STUDY OF THE HEPATIC GROWTH-HORMONE (GH) RECEPTOR AT DIFFERENT AGES IN CHICKENS SELECTED FOR A GOOD FEED CONVERSION (FC) AND A FAST WEIGHT-GAIN (GL)

The influence of genetic selection for improved feed conversion (FC) a nd fast growth (GL) in broiler chickens, on the hepatic growth hormone receptor was investigated as a function of age. Selection for improve d feed conversion resulted in lean chickens, whereas selection for fas t growth resulted in fat chickens. Growth hormone receptor characteris tics were determined in liver microsomal fractions and plasma GH conce ntrations were measured by radioimmunoassay. Embryos from the FC line showed higher specific GH binding than GL embryos at day 16 of embryon ic development only (P<0.05). On day 18 of embryonic development, the FC line had slightly more GH receptors than the GL line (P<0.05), wher eas simultaneous plasma GH concentrations were greater in the GL line. The effect of selection on hepatic growth hormone binding was greates t in 4 week old growing chicks at which age the GL line showed a 2- to 3-fold higher % of specific binding than the FC line (P<0.001). The d ifference at this stage is probably due to down regulation of the rece ptor by the significantly higher GH concentrations in the FC line. The adult GL chickens also showed higher specific binding than FC, but th e difference was less pronounced than in the growing stage (P<0.05). T his was due to a higher number of GH receptors, while receptor affinit y was unchanged. Because no line difference in plasma GH concentration s was found in adult hens, other unknown mechanisms probably play a ro le in determining differences in GH receptor binding between these sel ected lines at older ages.