Bioactive and immunoreactive variants of prolactin in milk and serum of lactating rats and their pups.

Prolactin (PRL)-like bioactivity (in Nb2 lymphoma assay) and immunoreactivity (in radioimmunoassay (RIA)) in rat milk, maternal and neonatal sera and in neonatal rat pituitary cultures were investigated. The PRL-like bioactivity in the water-soluble fraction of rat milk was high and exceeded its immunoreactivity 5.8-, 4.0- and 2.1-fold, on days 2, 12 and 22 of lactation respectively. The elevated bioactivity to immunoreactivity (B/I) ratio of PRL in milk was not due to the presence of interleukin-2 (IL-2) in milk, since the proliferation of the CTLL-2 murine T cells, which are not sensitive to PRL, was promoted by IL-2 but not by milk. Serum levels of immunoreactive PRL were low in sera of non-weaned rat pups on days 2, 12 and 22 postpartum. Similar to milk, the B/I ratio of PRL in sera of rat pups was high and decreased with time postpartum. Pituitary glands of pups obtained on days 2, 12 and 22 secreted progressively increasing amounts of PRL in vitro; the B/I ratio ranged between 1.2 and 2.1 without a significant change. The relative concentrations of size variants in milk were not proportional to those in serum of lactating rats on day 2 postpartum as assessed by Sepharcryl S-100 HR gel permeation chromatography and Nb2 bioassay or RIA. Size variants of biologically active PRL were abundant in early milk and gradually diminished as lactation progressed: a partially resolved peak representing monomeric to dimeric PRL variants (relative molecular weights ranging between 18 k and 42 k) became progressively narrower between days 2 and 22. Biologically active and immunoreactive PRLs displayed disparate elution profiles. The elution profile of PRL in sera of neonatal rats on day 2 post-partum was different from that of maternal serum or milk. The major immunological (and possibly biological) PRL-like activity eluted as two adjacent peaks at 2.2 k and 1.5 k, raising the possibility that fragments of milk-borne PRL were absorbed from the gut after partial proteolytic degradation. In contrast with PRL, GH (which is present in rat milk only in minute concentrations) did not show heterogeneity in sera of 2-day-old rat pups in gel permeation chromatography. The present results demonstrate that the concentrations of PRL-like activity in rat milk and newborn rat serum have been grossly underestimated because levels have been measured by RIA. The high B/I ratio of PRL in milk and neonatal sera is due to the presence of PRL-related compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Rat milk stimulates pituitary growth hormone secretion of neonatal pituitaries in vitro.

Aqueous extracts of rat milk stimulated growth hormone (GH) secretion from superfused pituitaries of two-day old rats. The GH stimulatory effect of milk increased with the time elapsed postpartum; growth hormone releasing hormone and thyrotropin releasing hormone seem to be the major milk borne GH releasing factors. These results indicate that milk intake may play a role in maintaining the high plasma GH levels observed in the neonatal period.

Data suggesting that milk of early lactation period might be involved in sexual differentiation of rat brain.

The contribution of a hypothetic milk factor in the masculinization process of gonadotropin secretion pattern was investigated using a cross-fostering model. Adult female rats whose nipples had been previously excised were mated. At the time of delivery their pups were given to recipient dams that had given birth one week earlier. Pups remaining with their own (intact) mother served as control group. At the age of 37-39 days (birth = day 0) male rats from the experimental and the control groups were castrated and also control females were ovariectomized. Ten days later gonadectomized animals received ovarian grafts excised from 20-day-old rats. Four and seven days after transplantation the grafts were processed for histology. Corpus luteum formation suggests that male rats nursed by recipient dams did not undergo the masculinization process normally occurring during the first few days of postnatal life. In a separate experiment, male pups nursed by dams being at the early lactation (control) or at the midlactation period were decapitated on postpartum day 1 and serum testosterone levels were measured by RIA. Mean testosterone concentration was almost twice as high in the control group than in pups nursed by recipient dams of the midlactation period. These data suggest that milk of the early lactation period might be necessary for the normal masculinization process of the male rat.