The levels of corticosterone-binding proteins in rat milk and coincidental serum, and the dissociation rates of the corticosterone.protein complexes.

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 corticosterone-binding 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 corticosterone-binding proteins in serum and whey may be factors influencing the uptake of the glucocorticoid by its target cells.