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Comparison of the Magnitude of the Selenite-Exchangeable Metabolic Pool and Whole Body Endogenous Selenium in Adult Rats¹

Clinical Nutrition Research Center, Department of Medicine, University of Chicago, Chicago, IL 60637

The quantitative relationship between the size of the selenite-exchangeable metabolic pool (W_Se-EMP) and whole body endogenous selenium (Se_end) 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 W_Se-EMP was determined from plasma or urine isotope ratios. In Experiment 1, with plasma as the sampling compartment, W_Se-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, W_Se-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.

KEY WORDS: • selenium • selenite • isotope dilution • stable isotopes • rats

¹ Work supported by public health service grants R01-CA38943 and DK-26678.

Manuscript received 8 May 1989. Revision accepted 2 October 1989.