pituitary glands of fetal rats from day 17 of gestation. The pituitary content of alpha-melanotrophin increased regularly, at a gradually decreasing rate, throughout gestation and in the postnatal period. Concentrations of alpha-melanotrophin in the plasma of fetal and newborn rats were below the detection limit of the radioimmunoassay (10 pmol/l). Detectable concentrations were first found in young rats on day 3 after birth and did not differ significantly from those in their mothers throughout the period of suckling. Plasma concentrations of alpha-melanotrophin were raised in pregnant rats during the last 4 days of gestation and after parturition. They returned to basal levels in the 2 weeks after delivery. After weaning at 3 weeks of age, a large increase in the plasma concentration of alpha-melanotrophin was detected in juvenile rats. Plasma levels had returned to the normal adult range by 6 weeks of age. The increases in alpha-melanotrophin in the blood were thought to be the result of non-specific stress effects. The data did not provide evidence for a role of alpha-melanotrophin in reproductive processes in the rat.