Transfer of milk prolactin to the plasma of neonatal rats by intestinal absorption.

Prolactin passes from the systemic circulation of lactating rats into the milk where it can be consumed by the young rats during suckling. 131I-Labelled rat prolactin was detected in the plasma of 9- to 14-day-old rats after being nursed by mothers previously injected with 131I-labelled rat prolactin and after the pups had received 131I-labelled rat prolactin by gastric intubation. It was estimated that 16% of the 131I-labelled rat prolactin given by gastric intubation subsequently appeared in the plasma of the neonate. Gastric administration of 10.5 or 21.0 microgram B-1 rat prolactin significantly raised the level of prolactin in the plasma of 13-day-old pups, but a similar increase was not observed when 27-day-old rats were given 46.2 microgram B-1 prolactin by gastric intubation. The concentration of prolactin in the plasma of 13- to 14-day-old rats rose to 55 ng/ml 30 min after the onset of nursing by mothers whose mammary glands were full of milk, whereas the concentration in the plasma of offspring suckled by mothers with empty mammary glands remained at basal values. It is concluded that the intestine of the newborn rat is permeable to prolactin and that milk may constitute an exogenous source of prolactin for the suckled offspring.