High Potassium Intake Enhances the Inhibitory Effect of 11,12-EET on ENaC

High dietary potassium stimulates the renal expression of cytochrome P450 (CYP) epoxygenase 2C23, which metabolizes arachidonic acid (AA). Because the AA metabolite 11,12-epoxyeicosatrienoic acid (11,12-EET) can inhibit the epithelial sodium channel (ENaC) in the cortical collecting duct, we tested whether dietary potassium modulates ENaC function. High dietary potassium increased 11,12-EET in the isolated cortical collecting duct, an effect mimicked by inhibiting the angiotensin II type I receptor with valsartan. In patch-clamp experiments, a high potassium intake or treatment with valsartan enhanced AA-induced inhibition of ENaC, an effect mediated by a CYP-epoxygenase–dependent pathway. Moreover, high dietary potassium and valsartan each augmented the inhibitory effect of 11,12-EET on ENaC. Liquid chromatography/mass spectrometry showed that the rate of EET conversion to dihydroxyeicosatrienoic acids (DHET) was lower in renal tissue obtained from rats on a high-potassium diet than from those on a control diet, but this was not a result of altered expression of soluble epoxide hydrolase (sEH). Instead, suppression of sEH activity seemed to be responsible for the 11,12-EET–mediated enhanced inhibition of ENaC in animals on a high-potassium diet. Patch-clamp experiments demonstrated that 11,12-DHET was a weak inhibitor of ENaC compared with 11,12-EET, whereas 8,9- and 14,15-DHET were not. Furthermore, inhibition of sEH enhanced the 11,12-EET–induced inhibition of ENaC similar to high dietary potassium. In conclusion, high dietary potassium enhances the inhibitory effect of AA and 11,12-EET on ENaC by increasing CYP epoxygenase activity and decreasing sEH activity, respectively.We previously demonstrated that cytochrome P450 (CYP) epoxygenase-dependent arachidonic acid (AA) metabolism inhibited epithelial sodium channel (ENaC) in the cortical collecting duct (CCD) and that 11,12-epoxyeicosatrienoic acid (11,12-EET) was responsible for mediating the effect of AA on ENaC.¹ Furthermore, the observation that AA failed to inhibit ENaC in the CCD of the mice with a low expression of CYP2C44 suggests that CYP2C44 and its orthologs may be responsible for mediating the inhibitory effect of AA.² The expression of CYP2C44 or its orthologs has been shown to be regulated by dietary Na intake: A high Na intake stimulates² whereas a low, Na intake suppresses the expression of CYP2C44 homologue.³ A large body of evidence has suggested that EET plays a role in the regulation of renal Na transport and salt-sensitive hypertension.^1,2,4,5 Inhibition of CYP epoxygenase-dependent AA metabolism results in the development of salt-sensitive hypertension^5,6; however, the BP returned to normal after the removal of the epoxygenase inhibitor, even when the animals were still kept on a high-Na diet. The high Na intake–induced increase in EET formation was defective in CYP4A10(−/−) mice.² Although CYP4A10 is not the enzyme responsible for generating EET, deleting the CYP4A10 gene impairs the expression of CYP epoxygenases, CYP2C44 in particular. Consequently, CYP4A10(−/−) mice had developed a salt-sensitive hypertension that was prevented by amiloride, suggesting that defective regulation of ENaC by CYP epoxygenase-dependent AA metabolism was responsible for the salt-sensitive hypertension in CYP4A(−/−) mice.² The expression of CYP2C44 or its ortholog is stimulated not only by high Na but also by high potassium (HK) intake.⁷ Because CYP2C44 is highly expressed in the connecting tubule and the CCD,¹ it is conceivable that a high expression of CYP2C44 should enhance the AA-induced inhibition of ENaC; therefore, the aim of this study was to examine whether HK intake enhances the CYP-epoxygenase–dependent inhibitory effect of AA on ENaC.