[13C]bicarbonate kinetics in humans: intra- vs. interindividual variations

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Abstract
A comprehensive multicompartmental analysis of HCO3- kinetics was carried out on 5 normal, resting, fasted adults on 3 separate occasions at 1-wk to 1-mo. intervals to obtain a set of bicarbonate kinetic parameters and estimates of their inter- and intraindividual variation for use in the design and analysis of future nutrient oxidation studies. Following a rapid bolus of NaH13CO3 (10 .mu.nol .cntdot. kg-1 i.v.), the decay of 13C enrichment of breath CO2 could be described by a 3 exponential decay process and a linearly time-dependent term that accounted for changes in the 13C enrichment of metabolic fuels. The data were fitted subsequently to a mammillary multicompartmental model that consisted of a central pool and 2 peripheral pools of 3310,3490 and 8070 .mu.mol .cntdot. kg-1 HCO3- CO2. Labeled CO2 was eliminated from the central pool by respiratory and nonrespiratory routes at rates of 101 and 97 .mu.mol .cntdot. kg-1 .cntdot. min-1, respectively. The within-subject and among-subject variances were similar for the amount of freely exchangeable bicarbonate (14,870 .mu.mol .cntdot. kg-1), CO2 output (101 .mu.mol .cntdot. kg-1 .cntdot. min-1), bicarbonate flux (198 .mu.mol .cntdot. kg-1 .cntdot. min-1) and the fraction of administered bicarbonate recovered in breath (0.51). Comparison of variances associated with the assigment of a population mean value to a new subject and the variance associated with the use of a value obtained in the same individual on another day indicated that there was no advantage to making a determination on an alternate day over using a population value. Efforts should be made to compile population values for bicarbonate kinetics in different age groups and metabolic, nutritional and pathological states for use in interpretation of nutrient oxidation data.