Clinical patterns of tubular dysfunction

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Distal renal tubular acidosis: pathogenesis and classification

Distal renal tubular acidosis results from ineffective addition of hydrogen ions to the lumen of the distal nephron. The syndrome is manifested by hyperchloremic metabolic acidosis often associated with hypokalemia. More recently, it has been recognized that hyperkalemia rather than hypokalemia can be a dominant feature of some cases of distal renal tubular acidosis. It has been generally accepted that all cases of this syndrome ultimately resulted from a similar mechanism. The prevailing view was that the abnormality underlying distal renal tubular acidosis was that of inability to either generate or maintain a steep pH gradient across the distal nephron. Recent advances in our understanding of the process of distal acidification have provided evidence that different mechanisms can alter distal hydrogen ion secretion. In this article, the significance of the various indices of urinary acidification and their use in the characterization of the mechanism underlying distal renal tubular acidosis are revised. A classification of distal renal tubular acidosis on the basis of mechanism is presented. The importance of plasma potassium and renal potassium excretion in the evaluation of patients with distal renal tubular acidosis is emphasized.

The distal tubular handling of ninhydrin-positive substances

The possible handling of ninhydrin-positive substances by the distal tubule was assessed by comparing the excretion of these substances from a kidney with damaged distal tubules with that from the undamaged contralateral kidney, which served as the control. The studies reveal that the distal tubule reabsorbs to a very limited extent alanine, asparagine, glutamic acid, glutamine, glycine, hydroxyproline, proline, possibly serine, and two unknown substances, all compounds which may take part in ammonia production.

The proximal tubular handling of amino acids and other ninhydrin-positive substances

The proximal tubular handling of a number of endogenous ninhydrin-positive substances in the dog was assessed by comparing the clearances of these substances following the injection of an area-specific nephrotoxin into one kidney, with the contralateral uninjected kidney serving as the control.The upper portion of the proximal tubule was primarily responsible for the reabsorption of most of the amino acids and certain other ninhydrin-positive substances. The lower portion of the proximal tubule seems to possess some secretory function (for cystine, lysine, glutamic acid, glutamine, and an unknown substance) and a fair degree of reabsorptive capacity for most of the amino acids.

The plasma levels, filtered loads, excretion rates, and clearances of a number of ninhydrin-positive substances by normal fasting dogs

Studies of the plasma levels, filtered loads, and excretion rates of 30 endogenous ninhydrin-positive substances in 10 normal female dogs were carried out to assess more accurately the renal clearances of these compounds under abnormal circumstances.The more prominent amino acids in the plasma, which account for 90% of the total amino acids measured, were glutamine, alanine, lysine, threonine, valine, glycine, serine, asparagine, proline, arginine, isoleucine, leucine, and histidine, in order of decreasing concentration. In the urine, glycine, alanine, glutamine, histidine, 1-methylhistidine, asparagine, threonine, serine, and 3-methylhistidine formed 90% of the total output.The percentage reabsorption by the tubules from the glomerular filtrate of β-alanine was 94.5, 1-methylhistidine 95.4, 3-methylhistidine 96.2, histidine 97.6, and glycine 98.4. Except for hydroxyproline, proline, citrulline, methionine, valine, phenylalanine, isoleucine, leucine, tryptophane, and ornithine, the reabsorption of which was complete or almost complete, the amino acids were reabsorbed by the tubules at a level generally greater than 99%.The renal clearances for most of the amino acids measured in the dog were between 0.5 and 0.2 ml per minute, except for β-alanine, 1-methylhistidine, 3-methylhistidine, histidine, taurine, and glycine, whose clearances ranged in decreasing order from 2.3 to 0.7 ml per minute, and also for those amino acids which were almost completely reabsorbed and whose clearances would therefore be rather low.

The Cerebro-ocular-renal Dystrophies: a new variant

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