Molecular heterogeneity and glycosylation modulation of rat pituitary prolactin isoforms synthesized and secreted in vitro in postnatal ontogeny, gestation, lactation and weaning.

The modulation of both the molecular size heterogeneity and the relative distribution of rat prolactin variants, synthesized and secreted in vitro by rat pituitary cells in the course of postnatal ontogeny and in gestation, lactation and weaning was investigated by SDS-PAGE, immunoblotting, radioimmunological techniques and O-sialoendopeptidase digestion. The outcome of the experiments is as follows: 1) from day 1 of postnatal life 20-, 23-, 26-, 40-44 kDa and oligomeric rat prolactin isoforms were stored and secreted; 2) perinatal life is characterized by a high degree of variability of prolactin size isoforms and their respective repartition in storage and release; in addition to the major variants, transient ones of M, 25-, 28-, 33- and 36 kDa were secreted and/or stored; 3) O-sialoglycoprotease digestion of pituitary cell lysate gave good evidence for 25 kDa prolactin being a glycoform; 4) at 1 month of age 16 kDa rat prolactin appeared and persisted over the whole postnatal span (1 day-->1 year) but only in stored form; 5) the physiology of gestation was essentially characterized by the M(r)-modulation of the glycoform (26 kDa-->26.3 kDa) and the virtual absence of stored 26 kDa rat prolactin at week 1 of pregnancy; 6) in lactation and weaning uncommon multiple banding was observed in secreted oligomeric prolactin; 7) in pregnancy, lactation and weaning the differential distribution of released and stored prolactin isoforms displayed a considerable intra- and intervariability; 8) in the vast array of size isoforms observed in all our experiments monomeric 23 kDa prolactin was always the dominating variant. In conclusion, the molecular size heterogeneity and the differential distribution of secreted and stored rat pituitary prolactin is considerably influenced by age and physiological stimuli. The nature of polymeric prolactin and of the transient variants is presently unclear, and the exact physiological role of molecular heterogeneity modulation is unknown, both in humans and rat, but the patterns of change we observed in definite stages of life, suggest that this phenomenon is important in the maturation of the hypothalamus-pituitary axis and in the metabolic and hormonal changes accompanying gestation.