MEASUREMENT OF DEVELOPMENTAL-CHANGES IN PLASMA INSULIN-LIKE GROWTH FACTOR-I LEVELS OF BROILER-CHICKENS BY RADIORECEPTOR ASSAY AND RADIOIMMUNOASSAY

The main purpose of this study was to examine the relationship between insulin-like growth factor-I (IGF-I) and growth hormone (GH) during embryonic and posthatching development of broiler chickens. Two heterologous assays were validated for measurement of IGF-I in chicken and turkey plasma. A radioreceptor assay (RRA), utilizing microsomal membranes prepared from human placenta, was modified and validated for measurement of IGF peptide (mainly IGF-I). A double-antibody radioimmunoassay (RIA) was validated for measurement of immunoreactive IGF-I levels in chicken and turkey plasma. In both assay systems, recombinant-derived human IGF-I was used for standards and trace hormone. Hypophysectomy in turkey poults reduced plasma levels of IGF (RRA) by 35% and IGF-I (RIA) by 59% as compared to intact control turkeys. In Experiment 1,14 chicken embryos were bled at 15, 17, 19, and 21 days of incubation and at 1 week of age to determine plasma levels of IGF-I and GH. Plasma IGF levels (RRA) remained constant during late incubation, but increased significantly (P < 0.05) at 1 week of age. Plasma IGF-I levels (RIA) declined 2 days before hatching; however, plasma levels of IGF-I were sharply elevated (P < 0.05) at 1 week of age. Plasma GH concentrations were low in embryos and were greatly elevated (P < 0.05) at hatching (21 days of incubation) and at 1 week of age. In Experiment 2, 12 different broiler cockerels were weighed and then bled by cardiac puncture each week from hatching (1 day of age) to 7 weeks of age. The plasma profiles of IGF, IGF-I, GH, triiodothyronine (T3), and thyroxine (T4) were each compared to relative growth rate by analysis of covariance. Plasma IGF and IGF-I levels increased progressively from 0 to 3 weeks of age and were maintained in a plateau from 3 to 7 weeks of age. Plasma GH levels reached a peak at 4 weeks of age, but declined sharply thereafter, while IGF and IGF-I levels remained elevated. Plasma T3 concentrations were progressively increased and reached peak concentrations at 3 weeks of age, while plasma T4 levels increased only at 6 and 7 weeks of age. There was a high correlation (P < 0.01) between relative growth rate and age-related changes in plasma levels of IGF (r = 0.96), IGF-I (r = 0.97), and T3 (r = 0.94); however, there was no correlation between relative growth rate and changes in plasma GH or T4. These observations suggest that thyroid activity could be more important than GH in maintaining normal growth rate and plasma IGF-I levels in broiler chickens. © 1990.