Rat Anterior Pituitary

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Abstract
Studies on the feedback inhibition of ACTH release by steroid hormones and on the binding of tritiated steroids by the pituitary have prompted the hypothesis that receptors in addition to or other than classical glucocorticoid receptors may mediate steroid hormone effects in this tissue. Accordingly, we have asked whether more than one glucocorticoid-binding species, distinct from corticosteroid binding globulin, can be found in rat anterior pituitary gland. In our study we have demonstrated high affinity (Kd 4°C ∼ 1 nM) binding sites for tritiated corticosterone (3H-B) in rat pituitary cytosol, distinct from classical glucocorticoid receptors and transcortin-like sites. Unlike 3H-B-transcortin complexes, 3H-B bound to such sites is adsorbed onto hydroxylapatite and is stabilized by sulphydryl group reducing agents. Sucrose density gradient analysis in low ionic strength buffer under equilibrium conditions (3H-B±nonradioactive competitors throughout) showed 3H-B to sediment as a single, ∼8S peak, from which 3H-B was consistently better displaced by B than dexamethasone (DM); 3H-DM similarly bound to an ∼8S peak, from which it was better displaced by DM than B. The existence of two species of pituitary glucocorticoid receptors is further supported by clear differences in specificity for a range of steroids, and in the differential depletion of cytoplasmic sites after in vivo DM administration. Similar “B-preferring” sites were not found in thymus cytosols. These results demonstrate that there exist in the pituitary high affinity intracellular binding sites for naturally occurring glucocorticoids, distinct from classical glucocorticoid receptors and transcortin-like sites. Physiological roles as glucocorticoid receptors remain to be established for these B-preferring sites.
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