| Abstract: | Glucocorticoid uptake by AtT-20/D-1 mouse pituitary adenocarcinoma cells grown in tissue culture was examined. The binding of triamcinolone acetonide, a potent synthetic glucocorticoid, by intact cells and by cell cytosol was studied at both 4 and 25 degrees. Specific binding of [3H]triamcinolone acetonide by intact cells was markedly different from cell-free cytosol binding at 4 degrees. Intact cells bound a relatively small amount of labeled steroid within 2 min, after which no further binding was observed. In contrast, the receptor in a cell-free cytosol preparation was capable of binding steroid progressively at 4 degrees, indicating that the limited binding by intact cells was not a consequence of receptor characteristics. At 25 degrees, uptake by intact cells and cytosol was nearly identical and appeared to be limited only by the binding kinetics of the cytosol receptor. Estradiol-17 beta, a nonglucocorticoid steroid, was not bound by the AtT-20/D-1 cell at 4 degrees. Triamcinolone was not bound significantly at 4 or 25 degrees by an adrenal carcinoma cell that does not appear to be a glucocorticoid target cell. An Arrhenius plot of cell steroid uptake vs. the reciprocal of absolute temperature revealed an abrupt change in slope at 16 degrees, which is compatible with the temperature-dependent mechanism involved in glucocortidoid uptake being associated with lipid constituents of the cell membrane. These data suggest that glucocorticoid uptake by this target cell involves a mechanism of specific, temperature-dependent transport through the cell membrane. |
