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...