Sex hormone-binding globulin (SHBG) is thought to mainly function as a transporter and reservoir for the estradiol and testosterone sex hormones. However it has also been demonstrated that SHBG can bind to a cell surface receptor (SHBG-R). The SHBG-R has not been completely characterized. A subset of steroids are able to bind to the SHBG/SHBG-R complex resulting in an activation of adenylyl cyclase and synthesis of the cAMP second messenger.  Hence the SHBG/SHBG-R complex appears to act as a transmembrane steroid receptor that is capable of transmitting signals to the interior of cells.
The potency of 17 derivatives of 16 alpha-gitoxin was tested in the isolated atrium and heart of the guinea pig and the contractility-increasing activity of the 16 alpha-gitoxin 16 alpha-acetate was compared with that of 16 alpha-gitoxin in the anesthetized dog. The potency of 16 alpha-gitoxin was increased by the substitution of 16 alpha-OH for 16 alpha-methyl ether, 16 alpha-acetate and 16 alpha-nitrate. Substitution of the 16 alpha-acetyl group for substituents with a higher molar volume diminished this enhanced potency. Variation in the digitoxose moiety caused an increase or decrease in potency depending on the position and number of the substituted OH groups. In spite of changes in 16 alpha-OH, the low influence on rhythmicity persisted, as was found in experiments in the dog.