ABSTRACT
Specific binding of 125I-labelled human somatotropin was demonstrated in isolated hepatocytes from male mice. In the presence of divalent cations (Ca2+ and Mg2+) the binding of 125I-labelled human somatotropin was competitive with ovine prolactin. Scatchard analysis of competition data indicated a KD of 1.4 +/- 0.2 nM and a binding capacity of 13 000 +/- 2000 sites/cell. In the absence of divalent cations and in the presence of EDTA, human and bovine somatotropins were found to be equally effective to displace bound 125I-labelled human somatotropin, while ovine prolactin showed a weak competition. In this case, the binding capacity was 8400 +/- 1500 sites/cell and the KD was 1.1 +/- 0.1 nM.
Subject(s)
Cations, Divalent/pharmacology , Liver/metabolism , Receptors, Cell Surface/drug effects , Animals , Binding Sites , Binding, Competitive , Edetic Acid/pharmacology , Growth Hormone/metabolism , Liver/cytology , Liver/drug effects , Male , Mice , Receptors, Prolactin , Receptors, SomatotropinABSTRACT
The distribution of human growth hormone labelled with 125I (125I-hGH) was studied in normal adult female and male mice. The radioactivity was basically concentrated by the liver and kidney reading a maximum 15 minutes after the labelled hormone injection. Only the liver showed a significant reduction of radioactivity uptake when 125I-hGH was injected together with an excess of unlabelled hormone. This reduction was dose-dependent and the amount of unlabelled hormone that prevented 50% of the liver uptake (ED50) of 125I-hGH was close to 3 micrograms for both female and male mice. Similar results were obtained in studies where bovine growth hormone labelled with 125I (125I-bGH) was injected, except that the maximum uptake value was significantly lower than that observed when 125I-hGH was used. This observation could be attributed to the difference that exists between the biological properties of both hormones since hGH has growth-promoting and lactogenic effects in rodents, whereas bGH exhibits exclusively somatotropic activity. In order to examine the nature of the radioactive material which localized in the liver soluble extracts were prepared using Triton X-100 and analyzed on Sepharose CL-6B. The majority of the radioactivity appeared as an homogeneous peak with KD = 0.31 which could be attributed to a molecular species of Stokes radius of approximately 64A. This magnitude is consistent with the effective molecular size reported for various hormone-receptor complexes.
