In-vivo uptake of human growth hormone in male rat liver.

125I-Labelled human GH (hGH) was injected i.v. to male rats and its subcellular distribution in the hepatocyte was examined using fractionation techniques. Uptake into liver homogenates was maximal by 15 min after injection and represented 24% of the injected radioactivity; it was markedly inhibited by coinjection of native hGH. 125I-Labelled hGH taken up by the liver underwent a time-dependent translocation process. The peak of specific labelling of plasma membranes occurred at 3 min whereas later on the radioactivity was concentrated in low-density structures present in Golgi-endosome fractions. To characterize the ligand-associated structures better, endosome-enriched fractions were prepared from a microsomal fraction by isopycnic centrifugation in a sucrose gradient and a Nycodenz gradient. The radioactivity was in one peak with a median density of 1.096 g/cm3 in the Nycodenz gradient fractions. The peak of radioactivity was distinct from that of galactosyltransferase activity which appeared at a median density of 1.114 g/cm3. The labelled material eluted from the various subcellular fractions appeared as intact hGH. Upon in-vivo interaction with male rat hepatocytes, 125I-Labelled hGH was internalized with a sequential association with plasma membranes and endocytic structures distinct from Golgi elements.

[Regulation of receptors and mechanism of action of growth hormone]. / Régulation des récepteurs et mécanismes d'action de l'hormone de croissance.

The initial step in the action of growth hormone (GH) is interaction with specific sites localized in target cell membranes. Growth hormone receptors have been demonstrated in different tissues and have been extensively studied in the liver. 125I-human growth hormone injected IV in the intact male rat is internalized in the hepatocytes by a receptor-mediated process; the radioactive material is sequentially associated with plasma membranes, endocytic structures and lysosomes. A dramatic decrease in the number of hepatic growth hormone receptors has been demonstrated in several situations of growth defect in the rat. GH, insulin and estrogens play a role in the regulation of hepatic growth hormone receptors, but their mechanism of action is not clear. GH receptors have been partially purified; the binding subunit has an approximate MW of 110,000 in rat hepatocytes and adipocytes and in IM-9 lymphocytes, a MV of 50 to 70,000 in rabbit liver. Monoclonal antibodies raised against the GH receptor will facilitate studies of receptor structure and function. The postreceptor events in growth hormone action are unknown. No second messenger or mediator has been demonstrated. Elucidation of the early events that follow activation of the growth hormone receptor is a priority.

The Brattleboro rat: normal growth hormone secretion, decreased hepatic growth hormone receptors and low plasma somatomedin activity.

Three-month-old male Brattleboro rats with hereditary diabetes insipidus (DI) present a growth defect; Brattleboro rats were studied together with age-matched Long-Evans (LE) rats. Pituitary growth hormone (GH) content was comparable in both groups of rats. Pulsatile GH release and mean 6 h GH plasma levels did not appear significantly different in chronically catheterized DI and control animals. In parallel with the growth defect, the plasma somatomedin bioactivity was significantly lower in DI than in LE rats. The specific binding of [125I]iodo-hGH to liver microsomal membranes of DI rats was 59.7% that of controls. The number of the GH binding sites rather than the affinity of the binding was decreased. The specific binding of [125I]iodo-insulin was oppositely affected by the DI state: it was 1.5 times higher in liver membranes of DI rats than in membranes of LE rats. These findings make a non-specific effect of the DI state on liver membrane proteins unlikely. The Brattleboro rats present a growth failure without reduction of their GH secretion. The decreased number of the hepatic GH receptors and the subsequent low plasma somatomedin activity could explain the growth retardation of the DI rats.