Red blood cell trapping in the kidney is caused by obstruction of the large renal veins

The pathogenesis of ischemic acute kidney injury remains unclear. Red blood cell (RBC) trapping presents as the accumulation of many, tightly packed, RBC in the outer-medullary plexus and is a hallmark of ischemic AKI. We have recently reported that extravasation of toxic heme from the RBC congested medullary capillaries is a cause of tissue injury. This suggests that preventing RBC trapping from occurring may limit tissue injury. The pathogenesis of RBC trapping, however, is poorly understood. While it is clear that RBC trapping occurs in response to occlusion of vessels that drain the renal medulla, the location, as well as the nature of the occlusion, remain unknown. The current study was designed to identify the location of the obstruction responsible for RBC trapping in the rat kidney following ischemia/reperfusion. Our study was performed in 10–12-week-old male and female Sprague Dawley rats. Rats were anesthetized with isoflurane (2-5%) and body temperature maintained at 37°C using a heated table and UV lamp. Flank incisions were performed, and renal ischemia caused by placing a clamp on one of the renal arteries for 45 minutes followed by a 1-hour reperfusion period. The contralateral kidney was used as a non-ischemic control. At the end of the reperfusion period, kidneys were harvested for histological analysis. To identify the location of the vascular obstruction in the kidney, prior to the end of the ischemic period, either 50mg/kg of the plasma protein marker Evans blue or the lipid dye Vybrant DIL (0.12mg) was injected into the tail vein. In non-ischemic control kidneys, both the cortical and medullary vasculature was stained. In contrast, following 1-hour of reperfusion from ischemia, the majority of the cortical glomeruli were unstained, suggesting poor re-flow in this region. In contrast the juxtamedullary glomeruli that supply the medulla with blood were stained, indicating reperfusion of these vessels. In kidneys in which significant RBC trapping occurred, tightly packed RBCs were observed in both the vasa recta bundles and surrounding outer-medullary plexus capillaries. RBC in the vasa recta bundles stained positive, indicating these RBC had entered the kidney on reperfusion. In contrast, most of the RBCs in the capillary plexus were unstained. The interlobular and arcuate veins that drain the outer medulla often contained what appeared to be sludged blood, which was unstained, indicating these vessels had not reperfused. In many cases, however, an internal core of tightly packed stained RBC could be observed within the center of these vessels, indicating that the obstruction had occurred further downstream. In some animals tightly packed, stained RBC could be observed as far downstream as the large interlobar and segmental veins. In these examples, trapped RBC were surrounded by a homogeneous material, devoid of significant structure or cells. Our data indicate that venous obstruction leading to RBC trapping is localized to the large veins of the kidney which drain both the cortex and medulla. The absence of RBC trapping in the cortex is likely due to failure of this region to reperfuse while the obstruction is present. The nature of the obstruction remains unclear. In some cases, unstained sludged blood appeared to line the vascular wall, perhaps preventing smaller veins from draining. In many cases, however, a homogeneous material, largely devoid of cells, appeared to prevent the flow of blood through the large renal veins. Further studies are required to identify the nature of the material obstructing the large veins following ischemia. 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.