Growth hormone binding proteins in pig adipose tissue: number, size and effects of pGH treatment on pGH and bGH binding.

The present study was undertaken to determine the number and size of growth hormone binding proteins present in pig adipose tissue, determine if there were differences in binding of pGH and bGH to adipose tissue membranes and establish the effects of pGH treatment on GH binding. Administration of pGH (0, 25, 50 or 100 micrograms pGH/kg BW/d) for 7 d did not affect binding of [125I]bGH to adipose tissue microsomes. Maximum binding of bGH was approximately 8-fold higher than that observed for pGH. Half-maximal inhibition of [125I]bGH binding was observed at 11 ng/ml of bGH. In contrast, a more than 10-fold greater concentration of pGH was required to half-maximally inhibit [125I]pGH binding. bGH and pGH both bound to the same GH binding proteins (Mr of 92,000, 73,000 and 53,000). The GH binding proteins appear to be produced by post-translational modification of a single GH receptor transcript rather than alternative splicing of a primary transcript since only one GH receptor mRNA transcript (4.2 kb) was detected on Northern analysis. Our findings indicate that: 1) bGH is the preferred ligand to use to study GH binding in pig adipose tissue membranes (or adipocytes); 2) exogenous pGH does not alter GH binding; and 3) only one GH receptor mRNA transcript is present in pig adipose tissue.