ENDOGENOUS EXCRETION AND TRUE ABSORPTION OF COBALT AS AFFECTED BY THEORAL SUPPLY OF COBALT

At the end of a 49-d experiment with 32 growing male rats, a period of 8 d was used to determine endogenous excretion and true absorption as well as apparent absorption and retention of cobalt with the aid of t he isotope dilution technique. For this purpose, a single im dose of C o-58 was applied at d 35 of the experiment. After that, urine and fece s were collected separately from d 8 to 15 after injection of the isot ope. The specific cobalt activity of the liver was used as an endogeno us reference source. The basal diet provided 5.9 ppb cobalt, the diffe rent treatment groups were obtained by supplementing the diet with 0, 10, 50, 250, or 1250 ppb cobalt. The different diets were offered from the beginning of the experiment. In the balance period, apparent and true absorption as well as fecal excretion behaved similar to cobalt i ntake, whereas urinary excretion increased more rapidly with increasin g cobalt supply. Endogenous fecal excretion accounted for 3.5 ng Co/d in the groups fed the diets without and with 10 ppb cobalt. An increas e was not observed until supplementing the diet with 50 ppb cobalt. Th is increase between 250 and 1250 ppb cobalt was higher than the corres ponding increase in the dietary cobalt supply. This indicates that end ogenous fecal excretion might be more important for homeostatic regula tion at a higher dietary cobalt concentration. Endogenous renal excret ion as calculated from the results of the isotope dilution technique s howed a similar kind of response to increasing cobalt supply as endoge nous fecal loss. Nevertheless, the elimination of excessive cobalt mai nly took place by adjusting urinary excretion, whereas the variations in true absorption and endogenous fecal excretion had no quantitative importance.