Amino acids and skeletal muscle growth in lambs : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
The objectives of this thesis were three-fold. Firstly, to identify whether reduced muscle growth in twin compared to singleton lambs during late pregnancy and early post-natal life was associated with changes in the concentration of intracellular free amino acids (FAA) that may play a role in the regulation of pathways involved in muscle growth. Secondly, to evaluate if supplementation with a specific amino acid improved muscle growth in twin fetuses/lambs, and thirdly to examine the role of mTOR signalling. The first objective was investigated by examining the differences in muscle FAA concentrations between singleton and twin fetuses in late pregnancy from either heavy or light ewes, under a maintenance or ad libitum feeding regimen. Twin fetuses had lower intracellular FAA concentrations of arginine (Arg), leucine, valine, glutamine, while muscle mass was positively associated only with Arg concentration. A further study characterised the FAA concentrations of singleton and twin well-fed lambs at 140 days pregnancy and at weaning. High levels of Arg and glutamine were associated with muscle growth during pregnancy; however several FAA appeared to be associated with muscle growth to weaning. Objective 2 was tested by examining the effects of maternal Arg administration on fetal muscle growth and mTOR signalling. Well-fed twin-bearing ewes, received either an intravenous bolus of Arg or saline solution 3 times daily from 100 days of pregnancy to parturition. Female lambs from supplemented ewes had increased birth weight and muscle mass at market weight, associated with increased ribosome number and mTOR abundance at P140 and increased ribosome number at weaning, compared to control females. An additional experiment supplemented twin-born lambs with Arg via fortification of colostrum and milk replacer from birth to 28 days or from birth to 70 days of life. Supplementation increased body growth between 7 and 21 days of life. Only supplemented females expressed higher muscle weight at 70 days, compared with control females. Collectively, these results indicate that singleton and twin muscle differs in Arg concentration, and the use of Arg during pregnancy and early neonatal life improves muscle growth in females. This action potentially occurs through mTOR signalling, and ameliorates reduced females weight in at birth and growth from birth to weaning.