Comparison of the Magnitude of the Selenite-Exchangeable Metabolic Pool and Whole Body Endogenous Selenium in Adult Rats

The quantitative relationship between the size of the selenite-exchangeable metabolic pool (WSe-EMP) and whole body endogenous selenium (Seend) was investigated in adult male rats. Two experiments based on multiple labeling with stable isotopes were performed. One focused on short-term (7 d, Expt. 1) and the other on long-term (60 d, Expt. 2) relationships. Rats were fed a Torula yeast diet and water supplemented with [76Se]selenite at 0.1 µg Se/mL; the in vivo [74Se]selenite tracer was administered orally. Groups of three or four animals were killed at timed intervals and whole carcass or selected organs were analyzed for the stable isotopes 74Se, 77Se and 82Se with hydride generation/inductively coupled plasma mass spectrometry. The value of WSe-EMP was determined from plasma or urine isotope ratios. In Experiment 1, with plasma as the sampling compartment, WSe-EMP at 24 h was 36.5 ± 1.2% of the baseline value of whole body endogenous selenium (Se·end) and 36.3 ± 1.8% at 7 d. When urine was the sampling compartment, the corresponding values were 3.9 ± 0.3% and 43.1 ± 2.8%, respectively. In Experiment 2, WSe-EMP (plasma) was 38.9 ± 1.3% of Se·end at 7 d, increasing to 45.5 ± 1.6% at 60 d. The corresponding values for urine as the sampling compartment were 45.5 ± 2.0% (7 d) and 61.5 ± 1.7% (60 d), respectively.