Measurement of developmental changes in plasma insulin-like growth factor-I levels of broiler chickens by radioreceptor assay and radioimmunoassay

Abstract

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 (T 3), and thyroxine (T 4) 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 T 3 concentrations were progressively increased and reached peak concentrations at 3 weeks of age, while plasma T 4 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 T 3 ( r = 0.94); however, there was no correlation between relative growth rate and changes in plasma GH or T 4. 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.