F. Carraro

Alanine kinetics in humans during low-intensity exercise

There is little doubt that pyruvate contributes to the increased alanine flux in exercise, but the role of protein breakdown is less clear. To quantify the relative contributions of pyruvate and protein breakdown to the increase in alanine flux observed in exercise, we used a primed, constant infusion of 15N-alanine and 1-13C-lactate. The rate of appearance of alanine, the de novo synthesis of alanine, and rate of alanine release from protein breakdown were determined in five healthy subjects at rest and during exercise. The exercise was performed for 120 min on a treadmill at 45% of the subjects VO2max. The total rate of appearance of alanine, calculated with the 15N-alanine tracer, increased significantly during exercise from 4.9 +/- 0.5 to 7.9 +/- 0.9 mumol.kg-1. The amount of alanine derived from pyruvate also significantly increased during exercise (3.2 +/- 0.3 vs 4.5 +/- 0.7), but the proportion of the total decreased from 65% at rest to 57% during exercise (statistically significant, P < 0.05). Consequently, the alanine derived from protein breakdown significantly increased (1.7 +/- 0.5 vs 3.4 +/- 0.8) and was also increased as percent of total alanine flux. Thus, we conclude that during low-intensity exercise, whole body protein catabolism is accelerated.

Isotopic determination of fibronectin synthesis in humans

Fibronectin is an opsonic protein that, among other functions, activates the reticuloendothelial system. Accurate measurement of its rate of synthesis is necessary to more fully understand its physiological role in normal and pathological conditions. We have determined the rate of fibronectin synthesis in three normal volunteers using a primed-constant infusion of 15N-glycine and 1,2-13C-leucine, and measuring the incorporation of the isotopes into the protein over 5 days of infusion. In nine additional subjects, the fractional synthetic rate (FSR) of fibronectin was calculated during a 24-hour infusion using urinary hippurate and plasma alpha-ketoisocaproic acid enrichment to represent the precursors for incorporation of labeled glycine and leucine, respectively, into fibronectin. The FSR using glycine and leucine was 1.56 +/- 0.14 and 1.29 +/- 0.04 (%/h), respectively, in the 5-day infusion study, and 1.56 +/- 0.10 versus 1.83 +/- 0.09 (%/h), respectively, in the 24-hour study. The results of the 5-day infusion of 15N-glycine justify the use of urinary hippurate to reflect the precursor enrichment for the determination of the FSR of fibronectin during a shorter (less than 24 hour) infusion period.