D2O as a substitute for 3H2O, as a reference indicator in liver multiple-indicator dilution studies.

The hepatic space of distribution and the processes underlying uptake of tracer substrate may be appraised by the multiple-indicator dilution technique after simultaneous injection of noneliminated vascular (51Cr-labeled red blood cells), extracellular 125I-labeled albumin and [14C]sucrose (or [58Co]EDTA) as high- and low-molecular-weight interstitial references, respectively], and cellular (3H2O or [14C]urea) indicators, together with the tracer-labeled substrate. The use of 3H2O or [14C]urea, with [14C]- or [3H]sucrose, however, precludes the simultaneous introduction and analysis of the behavior of 3H- and 14C-labeled substrate and metabolite. An assay for the quantitation of D2O in plasma by Fourier transform infrared spectrometry was therefore developed such that D2O could be used in lieu of 3H2O in multiple-indicator dilution studies in the blood-perfused rat liver. In experiments performed with an injection dose containing 51Cr-labeled red blood cells, 125I-labeled albumin, [14C]sucrose, 3H2O, and D2O, D2O was found to behave virtually identical to 3H2O in blood and liver; the accessible cellular water spaces were 0.625 and 0.621 ml/g liver for 3H2O and D2O, respectively, and the corresponding ratios of the sum of the cellular water plus the interstitial water space to the sinusoidal water space were 3.87 and 3.89. D2O was found to be an ideal substitute and is much superior to [14C]urea, which exhibits a small red blood cell carriage effect and which is slightly less dispersed than 3H2O.