Ontogeny of growth hormone receptor gene expression in tissue of growth-selected strains of broiler chickens.

The purpose of this study was to determine the relationship between genetic selection for growth traits and tissue expression of the chicken growth hormone receptor (cGHR) gene. Two different populations of broiler chickens were studied. One population consisted of strain (S) 80, selected for 14 generations for high 9-week body weight (BW), and its progenitor, S90 (a 1950's strain). The second population consisted of S21, selected for 10 generations for high 4-week BW and low abdominal fat, and its progenitor S20 (a 1970's strain). Tissue (liver, fat, breast and leg muscle) and blood samples were collected from six birds/strain at 2-week intervals between 1 and 11 weeks of age. An RNase protection assay was developed to measure mRNA levels of full-length cGHR (3.2 and 4.3 kb) transcripts and chicken glyceraldehyde 3-phosphate dehydrogenase (for normalization) in total RNA prepared from tissue. Analysis of the area-under-curve (AUC) was used for strain comparisons of certain developmental profiles (BW, plasma hormones and tissue cGHR mRNA). The BW AUC showed that the growth rates are different (P < 0.05) among the four strains (S21 > S20 > S80 > S90). Both slow-growing strains (S90 and S80) had a higher (P < 0.05) plasma GH AUC than the two fast-growing strains (S20 and S21). The plasma T3 AUC was highest (P < 0.05) in S90 due to maintenance of higher T3 levels after 3 weeks of age. At 11 weeks of age, hepatic and plasma GH-binding activities were positively related to growth rate (S21 > S20 > S80 > S90). However, the developmental increase in cGHR mRNA in liver and fat was similar among these different populations of growth-selected broiler chickens. Steady-state levels of cGHR mRNA increased in a developmental manner in the liver (5-fold at 9 weeks of age) and abdominal fat (4.5-fold at 11 weeks of age) of all strains. In contrast, there was no developmental increase or strain difference in cGHR mRNA levels in breast and leg muscle. There is a discrepancy between GH-binding activity in liver and plasma, which is different among strains, and steady-state levels of tissue cGHR mRNA which are similar among strains. These observations suggest that the cGHR is under translational or post-translational regulation which would determine the amount of cGHR protein available for GH binding.