GROWTH HORMONE-BINDING PROTEIN ENHANCES GROWTH-HORMONE ACTIVITY IN-VIVO

The decay curve of labeled growth hormone (GH) in the plasma followed a three-compartment model and could be described by the equation: conc entration = Ae(-alpha t) + Be-beta t+ Ce-gamma t, where A, B, and C ar e gamma-intercepts and alpha, beta, and gamma are compartments. When I -125-labeled ovine prolactin (oPRL) was injected, the decay curve coul d be described by the equation: concentration = Ae(-alpha t) + Ce-gamm a t. Formation of I-125-labeled bovine-GH-binding protein (GHBP) compl exes with somatogenic characteristics was demonstrated in the serum of both normal and GH transgenic mice. In contrast, I-125-oPRL was unabl e to form complexes of this type in any of the mice studied. Receptor- mediated liver uptake was found to be faster for PRL than for GH (5-6 min vs. 15-20 min). Liver uptake of radioactivity was significantly lo wer for PRL than for GH [liver to blood ratio (L/B) of 1.7 +/- 0.3 at 6 min vs. L/B of 3.7 +/- 0.6 at 20 min, respectively]. The presence of binding proteins for GH substantially reduces the clearance of this h ormone and consequently increases the liver uptake of GH (mediated by GH receptors). This suggests that GHBPs act to increase the biological activity of GH in vivo.