Specific Binding of Growth Hormone by Rat Adipocytes*

Abstract

Human GH (hGH) binding by isolated epididymal fat cells was studied in nonhypophysectomized Wistar rats. Total binding of [¹²⁵I]hGH was temperature and time dependent; nonspecific binding was maximal within 30 min of incubation at both 24 and 37 C. Equilibrium binding of [¹²⁵I]hGH by small adipocytes (mean volume, 40–70 pl) from young lean rats (150– 220 g BW) at 24 C was reached at 90 min and remained stable for at least 3 h. At 2 h, nonspecific binding was 30% of the total radioactivity bound. Specific binding was demonstrable and directly related (r = 0.998; P < 0.001) to the number of cells present in the flasks from 0.2 to 2.4 × 10⁶ fat cells/ml. The amounts of hGH specifically bound to small fat cells increased as the hGH concentration in the medium increased from 10 to 100 ng/ml, and remained at a constant level with further increments in medium hGH concentration. Several nonprimate GH preparations suppressed [¹²⁵I]hGH binding to adipocytes, but did so to a lesser extent than biologically equivalent amounts of hGH (hGH, 100%; bovine GH, 75%; ovine GH, 74%, rat GH, 73%; porcine GH, 57%). Non-GH peptide hormones (3 μg/ml) did not suppress the specific binding of [¹²⁵I]hGH to the fat cells. Large fat cells (400–500 pl) isolated from older, fatter rats (560–610 g BW) specifically bound one third to one half as much hGH at each concentration (from 10–234 ng/ml) as did an equal number of fat cells from the small rats. Scatchard analysis demonstrated that there was a single class of hGH-specific receptors in the fat cells. In small fat cells, the apparent K_a was 1.2 × 10⁹ M^-1 and the number of binding sites was 25,400/cell. In large fat cells, the K_a was 1.9 × 10⁸ M^-1 and the number of sites was 30,300/cell. Thus, fat cells isolated from nonhypophysectomized rats bind purified hGH in a specific, saturable, and temperature-, time-, and dose-related manner; the binding occurs at physiological GH concentrations. The ability of fat cells to bind hGH concurs with reported metabolic effects of GH on adipose tissue and isolated adipocytes, and parallels the previously observed reduced metabolic responsiveness of the large cells to the hGH effect on glucose metabolism.