BODY GROWTH, CARCASS COMPOSITION, AND ENDOCRINE CHANGES IN LAMBS CHRONICALLY TREATED WITH RECOMBINANTLY DERIVED INSULIN-LIKE GROWTH FACTOR-I

Castrate yearling male sheep were treated for 8 weeks with either 50-mu-g/kg body wt/8 hourly sc insulin-like growth factor-I (IGF-1) (n = 10) or with saline (n = 9). IGF-I treatment increased plasma IGF-I from 235 +/- 17 to 347 +/- 16 ng/ml (P < 0.001). There was a gradual divergence in body wt (P < 0.10) between treatment groups. Food intake did not change significantly. The weight of the spleen corrected for body wt increased by 40% (P < 0.001) and there was a marginal increase in adjusted kidney wt (P < 0.1). There was no effect of IGF-I on carcass weight or dimensions, or on long bone length, although the weight per unit length of the tibia (P < 0.05) and femur (P < 0.10) were increased. There was no effect on wool growth. Plasma IGF binding proteins (IGFBPs) were quantified by ligand blot analysis. In the IGF-I treated group,IGFBP-1 showed a transient increase (P < 0.05) at day 3 but was similar in both groups at day 55 of treatment. IGFBP-2 was suppressed (P < 0.05) by day 55 and IGFBP-3 and 4 did not change. Plasma glucose was elevated (P < 0.05) and plasma insulin was suppressed (P < 0.01) from 280 +/- 32 pg/ml to 124 +/- 30.4 pg/ml, plasma urea (P < 0.01) and creatinine (P < 0.05) were reduced in the IGF-I treated group. The somatogenic effect of IGF-I in this study was minimal suggesting that in the well fed animal with an intact somatotropic axis IGF-I treatment at doses which double plasma IGF-I does not enhance somatic growth performance. However, the marked splenomegaly shows the sensitivity of splenic growth to systemic IGF-I. The suppression of insulin with chronic IGF-I treatment was accompanied by hyperglycaemia-this may explain in part the lack of a significant anabolic response and may limit the utility of IGF-I therapy unless higher doses with insulin-like effects are used.