A Compartmental Model Depicting Short-Term Kinetic Changes in Selenium Metabolism in Ewes Fed Hay Containing Normal or Inadequate Levels of Selenium

Changes in Se metabolism were studied in ewes fed hay containing normal or inadequate levels of Se. After intravenous injection of 75 Se-sodium selenite, blood, feces and urine were collected at different times, and the concentrations of labeled and unlabeled Se were determined. Ewes were killed 1, 5, 9 or 14 d after tracer injection, and tissues were obtained for determination of radioactivity and Se concentration. The data were fitted to a compartmental model using the SAAM/CONSAM computer program, and kinetic parameters and steady-state transport rates were estimated. Daily Se intake (V,) and fecal excretion (V F ) were significantly (P < 0.001) higher in the ewes fed normal hay (6.06 ± 1.09 and 3.36 ± 0.88 μmol/d, respectively) than in those fed Se-deficient hay (0.64 ± 0.18 and 0.26 ± 0.15 μmol/d). The net absorption (V a ) of Se was significantly higher in ewes fed normal hay [3.19 ± 0.82 μmol/d by the balance method, V a = V 1 - (V F - V f ) (V f = endogenous fecal Se) and 1.05 ± 0.38 μmol/d by using the model (plasma entry rate, U(1))] than in those fed hay deficient in Se [0.57 ± 0.33 μmol/d (balance) and 0.28 ± 0.08 μmol/d (model)]. The efficiency of absorption [α = U(1)? V i ] was significantly higher (0.46 ± 0.19) in ewes fed Se-deficient hay than in those fed normal hay (0.18 ± 0.09). Simultaneous fitting of the tracer data of both the groups showed that changes in hepatic extraction and urinary and fecal excretion were sufficient and necessary to account for the kinetic differences observed between treatments.