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 immunorea ctivity (in radioimmunoassay (RIA)) in rat milk, maternal and neonatal sera and in neonatal rat pituitary cultures were investigated. The PR L-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 immun oreactivity (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 mu rine 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 n on-weaned rat pups on days 2, 12 and 22 postpartum. Similar to milk, t he B/I ratio of PRL in sera of rat pups was high and decreased with ti me postpartum. Pituitary glands of pups obtained on days 2, 12 and 22 secreted progressively increasing amounts of PRL in vitro; the B/I rat io ranged between 1.2 and 2.1 without a significant change. The relati ve concentrations of size variants in milk were not proportional to th ose in serum of lactating rats on day 2 postpartum as assessed by Seph arcryl 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 molecul ar weights ranging between 18 k and 42 k) became progressively narrowe r between days 2 and 22. Biologically active and immunoreactive PRLs d isplayed disparate elution profiles. The elution profile of PRL in ser a of neonatal rats on day 2 postpartum was different from that of mate rnal 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, rai sing the possibility that fragments of milk-borne PRL were absorbed fr om 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 concentratio ns of PRL-like activity in rat milk and newborn rat serum have been gr ossly underestimated because levels have been measured by RIA. The hig h B/I ratio of PRL in milk and neonatal sera is due to the presence of PRL-related compounds. The difference between the ontogeny in the B/I ratio of serum and in-vitro secreted PRL might be related to absorpti on of PRL variants from milk during the early postpartum period. The d ata suggest that PRL might be modified by the mammary gland and the ne onatal gut during its passage from the circulation of the mother to. t hat of the neonatal rat.