CONCANAVALIN-A-BOUND AND CONCANAVALIN-A-UNBOUND PROLACTIN IN NORMAL AND HYPERPROLACTINEMIC RATS

Concanavalin-A (Con-A)-bound and -unbound forms of prolactin were studied in female Wistar-Furth rats, both normal and with hyperprolactinaemia induced by treatment with oestrogen or a prolactinoma graft. In normal rats, Con-A-bound prolactin was the major circulating form (more than 50%) and a minor pituitary component (less than 10%), essentially as 25 kDa prolactin. In oestrogen-treated rats, plasma prolactin levels were 100-fold higher and pituitary weight was fivefold higher than in the controls, but total pituitary prolactin content was unmodified. Under oestrogen, Con-A-bound prolactin represented about one-third of the total hormone levels in the plasma and less than 10% in the pituitary. In the pituitary, bound prolactin was found essentially as 25 kDa and unbound prolactin as 22, 30 and 40-45 kDa. A similar increase in plasma prolactin levels was induced 6 months after the graft of a prolactinoma. Pituitary weights and total pituitary prolactin contents were slightly decreased. Plasma and pituitary Con-A-bound prolactin levels were similar to those observed in oestrogen-treated rats. On the other hand, unbound prolactin was only present as a 22 kDa monomer. In the tumour, Con-A-bound prolactin (essentially as 25 kDa prolactin) represented one-third of the total hormone level and unbound prolactin was composed of the 22 and 45 kDa forms, this latter form being partially transformed into 22 kDa by heating. As Con-A-bound prolactin was previously characterized as glycosylated prolactin, these data suggest that glycosylated prolactin is present in the plasma of the normal rat as a major circulating form, unglycosylated 40-45 kDa is only present when the rate of the prolactin synthesis is high (in the pituitary of oestrogen-treated rats and in the tumour of grafted rats); this result supporting the hypothesis of a precursor product relationship between dimeric and monomeric prolactin. The manners in which prolactin is synthetized are not comparable in normal and tumour cells, percentages of glycosylated prolactin being different in normal pituitary and prolactinoma.