ONTOGENY OF HEPATIC BOVINE GROWTH-HORMONE RECEPTORS IN CATTLE

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 ([I-125]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 [I-125]rbGH from bovine hepatocyte membranes. Only 50% of [I-125]rbGH was bound reversibly to hepatic microsomes. Addition of dithiothreitol (DTT) to the receptor-assay buffer increased the binding of [I-125]rbGH to hepatic membranes in a time-dependent manner. Moderate concentrations of Ca++ and Mg++ in the receptor-assay buffer had no detectable effects on binding of [I-125]rbGH to hepatic microsomes. In growing dairy bulls, specific binding of [I-125]rbGH per milligram of membrane protein increased from 1.9 +/- 1.8% at 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 concentations of specific bGH receptors in the bovine liver may play a key role in regulating postnatal growth in cattle.