Diurnal variation in urine [15N]urea content, estimates of whole body protein turnover, and isotope recycling in healthy meal-fed children with cystic fibrosis.
The pattern of urinary urea excretion and labelling with 15N was examined in eight meal-fed 6 to 9 year old children, over a 3-day period using a simulated constant infusion of the label. The children had cystic fibrosis but were healthy and in a good nutritional status at the time of the study. Reciprocal diurnal patterns of urea excretion and [15N]urea enrichment were noted and found to be suitable for mathematical description. Urea excretion was maximal in the evening at approximately 2000 and minimal at 0800, whereas the [15N]urea enrichment was maximal at about 0800 and minimal at 2000. In addition to the diurnal variation the [15N]urea enrichment increased exponentially to a plateau or isotopic steady state. The diurnal variation in [15N]urea enrichment resulted in large diurnal changes in the calculated rates of whole body amino nitrogen flux, synthesis, and breakdown. Flux rates were approximately 44% higher in the evening than in the morning. Synthesis rates were 19% higher in the evening, whereas breakdown rates were 27% greater in the morning. Mean amino nitrogen flux rates were 1.28 (SD 0.13) . g N . kg-1 . day-1. Isotope recycling was estimated from the slope of the [15N]urea enrichment curve between 30 and 54 h from the start of the study. There was a wide range in recycling, 2.9 - 19.4% (mean 11.4, SD 5.4). Some of the biological and pharmacological importance of the diurnal variation in the protein metabolism is discussed.
D000596:Amino Acids / Q000097:blood; D002648:Child; D002940:Circadian Rhythm; D003550:Cystic Fibrosis / Q000378:metabolism*; D004044:Dietary Proteins / Q000008:administration & dosage; D005260:Female; D006801:Humans; D008297:Male; D009587:Nitrogen Isotopes; D011506:Proteins / Q000378:metabolism*; D014508:Urea / Q000652:urine*
D000596:Amino Acids; D004044:Dietary Proteins; D009587:Nitrogen Isotopes; D011506:Proteins; D014508:Urea