HORMONAL PROFILES OF CALLIPYGE AND NORMAL SHEEP

Five sheep expressing the callipyge gene, which causes muscle hypertro phy, were compared with five normal sheep to determine whether endocri ne differences existed between genotypes. Blood samples were taken at 15-min intervals for 6 h to measure serum concentrations of growth hor mone and insulin. Thyroxine and IGF-I levels were determined in single samples. No differences were found in mean serum growth hormone conce ntrations, growth hormone pulse amplitude, or pulse frequency (P > .3) . Insulin concentrations were not different between genotypes before o r after feeding (4.5 +/- 1.3 ng/mL callipyge vs 4.9 +/- 1.7 ng/mL norm al, P > .4). The IGF-I concentrations did not differ (273.8 +/- 17.6 n g/mL callipyge vs 261.4 +/- 12.3 ng/mL normal). Serum thyroxine concen trations also were not different (5.9 +/- 2.3 mu g/mL for callipyge vs 5.1 +/- 2.1 mu g/mL normal, P > .3). Tn a separate experiment, five e we lambs with and five without the callipyge gene were stressed to det ermine 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 cortis ol concentrations in both groups (P < .001), but genotypes did not dif fer at any time (P > .3). These results suggest that differences in mu scling are not due to differences in systemic hormone secretion. The r esults of the second experiment indicate that callipyge and normal she ep have similar adrenocortical responses to stress.