Kinetic modeling of selenium metabolism in nonpregnant ewes

The kinetics of selenium metabolism in three nonpregnant ewes were studied by the intravenous injection of 75Se-sodium selenite and measurement of radioactivity responses in blood, tissues and excreta. Stable selenium measurements were also made to determine selenium intake, excretion in feces and urine, and mass of selenium in tissues. Immediately following tracer injection, there was a rapid disappearance of radioactivity from plasma reflecting the uptake of the element by the liver and blood cells. The decrease in plasma radioactivity ceased abruptly by 30-45 min, and was followed by an increase to a peak by 3-4 h and a more gradual biphasic decline thereafter. A kinetic model of selenium metabolism in the whole animal was constructed employing the SAAM/CONSAM computer program. The multiphasic response of plasma radioactivity during a physiological steady state was explained on the basis of rapid hepatic uptake of selenium and its subsequent reappearance in the circulation in protein-bound form followed by further metabolism and excretion of the element. The model provides reference parameter values for 75Se-sodium selenite kinetics in selenium-replete, mature nonpregnant ewes for comparison with the kinetics in animals whose selenium status may be altered.