Rapid and opposite effects of cortisol and estradiol on human erythrocyte Na+,K+-ATPase activity:: Relationship to steroid intercalation into the cell membrane

We determined whether two naturally occurring steroids, cortisol and 17 beta-estradiol (E-2), can rapidly modulate the activity of an important membrane protein, human erythrocyte (RBC) Na+,K+-ATPase, an enzyme that does not bind either hormone directly. We also determined the membrane binding locations for cortisol and E-2 and their effects on membrane molecular structure and fluidity. Direct application of both steroids to intact human RBC significantly altered maximum ouabain-sensitive Rb-86 uptake within 5 min: Cortisol decreased it by 24% whereas E-2 increased it by 18%. As determined by small angle x-ray diffraction, these steroids occupied distinct time-averaged binding locations in the RBC membrane, cortisol localizing near the bilayer surface, 14-29 Angstrom from the bilayer center, and E-2 localizing deep within the hydrocarbon core, 0-7 Angstrom from the bilayer center. Neither steroid significantly changed overall bilayer width or membrane fluidity. These data suggest that cell membrane protein function can be altered rapidly and differentially by naturally occurring steroids. This effect did not appear to be related to the different binding locations of the steroids in the membrane or to their influence on membrane fluidity.