Virtual absence of turnover in cadmium metabolism: Cd109 studies in the mouse

Following injection, radioisotopes of physiologically occurring elements are excreted at rates that increase upon metabolic loading with stable carrier. In contrast, Cd109 showed negligible total-body turnover in the mouse regardless of challenges with Cd or Zn, and alimentary absorption occurred irrespective of the body's Cd burden. As opposed to certain bone-seeking nonphysiological radioisotopes which also show this behavior, Cd109 concentrated primarily in nonosseous soft tissue. Intracellularly the distribution of Cd109 resembled that of Zn65, except for a lower uptake of Cd109 by the nuclear fraction. Other dissimilarities between Cd and Zn metabolism included the presence of a well-defined excretory pathway only for Zn and different effects of Cd and Zn carriers upon the organ distribution of Cd109. These findings suggest the following: a) Cd does not obey homeostatic control; b) similarities between Cd and Zn metabolism imply that Cd may function, in part, as an antimetabolite of Zn; c) significant physiological discrimination between Cd and Zn exists, at least at the level of excretory processes.