Calcium reabsorption in proximal tubule of the dog nephron

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Abstract
This experiment was performed to study the reabsorption of calcium in the dog proximal tubule and further evaluate the relationship between sodium and calcium transport. A microadaptation of a fluoro-metric method was developed to determine the concentration of calcium in tubule fluid. Calcium concentration was measured in tubule fluid samples and compared with simultaneous values in serum ultra-filtrates in normal, and in mannitol- and saline-loaded dogs. No correlation was found between TF/UF calcium ratio and location along the proximal tubule. The mean TF/UF calcium concentration ratio in nondiuretic dogs was 1.08 (SD [plus or minus] 0.12). The mean TF/UF calcium concentration ratios obtained in mannitol- and saline-loaded dogs was 0.92 (SD [plus or minus] 0.14) and 0.91 (SD [plus or minus] 0.05), respectively, both of which are significantly less than that observed in nondiuretic dogs. It was also demonstrated that the urinary excretion of calcium is dependent on the filtered load and closely follows the pattern of sodium excretion. Tubular inhibition of sodium transport in the proximal tubule during isotonic saline loading was accompanied by a smaller but significant inhibition of calcium reabsorption which was sufficient to account for the increase in calcium excretion in the urine.

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