Nitrogen Metabolism

Effects of pregnancy on maternal nitrogen metabolism have been less studied than those on carbohydrate and lipid supply and utilization. Increased predisposition to the mobilization of amino acids from skeletal muscle during late pregnancy can be inferred from measurements of tissue nitrogen balance in ewes.[8] This response may be permitted by the waning influences of insulin and other anabolic factors, including insulin-like growth factor 1 (IGF-1) as term approaches.[9] Coincidentally, hepatic protein synthesis is upregulated in dairy cows at a time when voluntary intake of dry matter and dietary protein is declining in the prepartum period.[10]

The dam also must dispose of the significant increase in products of nitrogen catabolism generated by conceptus tissues during late pregnancy. Fetal urea synthesis is substantial because of extensive oxidative deamination of amino acids in fetal liver and other tissues. This urea is efficiently transported by the placenta to the maternal circulation and adds directly to maternal urea destined for renal clearance and excretion. The placenta, which has little urea cycle enzyme activity, also deaminates amino acids to an extent that causes perceptible increases in maternal blood-ammonia concentrations and the need for hepatic detoxification.[3]

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