Prolactin and other Protein Hormones in Milk12

Pituitary prolactin has been quantified by radioimmunoassay in whole milk obtained from cattle, goats, sheep and rats. Prolactin concentrations in milk samples obtained following the completion of lactogenesis approximate concentrations of the hormone in blood plasma or serum. However, concentrations of prolactin in prepartum mammary secretions were much higher than plasma prolactin in prepartum dairy cows. This observation was consistent with the hypothesis that during mammary lactogenesis, endogenous milk prolactin in the alveolar lumen may be an additional source of biologically active prolactin. The value of milk prolactin to neonatal animals remains unknown. Experiments with milk-fed calves and suckling rats failed to demonstrate absorption of the intact molecule into the neonate's blood. Further research is needed to determine the role, if any, that maternal prolactin consumed in milk plays in neonatal physiology. Measurements of milk prolactin seem to be highly predictive of the average blood prolactin concentration. Milk prolactin can probably be used in lactating females to predict average plasma prolactin in a manner that is relatively independent of stress- or milking-induced increases in pituitary release of the hormone.