Uptake of corticosterone into isolated rat liver cells: Possible involvement of Na/K-ATPase

Isolated rat hepatocytes posses a saturable glucocorticoid uptake system with high affinity (K_d value = 2.8 ± 0.7 × 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 × 10⁻⁷ and 3 × 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- and 5β-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.