Uptake of cortisol by isolated rat liver cells. A phenomenon indicative of carrier-mediation and simple diffusion.

The uptake of cortisol by isolated rat liver cells was studied. Cortisol was taken up rapidly; the uptake increased with increasing temperature and reached a plateau after 45 s at 37 degrees, C, after 60 s at 27 degrees C, and after 90 s at 22 degrees C; at 5 degrees C the uptake increased linearly with time. The uptake was linear up to 1.5 mg of cell protein. Analysis of uptake as a function of increasing concentration of cortisol in the external medium indicated the presence of two saturable systems: a high-affinity system with an apparent Km value of 190 +/- 25 nM and a low-affinity system with an apparent Km value of 2200 +/- 180 nM. Above 600 nM, the rate of uptake of cortisol increased almost linearly with increasing cortisol concentration. Treatment of cells with KCN or 2,4-dinitrophenol inhibited the two saturable components, leaving the nonsaturable system unaffected. The affinity constants, Ka, were 6 X 10(6) M-1 and 0.6 X 10(6) M-1 for the high and low affinity components, respectively. These values increased approximately two-fold when uptake rates were corrected for diffusion. Cortisone and corticosterone inhibited the uptake of cortisol by liver cells competitively; dexamethasone inhibited cortisol uptake noncompetitively. Similarly, oestrone, oestradiol and testosterone decreased the uptake of cortisol, at a concentration of 2000 nM in the external medium, by 20, 49 and 35 percent, respectively; the inhibition was noncompetitive. p-Chloromercuribenzoate, N-ethylmaleimide and 1-fluoro-2,4-dinitrobenzene decreased the uptake of cortisol. Ouabain did not influence the uptake of cortisol; varying the external sodium concentration also did not affect uptake of cortisol. Cyclic-3',5'-adenosine monophosphate had a stimulatory effect. The results show that the first step, before cortisol is bound to intracellular binding proteins, is the uptake of cortisol by proteins in the plasma membrane. At lower concentrations of cortisol, uptake takes place by saturable processes; at higher concentrations saturation is not achieved, indicating that simple diffusion becomes the major route of transport into the cell. The proteins in the plasma membrane probably function as carriers to transport the glucocorticoid into the cell.