Studies on the Uptake of Human Chorionic Gonadotropin and its Subunits by Rat Testicular Homogenates and Interstitial Tissue1

Abstract

The binding of human chorionic gonadotropin (hCG) and its α– and β–subunits, by rat testicular homogenates (RTH) and rat testicular interstitial tissue (RTIT) preparations was investigated by direct uptake studies, using fractions labeled with m I by the chloramine—T procedure, as well as by uptake inhibition studies. We found that the ratio (w/w) of chloramine—T to protein as well as length of exposure to chloramine— T before termination of the reaction with sodium metabisulfite, were critically important in determining the binding properties of the labeled hormone or subunit. The direct uptake of suitably labeled hCG could be inhibited by preincubation of RTH or RTIT with cold hCG, human LH or bovine LH. Inhibition was also noted following preincubation of RTH with the α– or β–subunits of hCG, human TSH or human FSH. This could be explained, however, on the basis of contamination of these preparations with intact hCG or LH. The uptake of labeled hCG by RTH or RTIT was considerably greater than that found for liver, kidney and heart muscle homogenates. In the latter instances, uptake was not inhibited by preincubation with cold hormone, suggesting a nonspecific binding phenomenon probably related to hormone damage during iodination. The uptake of iodinated hCG—subunits by RTH or RTIT was either low or could not be inhibited by preincubation with cold subunits or hCG. The dissociation constant (Kd) of the hCG—RTH receptor was estimated on the basis of 4 separate analyses to be 3.05 ± 0.4 X 1 0 -10M. RTH and RTIT appear to be equally suitable as systems amenable to study of hCG and hCG—subunit binding phenomena. (Endocrinology91: 135, 1972