Fate of Injected Human Growth Hormone in the Female Rat Liver in Vivo*

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Abstract
[125I]Iodohuman GH ([125I]iodo-hGH) was injected iv to female rats, and its subcellular distribution was studied with time using fractionation techniques. Uptake in the total particulate fraction was maximal by 15 min after injection; at that time, 1% of the radioactivity injected was recovered per g liver. Liver uptake of [125I]iodo-hGH was markedly inhibited by coinjection of native hGH. [125I]Iodo-hGH taken up by the liver underwent a time-dependent translocation process. In the first 5 min, the radioactivity was associated with membranes of high density sedimenting in the nuclear and microsomal fractions. Later on, it was progressively associated with microsomal subtractions of lower density, the Golgi fractions; labeling of these was maximum at 15 min. Fifteen and 30 min after injection, labeled material was recovered in the mitochondrial-lysosomal fraction. Further fractionation of the latter by centrifugation on a metrizamide gradient showed that all of the radioactivity was associated with lysosomal subtractions, with virtually no radioactivity associated with mitochondria. On linear sucrose density gradients, the radioactivity exhibited a broad, somewhat bimodal distribution; the component of highest density coincided with the lysosomal enzyme acid phosphatase. Triton WR 1339 treatment of rats resulted in a shift of the radioactivity and acid phosphatase toward lower densities, indicating that a high proportion of the former was associated with lysosomes. The labeled material eluted from the subcellular fractions prepared at different times appeared as intact hGH, as judged by trichloroacetic acid precipitability and binding to membranes. Upon in vivo interaction with liver cells, [125I]iodo-hGH is internalized, with a sequential association with plasma membranes, Golgi elements, and lysosomes.