Developmental changes in the concentrations of glutamine and other amino acids in plasma and skeletal muscle of the Standardbred foal.

Glutamine is concentrated within skeletal muscle, where it has been proposed to play a regulatory role in maintaining protein homeostasis. The work presented here addressed the hypothesis that glutamine would be the most abundant free alpha-AA in plasma and skeletal muscle in the foal during the first year of life. Glycine, however, was the most abundant free alpha-AA in plasma at birth and between 3 and 12 mo of age. The concentration of glutamine, the second most abundant AA at birth, increased through the first 7 d (P < 0.05) and then returned to values similar to those at birth. This resulted in glutamine being the most abundant free alpha-AA in plasma from 1 d through 1 mo of age. The most abundant free alpha-AA in skeletal muscle at birth was glutamine, but the concentration fell by more than 50% by d 15 and continued to decrease, reaching about one-third of the original values by 1 yr of age (P < 0.05). Glutamine synthetase was barely detectable in skeletal muscle at birth, but the abundance increased rapidly within 15 d of birth. The concentration of glycine, the second most abundant alpha AA in muscle at birth, decreased by about 40% by d 15 (P < 0.05) and then stabilized at this value throughout the year. In contrast, glutamate, alanine, and serine concentrations, the third, fourth, and fifth most abundant free alpha-AA in muscle at birth, respectively, increased to new stable concentrations between 3 and 6 mo of age (P < 0.05). This resulted in alanine being the most abundant free alpha-AA in skeletal muscle at 12 mo of age, followed by glutamate, glutamine, and glycine. The decrease in intramuscular glutamine content, particularly during the first 2 wk after birth, is not compatible with a regulatory role for glutamine in muscle protein synthesis because it occurred at the time of maximum growth in these animals. The findings that, at certain times of development, glutamine was not the most abundant free alpha-AA in the foal is novel and signifies that intramuscular glutamine may have functions specific to muscle type and mammalian species.

Changes in glutamine metabolism indicate a mild catabolic state in the transition mare.

Glutamine is the most abundant free alpha-AA in the mammalian body, and large amounts of glutamine are extracted by both the fetus during pregnancy and the mammary gland during lactation. The work presented here addressed the hypothesis that there would be major changes in glutamine metabolism in the mare during the transition period, the time between late gestation, parturition, and early lactation. Eight foals were born to Standardbred mares provided with energy and protein at 10% above NRC recommendations, and foals remained with mares for 6 mo. During lactation, lean body mass decreased by 1.5% (P < 0.05), whereas fat mass was unchanged throughout gestation and lactation. There was a sharp increase in the concentration of most plasma metabolites and hormones after birth, which was due in part to hemoconcentration because of fluid shifts at parturition. Plasma glutamine concentration, however, was maintained at greater concentrations for up to 2 wk postpartum but then began to decrease, reaching a nadir at approximately 6 wk of lactation. Skeletal muscle glutamine content did not change, but glutamine synthetase expression was decreased at the end of lactation (P < 0.05). Free glutamine was highly abundant in milk early in lactation, but the concentration decreased by more than 50% after 3 mo of lactation and paralleled the decrease in plasma glutamine concentration. Thus, lactation represents a mild catabolic state for the mare in which decreased glutamine concentrations may compromise the availability of glutamine to other tissues such as the intestines and the immune system.