AJP - Endocrinology and Metabolism, Vol 258, Issue 6 E1006-E1013, Copyright © 1990 by American Physiological Society
Estimation of glucose kinetics in fetal-maternal studies: potential errors, solutions, and limitations
R. K. Menon, C. A. Bloch and M. A. Sperling
Division of Endocrinology, Children's Hospital Medical Center, Cincinnati, Ohio.
We investigated whether errors occur in the estimation of ovine
maternal-fetal glucose (Glc) kinetics using the isotope dilution technique
when the Glc pool is rapidly expanded by exogenous (protocol A) or
endogenous (protocol C) Glc entry and sought possible solutions (protocol
B). In protocol A (n = 8), after attaining steady-state Glc specific
activity (SA) by [U-14C]glucose (period 1), infusion of Glc (period 2)
predictably decreased Glc SA, whereas. [U-14C]glucose concentration
unexpectedly rose from 7,208 +/- 367 (means +/- SE) in period 1 to 8,558
+/- 308 disintegrations/min (dpm) per ml in period 2 (P less than 0.01).
Fetal endogenous Glc production (EGP) was negligible during period 1 (0.44
+/- 1.0), but yielded a physiologically impossible "negative" value of -2.1
+/- 0.72 mg.kg-1.min-1 during period 2. When the fall in Glc SA during Glc
infusion was prevented by addition of [U-14C]glucose admixed with the
exogenous Glc (protocol B; n = 7), EGP was no longer "negative." In
protocol C (n = 6), sequential infusions of four increasing doses of
epinephrine serially decreased SA, whereas tracer Glc increased from 7,483
+/- 608 to 11,525 +/- 992 dpm/ml plasma (P less than 0.05), imposing an
obligatory underestimation of EGP. Thus a tracer "mixing" problem leads to
erroneous estimations of fetal Glc utilization and Glc production via the
three-compartment model in sheep when the Glc pool is expanded exogenously
or endogenously. These errors can be minimized by maintaining the Glc SA
relatively constant.
