The transport of steroids into animal cells in culture

The movement of progesterone, glucocorticoids, free and conjugated estrogens, and androgens into hamster fibroblasts (NIL 8) and rat hepatoma cells (HTC) grown in monolayer has been investigated. The experiments have ascertained that the movement of free steroids into the cultured cells occurs by simple diffusion, since the rate of uptake is unsaturable and there is no competition between steroids for entry into the cells. Furthermore, the velocity of passage through the plasma membrane (permeability coefficient) decreases from the least to the most polar steroids and parallels the solubility of the compounds in organic solvents (partition coefficient). Indeed, the variations of the permeability coefficients for both free steroids and representative nonsteroidal compounds (urea, thiourea, glycerol, and antipyrine) closely resemble the variations in the coefficients of partition in n-octanol. On the assumption that noctanol models the solubility properties of the membrane lipids, we have used the pa tition coefficients in this solvent of free steroids and of the four nonsteroidal compounds to estimate diffusion coefficients in the membrane. Thus, the diffusion coefficients are inversely proportional to a high power of the molecular weight of the permeants, as would be anticipated if passage of the permeants across the plasma membrane was taking place by simple diffusion through the lipid bilayer. Uptake of conjugated steroids, on the other hand, does not conform to this mode of movement by simple diffusion. By comparing the velocity of passage through the plasma membrane and the half-time of equilibration of the steroids with hamster fibroblasts and hepatoma cells, we have shown that movement across the membrane does not limit the rate of interaction between steroids and either specific receptors or other cellular components that retain the hormones. Velocity of passage of all of the free steroids is indeed so fast (of the order of 10^-4 cm sec^-1) that it seems unlikely that differences in permeability to any one hormone between two cell lines may be of importance in determining differential cellular responses. The physiological implications of the above findings are briefly discussed.