Metabolic alkalosis in models of primary and secondary hyperparathyroid states.

Hyperchloremic metabolic acidosis has been reported in clinical states of primary and secondary hyperparathyroidism (HPT). Acute administration of parathyroid hormone (PTH) decreases renal acidification in humans and dogs, but the renal and systemic acid-base effects of chronic HPT have not been extensively investigated. In chronically thyroparathyroidectomized (TPTX) dogs (group I), bPTH 1-5 U/kg twice daily resulted in sustained hypophosphatemia, hypercalcemia, and Cl- -resistant metabolic alkalosis that was of renal origin at least in part: delta [HCO3-]p + 4.1 +/- 0.8 meq/liter, P less than 0.01; delta [H+]p -4 +/- 1 neq/liter, P less than 0.001, days 10-12. The cumulative change (sigma delta) in net acid excretion (NAE) was +44 meq (day 9, P less than 0.05). Similarly, metabolic alkalosis of renal origin, at least in part, occurred when PTH was administered by chronic continuous intravenous infusion (group II). Since chronic administration of calcitriol in dogs results in metabolic alkalosis, plasma calcitriol concentration was measured and found not to be increased by chronic intravenous PTH administration. In intact dogs (group III), a continuous chronic intravenous infusion of the Ca2+ chelator, Na4EGTA (3.0 mmol/kg daily), substituted for an equimolar amount of prechelated EGTA (CaNa2EGTA), resulted in a model of hypocalcemic HPT and severe Cl- -resistant metabolic alkalosis: delta [HCO3-]p +9.1 +/- 1.9 meq/liter, P less than 0.05; delta [H+]p -5 +/- 1 neq/liter, P less than 0.01, days 6-8. NAE decreased significantly. Thus, whereas metabolic alkalosis induced by PTH administration could be accounted for by increased NAE (group I), EGTA-induced metabolic alkalosis was accounted for by an extrarenal mechanism of base input to extracellular fluid (group III). Neutralization of the extrarenal base input by chronic administration of HCl during the period of EGTA-induced HPT did not preclude the development of metabolic alkalosis (group V), suggesting that a renal component was present in EGTA-induced metabolic alkalosis as well as in models of primary HPT (groups I and II). During the steady state, in this group as in the groups administered PTH, the net endogenous load of acid to the systemic circulation requiring renal excretion was unchanged from control, as indicated by stable values of NAE not significantly different from control. Yet metabolic alkalosis persisted in the steady state.(ABSTRACT TRUNCATED AT 400 WORDS)

Renal and systemic acid-base effects of chronic hypoparathyroidism in dogs.

MEtabolic alkalosis has been reported in patients with chronic hypoparathyroidism under conditions of uncontrolled diet and medication intake. Hypoparathyroidism has also been reported to result in increased renal bicarbonate reabsorptive capacity in acutely bicarbonate-loaded dogs. However, the acid-base effects of experimentally induced chronic hypoparathyroidism have not been investigated in any species. Accordingly, we investigated the chronic effects of hypoparathyroidism by thyroparathyroidectomy (TPTX) plus thyroxine replacement on renal regulation of plasma acid-base composition under metabolic balance conditions of normal dietary acid load (group I) and alkali load (group II, 9.0 meq/kg HCO3(-) daily) in dogs ingesting a normal Cl-, high Ca2+ diet throughout study. For groups I and II pre-TPTX: [HCO3(-)]p, 19.7 +/- 1.0, 20.1 +/- 0.9 meq/liter. Plasma acid-base composition (days 5-10) was unchanged by TPTX: delta [HCO3(-)]p, -0.7 +/- 0.4, 0.0 +/- 0.2 meq/liter; delta [H+]p, 0 +/- 1, -1 +/- 0 neq/liter, NS from control. A reduction in plasma total calcium concentration ([CaT]p) occurred and persisted (group I: [CaT]p, -1.6 +/- 0.2 mg/100 ml, P less than 0.01, day 1 and -1.2 +/- 0.9, days 5-10; group II: -1.4 +/- 0.3 mg/100 ml, P less than 0.01, day 1 and -2.3 +/- 0.4, days 5-10). No significant change in net acid or Cl- excretion occurred following TPTX. Thus, chronic hypoparathyroidism characterized by a chronic reduction in [CaT]p does not result in significant alterations in renal regulation of plasma acid-base composition in the dog.

Pathophysiology of chronic renal tubular acidosis induced by administration of amiloride.

Amiloride is a "potassium-sparing" diuretic agent of moderate natriuretic potency with site of action in postmacula densa segments of the distal nephron. In isolated segments of mammalian cortical distal nephron, amiloride diminishes sodium reabsorption and transtubular electrical PD and inhibits potassium secretion. We investigated the effects of long-term administration of a demonstrably maximal dose of amiloride (1.0 mg/kg b.i.d.) on plasma and urine acid-base and electrolyte composition in fixed steroid-replaced ADX dogs. Amiloride administration resulted in potassium retention and hyperkalemia and reduced net acid excretion and caused chronic hyperchloremic metabolic acidosis. The cumulative reduction in net acid excretion and severity of systemic acidosis were not significantly different in additional groups in which potassium retention was prevented by restriction of dietary potassium during amiloride administration or in which amiloride was administered to animals with pre-existing dietary potassium depletion. The response of urine pH and ammonium excretion, however, differed among groups. In the steady state of chronic acidosis, urine pH and ammonium concentration were lowest in the hyperkalemic group and highest in the hypokalemic group, and among the three groups pH and ammonium were positively correlated (r = 0.67, p less than 0.001). Ammonium concentration varied inversely with plasma potassium concentration. Net acid excretion rates returned to control levels during the steady state of chronic amiloride-induced acidosis in the three groups. During continued amiloride administration, sustained correction of acidosis by long-term oral administration of sodium bicarbonate did not result in negative values of net acid excretion; that is, amiloride did not cause net wasting of base at normal plasma bicarbonate concentration. The results of these studies suggest that chronic amiloride administration results in a sustained impairment of renal hydrogen ion secretion restricted to the distal nephron and not dependent on alterations in potassium balance. Differences in potassium balance (positive or negative) appeared to influence only the availability of ammonia for diffusion into urine and steady-state urine pH, but not the steady-state net rate of renal hydrogen ion secretion during amiloride. These studies identify an experimental model of chronic distal renal tubular acidosis in which external hydrogen ion balance is re-established during chronic acidosis even when the availability of ammonia for excretion is decreased.

Correction of metabolic acidosis by the kidney during isometric expansion of extracellular fluid volume.

In dogs with chronic hypochloremic metabolic alkalosis associated with ECFV contraction, plasma [HCO-3] ([HCO-3]p) normalizes during expansion of ECFV with a solution containing Cl- and HCO-3 in concentrations duplicating those in the plasma before expansion (isometric expansion). The kidney selectively rejects administered HCO-3 and retains Cl-. If this preferential Cl- less than HCO-3 reabsorptive selectivity were a characteristic renal response to ECFV expansion, isometric expansion during hyperchloremic acidosis would exacerbate the acid-base disturbance rather than correct it as it does in alkalosis. We examined the effect of isometric expansion in dogs with chronic hyperchloremic metabolic acidosis induced by HCl feeding or mineralocorticoid hormone deficiency. During expansion, as the expected decrease occurred in the fractional reabsorption of Na+, a lesser decrease occurred in fractional reabsorption of HCO-3, whereas a greater decrease occurred in fractional reabsorption of Cl-. The kidney selectively retained administered HCO3 and rejected Cl-. [HCO-3]p normalized. The shift to bicarbonate-selective from chloride-selective anion reabsorption during ECFV expansion in metabolic acidosis vs. metabolic alkalosis indicates that in response to ECFV expansion- the kidney selectively alters the ratio of bicarbonate to chloride concentration in the tubular reabsorbate in the direction that tends to normalize plasma acid-base composition, irrespective of the direction of deviation of the initial plasma bicarbonate concentration. The signal that initiates the shift in anion reabsorptive selectivity remains to be identified.

Impaired renal H+ secretion and NH3 production in mineralocorticoid-deficient glucocorticoid-replete dogs.

When the administration of exogenous mineralocorticoid hormones was discontinued in adrenalectomized dogs maintained on glucocorticoid, net acid excretion decreased due largely to a reduction in urinary ammonium excretion (UNH4+V), and hyperchloremic hyperkalemic metabolic acidosis occurred and persisted. The reduction in UNH4+V was not associated with an increase in urine pH (UpH) or a decrease in urine flow, but correlated with the severity of hyperkalemia and was mitigated by dietary potassium restriction. UpH decreased to values as low as 5.3. During acidosis, UpH varied directly with UNH4+V, but in relation to UNH4+V, UpH exceeded that in acid-fed mineralocorticoid-replete dogs. Extrapolated to UNH4+V=0, however, UpH was not significantly different in the two groups (5.27 vs. 5.44). When distal delivery of sodium was increased by infusion of sodium phosphate, titratable acid excretion increased in both groups but pateaued at lower rates in the mineralocorticoid-deficient dogs. These results suggest that in mineralocorticoid-deficient dogs, renal ammonia production is diminished, in part due to potassium retention and hyperkalemia; renal hydrogen ion secretory capacity is reduced even when sodium and buffer delivery to the distal nephron is not reduced; and the ability of the kidney to generate normally steep urine-to-blood hydrogen ion concentration gradients is unimpaired.