SUMMARYThe physiological consequences of selection for weaning weight were investigated using 14-month-old castrated male Merino lambs from lines selected for (W +) and against (W – ) weight at weaning for over 10 generations. The selection experiment commenced in 1953 and was conducted at Trangie, NSW, Australia. Lambs from these lines now differ by 42% in weight at weaning and maturity, and have a proportionately similar composition of protein, fat and ash in their bodies at similar stages of maturity. In the whole body, there were no apparent between-strain differences in protein synthesis, but W + lambs had lower (P < 0·05) rates of phenylalanine oxidation than W – lambs. In hind limb muscle, protein metabolism of W + lambs was more responsive to food intake than in W – lambs. In particular, the rate of change of protein degradation differed (P < 0·05) between the lines in response to food intake per unit liveweight with protein degradation decreasing with increased feed intake in W + lambs. W + lambs had lower (P < 0·05) rates of blood flow to, and used less (P < 0·05) oxygen per kg hind limb muscle than W— lambs. Endocrine differences between the lines included increased (P < 0·05) plasma insulin-like growth factor-I (IGF-1) concentration in W + compared to W— lambs and, from regression analysis, an indication that there was a higher (P < 0·01) response in hind limb protein gain to insulin in the W + lambs.These results suggest that selection for weaning weight has altered the dynamics of protein metabolism in lambs, principally through alterations in protein breakdown in muscle. Moreover, they suggest that there are associated genetic differences in endocrine control of protein metabolism which involve insulin and possibly IGF-I.
A Novel Intronic Single Nucleotide Polymorphism in theMyosin heavy polypeptide 4Gene Is Responsible for the Mini-Muscle Phenotype Characterized by Major Reduction in Hind-Limb Muscle Mass in Mice
