Influence of continuous growth hormone or insulin-like growth factor I administration in adult female chickens

A series of studies was conducted to determine whether growth hormone (GH) exerts effects on adult female chickens. Recombinant chicken GH (rcGH) was administered continuously via osmotic minipumps. No consistent effects of r cGH treatment were observed on reproductive indices. Hens receiving rcGH tr eatment for 10 days exhibited hepatomegaly and showed a tendency (P < 0.1) for increased spleen and thymus weights. Moreover, there were increases in the circulating concentrations of insulin-like growth factor-I (IGF-I) and IGF-binding proteins (IGF-BPs) (22-kDa IGF-BP after 2, 5, and 10 days; 28-k Da IGF-BP after 5 and 10 days; and 36-kDa IGF-BP after 10 days) with rcGH t reatment. To determine whether the changes in IGF-BPs were due directly to GH or indirectly via IGF-I, the effects of the continuous administration of rcGH or recombinant human IGF-I (rhIGF-I) were compared. While rcGH again elevated the circulating levels of 28- and 36-kDa IGF-BPs, no such effect w as observed with rhIGF-I treatment. However, both treatments exerted simila r effects in depressing pituitary GH mRNA levels and elevating plasma conce ntrations of IGF-I. It is concluded that GH directly elevates circulating c oncentrations of IGF-I and IGF-BPs, bur the negative feedback effect on GH synthesis is mediated via IGF-I.