Pathogenesis of renal hyperchloremic acidosis resulting from dietary potassium restriction in the dog: role of aldosterone

Regulation of collecting tubule adenosine triphosphatases by aldosterone and potassium

To examine the precise role of potassium and aldosterone on acid-base composition and on collecting tubule ATPases, glucocorticoid-replete adrenalectomized rats were replaced with zero, physiological, or pharmacological doses of aldosterone and were fed varying potassium diets to produce hypokalemia, normokalemia, or hyperkalemia. Radiochemical measurement of ATPase activities showed that collecting tubule H/K-ATPase changed inversely with potassium and not with aldosterone whereas H-ATPase changed directly with aldosterone but not with potassium. When both enzymes changed in the same direction, alterations in acid-base composition were profound; however, when these two acidifying enzymes changed in opposite directions or when only one enzyme changed, the effect on acid-base balance was modest. Serum bicarbonate was approximately 45 meq/liter when aldosterone was high and potassium was low; it was only 29 meq/liter when aldosterone was high but potassium was normal or when aldosterone was normal and potassium was low. Our observations may help explain the metabolic alkalosis of primary aldosteronism in which aldosterone excess and hypokalemia are combined and the metabolic acidosis of aldosterone deficiency in which hypoaldosteronism and hyperkalemia are paired. The present study also demonstrated that aldosterone plays the major role in controlling Na/K-ATPase activity in cortical collecting tubule. Hypokalemia stimulates Na/K-ATPase activity in the medullary collecting tubule; this stimulatory effect of hypokalemia supports the hypothesis that the enzyme is present on the apical membrane at this site.

Chronic metabolic acidosis increases the serum concentration of 1,25-dihydroxyvitamin D in humans by stimulating its production rate. Critical role of acidosis-induced renal hypophosphatemia

Chronic metabolic acidosis results in metabolic bone disease, calcium nephrolithiasis, and growth retardation. The pathogenesis of each of these sequelae is poorly understood in humans. We therefore investigated the effects of chronic extrarenal metabolic acidosis on the regulation of 1,25-(OH)2D, parathyroid hormone, calcium, and phosphate metabolism in normal humans. Chronic extrarenal metabolic acidosis was induced by administering two different doses of NH4Cl [2.1 (low dose) and 4.2 (high dose) mmol/kg body wt per d, respectively] to four male volunteers each during metabolic balance conditions. Plasma [HCO3-] decreased by 4.5 +/- 0.4 mmol/liter in the low dose and by 9.1 +/- 0.3 mmol/liter (P < 0.001) in the high dose group. Metabolic acidosis induced renal hypophosphatemia, which strongly correlated with the severity of acidosis (Plasma [PO4] on plasma [HCO3-]; r = 0.721, P < 0.001). Both metabolic clearance and production rates of 1,25-(OH)2D increased in both groups. In the high dose group, the percentage increase in production rate was much greater than the percentage increase in metabolic clearance rate, resulting in a significantly increased serum 1,25-(OH)2D concentration. A strong inverse correlation was observed for serum 1,25-(OH)2D concentration on both plasma [PO4] (r = -0.711, P < 0.001) and plasma [HCO3-] (r = -0.725, P < 0.001). Plasma ionized calcium concentration did not change in either group whereas intact serum parathyroid hormone concentration decreased significantly in the high dose group. In conclusion, metabolic acidosis results in graded increases in serum 1,25-(OH)2D concentration by stimulating its production rate in humans. The increased production rate is explained by acidosis-induced hypophosphatemia/cellular phosphate depletion resulting at least in part from decreased renal tubular phosphate reabsorption. The decreased serum intact parathyroid hormone levels in more severe acidosis may be the consequence of hypophosphatemia and/or increased serum 1,25-(OH)2D concentrations.

Oral intake of phosphorus can determine the serum concentration of 1,25-dihydroxyvitamin D by determining its production rate in humans

Changes in the oral intake of phosphorus could induce the reported changes in the serum concentration of 1,25-dihydroxyvitamin D (1,25-(OH)2D) by inducing changes in its production rate (PR) or metabolic clearance rate (MCR), or both. To investigate these possibilities, we employed the constant infusion equilibrium technique to measure the PR and MCR of 1,25-(OH)2D in six healthy men in whom the oral intake of phosphorus was initially maintained at 1,500 mg/70 kg body weight per d for 9 d, then restricted to 500 mg/d (coupled with oral administration of aluminum hydroxide) for 10 d, and then supplemented to 3,000 mg/d for 10 d. With phosphorus restriction, the serum concentration of 1,25-(OH)2D increased by 80% from a mean of 38 +/- 3 to 68 +/- 6 pg/ml, P less than 0.001; the PR increased from 1.8 +/- 0.2 to 3.8 +/- 0.6 micrograms/d, P less than 0.005; the MCR did not change significantly. The fasting serum concentration of phosphorus decreased from 3.5 +/- 0.2 to 2.6 +/- 0.2 mg/dl, P less than 0.01. With phosphorus supplementation, the serum concentration of 1,25-(OH)2D decreased abruptly, reaching a nadir within 2 to 4 d; after 10 d of supplementation, the mean concentration of 27 +/- 4 pg/ml was lower by 29%, P less than 0.01, than the value measured when phosphorus intake was normal. The PR decreased to 1.3 +/- 0.2 micrograms/d, P less than 0.05; the MCR did not change significantly. The fasting serum concentration of phosphorus increased significantly, but only initially. These data demonstrate that in healthy men, reductions and increases in the oral intake of phosphorus can induce rapidly occurring, large, inverse, and persisting changes in the serum concentration of 1,25-(OH)2D. Changes in the PR of 1,25-(OH)2D account entirely for the phosphorus-induced changes in serum concentration of this hormone.