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1 Metabolism Unit, Shriners Burns Institute, and University of Texas Medical Branch, Galveston, Texas 77550-2725; and 2 Institute of Clinical Physiology, National Research Council, Pisa 56100, Italy
The most common approach for estimating
substrate rate of appearance
(Ra) is use of the single-pool
model first proposed by R. W. Steele, J. S. Wall, R. C. DeBodo, and N. Altszuler. (Am. J. Physiol. 187: 15-24, 1956). To
overcome the model error during highly non-steady-state conditions due
to the assumption of a constant volume of distribution (V), two
strategies have been proposed: 1)
use of a variable tracer infusion rate to minimize tracer-to-tracee
ratio (TTR) variations (fixed-volume approach) or
2) use of two tracers of the same
substrate with one infused at a constant rate and the other at a
variable rate (variable-volume approach or approach of T. Issekutz, R. Issekutz, and D. Elahi. Can. J. Physiol. Pharmacol. 52:
215-224, 1974). The goal of this study was to compare the results
of these two strategies for the analysis of the kinetics of glycerol
and glucose under the non-steady-state condition created by a constant
infusion of epinephrine (50 ng · kg
1 · min
1)
with the traditional approach of Steele et al., which uses a constant
infusion and fixed volume. The results showed that for glucose and
glycerol the estimates of Ra
obtained with the constant and the variable tracer infusion rate and
the equation of Steele et al. were comparable. The variable tracer
infusion approach was less sensitive to the choice of V in estimating
Ra for glycerol and glucose,
although the advantage of changing the tracer infusion rate was greater
for glucose than for glycerol. The model of Issekutz et al. showed
instability when the ratio
TTR1/TTR2
approaches a constant value, and the model is more sensitive to
measurement error than the constant-volume model for glucose and
glycerol. We conclude that the one-tracer constant-infusion technique
is sufficient in most cases for glycerol, whereas the one-tracer variable-infusion technique is preferable for glucose. Reasonable values for glucose Ra can be
obtained with the constant-infusion technique if V = 145 ml/kg.
stable isotopes; epinephrine; glycerol; glucose
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