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The effect of glucocorticoids (dexamethasone) on circulating concentrations of insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBPs) was examined in young (2-week-old) chickens. Increasing dosages (6, 60, and 600 microg/kg) of dexamethasone resulted in progressive growth retardation as evidenced by decreased body-weight gain, reduced bone growth, and breast muscle weights, together with immunosuppression as indicated by decreased thymus and bursa weights. Plasma concentrations of IGF-I were progressively reduced with increased doses of dexamethasone. There were no changes in circulating IGFBPs except on Day 7 when binding activity of the 36-kDa IGFBP was increased by dexamethasone (6 microg/kg). It is speculated that similarities exist between the 36-kDa IGFBP and the mammalian IGFBP-1 based on the response to dexamethasone treatment.

The effect of dietary protein restriction (5% and 10% compared to control 20%) on circulating concentrations of insulin-like growth factor (IGF)-1 and IGF binding proteins (IGFBPs) was examined in young (18-day-old) chickens during a 2-week period. Reductions in dietary protein caused progressive growth retardation as evidenced by decreased body-weight gain and reduced bone growth. The decrease in plasma concentrations of IGF-I appeared to be directly related to dietary protein levels (i.e., the lower the amount of dietary protein, the greater the reduction in the circulating concentrations of this growth factor). Three IGFBPs with MWs of 30, 36, and 40 kDa were detected by radioligand assay following separation by SDS-electrophoresis. Binding activity of the 30-kDa IGFBP was transiently increased on Day 3 by protein restriction (both 5% and 10%). The 36-kDa IGFBP was also affected by protein restriction with binding activity of this IGFBP decreased throughout the study. The binding activity of the 40-kDa IGFBP was transiently increased on Days 3 and 7 but subsequently decreased on Days 10 and 14 in the 10% protein group. In the 5% protein group, binding activity of the 40-kDa IGFBP was decreased throughout the study. The decrease in circulating concentrations of IGF-I appeared to be inversely related to the initial increase in the binding activity of the 30-kDa IGFBP on Day 3 (5% and 10% protein) and to the increased binding activity of the 40-kDa IGFBP on Days 3 and 7 (10% protein). This may suggest that bioavailability of plasma IGF-I is decreased initially due to increased binding with these IGFBPs. However, binding activity of all three IGFBPs then decreased in a manner directly related to the decreasing IGF-I plasma concentrations for the remainder of the experiment. The initial increase in binding activity observed with the 30-kDa IGFBP is similar to that observed with IGFBP-1 in mammals in that both of these IGFBPs respond rapidly to nutritional deprivation. Decreased protein intake would certainly have an impact on the amount of available proteins required for the synthesis of these growth factors.