What is the pathophysiology of the initiation phase of acute tubular necrosis (ATN)?

Updated: Mar 15, 2021
  • Author: Sangeeta Mutnuri, MBBS; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Ischemic ATN is often described as a continuum of prerenal azotemia. Indeed, the causes of the two conditions are the same. Ischemic ATN results when hypoperfusion overwhelms the kidney’s autoregulatory defenses. Under these conditions, hypoperfusion initiates cell injury that often, but not always, leads to cell death.

Injury of tubular cells is most prominent in the straight portion of the proximal tubules and in the thick ascending limb of the loop of Henle, especially as it dips into the relatively hypoxic medulla. The reduction in the glomerular filtration rate (GFR) that occurs from ischemic injury is a result not only of reduced filtration due to hypoperfusion but also of casts and debris obstructing the tubule lumen, causing back-leak of filtrate through the damaged epithelium (ie, ineffective filtration).

The earliest changes in the proximal tubular cells are apical blebs and loss of the brush border membrane followed by a loss of polarity and integrity of the tight junctions. This loss of epithelial cell barrier can result in the above-mentioned back-leak of filtrate.

Another change is relocation of Na+/K+-ATPase pumps and integrins to the apical membrane. Cell death occurs by both necrosis and apoptosis. Sloughing of live and dead cells occurs, leading to cast formation and obstruction of the tubular lumen (see the image below). Activation of the renal immune system—with damage to tubular cells stimulating local secretion of proinflammatory cytokines—in turn induces further necrosis. [3]

Acute tubular necrosis. Photomicrograph of a kidne Acute tubular necrosis. Photomicrograph of a kidney biopsy specimen shows renal medulla, which is composed mainly of renal tubules. Features suggesting acute tubular necrosis are the patchy or diffuse denudation of the renal tubular cells with loss of brush border (blue arrows); flattening of the renal tubular cells due to tubular dilation (orange arrows); intratubular cast formation (yellow arrows); and sloughing of cells, which is responsible for the formation of granular casts (red arrow). Finally, intratubular obstruction due to the denuded epithelium and cellular debris is evident (green arrow); note that the denuded tubular epithelial cells clump together because of rearrangement of intercellular adhesion molecules.

In addition, ischemia leads to decreased production of vasodilators (ie, nitric oxide, prostacyclin [prostaglandin I2, or PGI2]) by the tubular epithelial cells, leading to further vasoconstriction and hypoperfusion.

On a cellular level, ischemia causes depletion of adenosine triphosphate (ATP), an increase in cytosolic calcium, free radical formation, metabolism of membrane phospholipids, and abnormalities in cell volume regulation. The decrease or depletion of ATP leads to many problems with cellular function, not the least of which is active membrane transport.

With ineffective membrane transport, cell volume and electrolyte regulation are disrupted, leading to cell swelling and intracellular accumulation of sodium and calcium. Typically, phospholipid metabolism is altered, and membrane lipids undergo peroxidation. In addition, free radical formation is increased, producing toxic effects. Damage inflicted by free radicals apparently is most severe during reperfusion.


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