Influence of hypothyroidism on growth hormone binding by rat liver.
The binding of 125I-iodinated human GH ([125I]iodo-hGH) to crude, membrane-rich preparations from the livers of thyroidectomized male rats was examined to determine if alterations in GH binding might explain hypothyroid-induced growth failure. Membrane preparations from chronically hypothyroid rats bound nearly twice as much labeled hGH (12.8 +/- 2.5%; mean +/- 1 SD) as those from normal rats (6.6 +/- 1.4%) or thyroxine-treated thyroidectomized animals (7.0 +/- 1.1%). Binding by membrane preparations from hypothyroid rats treated with hGH for 2 weeks was 12.3%. By Scatchard analysis the apparent affinities of the membrane preparations for hGH were relatively constant (Ka = 2.30-2.88 X 10(9) M-1) among the experimental groups. Ovine PRL was only 1.4% as potent as hGH in displacing [125I]iodo-hGH from the liver preparations, indicating that hGH was bound primarily to somatogenic sites. Since administration of hGH did not reduce the binding of [125I]iodo-hGH, it is unlikely that the increase in hGH binding in hypothyroidism is mediated by a reduction in the ambient GH concentration. Furthermore, this increase in [125I]iodo-hGH binding indicates that an alteration in binding of GH to its receptor probably does not mediate either the low somatomedin levels or the growth failure that result from hypothyroidism.
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D013006:Growth Hormone / Q000378:metabolism* / Q000494:pharmacology; D007037:Hypothyroidism / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D011388:Prolactin / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism; D011986:Receptors, Somatotropin; D013965:Thyroidectomy
D011956:Receptors, Cell Surface; D011986:Receptors, Somatotropin; D011388:Prolactin; D013006:Growth Hormone