Early segmental changes in ischemic acute tubular necrosis of the rat kidney

Functionally induced changes in water transport in the proximal tubule segment of rat kidneys

To eliminate freezing artifacts in the proximal tubule cells, two cryotechniques were applied to normal rat kidneys, ie, freeze substitution and special freeze drying. In addition, salt depletion and salt loading were applied to groups of rats to evaluate whether the segmental structure of the proximal tubule could be altered. In the superficial part of the renal cortex of normal kidneys, the typical first segment structure in the proximal tubule was generally present in the early postglomerular fraction of the tubule. However, in the second segment, a special cellular phenomenon was constantly present, comprising a significant intercellular space that was easily identified using a light microscope. In the third segment, in which the presence of basolateral interdigitations is minimal, the small lateral space, which was found to be present in cryopreparations between neighboring cells from the normal kidney, was found to be enlarged by heavy salt loading of short duration. It is concluded that these cryotechniques demonstrate quantitative structural variations between superficial and deep nephrons, as well as the presence of extracellular areas between the cells of the second and the third segment, representing a structural background for the essential transport of water from the proximal tubules to the peritubular capillaries.

Early monitoring of acute tubular necrosis in the rat kidney by 23Na-MRI

Reabsorption of water and other molecules is dependent on the corticomedullary sodium concentration gradient in the kidney. During the early course of acute tubular necrosis (ATN), this gradient is altered. Therefore, 23Na magnetic resonance imaging (MRI) was used to study the alterations in renal sodium distribution in the rat kidney during ischemia and reperfusion (IR) injury, which induces ATN. In-magnet ischemia was induced for 0 (control), 10, 20, 30 or 50 min in Wistar rats. 23Na images were collected every 10 min during baseline, ischemia, and 60-min reperfusion periods. T1 and T2 relaxation times were measured by both 23Na-MRI and -MRS on a separate cohort of animals during ischemia and reperfusion for correction of relaxation-related tissue sodium concentration (TSC). A marked decrease was observed in the medulla and cortex 23Na-MRI signal intensity (SI) during the early evolution of ATN caused by IR injury, with the sodium reabsorption function of the kidney being irreversibly damaged after 50 min of ischemia. Sodium relaxation time characteristics were similar in the medulla and cortex of normal kidney, but significantly decreased with IR. The changes in relaxation times in both compartments were identical; thus the medulla-to-cortex sodium SI ratio represents the TSC ratio of both compartments. The extent of IR damage observed with histological examination correlated with the 23Na-MRI data. 23Na-MRI has great potential for noninvasive, clinical diagnosis of evolving ATN in the setup of acute renal failure and in differentiating ATN from other causes of renal failure where tubular function is maintained.

Chordin-like 1 and twisted gastrulation 1 regulate BMP signaling following kidney injury

Stimulation of the bone morphogenetic protein (BMP) pathway protects the kidney from acute and chronic injury. Numerous regulators in the kidney control BMP signaling, offering many targets for therapeutic manipulation. Here, we screened for modulators of BMP signaling in the ischemia-sensitive S3 segment and found that Chordin-like 1 is expressed in this segment of both the mouse and human nephron. Chordin-like 1 specifically antagonizes BMP7, which is expressed in the neighboring distal nephron, and this depends on the presence of the protein Twisted gastrulation. Upon ischemia-induced degeneration of the S3 segment, we observed a reduction in Chordin-like 1 expression coincident with intense BMP signaling in tubules of the recovering kidney. Restored expression accompanied proximal tubule epithelia redifferentiation, again coincident with decreased BMP signaling. We propose that Chordin-like 1 reduces BMP7 signaling in healthy proximal tubules, and the loss of this activity upon sloughing of injured epithelia promotes BMP7 signaling in repopulating, dedifferentiated epithelia. As regenerating epithelia differentiate, Chordin-like 1 is again expressed, antagonizing BMP7. These data suggest a mechanism for dynamic regulation of renoprotective BMP7 signaling in the S3 segment of the proximal tubule.

Comparison of prolonged warm and cold ischemia on the solitary kidney during partial nephrectomy in a rabbit model

The aim of this study was to investigate the patterns of renal function recovery during partial nephrectomy (PN) on an experimental solitary kidney rabbit model and establish the upper tolerable time limits of applied ischemia. Forty-eight New Zealand rabbits underwent an open right nephrectomy and after 30 days, the animals were clustered into five groups (A, B, C, D, E). The first four groups received an open left PN, under different types of ischemia. Groups A (n = 8) and B (n = 10) were subjected to 90 and 60 min of warm ischemia (WI), respectively, while groups C (n = 10) and D (n = 10) received 90 and 120 min of cold ischemia (CI) with ice-slush cooling. Group E (n = 10) served as sham group. Serum determinations of creatinine (SCr) and BUN were recorded preoperatively and on postoperative days (POD) 1, 3, 6 and 15. The animals were euthanized and the remaining kidneys were harvested and evaluated microscopically. The type and duration of ischemia were statistically significant parameters (P < 0.001). Groups B, C and D exhibited a similar pattern of recovery from trial initiation to the 15th POD (P = 0.788 and P = 0.068, respectively). Group A was extremely differentiated, with 100% mortality caused by uremia. The microscopic findings were consistent to the serum biochemistry. In our solitary kidney rabbit model, the upper limits of tolerable WI seem to be set on 60 min. CI can safely preserve the model's renal function--even up to 120 min.

Aquaporin-1 Facilitates Epithelial Cell Migration in Kidney Proximal Tubule

Aquaporin-1 (AQP1) is the principal water-transporting protein in cell plasma membranes in kidney proximal tubule, where it facilitates transepithelial water transport. Here, a novel role for AQP1 in kidney involving the migration of proximal tubule cells is reported. Migration was compared in primary cultures of proximal tubule cells from wild-type and AQP1 null mice. Cell cultures from AQP1 null mice were indistinguishable from those of wild-type mice in their appearance, growth/proliferation, and adhesiveness, although, as expected, they had reduced plasma membrane water permeability. Migration of AQP1-deficient cells was reduced by >50% compared with wild-type cells, as measured in a Boyden chamber in the presence of a chemotactic stimulus. Comparable slowing of migration of AQP1-deficient cells was also found in an in vitro scratch assay of wound healing, with reduced appearance of lamella-like membrane protrusions at the cell leading edge. Adenoviral-mediated expression of AQP1 in the AQP1-deficient cells, which increased their water permeability to that of wild-type cells, corrected their migration defect. The potential relevance of these in vitro findings to the intact kidney was tested in an in vivo model of acute tubular injury caused by 30 min of renal artery occlusion. At 3 to 5 d after ischemia-reperfusion, kidneys in AQP1 null mice showed remarkably greater tubular injury and cellular actin disorganization than kidneys in wild-type mice. These results provide evidence for the involvement of AQP1 in migration of proximal tubule cells and possibly in the response of the proximal tubule to injury.