Evidence for the Existence of Non-exchangeable Calcium in Human Urine

Abstract

IT is customary to interpret the results of kinetic metabolic studies with radioactive tracers in terms of compartmental models. In the case of calcium, it is usual to assume the existence of a central pool of exchangeable calcium the specific activity of which is equal to that of the plasma. Several workers have found discrepancies between specific activities of plasma and urine samples in various species^1–5 though others have not^6,7. As far back as 1930 Brull⁸ suggested that “what is excreted and concentrated by the kidney is the very part of the calcium which is diffusible but inactive and useless”. If this were the case, it would seem likely that any differences between urinary and plasma calcium specific activities might be attributable to the presence of calcium in the central pool in chemical forms which do not exchange, or exchange only slowly, with ionized calcium. Recently, Giese and Comar⁵ have demonstrated that there is, in fact, a non-exchangeable calcium fraction in sheep plasma. In the present work, it is shown that a similar non-exchangeable fraction occurs in normal human urine and presumably in the plasma.