Extracellular water measurements: organ tracer kinetics of bromide and sucrose in rats and man.

Bromide and sucrose distributions were measured as functions of time after tracer injection into 14 rat organs that accounted for 93% of body wt, with the goal of evaluating the use of bromide and sucrose as tracers for the extracellular water volume (ECW). The tracers, Na, 82Br, 125I-labeled human serum albumin, [14C]sucrose, and 3H2O, were used to calculate bromide and sucrose content in red cells, plasma, and 13 blood-free organs. Selective concentration of Br- occurs in RBC, stomach, and skin, accounting in part for the discrepancy between the Br- space and the smaller ECW volume as derived from other methods. Sucrose is rapidly metabolized in the rat and its 14C tracer cannot be used for ECW determination in this species. The kinetics of Br- distribution were estimated in rats and in 16 human subjects by measuring plasma disappearance values and specific organ uptakes. A pharmacokinetic compartmental model was derived, containing explicit parameters for blood flow, diffusion constants, and ECW spaces separately for each organ. Precise fitting of experimental bromide data was achieved for the rat; satisfactory fitting was also achieved in man from more limited plasma and biopsy data.