GLUCOCORTICOID BINDING TO PLASMA MEMBRANES OF THE ADENOHYPOPHYSIS

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Abstract
Using isolated cells and plasma membrane fractions, indirect evidence suggests attachment of the transcortin-like material to the cellular membrane of [rat] pituitary cells. Cytosol derived from pituitary cells exhibited a transcortin-like component, whose apparent Kd was similar to the value (4.9 nmol/l) obtained using cytosol from whole glands. The concentration of binding sites was reduced in that case, probably as a result of the trypsin treatment of the pituitary glands. The transcortin-like material is probably not just confined in extracellular spaces, but is intrinsic to the cell and located within the cell and/or attached to the plasma membrane. To investigate the latter hypothesis, binding properties of the plasma membranes were examined and competition studies were carried out using natural and synthetic labeled glucocorticoids, and various unlabeled steroids. Non-radioactive corticosterone and progesterone competed to a significantly greater extent (P < 0.01, Duncan''s multiple range test) than dexamethasone for corticosterone-binding sites. Dexamethasone, which does not bind to transcortin and to the transcortin-like component, displaced radioactive corticosterone significantly (P < 0.01). This finding might be due to non-specific uptake of [3H] dexamethasone, or possibly [in addition to the transcortin-like compound], the true receptor (which binds both glucocorticoids) partly adheres to the cellular membrane.

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