Five sheep expressing the callipyge gene, which causes muscle hypertrophy, were compared with five normal sheep to determine whether endocrine differences existed between genotypes. Blood samples were taken at 15-min intervals for 6 h to measure serum concentrations of growth hormone and insulin. Thyroxine and IGF-I levels were determined in single samples. No differences were found in mean serum growth hormone concentrations, growth hormone pulse amplitude, or pulse frequency ( P > .3). Insulin concentrations were not different between genotypes before or after feeding (4.5 ± 1.3 ng/mL callipyge vs 4.9 ± 1.7 ng/mL normal, P > .4). The IGF-I concentrations did not differ (273.8 ± 17.6 ng/mL callipyge vs 261.4 ± 12.3 ng/mL normal). Serum thyroxine concentrations also were not different (5.9 ± 2.3 μg/mL for callipyge vs 5.1 ± 2.1 μg/mL normal, P > .3). In a separate experiment, five ewe lambs with and five without the callipyge gene were stressed to determine whether the adrenocortical response to stress differed between genotypes. Blood samples were taken at 15-min intervals for 2 h before, during, and after restraint stress. Restraint increased serum cortisol concentrations in both groups (P < .001), but genotypes did not differ at any time (P > .3). These results suggest that differences in muscling are not due to differences in systemic hormone secretion. The results of the second experiment indicate that callipyge and normal sheep have similar adrenocortical responses to stress.
- Insulin-Like Growth Factor