Ontogeny of hepatic bovine growth hormone receptors in cattle2

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Abstract
A series of studies examined the binding characteristics and ontogeny of hepatic growth hormone binding sites in dairy bulls on d 2, 30, 180, and 365 of age. Binding of iodinated recombinant bovine growth hormone ([125I]rbGH) to liver membrane receptors was membrane protein-dependent. Receptors were considered growth hormone-specific, because physiological concentrations of bovine prolactin (bPRL) failed to displace [125I]rbGH from bovine hepatocyte membranes. Only 50% of [125I]rbGH was bound reversibly to hepatic microsomes. Addition of dithiothreitol (DTT) to the receptor-assay buffer increased the binding of [125I]rbGH to hepatic membranes in a time-dependent manner. Moderate concentrations of Cg++' and Mg++ in the receptor-assay buffer had no detectable effects on binding of [125I]rbGH to hepatic microsomes. In growing dairy bulls, specific binding of [125I]rbGH per milligram of membrane protein increased from 1.9 ± 1.8% at d 2 to 14.1 ± 1.8% at d 180 and then declined to 5.2 ± 1.6% at d 365. Likewise, concentration of insulin-like growth factor (IGF)-I in serum was low during the 1st mo of age (d 2,13.3 ± 8.8 ng/ml; d 30,9.7 ± 8.8 ng/ml), but it became maximal at d 180 (151.0 ± 8.8 ng/ml). Circulating concentrations of IGF-II increased linearly during the 1st yr of growth. Serum concentrations of GH, triiodothyronine, and thyroxine declined from 39.9 ± 6.5, 2.7 ± .2, and 75.4 ± 4.6 ng/ml at d 2 to 16.5 ± 6.5, 1.3 ± .2, and 53.4 ± 4.6 ng/ml at d 30, respectively, and remained low through 1 yr of age. Insulin concentration in serum did not change significantly with development. Results indicated that increasing concentrations of specific bGH receptors in the bovine liver may play a key role in regulating postnatal growth in cattle.

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