ABSTRACT
Isolated rat hepatocytes possess a saturable glucocorticoid uptake system with high affinity (Kd value = 2.8 +/- 0.7 x 10(-8) M; 318,000 +/- 80,000 binding sites per cell; 317 fmol/mg protein). The initial rates of uptake decrease by about 30-40% if the cells are incubated simultaneously with [3H]corticosterone and either SH-reagents (N-ethylmaleimide and p-chloromercuriphenylsulphonate, 1 mM), metabolic inhibitors (2,4-dinitrophenol, 1 mM; and antimycin, 0.1 mM) or the Na+/K(+)-ATPase-inhibitors, ouabain and quercetine. These Na+/K(+)-ATPase-blockers exert half-maximal inhibition at 3 x 10(-7) and 3 x 10(-6) M, respectively. A slight increase in K+ concentration and a corresponding decrease in Na+ in the medium leads to a significant reduction in the initial uptake rate. The uptake system from the rat hepatocytes shows a clear steroid specificity, being different from the intracellular receptor. Corticosterone and progesterone are the strongest competitors, cortisol, 5 alpha- and 5 beta-dihydrocorticosterone, 11-deoxycorticosterone, cortisone and testosterone have an intermediate effect and only weak competition is exerted by dexamethasone and by the mineralocorticoid, aldosterone. Estradiol and estrone sulphate as well as the synthetic glucocorticoid triamcinolone acetonide are unable to inhibit initial corticosterone uptake.