Properties of thyrotropin (TSH) binding to plasma membranes prepared from rat fat tissues

Rat fat membranes were prepared from male Sprague-Dawley rats. At 37 degrees C, TSH binding to rat fat membranes was rapid and unstable, although the binding reached a steady state after 2 hr and unchanged up to 24 hr at 4 degrees C. The binding to rat fat membranes was significantly inhibited by 50 mM NaCl and almost completely inhibited by 150 mM NaCl. TSH binding to rat fat membranes was not affected by 10 mM dithiothreitol (DTT) or 1 mM diamide although the binding to human thyroid membranes was inhibited by 10 mM DTT significantly. Conventional Scatchard analysis revealed a single class of binding site which had lower Ka value (2.6 X 10(8) M-1) than that of high affinity binding site of human thyroid membranes (9.3 X 10(8) M-1). Immunoglobulin G (IgG) from patients with Graves' disease inhibited the binding of TSH to rat fat membranes. A significant correlation was observed between the inhibiting activity of Graves' IgG measured with rat fat and human thyroid membranes (r = 0.82, p less than 0.01). In conclusion, TSH receptors on rat fat membranes were not identical to those on human thyroid membranes, but TSH receptor antibodies crossreacted with TSH receptors in rat fat tissue.