Molecular heterogeneity of prolactin in lactating rats and their pups: biological and immunological activities in pituitary gland, serum and milk.
Prolactin (PRL)-like bioactivities (in Nb2 lymphoma assay), immunoreactivities (in RIA) and B/I ratios in rat milk, maternal and neonatal pituitary glands and sera were investigated. The PRL-like bioactivity in the water-soluble fraction of rat milk (infranatant prepared by ultracentrifugation) exceeded its immunoreactivity 3-7-fold. The elevated B/I ratio was in part due to the presence of a glycosalated PRL (G-rPRL)-like material, since 5-70% of the PRL-like bioactivity was recovered from the glycosylated fraction of rat milk infranatant prepared by concanavalin-A affinity chromatography. We were unable to detect PRL-like immunoreactivity in the glycosylated fraction of rat milk, and calculated that the maximal cross-reactivity of G-rPRL in the RIA is less than 3.8%. In day 12 milk, over 80% of the G-rPRL-like bioactivity eluted from a Sephadex G-100 column as a high apparent molecular weight (Mr) substance (approximately 50 kD), while the rest eluted as a monomeric G-rPRL (24-25 kD). The PRL-like bioactivity in the nonglycosylated fraction eluted in three peaks (Mr: 50, 24 and 16 kD), while two immunoreactive peaks occurred (Mr: 24 and 8 kD). The concentration of rPRL-like immunoreactivity in rat milk increased during the first days of lactation, remained high in midlactation, and declined by the end of lactation. The PRL-like bioactivity in the nonglycosylated fraction of rat milk displayed a similar timecourse. G-rPRL-like bioactivity in rat milk, however, changes inversely, i.e. decreased between days 2 and 18 postpartum then increased by day 22. The concentration of high Mr PRL-like bioactivity in rat milk was greatly reduced by day 22 from day 2 postpartum. No PRL-like bioactivity or immunoreactivity was recovered from the IgG fraction (prepared by protein A affinity chromatography) of rat milk. The B/I ratio in day 2 maternal pituitary glands was close to 1. In neonatal pituitaries and in maternal sera, however, the B/I ratio was slightly elevated (2-3). The B/I ratio in day 2 neonatal serum was between 6 and 22, while the B/I ratio of PRL secreted by day 2 neonatal pituitary glands in vitro was 1. The present results demonstrate that the concentrations of PRL in rat milk and neonatal serum have been grossly underestimated because levels were detected by RIA. The high B/I ratio reflects the presence of PRL variants. Milk appears to be the most likely source of PRL variants in the circulation of the neonate.(ABSTRACT TRUNCATED AT 400 WORDS)
Kacsóh
B
B
Department of Molecular and Cell Biology, Pennsylvania State University, University Park 16802.
Tóth
B E
BE
Avery
L M
LM
Yamamuro
Y
Y
Grosvenor
C E
CE
D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D002846:Chromatography, Affinity; D005260:Female; D007074:Immunoglobulin G / Q000032:analysis; D007774:Lactation / Q000378:metabolism*; D008223:Lymphoma / Q000378:metabolism / Q000473:pathology; D008892:Milk / Q000032:analysis* / Q000276:immunology; D009924:Organ Culture Techniques; D010903:Pituitary Gland, Anterior / Q000737:chemistry* / Q000276:immunology; D011388:Prolactin / Q000032:analysis* / Q000097:blood / Q000276:immunology; D011863:Radioimmunoassay; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors; D014407:Tumor Cells, Cultured / Q000378:metabolism / Q000473:pathology; D014461:Ultracentrifugation
D007074:Immunoglobulin G; D011388:Prolactin