Loss of autoregulation and increased renal vasoconstriction: the role of increased cytosolic and mitochondrial calcium. Acute ischemic injury has been shown in experimental animals to be associated with a loss of renal autoregulation (17). Moreover, rather than the normal autoregulatory renal vasodilation that occurs during a decrease in renal perfusion pressure, there is evidence that renal vasoconstriction actually occurs in the ischemic kidney. An increase in the response to renal nerve stimulation has also been observed in association with an acute ischemic insult (17). Moreover, the vasoconstrictor response to exogenous norepinephrine and endothelin has been observed to be increased in the acutely ischemic kidney (18). These vascular abnormalities observed in the ischemic kidney may be related to the resultant increase in cytosolic calcium observed in the afferent arterioles of the glomerulus. The observation that intrarenal calcium channel blockers can reverse the loss of autoregulation and the increase in sensitivity to renal nerve stimulation (17) supports a pathogenetic role of increased cytosolic calcium concentration in the afferent arteriole of the ischemic kidney. The mitochondrial calcium accumulation in the ischemic kidney has also been reversed by administration of calcium channel blockers (19–21). Moreover, calcium channel blockers have been shown to attenuate renal dysfunction and toxicity associated with the immunosuppressive drug cyclosporine following cadaveric renal transplantation, when administrated prior to the drug and ischemic insults (22).
Outer medullary congestion. Outer medullary congestion of the kidney is another vascular hallmark of acute renal ischemia. This congestion has been proposed to worsen the relative hypoxia in the outer medulla and thus the hypoxic injury in the S3 segment of the proximal tubule and the thick ascending limb of the Henle loop (Figure 1) (23). Upregulation of adhesion molecules has been implicated in this outer medullary congestion, and antibodies to ICAMs and P-selectin have been shown to afford protection against acute ischemic injury (24–27). Acute renal ischemia has also been shown to be associated with endothelial damage, perhaps at least in part because of increased oxidant injury. Some evidence in support of this possibility is the observation that activated leukocytes enhance the renal ischemic injury, and this observation could not be duplicated using leukocytes from patients with chronic granulomatous disease that do not produce reactive oxygen species (28). Oxidant injury may also lead to a decrease in eNOS and vasodilatory prostaglandins as well as an increase in endothelin, all of which may enhance the renal vasoconstrictor effect of circulating pressor agents present in ARF (29–31).