The permeability of the human red cell membrane to steroid sex hormones.

The release rates of the 3H-labeled steroid sex hormones estrone, estradiol, estriol, progesterone, and testosterone from the human red blood cell and resealed red cell ghosts were studied at 38 degrees C and pH 7.2 by means of the rapid continuous flow tube method which has a time resolution of a few milliseconds. Further, the equilibria between unbound hormone and hormone bound to red cells, resealed red cell ghosts and albumin were studied by partitioning analysis of trace amounts of labeled hormones. The half-times for release from erythrocytes under physiological conditions ranged from 4 ms (testosterone) to 150 ms (estriol). The release from ghosts was significantly faster than from cells preincubated with hormones at unphysiological high concentrations. Affinities of hormone binding to cells and hormone indicate that as much as 15-35% of the total hormone content in whole blood is confined to red cells. The ratio between bound and free hormone in the cell ranged from 5 to 10, and the ratio between cytoplasma-bound and membrane-bound hormone ranged between 3 and 9. The results are compatible with a model of fast transition of hormone through the red cell membrane and intracellular binding of hormone. We suggest that red cells function as carriers of sex hormones in the bloodstream in a manner similar to that of albumin, and that red cells may be responsible for 5-15% of sex hormone delivery to target tissues.