Defects of Hydrogen Ion Secretion & Bicarbonate Reabsorption (Classic Renal Tubular Acidosis, Type I)
Failure to secrete hydrogen ion and to form ammonium ion results in loss of “fixed base” sodium, potassium, and calcium. There is also a high rate of excretion of phosphate. Vomiting, poor growth, and symptoms and signs of chronic metabolic acidosis are accompanied by weakness due to potassium deficit and bone discomfort due to osteomalacia.
Nephrocalcinosis, with calcification in the medullary portions of the kidney, occurs in about one-half of cases. The urine is alkaline and contains larger than normal quantities of sodium, potassium, calcium, and phosphate.
An abnormality in urinary anion gap (U.Na+ + U.K+ – U.Cl–) is noted (low), which is associated with the reduced NH4+ production. This abnormality differentiates this condition from type II RTA and from the metabolic acidosis seen with diarrhea. The blood chemistry findings are those of metabolic acidosis with low serum potassium.
Treatment consists of replacing deficits and increasing the intake of sodium, potassium, calcium, and phosphorus. Sodium and potassium should be given as bicarbonate or citrate. Additional vitamin D may be required.
Excess Potassium Secretion (Potassium “Wastage” Syndrome)
Excessive renal secretion or loss of potassium may occur in 4 situations: (1) moderate renal insufficiency with diminished H+ secretion; (2) RTA (proximal and distal RTA); (3) hyperaldosteronism and hyperadrenocorticism; and (4) tubular secretion of potassium, the cause of which is unknown. Hypokalemia indicates that the deficit is severe. Muscle weakness and polyuria and dilute urine are signs attributable to hypokalemia. Treatment consists of correcting the primary disease and giving supplementary potassium.
Reduced Potassium Secretion
Reduced potassium secretion is noted in conditions in which extrarenal aldosterone is reduced or when intrarenal production of renin (and secondary hypoaldosteronism) occurs. The latter condition is termed RTA, type IV, and is associated with impaired H+ and K+ secretion in the distal tubule. Drug-induced interstitial nephritis, gout, and diabetes mellitus are clinical circumstances that may produce type IV RTA and resulting hyperkalemia and mild metabolic acidosis. Treatment is to promote kaliuresis (with loop diuretics) to prescribe potassium-binding gastrointestinal resins (Kayexalate), or to provide the patient with a mineralocorticoid, fludrocortisone acetate.
Defects of Water Absorption (Renal Diabetes Insipidus)
Nephrogenic diabetes insipidus occurs more frequently in males than females. Unresponsiveness to antidiuretic hormone is the key to differentiation from pituitary diabetes insipidus.
In addition to congenital refractoriness to antidiuretic hormone, obstructive uropathy, lithium, methoxyflurane, and demeclocycline also may render the tubule refractory to vasopressin.
Symptoms are related to an inability to reabsorb water, with resultant polyuria and polydipsia. The urine volume approaches 12 L/d, and osmolality and specific gravity are low.
Treatment consists primarily of an adequate water intake. Chlorothiazide may ameliorate the polyuria; the mechanism of action is unknown, but the drug may act by increasing isosmotic reabsorption in the proximal segment of the tubule.
In idiopathic hypercalciuria, decreased reabsorption of calcium predisposes to the formation of renal calculi. Serum calcium and phosphorus are normal. Urine calcium excretion is high; urine phosphorus excretion is low. Microscopic hematuria may be present. See treatment of urinary stones containing calcium.