The Renal Excretion of Strontium and Calcium in Dogs

Renal-excretion and plasma-clearance rates of ⁸⁵Sr and ⁴⁵Ca were determined in trained female dogs 18 to 24 hours after the simultaneous intravenous or subcutaneous administration of these two isotopes. Tubular reabsorption of ⁴⁵Ca in the normal dog was nearly complete, being more than 99 per cent of that filtered at the glomeruli. The reabsorption of ⁸⁵Sr was consistently less than that of calcium, averaging less than 98 per cent. Clearance measurements, using endogenous creatinine for determining the glomerular filtration rate, indicated a more rapid clearance from the plasma of the strontium isotope (0·91 ml/min for ⁸⁵Sr and 0·17 ml/min for ⁴⁵Ca). As a result of these differences, the excretion of ⁸⁵Sr was 5 to 6 times more rapid than that of ⁴⁵Ca.

Drastic changes in plasma-calcium levels, as achieved by thyropara-thyroidectomy and parathyroid-extract injection, did not appreciably alter the ratio of the excretion rates despite marked changes in the absolute values. The infusion of sodium citrate caused large increases in the urinary excretion of both strontium and calcium and countered the renal discrimination process. The excretion of both ions was also significantly increased during alkalosis brought about by the infusion of sodium bicarbonate. Acidosis was without effect.