Effects of ACTH on the ligand-binding properties of the glucocorticoid receptor exerted not only via elevated levels of corticosteroids

Treatment of mice with ACTH not only increased plasma corticosterone levels, but also reduced the ligand-binding capacity of the glucocorticoid receptor in skeletal muscle cytosol. Non-linear Scatchard plots were obtained for the glucocorticoid receptor following ACTH treatment. This upward concave curvilinearity could not be reproduced by simply adding to the cytosol an amount of corticosterone corresponding to the increase, even though this treatment resulted in reduced binding capacity. The addition of corticosterone resulted in a slower formation of ligand-receptor complexes (lower association rate constant), which offered a partial explanation for the low binding. In addition, ACTH treatment was also found to reduce the binding capacity in adrenalectomized mice that did not respond with corticosterone elevation. However, in this case Scatchard plots were linear.