The excretion of selenium in bile and urine of steers: the influence of form and amount of Se salt.

1. The excretion of 75Se and stable Se in bile and urine was measured in four steers during 6 h after intravenous injections of 75Se as either selenite or selenate containing either 5 or 5000 microgram carrier Se. 2. When 5000 microgram Se were given, the rate of urinary excretion and plasma clearance of 75SE was similar for both salts. Approximately 23% was excreted in urine and plasma clearance was triexponential, the mean half-life (t 1/2) of the successive components, alpha, beta, and gamma, being 2.3, 15-2 and 465 min respectively. The amount of 75Se excreted in bile was small; 1.94% of the 75SeO3(2-) and 0.86% of the 75SeO4(2-) dose. 3. When 5 microgram Se were given the plasma clearance of 75Se was initially biexponential but the entry of 75Se-labelled protein from the liver caused an increase in plasma radioactivity after 30-40 min. The effect was most marked after 5 microgram 75SeO3(2-) when plasma 75Se radioactivity returned to 69% of the activity present at 2 min. Values for t 1/2 of the two components of clearance for 75SEO4(2-) were respectively alpha 2.6 and 2.5 min, and beta 15.9 and 36.6 min. Similar amounts of 75Se appeared in bile (0.2% of the dose) after injection of either salt but much less 75Se was excreted in urine after 75SeO3(2-) (6%) than after 75SeO4(2-) (17%). 4. At low dosage rates (5 microgram) Se is more readily incorporated into tissues from SeO3(2-) than from SeO4(2-).