Calcium clearance as a function of sodium clearance in the dog

Abstract

During diuresis induced in dogs by water, sodium chloride, sodium bicarbonate, mannitol, glucose, or sucrose, the plasma calcium clearance is, on the average, half the sodium clearance. Free calcium ion clearance therefore equals sodium clearance. This relationship is not altered by reducing calcium or sodium intake, by varying urinary flow or urinary ionic strength, or by varying chloride excretion, independently of sodium excretion. It is only slightly affected by urinary pH, and is not distinctly altered by potassium infusion. Sodium and calcium ions are thus reabsorbed in the proportions in which they are present in plasma. However, previous observations made during sulfate diuresis, when approximately half of the urinary calcium is complexed by sulfate, suggest that the tubular cells tend to maintain a constant ratio of sodium to free calcium ions in the tubular fluid, rather than to reabsorb proportionate quantities of each. It is speculated that this may reflect competitive binding of calcium and sodium at the cell membrane.