Thallium reabsorption via NKCC2 causes severe acute kidney injury with outer medulla-specific calcium crystal casts in rats.

Thallium (Tl) is one of the most toxic heavy metals, associated with accidental poisoning and homicide. It causes acute and chronic systemic diseases, including gastrointestinal and cardiovascular diseases and kidney failure. However, few studies have investigated the mechanism by which Tl induces acute kidney injury (AKI). This study investigated the toxic effects of Tl on the histology and function of rat kidneys using biochemical and histopathological assays after intraperitoneal thallium sulfate administration (30 mg/kg). Five days post-administration, rats exhibited severely compromised kidney function. Low-vacuum scanning electron microscopy revealed excessive calcium (Ca) deposition in the outer medulla of Tl-loaded rats, particularly in the medullary thick ascending limb (mTAL) of the loop of Henle. Tl accumulated in the mTAL, accompanied by mitochondrial dysfunction in this segment. Tl-loaded rats showed reduced expression of kidney transporters and channels responsible for Ca2+ reabsorption in the mTAL. Pre-administration of the Na-K-Cl cotransporter 2 (NKCC2) inhibitor furosemide alleviated Tl accumulation and mitochondrial abnormalities in the mTAL. These findings suggest that Tl nephrotoxicity is associated with preferential Tl reabsorption in the mTAL via NKCC2, leading to mTAL mitochondrial dysfunction and disrupted Ca2+ reabsorption, culminating in mTAL-predominant Ca crystal deposition and AKI. These findings on the mechanism of Tl nephrotoxicity may contribute to the development of novel therapeutic approaches to counter Tl poisoning. Moreover, the observation of characteristic Ca crystal deposition in the outer medulla provides new insights into diagnostic challenges in Tl intoxication.

Near-drowning: new perspectives for human hypoxic acute kidney injury.

Concepts regarding hypoxic acute kidney injury (AKI) are derived from widely used warm ischemia-reflow (WIR) models, characterized by extensive proximal tubular injury and associated with profound inflammation. However, there is ample clinical and experimental data indicating that hypoxic AKI may develop without total cessation of renal blood flow, with a different injury pattern that principally affects medullary thick limbs in the outer medulla. This injury pattern likely reflects an imbalance between blood and oxygen supply and oxygen expenditure, principally for tubular transport. Experimental models of hypoxic AKI other than WIR are based on mismatched oxygen delivery and consumption, particularly within the physiologically hypoxic outer medulla. However, evidence for such circumstances in human AKI is lacking. Recent analysis of the clinical course and laboratory findings of patients following near-drowning (ND) provides a rare glimpse into such a scenario. This observation supports the role of renal hypoxia in the evolution of AKI, as renal impairment could be predicted by the degree of whole-body hypoxia (reflected by lactic acidosis). Furthermore, there was a close association of renal functional impairment with indices of reduced oxygen delivery (respiratory failure and features of intense sympathetic activity) and of enhanced oxygen consumption for active tubular transport (extrapolated from the calculated volume of consumed hypertonic seawater). This unique study in humans supports the concept of renal oxygenation imbalance in hypoxic AKI. The drowning scenario, particularly in seawater, may serve as an archetype of this disorder, resulting from reduced oxygen delivery, combined with intensified oxygen consumption for tubular transport.

Role of Nox4 and p67phox subunit of Nox2 in ROS production in response to increased tubular flow in the mTAL of Dahl salt-sensitive rats.

Nox4 and Nox2 are the most abundant NADPH oxidases (Nox) in the kidney and have been shown to contribute to hypertension, renal oxidative stress, and injury in Dahl salt-sensitive (SS) hypertensive rats. The present study focused on the role of Nox4 and p67phox/Nox2 in the generation of H2O2 and O2 (·-) in the renal medullary thick ascending limb of Henle (mTAL) of SS rats in response to increasing luminal flow (from 5 to 20 nl/min). Nox4 and p67phox/Nox2 genes were found to be expressed in the mTAL of SS rats. Responses of SS rats were compared with those of SS rats with knockout of Nox4 (SS(Nox4-/-)) or functional mutation of p67phox (SS(p67phox-/-)). Nox4 was the dominant source of increased intracellular H2O2 production in response to increased luminal flow as determined using the fluorescent dye peroxyfluor 6-AM (PF6-AM). The rate of mitochondrial H2O2 production [as determined by mitochondria peroxy yellow 1 (mitoPY1)] was also significantly reduced in SS(Nox4-/-) compared with SS rats, but not in SS(p67phox-/-) rats. In contrast, intracellular superoxide (O2 (·-)) production (the ratio of ethidium to dihydroethidium) in the mTAL of SS(Nox4-/-) rats was nearly identical to that of SS rats in response to luminal flow, indicating that Nox4 made no measurable contribution. mTAL O2 (·-) production was reduced in SS(p67phox-/-) compared with SS rats at the lower luminal flow of 5 nl/min and progressively increased when perfusion was changed to 20 nl/min. We conclude that increased mTAL luminal flow results in increases in intracellular and mitochondrial H2O2, which are dependent on the presence of Nox4, and that p67phox/Nox2 accounts solely for increases in O2 (·-) production.

Ultrastructure of mitochondria and the endoplasmic reticulum in renal tubules of Dahl salt-sensitive rats.

Metabolic and functional abnormalities in the kidney precede or coincide with the initiation of overt hypertension in the Dahl salt-sensitive (SS) rat. However, renal histological injury in SS rats is mild before the development of overt hypertension. We performed electron microscopy analysis in 7-wk-old SS rats and salt-insensitive consomic SS.13(BN) rats and Sprague-Dawley (SD) rats fed a 4% NaCl diet for 7 days. Long mitochondria (>2 µm) accounted for a significantly smaller fraction of mitochondria in medullary thick ascending limbs in SS rats (4% ± 1%) than in SS.13(BN) rats (8% ± 1%, P < 0.05 vs. SS rats) and SD rats (9% ± 1%, P < 0.01 vs. SS rats), consistent with previous findings of mitochondrial functional insufficiency in the medulla of SS rats. Long mitochondria in proximal tubules, however, were more abundant in SS rats than in SS.13(BN) and SD rats. The width of the endoplasmic reticulum, an index of endoplasmic reticulum stress, was significantly greater in medullary thick ascending limbs of SS rats (107 ± 1 nm) than in SS.13(BN) rats (95 ± 2 nm, P < 0.001 vs. SS rats) and SD rats (74 ± 3 nm, P < 0.01 vs. SS or SS.13(BN) rats). The tubules examined were indistinguishable between rat strains under light microscopy. These data indicate that ultrastructural abnormalities occur in the medullary thick ascending limbs of SS rats before the development of histological injury in renal tubules, providing a potential structural basis contributing to the subsequent development of overt hypertension.