EFFECT OF EXOGENOUS CHICKEN GROWTH-HORMONE (CGH) ADMINISTRATION ON INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) GENE-EXPRESSION IN DOMESTIC-FOWL

The effects of chicken growth hormone (cGH) infusion on insulin-like g rowth factor (IGF-I) gene expression in rapidly-growing, meat-type chi ckens was investigated. Chicken GH was infused either continuously or in a pulsatile fashion to 8-week-old birds during a 7-day period. Foll owing cGH infusion, both IGF-I peptide and IGF-I mRNA content were mea sured in selected tissues. Steady-state IGF-I mRNA abundance was deter mined by a solution hybridization nuclease protection assay using tota l cellular RNA obtained from liver, heart, kidney, spleen, epiphyseal growth plate cartilage, gastrocnemius and pectoralis muscles. Continuo us infusion of cGH elicited a two-fold increase in IGF-I peptide conce ntration in the kidney (P < 0.05), while all other tissues remained un changed by cGH treatment under this infusion pattern. Pulsatile cGH in fusion produced a two-fold increase in IGF-I peptide content in the li ver, gastrocnemius, and pectoralis muscles (P < 0.05). In contrast wit h the levels of IGI-I peptide, relative steady-state IGF-I mRNA conten t was two-fold higher in liver and spleen of birds treated continuousl y with cGH, but was decreased to 35 and 55% of control birds in heart and pectoralis muscle. Pulsatile cGH infusion resulted in a 64% increa se in IGF-I mRNA in the liver and remained unchanged in other tissues. Under both patterns of administration, changes in IGF-I mRNA were not reflected by changes in tissue IGF-I peptide levels. Overall correlat ions between tissue IGF-I mRNA and peptide levels were low and not sig nificant in the tissues studied, except for liver under pulsatile infu sion, in which IGF-I peptide levels paralleled changes in IGF-I mRNA. We conclude that, in chickens, exogenous cGH treatment stimulates hepa tic IGF-I transcription and translation only when the pattern of infus ion mimics the natural episodic pattern of GH secretion. The low corre lation between IGF-I peptide and mRNA levels in extra-hepatic tissues may indicate differential responsiveness to GH in birds, and that in s ome tissues IGF-I levels are under GH-independent transcriptional cont rols.