The Levels of Corticosterone-Binding Proteins in Rat Milk and Coincidental Serum, and the Dissociation Rates of the Corticosterone • Protein Complexes*

Publisher Website
Google Scholar
Abstract
The corticosterone-binding protein present in rat whey was further characterized by determining, with the aid of a dextran-coated charcoal procedure, the apparent rate of dissociation of the corticosterone • protein complex. The half-time values for the dissociation of the corticosterone • protein complexes in rat whey and serum were compared and found to be identical, i.e. 23 min at 0 C, when the measurements were made over a period of 40 min. The possible presence in small amounts of a corticosterone-protein complex in whey with the much slower dissociation rate characteristic of mammary glucocorticoid receptor could not be detected even when the dissociation was followed over a much longer period. The charcoal adsorption method also provided independent estimates of the molar concentrations of the corticosterone-binding proteins in rat serum and whey. The mean concentration of corticosterone-binding protein in whey was found to be 15% of that in coincidental serum during early lactation. The serum levels of corticosteronebinding protein decline markedly at parturition and then rise from day 2 to day 6 of lactation in rats with small litters. The results of this and a previous study suggest that the corticosterone-binding protein in whey is probably derived from that in serum. The mode of transport of the corticosterone-binding protein from the bloodstream across the mammary epithelium into milk as well as the concentrations of the corticosteronebinding proteins in serum and whey may be factors influencing the uptake of the glucocorticoid by its target cells. (Endocrinology 108: 741, 1981)