Presence of estradiol-binding proteins in gastrointestinal tract of male rats

In the cytosol from pancreas, fundus, and antrum of rats, two classes of specific, estradiol (E₂)-binding proteins (EBPs) were observed, with 5- to 10-fold differences in their binding affinities, and 10- to 20-fold differences in their binding capacities. EBPs, both type I [high affinity, dissociation constant (K_d) < 1.0 nm] and type II (low affinity, K_d > 5 nm), in the fundus were found to be specific for estrogens. None of the corticoids, or androgenic and progestogenic steroids tested demonstrated any significant inhibition of binding of radiolabeled E₂ to EBP at doses as high as 5 × 10⁻⁶ to 5 × 10⁻⁵ M. At high concentrations of E₂ (>2–5 nm), nonsaturable EBPs were additionally observed in the pancreatic cytosol. In the intestine, however, EBP was not detected at E₂ concentrations of less than 2–5 nm. Similarly, progesterone-binding proteins were nonmeasurable in either pancreas or gastrointestinal tissues at less than 5 nm concentrations of the ligand, indicating questionable physiological significance of the latter binding sites. The two classes of EBPs in the pancreas and stomach of rats, measured at near physiological E2 concentrations of less than 2 nm, however, may play a significant role in gastric and pancreatic functions, since sex steroids have been documented to have specific effects on structure and function pf pancreas and stomach.