An epoxygenase metabolite of arachidonic acid 5,6 epoxy-eicosatrienoic acid mediates angiotensin-induced natriuresis in proximal tubular epithelium.

Several laboratories have documented that angiotensin II (AII) induces natriuresis in proximal tubular epithelium by a mechanism that has as yet not been disclosed. The present studies were designed to test the hypothesis that cytochrome P450-dependent metabolites of arachidonic acid mediate this effect. In cultured rabbit proximal tubule a ketoconazole-sensitive product comigrating with 5,6-epoxy-eicosatrienoic acid (5,6-EET) on reverse phase and normal phase HPLC was stimulated two-fold by AII. We employed cultures of early S1 segments on a modified Ussing chamber as a bioassay to evaluate the effects of AII and 5,6-EET on unidirectional 22Na (apical to basolateral) flux (JNa). AII inhibited JNa, an effect that was abolished by ketoconazole. Furthermore, 5,6-EET decreased JNa in a manner analogous to AII, and the effect was potentiated by inhibition of endogenous 5,6-EET production. Employing the Ca2(+)-sensitive fluorescent probe, Fura 2, we observed a dose-dependent increase in [Ca2+]i with nM to microM 5,6-EET, effects that were abolished by depletion of extracellular Ca2+ and voltage-sensitive Ca-channel blockers. These observations support the hypothesis that 5,6-EET represents the second messenger that mediates AII-induced natriuresis via stimulation of Ca2+ influx through voltage-sensitive channels.