EFFECT OF HIGH-PROTEIN FEED SUPPLEMENTS ON CONCENTRATIONS OF GROWTH-HORMONE (GH), INSULIN-LIKE GROWTH FACTOR-I (IGF-I) AND IGF-BINDING PROTEIN-3 IN PLASMA AND ON THE AMOUNTS OF GH AND MESSENGER-RNA FOR GH IN THE PITUITARY-GLANDS OF ADULT RAMS

Three groups of mature rams were maintained on diets of hay, hay + 2% lupin or hay + 2% cowpea for 11 weeks. Serial blood samples were taken at 15-min intervals for 12 h for the determination of GH and IGF-I co ntent by radioimmunoassay and for IGF binding protein-3 (IGFBP-3) leve ls by Western blotting. The rams were killed after 77 days of suppleme ntary feeding and their pituitary glands analysed for content of GH an d GH mRNA. Mean plasma GH and baseline GH levels were significantly (P <0.01) decreased in the rams fed lupin and cowpea compared with contro ls fed hay and GH pulse amplitude was significantly (P<0.001) decrease d in the group fed the cowpea diet. The frequency of GH pulses was not significantly altered by either treatment. Plasma concentrations of I GF-I were elevated in rams fed lupin (P<0.001) or cowpea (P<0.05). IGF BP-3 levels were not significantly (P>0.05) altered by either treatmen t. There were no significant differences in pituitary content of GH mR NA but pituitary content of GH was increased in rams fed lupin (P<0.05 ) and cowpea (P=0.07). In conclusion, a high-protein diet decreases pl asma GH levels and increases IGF-I without changing plasma IGFBP-3 lev els in rams. Thus ongoing synthesis of GH, as indicated by the mRNA le vels, may cause a build up of GH stores in the pituitary gland.