Divalent ion transport in dogs with experimental chronic renal failure

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Abstract
Micropuncture studies were performed to examine the segmental reabsorption of sodium, calcium, and magnesium in the normal dog kidney (stage I) and in the remnant kidney both in the presence (stage II) and in the absence (stage III) of the contralateral normal kidney. The protocol consisted of an initial phase of hydropenia, followed by 5% extracellular fluid volume expansion in the second phase, followed by parathyroid hormone administration in the final phase. In stage II dogs during hydropenia, proximal and distal transport of sodium, calcium, and magnesium were similar to those of normal dogs (stage I). Following 5% body weight volume expansion, fractional deliveries to both the proximal and distal puncture sites were increased similarly in stage 1 and stage II, with a slightly greater increase in stage II animals. In stage III dogs, proximal fractional reabsorption was depressed, as reflected by a marked reduction in proximal tubule fluid to plasma inulin ratios during hydropenia, and the response to volume expansion was accentuated. In the loop segment a constant fraction of the augmented load of sodium, calcium, and magnesium was reabsorbed in stage III. The percentage of the delivered load that was reabsorbed by this segment was similar in all three stages. The diminution in proximal reabsorption in stage III resulted in greater delivery to the distal nephron. The distal reabsorption of a constant fraction of delivered solute resulted in an increase in fractional urinary excretion of sodium, calcium, and magnesium in stage III. Parathyroid hormone significantly reduced the renal excretion of calcium and magnesium in the stage III dogs, indicating the preservation of the renal response to parathyroid hormone in azotemia.