Studies on the Renal Clearance of Diffusible Calcium and the Role of the Parathyroid Glands in its Regulation §

Calcium is excreted by the kidneys after tubular reabsorption of its diffusible (non-protein-bound) component. There is no evidence to date to suggest that tubular secretion of calcium occurs in the mammalian nephron. In fact, recent studies utilizing the stop-flow technique indicate that calcium is reabsorbed in both the proximal and distal portion of the nephron.' The stop-flow studies1 together with observations on the effect of osmotic diuretics (mannitol, sucrose, NaCl, and NaHCO3) and metabolic inhibitors (phloridzin, dinitrophenol, sodium azide) on calcium excretion, indicate that this ion is reabsorbed by a process of active tubular transport.23"' The complexing of calcium ion in the extracellular fluids (and therefore in the glomerular filtrate) by organic or inorganic anions such as sulfate,4 phosphate,2'3 citrate,5 or versenate,2'8 markedly decreases the tubular reabsorption of calcium and may increase calcium excretion from a normal value of 1-5 per cent to 60-80 per cent of the filtered load.2'" These observations