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

Abstract

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.