Tubular function by lithium clearance, plasma amino acids and hormones following a meat meal in childhood

Tubular function was measured by lithium clearance (CLi) and by its derived formulae before and after the transient increase (lasting 90 min) in glomerular filtration rate (GFR) following a meat meal (2g protein/kg body weight) in 12 normal children. Three baseline and 4 clearances after the meal were obtained, each lasting 30 min. The mean baseline CLi was 23.1 +/- 1.64 ml/min/1.73 m2. At peak GFR response (60 min from starting the meal), CLi averaged 27.6 +/- 2.4 ml/min/1.73 m2 (p less than 0.025 vs. baseline) and it was further increased (32.2 +/- 5.04 ml/min/1.73 m2, p less than 0.01 vs. baseline) 120 min after starting the meal, while GFR returned to baseline values. Fractional lithium excretion averaged 0.23 +/- 0.04 at baseline and increased continuously after the meat meal and, at completion of the study, it averaged 0.38 +/- 0.07 (p less than 0.025 vs. baseline). The distal absolute and fractional sodium reabsorption increased throughout the studies following the meal and peaked at 120 min. The functional changes were associated with a statistically significant increase in the plasma concentration of insulin, glucagon, and total amino acids after the meal. The latter at the end of the study was almost doubled (5,600 +/- 780 versus 3,200 microM at baseline, p less than 0.01). The data indicate that the tubulo glomerular feedback mechanism operates normally after a meat meal. The finding on increased distal sodium reabsorption might point to the existence of an insulin-dependent mechanism.