Acute down-regulation of the somatogenic receptors in rat liver by a single injection of growth hormone.

Prolonged continuous administration of GH induces somatogenic receptors in rat liver. However, because GH secretion is pulsatile and the effect of acute changes in serum GH concentrations on liver GH receptors is unknown, we measured total (MgCl2-treated homogenates) and free (water-treated homogenates) GH-binding sites in the livers of hypophysectomized (hypox) rats killed between 1 and 24 h after a single sc injection of rat GH (100 micrograms/100 g BW; n = 29). Control hypox rats (n = 10) were studied immediately or 3 h after injection of vehicle. GH injection caused profound decreases in both total and free liver GH receptors, but these changes followed different kinetic patterns. Free receptors declined rapidly (to 17% of control), reaching a nadir at the same time (1 h) as the maximal GH concentration in serum. These free receptors then increased, returning to normal 12 h after GH injection. In contrast, total GH receptors were slightly increased at 1 h, decreased to their minimal value at 6 h (53% of control), and returned to normal at 12 h. Serum immunoreactive somatomedin-C/insulin-like growth factor I concentrations peaked 12 h after GH injection. Total and free liver GH receptors were quantitated in hypox rats that had been injected 3 h previously with doses of rat GH from 2.5-500 micrograms/100 g BW or with vehicle. Both total and free binding sites decreased in a dose-dependent manner; the maximal responses were 40% and 90% below control values, respectively. Half-maximal reductions in GH binding were achieved when 10 micrograms GH/100 g BW were given. These data suggest that a surge of GH in serum leads to a time- and dose-dependent down-regulation of the liver somatogenic binding sites and are consistent with ligand-induced internalization and degradation of the receptor.