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 rcGH treatment were observed on reproductive indices. Hens receiving rcGH treatment 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-kDa IGF-BP after 5 and 10 days; and 36-kDa IGF-BP after 10 days) with rcGH treatment. 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 was observed with rhIGF-I treatment. However, both treatments exerted similar effects in depressing pituitary GH mRNA levels and elevating plasma concentrations of IGF-I. It is concluded that GH directly elevates circulating concentrations of IGF-I and IGF-BPs, but the negative feedback effect on GH synthesis is mediated via IGF-I.