Pituitary-Dependent Growth Hormone Receptors in Rabbit and Sheep Liver*

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Abstract
Studies were undertaken to examine the pituitary dependence of hepatic GH receptors in rabbits and lambs. Young (47 days old) and older (116 days old) rabbits were hypophysectomized (hypox) or sham operated (sham). Hypophysectomy completely arrested the growth of the older rabbits but only reduced it by about 50% in the young. After 21 days, the specific binding of 125I-labeled ovine PRL (oPRL), bovine GH (bGH), human GH (hGH), and insulin was measured in liver microsomes from hypox and sham rabbits. The percentages of specific binding (mean ± SE) in older (116 days old) hypox us. sham microsomes (150 μg protein/tube) were: bGH, 2.0 ± 0.4 vs. 26.8 ± 1.4; hGH, 7.4 ± 0.5 vs. 49.4 ± 3.3, oPRL, 4.0 ± 0.4 vs. 10.7 ± 1.1, and insulin, 9.3 ± 0.7 vs. 7.5 ± 1.2. In younger, (47 days old) hypox vs. sham animals, the results were: bGH, 3.6 ± 0.3 vs. 17.2 ± 2.6; hGH, 17.6 ± 1.9 vs. 38.7 ± 1.5; oPRL, 3.5 ± 0.5 vs. 7.6 ± 0.6; and insulin, 6.9 ± 0.4 vs. 4.5 ± 0.5. Similar studies in lambs showed that growth was still about 50% of that in sham animals after hypophysectomy. The percentages of specific binding in hypox vs. sham liver microsomes (300 μg protein/tube) were: bGH, 2.0 ± 0.2 vs. 11.6 ± 1.0; hGH, 4.8 ± 0.4 vs. 19.0 ± 1.2; oPRL, 1.3 ± 0.4 vs. 1.1 ± 0.1; and insulin, 14.6 ± 1.8 vs. 12.8 ±1.2. Six hypox lambs were treated with ovine or bovine GH (1 mg/kg every other day) for 19 days. The percentages of specific binding of bGH and hGH were significantly increased to 6.0 ± 0.8% and 11.2 ± 1.0%, respectively. No significant change in insulin binding was observed. Inhibition dose-response curves showed that the concentration of unlabeled bGH (16 ng/ml; 10-9 M) which inhibited [125I]iodo-bGH binding by 50% did not change with hypophysectomy or GH treatment. These studies demonstrate that hepatic GH receptors in both rabbits and sheep are pituitary dependent. Since GH receptor levels in lambs were partially maintained by GH treatment, it is possible that GH plays a role in inducing and/or maintaining its own hepatic receptors.