Isotopic determination of amino acid-urea interactions in exercise in humans

Abstract

It was recently recently reported that in light exercise (30% .ovrhdot.VO2 max), the oxidation of [1-13C]leucine was significantly increased but the rate of urea production was unchanged. Three possible explanations for this apparent incongruity were tested here. NaH13CO3 was infused throughout rest and exercise and it was found that, although altered bicarbonate kinetics in exercise resulted in greater recovery of 13CO2, the difference between rest and recovery was small compared with the increase in the rate of 13CO2 excretion during exercise when [1-13C]leucine was infused. [13N]leucine was infused and plasma urea N was isolated to determine directly the rate of incorporation of the 15N. During exercise there was no increase in the rate of 15N incorporation. Simultaneously, [2,3-13C]alanine was infused and the rate of incorporation of 15N in alanin was quantified. [15N]alanine production from [15N]leucine more than doubled in exercise, and by deduction, alanine production from other amino acids also doubled. The previous assumption that [1-13C]leucine metabolism in exercise was representative of the metabolism of other essential amino acids was tested by infusing [1-13C] and [.alpha.-15N]lysine throughout rest and exercise was not increased in a manner comparable to that of leucine. These data confirm the original findings that leucine decarboxylation is enhanced in light exercise but urea production is unchanges. Apparently, in exercise, leucine metabolism is not representative of the metabolism of all essential amino acids and a model of whole-body protein synthesis and catabolism based on that assumption is not valid.