Influence of ovariectomy on extracellular fluid volume in rats: assessment of extracellular fluid volume by means of bromide

There is considerable evidence that the extracellular fluid volume (ECV) may change in disease states or during longitudinal Intervention studies. Therefore, the measurement of ECV is Important for studying body composition in patients and laboratory animals. We present a modified plasma bromide (Br-, non-radioactive) assay using anion-exchange chromatography, in which a small blood sample of 200 microL (100 microL of plasma) appeared to be enough to reproducibly measure ECV. The inter- and intra-assay accuracy of the Br- analysis ranged from -1.6% to 0.9% and from -0.5% to 0%, respectively. The inter- and intra-assay precision ranged from 1.3% to 1.7% and from 0.6% to 1.2%, respectively. This modified precise and accurate Br- analysis in a small blood sample was applied to investigate whether the ECV changed in rats after ovariectomy. Ovariectomy significantly (P < .05) reduced the ECV as compared with results in SHAM rats. This observation indicates that a change in clinical condition may change ECV, which has consequences for the determination of, for example, the fractional absorption and the relative bioavailability of compounds principally distributed through the ECV.