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Anesthetized rats and hamsters were given Ca 45 intravenously, and fluid was subsequently collected by micropuncture from glomeruli and surface tubules in the rats, and from loops of Henle in the hamsters. In nondiuretic animals, fluid:plasma calcium ratios averaged 0.71 in the glomerulus; 0.76 in the proximal tubule; 2.0 in the loop of Henle; 0.47 in the distal convolution; and 0.9 in ureteral urine. In mannitol diuresis, the calcium ratio of glomerular fluid was unchanged, but ratios as low as 0.21 were noted in the proximal tubule. In this circumstance, the average proximal ratio was 0.61, and the distal ratio 0.07. These results indicate active transport of calcium out of all major parts of the nephron, with the bulk of calcium reabsorption occurring in the convoluted portion of the proximal tubule. Furthermore, the pattern of tubular reabsorption of calcium is similar to that of sodium, suggesting that the two are related.
Lassiter et al. (Wed,) studied this question.