What causes distal renal tubular acidosis (dRTA)?

Updated: Oct 18, 2018
  • Author: Sai-Ching Jim Yeung, MD, PhD, FACP; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Answer

The distal nephron, primarily the collecting duct (CD), is the site at which urine pH reaches its lowest values. Inadequate acid secretion and excretion produce a systemic acidosis. A metabolic acidosis occurring secondary to decreased renal acid secretion in the absence of marked decreases in GFR and characterized by a normal AG is due to diseases that are usually grouped under the term dRTA. These are further classified into hypokalemic (type I) and hyperkalemic (type IV) RTA.

Until the 1970s, dRTA was thought to be a single disorder caused by an inability to maintain a steep H+ gradient across the distal nephron, either as a failure to excrete H+ or as a result of increased back-diffusion of H+ through an abnormally permeable distal nephron. Structural damage to the nephron from a variety of sources has been shown to result in different pathogenic mechanisms.

Excretion of urinary ammonium (NH4+) accounts for the largest portion of the kidneys' response to the accumulation of metabolic acids. Patients with dRTA are unable to excrete ammonium in amounts adequate to keep pace with a normal rate of acid production in the body. In some forms of the syndrome, maximally acidic urine can be formed, indicating the ability to establish a maximal H+ gradient. However, despite the maximally acidic urine, the total amount of ammonium excretion is low. In other forms, urine pH cannot reach maximal acidity despite systemic acidemia, indicating low H+ secretion capacity in the collecting duct.

In the presence of systemic acidemia, a low rate of urinary ammonium secretion is related either to decreased production of ammonia by the cells of the PCT or to failure to accumulate ammonium in the distal convoluted tubule (DCT) and excrete it in the urine. Decreased ammonium production is observed in hyperkalemic types of dRTA, also known as type IV RTA, because hyperkalemia causes an intracellular alkalosis with resultant impairment of ammonium generation and excretion by renal tubular cells. Acid secretion is thus reduced because of the deficiency of urinary buffers. This type of acidosis is also observed in early renal failure, due to a reduction in renal mass and decreased ammonium production in the remaining proximal tubular cells.


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