Disorders of distal nephron function

Renal tubular acidosis type IV in hyperkalaemic patients--a fairy tale or reality?

OBJECTIVE

Hyperkalaemia is a common feature in hospitalized patients and often attributed to drugs antagonizing the renin-angiotensin-aldosterone system (RAAS) and/or acute kidney injury (AKI), despite significantly preserved glomerular filtration rate (GFR). The objective of this study was to determine the prevalence and role of renal tubular acidosis type IV (RTA IV) in the development of significant hyperkalaemia.

DESIGN

A single-centre retrospective study.

PATIENTS

Patients admitted to a University Hospital over 12 months.

MEASUREMENTS

Patients with a potassium value > 6·0 mm were identified. Clinical and laboratory data were revisited, and patients with a normal anion gap metabolic acidosis were evaluated for the existence of RTA IV.

RESULTS

A total of 57 patients having significant hyperkalaemia (>6·0 mm) were identified. Twelve patients had end-stage renal disease, while 21 patients had solely AKI or progressive chronic renal failure. RTA IV was present in 24 patients (42%), of whom 71% had pre-existing renal insufficiency because of diabetic nephropathy or tubulointerstitial nephritis. All hyperkalaemic patients with urinary/serum electrolytes suggestive of RTA IV had evidence of AKI, but creatinine levels were significantly lower (P < 0·05), while the number of drugs antagonizing the RAAS was comparable.

CONCLUSION

We demonstrated that RTA IV (i) is very common in patients with hyperkalaemia; (ii) should always be suspected in hyperkalaemic patients with only moderately impaired GFR; and (iii) may result in significant hyperkalaemia in the presence of both AKI and drugs antagonizing the RAAS.

Induction of Anti-Carbonic-Anhydrase-II Antibody Causes Renal Tubular Acidosis in a Mouse Model of Sjögren’s Syndrome

BACKGROUND/AIM

We recently reported that renal tubular acidosis (RTA) in Sjogren's syndrome (SjS) is associated with high titers of an autoantibody against carbonic anhydrase (CA) II, an important enzyme in renal acid-base regulation. The purpose of this study was to determine whether a CA-II antibody could cause RTA in a mouse model of SjS.

METHODS

PL/J mice were immunized with human CA II to induce CA II antibody formation, whereas controls were injected with phosphate-buffered saline and adjuvant. After 6 weeks, anti-CA-II antibody titers were measured, then ammonium chloride was administered orally for 1 week to detect any acidification defect.

RESULTS

CA-II-immunized mice showed higher anti-CA-II antibody titers than control mice. Pathologically, lymphocytic and plasma cell infiltration was seen in the salivary glands and kidneys of CA-II-immunized mice, but not in controls. On acid loading, blood pH and urine pH decreased in both groups of mice, but the slope of urine pH versus blood pH was less steep in the CA-II-immunized mice, suggesting that these mice had an impaired ability to reduce their urine pH in the face of metabolic acidosis.

CONCLUSION

CA-II-immunized mice had a urinary acidification defect, which may be similar to that seen in patients with SjS.

Early childhood hyperkalemia: variety of pseudohypoaldosteronism

Fractional excretion of electrolytes, renal acidification capacity and the renin-aldosterone system have been studied in 5 non-azotemic children, 19-25 months old, with mineralocorticoid resistant hyperkalemia, discovered in the first month of life. Although fractional potassium excretion was similar in patients and in a group of control healthy children (13.8 +/- 5.2% vs. 8.7 +/- 6.4%) it was inappropriately low in the patients for their higher potassium concentration. Fractional sodium excretion was significantly increased in the patients (1.6 +/- 0.3% vs. 0.67 +/- 0.4, p less than 0.02). Normal net acid and ammonium excretion and intact ability to lower urinary pH during acid loading were observed in all patients. Mean values for plasma aldosterone (37.0 +/- 9.1 vs. 13.9 +/- 11.2 ng/dl), plasma renin activity (12.5 +/- 3.9 vs. 8 +/- 2.8 ng/ml/h) and plasma aldosterone/plasma potassium ratio (7.11 +/- 1.5 vs. 3.08 +/- 1.7) were higher in the patients than in the control subjects (all p less than 0.001). These data support the hypothesis that a partial lack of response of the renal tubule to endogenous mineralocorticoids was present in the patients. This type of pseudohypoaldosteronism is less severe than that described for the classic form and for early childhood renal acidosis.

[Primary hypoaldosteronism, pseudo-hypoaldosteronism and distal tubular acidosis]

Aldosterone deficiency is caused by various defects of aldosterone biosynthesis in the adrenal gland or hyporeninism. The most important symptoms are hyponatremia and hyperkalemia. These electrolyte disturbances are also found in pseudohypoaldosteronism. Pseudohypoaldosteronism type I is characterized by insensitivity of the distal nephron for aldosterone. Hyperabsorption of chloride in the distal nephron leads to pseudohypoaldosteronism type II, which is linked with hypertension, whereas blood pressure in the other mentioned disorders is decreased. Renal tubular acidosis, mainly type 4, with impaired production of ammonia due to hyperkalemia, is frequently observed in hypoaldosteronism and both types of pseudohypoaldosteronism as well. The therapeutic regimen is different: low doses of fludrocortisone in hypoaldosteronism, potassium restriction, sodium bicarbonate and loop diuretics in type I of pseudohypoaldosteronism, and sodium restriction and chloruretic diuretics (thiazide) in type II of pseudohypoaldosteronism. In some cases hyperkalemia requires the use of potassium-binding resins.

[Primary hypoaldosteronism and secondary pseudo-hypoaldosteronism]

We observed a 23-year-old man with pronounced hyperkalemia (max. 6.8 mmol/l) and hyponatremia (min. 112 mmol/l), which had been existent for 3 years without complaint except a transitory psychorganic syndrome due to hyponatremia. Physical examination showed no abnormality except hypotension (blood pressure 100/70 mmHg). Renal function tests were normal. Fractional clearance of sodium was significantly increased (0.8%), whereas that of potassium was decreased (2.4%). Plasma renin activity was tripled and rose after furosemide. Plasma aldosterone was lowered and showed no rise after furosemide. Suppression of plasma renin and aldosterone by saline infusion was normal. Pressor dose of angiotensin II was increased (17,9 ng AT II/kg/min). Urinary excretion of aldosterone and its conjugates was below normal, and aldosterone precursors were within normal range. The findings were interpreted as selective primary hypoaldosteronism caused by corticosterone methyl oxidase defect type II. However, neither fludrocortisone (0.5 mg/day) nor sodium chloride (200 mmol/day) led to a normalization of sodium and potassium in plasma. Additional pseudohypoaldosteronism was thus assumed. Aldosterone infusion (3 mg in 1 h) decreased renal excretion of sodium; potassium excretion failed, however, to increase in contrast to its pattern in normal man. These findings resemble additional pseudohypo-aldosteronism of type II. After 8 weeks' application of additional 80 mmol sodium (as sodium bicarbonate) plasma sodium and potassium showed normal values under combined treatment with fludrocortisone (0.1 mg/day) and sodium bicarbonate (80 mmol/day). It is to be assumed that the patient suffers from a reduced aldosterone biosynthesis in the presence of an additional transitory secondary pseudohypoaldosteronism.

Acid-base physiology in uremia

Chronic renal failure represents the most common disorder responsible for chronic stable metabolic acidosis. This type of metabolic acidosis has been characterized as "hyperchloremic" in pre-end-stage disease and the "anion-gap" form as the GFR falls below 20 ml/min. The early hyperchloremic, hyperkalemic variety may result from disease of the juxtaglomerular apparatus, a distal acidification defect, or volume depletion. The anion-gap acidosis of advanced renal disease occurs as a result of the inability of the diminished nephron mass to keep pace with the metabolic acid load which depletes extracellular fluid bicarbonate. Total ammonium excretion diminishes despite an adaptive increase in ammonia production per nephron. The observation that the serum bicarbonate rarely falls below 15 mEq/L and the anion gap stays below 20 mEq/L despite positive hydrogen ion balance attests to the important role of extrarenal buffers. Bone buffers, primarily calcium carbonate, titrate a portion of the excess hydrogen ions at the expense of progressive loss of bone salts. Parathyroid hormone (PTH) appears to be involved in the control of bone buffering capacity. Both PTH-dependent and PTH-independent mechanisms must therefore be considered. PTH mediates bone buffering capacity by activating intracellular shifts of calcium, phosphorus, and carbonate or by stimulation of bone carbonic anhydrase. A direct effect of pH on bone mineral mobilization has been demonstrated. Adequate alkali therapy to maintain serum bicarbonate levels of 20-22 mEq/L may prevent bone dissolution and minimize risk of volume overload.