Comparison of growth hormone binding and metabolic response in rat adipocytes of epididymal, subcutaneous, and retroperitoneal origin

We undertook a comparison of human growth hormone (hGH) binding and metabolic responses in rat adipocytes of epididymal, subcutaneous, and retroperitoneal origin to determine whether the site of fat depot biopsy might affect the response to hGH stimulation. The results showed highest specific binding in epididymal (3.6%), followed by subcutaneous (2.3%) and retroperitoneal adipocytes (1.5%); half-maximal binding was achieved at 14-18 ng/ml hGH for the three sites. Scatchard analysis of the binding data from each site was linear; there was no significant difference in binding affinities (2.1 to 3.3 x 10⁹, M^-1), but the number of binding sites was statistically higher in epididymal (9.8 x 10³) as compared to subcutaneous (7.5 x 10³, P < 0.05) and retroperitoneal cells (3.3 x 10³, P < 0.01). Stimulation with 5 to 2500 ng pituitary hGH produced a dose-related increase in glucose incorporation, with the largest increase in epididymal fat cells (31%, P < 0.05) followed by subcutaneous cells (18%, P < 0.05); no significant increase was seen with retroperitoneal cells. Biosynthetic hGH produced a similar pattern of glucose incorporation in the three sites. Addition of hGH antibodies blocked the glucose incorporation in epididymal adipocytes using both pituitary-derived and biosynthetic hGH. It seems clear that this insulin-like effect is caused by hGH, not an insulin-like impurity. We conclude that the number of binding sites, perhaps related to adipose cell size, differs in adipose tissue from different locations and this influences the metabolic response to hGH stimulation.