This study examined the function of 20-HETE in renal I/R injury. The results indicate that blockade of the formation of 20-HETE with HET0016 exacerbated renal dysfunction following I/R injury whereas treatment of rats with two analogues of 20-HETE mitigated renal dysfunction and tubular injury compared to control or HET0016-treated rats. Although the mechanism of action remains to be determined, this study indicates that the 20-HETE analogue, 5,14−20-HEDGE, promotes sodium excretion in the absence of changes in MAP, RBF, or GFR and it prevents the postischemic fall in MBF. Taken together, the present findings suggest that 20-HETE analogues may mitigate renal ischemia by inhibition of renal tubular sodium transport and by preserving postischemic MBF. Notably, 5,14−20-HEDGE was more efficacious than 5,14−20-HEDE even though it was administered at a 10-fold lower dose. The increased efficacy of 5,14−20-HEDGE might be related to the addition of the glycine group that may help the compound enter cells or prolong its action by protecting it from β-oxidation.
Previous studies have demonstrated a pathologic function for 20-HETE and CYP ω-hydroxylase in ischemic injury to the heart and brain. In this regard, selective inhibition of CYP ω-hydroxylase protects myocardium from ischemic injury and inhibition of 20-HETE production counteracts cerebral vasospasm and reduces infarct size in animal models of ischemic stroke.10-13
On the other hand, the function of CYPAA metabolites and 20-HETE in acute kidney injury has not been well characterized. Prior studies in the rat have shown that I/R results in a decrease in renal CYP4A2 ω-hydroxylase expression and microsomal conversion of AA to 20-HETE.16,17
This was suggested to be an adaptive response that might attenuate postischemic vasoconstriction as 20-HETE potentiates vasoconstriction of preglomerular vessels.17
However, these studies did not associate the reductions in CYP ω-hydroxylase activity with the severity of renal dysfunction or injury following ischemia. On the other hand, Portilla et al
demonstrated that pretreatment of rats with clofibrate to induce CYP4A expression and renal 20-HETE production, reduced renal dysfunction following I/R injury. These findings were the first to suggest a protective function of 20-HETE in renal I/R injury. However, clofibrate is also an activator of peroxisome proliferator-activated receptor-α that may afford protection from I/R by inhibiting neutrophil infiltration, cytokine production, and oxidative stress within the injured kidney.19
The present results are consistent with the findings of Portilla et al
. and support a distinct protective function for 20-HETE in renal I/R and suggest that the decrease in renal CYP ω-hydroxylase activity and 20-HETE production following ischemia is maladaptive and contributes to renal injury and dysfunction.
The protective effect of 20-HETE analogues may result from more than one mechanism as administration of 5,14−20-HEDGE mitigated renal I/R when given before, or after, the ischemic interval. 20-HETE modulates vascular responsiveness to vasoconstrictors and administration of a 20-HETE analogue would be expected to reduce RBF and have a detrimental effect on renal I/R injury. However, the effects of 20-HETE may differ in the cortical and medullary circulation. In this regard, 20-HETE has been shown to increase renal MBF in a dose-dependent manner in rats.20
Furthermore, 20-HETE had a greater effect on medullary vasodilation than on cortical vasoconstriction.20
Several studies have established the importance of impaired MBF in the pathogenesis of renal I/R injury in the rat. For example, Vetterlein et al
utilized fluorescent tracers to assess postischemic blood flow and demonstrated a marked reduction in medullary plasma flow 1 h after of reperfusion. The present findings are consistent with prior studies that utilized LDF to assess postischemic MBF. Olof et al
found that MBF markedly decreased following 60 min of renal ischemia. Conesa et al
also demonstrated a decrease in MBF after renal ischemia. Furthermore, they demonstrated that N
-cysteine improved MBF after reperfusion and this directly correlated with preservation of renal function.22
In this context, we speculate that preservation of post-ischemic MBF is at least partially responsible for the protective effect of 20-HETE analogues demonstrated in this study.
Prevention of renal medullary hypoxia by inhibition of tubular sodium transport might also contribute to the renoprotective effect of 20-HETE analogues in I/R injury. In this study, administration of 5,14−20-HEDGE to rats resulted in a significant increase in urine sodium excretion without changing GFR or MAP, suggesting that it inhibits tubular reabsorption of sodium. These data are consistent with published reports demonstrating that inhibitors of 20-HETE synthesis blunt pressure natriuresis and promote sodium retention.23
Furthermore, 20-HETE has been shown to inhibit Na+
-ATPase and sodium transport in both the proximal tubule and medullary thick ascending loop of Henle.20,24,25
Thus, it is possible that 20-HETE analogues attenuate medullary hypoxia by decreasing oxygen demand during the ischemic period, thereby ameliorating renal dysfunction in a manner similar to other inhibitors of tubular electrolyte transport.26
For example, the selective dopamine receptor-1 agonist Fenoldopam increases sodium excretion similar to 20-HETE, but in contrast, is vasodilatory, decreasing systemic blood pressure while increasing RBF.27
Fenoldopam has been clinically tested for the prevention or treatment of acute kidney injury but the results have been inconclusive.28
This study did not explore and cannot exclude other mechanisms that may contribute to the renal protective actions of 20-HETE analogues in renal I/R injury. Administration of a 20-HETE analogue might be expected to mitigate inflammation following I/R as 20-HETE was recently shown to activate proliferator-activated receptor-α.29
Other pleiotropic actions of 20-HETE may include modulation of the inflammatory response to I/R by blocking platelet and neutrophil aggregation within the injured kidney as well mediating vasoprotection by stimulating mitogenesis and angiogenesis.9,30