The release of corticosterone and a corticosterone-binding protein by incubated rat adrenal slices.

Stimulation of incubated rat adrenal slices with ACTH(1-24) resulted in an increase in the release of both corticosterone and specific corticosterone-binding protein into the incubation medium. The release of corticosterone and binding protein was dose and calcium dependent with adrenals from animals pretreated with betamethasone. While the secretion of corticosterone was continuous throughout the incubation period, there appeared to be a limit to the increase in binding capacity. The specificity of steroid binding to the adrenal protein showed a similar profile to that of corticosteroid-binding globulin (CBG) in rat serum. A Western blot analysis using anti-rat CBG as the primary antiserum, showed that the adrenal protein was not CBG. [3H]corticosterone binding with disc electrophoresis, run at 2 degrees C, gave a single peak with approximately the same Rf value for rat serum, purified CBG, and adrenal incubate; at 22 degrees C peaks were only seen for rat serum or purified CBG. The data presented provides further evidence for the existence of a specific corticosterone-binding protein of adrenal origin released in conjunction with corticosterone. The adrenal protein would appear to have a lower affinity for corticosterone than does CBG, and to be functionally more labile. It is possible that the adrenal protein may be CBG that has been internalized, modified and released with corticosterone.