EFFECTS OF METABOLIC-ACIDOSIS AND ALKALOSIS ON SODIUM AND CALCIUM-TRANSPORT IN THE DOG KIDNEY

Clearance and micropuncture studies have been performed in dogs to examine the effects of acute and chronic metabolic acidosis and acute alkalosis on tubular sodium and calcium transport. Acute metabolic acidosis, induced by the infusion of hydrochloric acid, decreased proximal fluid reabsorption and increased the fractional deliery of sodium and calcium to the distal tubule, but not to the final urine. In comparison with normal dogs, dogs with chronic metabolic acidosis (induced by feeding ammonium chloride) showed an increase in proximal fluid reabsorption more distally, leading to an increased delivery of calcium relative to sodium at the distal tubule and in the final urine. The infusion of sodium bicarbonate to correct chronic metabolic acidosis, both in intact and thyroparathyroidectomized (TPTX) dogs, reduced proximal fluid reabsorption and caused a selective enhancement of calcium reabsorption relative to sodium in the more distal nephron, resulting in a reversal of the dissociation observed in acidosis, both at the distal tubule and in the final urine. By contrast, infusion of sodium chloride in parathyroid-intact acidotic dogs did not reduce proximal fluid reabsorption or enhance tubular calcium reabsorption. In nonacidotic dogs, both intact and TPTX, infusion of sodium bicarbonate to induce acute alkalosis resulted in selective enhancement of calcium over sodium reabsorption in the distal nephron segments. These data demonstrate the presence of a component of tubular calcium reabsorption situated beyond the proximal tubule, which is inhibited by chronic (but not acute) metabolic acidosis and enhanced by metabolic alkalosis (or bicarbonate infusion) independently of parathyroid hormone.