Hidden in Plain Sight: Does Medullary Red Blood Cell Congestion Provide the Explanation for Ischemic Acute Kidney Injury?

Acute kidney injury (AKI) represents a sudden reduction in renal function and is a major clinical problem with a high mortality rate. Despite decades of research, there are currently no direct therapies for AKI. The failure of therapeutic approaches identified in rodents to translate to human beings has led to questions regarding the appropriateness of these models. Our recent data indicate that there are two distinct processes driving tubular injury in the commonly used rat model of warm bilateral renal ischemia reperfusion injury, which often is used to mimic ischemic AKI. One results from the period of warm ischemia, manifesting as sublethal injury and coagulative necrosis of the proximal tubules in the renal cortex. This is the predominate type of injury observed 24 hours after reperfusion and the most well studied. The other results from red blood cell congestion of the outer medullary vasculature. This type of injury manifests as cell sloughing, along with the later formation of heme casts that fill distal nephron segments. Cell sloughing from congestion is most prominent in the early hours after reperfusion and often is masked by regeneration of the tubular epithelium by 24 hours postischemia. In this review, we argue that injury from outer medullary red blood cell congestion reflects the pathology observed in human kidneys and likely is representative of injury in most cases of ischemic AKI after shock. Greater focus on this congestive injury is likely to lead to improved translation in AKI.