During reperfusion from ischemia, extravasation of plasma results in rapid increases in volume and pressure in the kidney

The pathogenesis of ischemic acute kidney injury remains unclear. While tissue injury in ischemic AKI is currently thought to result primarily from hypoxia, we have argued that most injury may be secondary to transient venous obstruction at the onset of reperfusion, which results in the extravasation of plasma and toxic heme proteins into the surrounding tissue. Consistent with this, the weight of ischemic kidneys is known to be increased, however, it is unknown if this increase in kidney weight occurs at the onset of reperfusion or later as part of an inflammatory response. In the current study, we tested the hypothesis that extravasation of plasma occurs at the onset of reperfusion, resulting in rapid increases in kidney volume and interstitial pressure, prior to initiation of an inflammatory response. Our study was performed in 10–12-week-old male and female Sprague Dawley rats. Rats were anesthetized with isoflurane (2-5%). In experiment 1 (n=18), a midline incision was performed and a loose tie placed around the abdominal aorta superior to the left renal artery. The wound was then closed, and the left kidney imaged by ultrasound (Vevo3100). Rats were maintained at 37°C using a heated table and UV lamp. After a baseline period, the left kidney was made ischemic for 45 minutes by retracting the tie around the aorta before releasing the tie for a further 1-hour reperfusion period. 3D scans of the kidney were performed to determine kidney volume and percent vascularity (Doppler positive) at baseline, at the initiation of ischemia, at the end of ischemia, at the initiation of reperfusion and at 1 hour of reperfusion before the animals were humanely sacrificed and kidneys taken for histology. In experiment 2, flank incisions were performed and renal ischemia caused by placing clamps on both renal arteries. Prior to the end of the ischemic period, 50mg/kg of the plasma protein marker Evans blue was injected into the tail vein. Following reperfusion, urine was collected from the bladder and the kidneys harvested for histology. Experiment 3, mirrored experiment 2, except the left kidney was placed in a cup to prevent kidney movement and a pressure probe inserted into the kidney (Mikro-tip, Millar) to measure interstitial pressure. At baseline kidney volume averaged 616mm 3 and vascularity was 11.45% of kidney volume. Kidney volume decreased to 413mm 3 and vascularity to 3.6% at the onset of ischemia. Kidney volume increased across the ischemic period to 568mm 3 while vascularity remained at 3.7%. At the onset of reperfusion there was an initial decline in kidney volume to 542mm3 before increasing to 643mm 3 by 1-hour post-reperfusion. Vascularity at the onset of reperfusion was 7.8% before increasing to baseline levels at 1-hour post reperfusion. Kidneys with the greatest % increase in volume on reperfusion has significantly greater tubular injury than those that remained near baseline levels. Consistent with the increase in kidney volume resulting from extravasation of plasma, kidneys with large increase in volume demonstrated significant extravasation of Evan’s blue labelled blood proteins, with the surface of the kidney turning dark blue. Changes in interstitial pressure were consistent with the volume changes observed. Our data indicate that there is significant extravasation of blood proteins into the renal parenchyma at the onset of reperfusion and that this results in marked increases in kidney volume and pressure. As this occurs prior to the infiltration of significant numbers of immune cells, and kidneys with the largest increases in volume had the most severe tubular injury, these results are consistent with venous obstruction at the onset of reperfusion as a primary cause of cortical edema and tubular injury. NIDDK 1R01DK138150-01A1 to Paul O'Connor AHA Transformational Project Award to Paul O'Connor APS SURF fellowship to Angela Ajith This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.