Size and Charge Heterogeneity of Pituitary and Plasma Prolactin in the Male Rat

Abstract

Although pituitary prolactin (PRL) exhibits size and charge variability, questions concerning the structural heterogeneity and biological potency of hormonal forms secreted in vivo remain. In the present studies, monomeric PRL in male rat pituitaries and plasma was fractionated by Sephacryl S-100 size exclusion chromatography and aqueous chromatofocusing to resolve size and charge forms under conditions compatible with optimum preservation of biological activity. Individual hormonal variants were subsequently evaluated for their ability to stimulate the growth of PRL receptor-bearing rat lymphoma cells in vitro. Pituitary elution profiles contained several cross-reactive size variants ranging from 30.4 to 21.5 kD in M_r; major hormonal peaks were eluted at 25.6, 24.3, and 23.6 kD. Multiple size forms of PRL were also detected in plasma profiles, with predominant peaks eluting between 26.5 and 21.5 kD M_Γ in size. Mean B/I ratios, established as an index of relative biopotency, varied significantly between size variants obtained from pituitary and plasma. Pituitary PRL size variants of 27.1 24.3, and 21.5 kD exhibited greatest potency in the in vitro bioassay, whereas the 25.6- and 23.6-kD forms were least potent under these conditions. Of the PRL size variants detected in peripheral plasma, those of 24.3 and 21.5 kD size were characterized by highest mean B/I ratios. Pituitary 24.3-kD PRL was chromatofocused as five charge variants of pI 5.34, 5.31, 5.26, 5.20, and 5.14; only some of these isomers are apparently secreted in vivo, since pI values for plasma charge isomers ranged from 5.26 to 5.14. Charge isomers of pituitary 24.3-kD PRL exerted variable mitogenicity in the Nb₂ in vitro bioassay. The highest mean B/I ratio was associated with the relatively basic 24.3-kD isomer of pI 5.31; the relative biopotency of more acidic charge isomers was progressively diminished with increasing acidic charge. In summary, the present findings show that both size and charge variants of male rat pituitary PRL exhibit differential biopotency in vitro, indicative of their functional heterogeneity. The demonstration of multiple hormonal forms in peripheral plasma suggests that the net biopotency of circulating PRL reflects the sum of activity of structurally and functionally diverse molecules.

Prolactin (PRL) induction of cyclooxygenase 2 (COX2) expression and prostaglandin (PG) production in hamster Leydig cells

“…It is known that carbohydrate side-chains and sialylation are essential for biological activity. Although several reports indicate that more acidic/sialylated variants of the pituitary hormones exhibit a lower in vitro biological potency than their less acidic counterparts (Cerpa-Poljak et al, 1993;Briski et al, 1996;Zambrano et al 1996), other authors point out opposite effects (Matzuk et al, 1990;Fares et al, 1992;Lambert et al, 1998). Such discrepancies might be explained by variations in the amount of agonist used, in the density of receptors and/or in the receptor isoforms expressed.…”

Growth Hormone Size Variants: Changes in the Pituitary During Development of the Chicken

“…Analogous shifts in both glycosylated and nonglycosylated forms of both PRL and GH in plasma and pituitary have been observed during development/growth and other physiological states (e.g., pregnancy and lactation) in pig (45, 46). In view of the evidence of difference in biological activities for variants of GH and PRL (5–7, 28, 35, 45–48), it is a distinct possibility that GH and PRL actually each represent a spectrum of hormonal activities.…”