PRESENCE OF GROWTH HORMONE-BINDING PROTEINS IN CATTLE PLASMA AND MILK

The presence of GH-binding proteins (GHBPs) in the plasma of adult cat tle was investigated using Sephadex G-200 filtration, Western ligand b lotting and Western blotting. The changes in the concentration of GHBP in the plasma of dairy half-sister heifers during the first year of l ife as well as the presence of GHBP in milk were also investigated. Wh en analytical chromatography (on a 1.6 x 100 cm column) was performed, five peaks of recombinant bovine GH (rbGH)-associated radioactivity w ere revealed in cattle plasma; the first peak, which appeared near the void volume, was presumed to represent aggregates, the second (M(r) 2 90 kDa) and the third peaks (M(r) 75 kDa) corresponded to specific rbG H-GHBP complexes; the last two peaks representing free I-125-labelled rbGH and Na[I-125]. Western ligand blotting revealed multiple GHBPs. T hree major bands were observed at approximately 190, 58 and 31 kDa; an excess of unlabelled hormone blocked the binding of I-125-labelled rb GH. Minor non-specific binding bands were also detected in cattle plas ma with molecular weights between 40 and 136 kDa. One monoclonal antib ody (8H7) produced against synthetic peptide (amino acids 54-63 of the extracellular domain of the bovine GH receptor) specifically interact ed with 190 and 58 kDa bands while the 31 kDa band was not recognized. Finally, Western ligand blots were performed to evaluate the changes in plasma GHBP during the first year of life in 55 dairy half-sister h eifers and to identify GHBP in milk. In plasma, the intensity of the 3 1 kDa band varied greatly between animals while the other specific ban ds remained stable. In milk, all specific GHBP bands observed by Weste rn ligand blotting disappeared a few days after parturition. In conclu sion, these results demonstrate the presence of GHBPs in cattle plasma and milk. There were important variations in GHBP between dairy half- sister heifers during the first year of life but the potential role of these binding proteins in the regulation of the biological activity o f GH in ruminants and their functions in milk remain to be determined.