Cytochrome P450 metabolites of arachidonic acid are potent inhibitors of vasopressin action on rabbit cortical collecting duct

AA is metabolized by a cytochrome P450, NADPH-dependent epoxygenase to four regioisomeric epoxyeicosatrienoic acids (EETs). The EETs are further hydrated enzymatically to their respective diols, vic-dihydroxyeicosatrienoic acids (DHETs). We studied the effect of pretreatment with DHETs on 10 μU/cm² arginine vasopressin (AVP)-stimulated hydraulic conductivity (L(p)) (L(p) x 10^-7 cm/atm/s, mean ± SE) in rabbit cortical collecting ducts (CCDs) perfused in vitro at 37°C. At 10^-6 M all four DHETs were potent inhibitors of the hydroosmotic effect of AVP. 14,15-DHET was the most potent isomer; it reduced AVP-induced L(p) from a control value of 234.75 ± 11.7, n = 17, to a value of 95.2 ± 8.39, n = 5, P < 0.0001, a reduction of AVP-mediated water flow of 60%. The inhibitory effect of 14,15-DHET was dose dependent and significant to nanomolar concentrations. 14,15-DHET at 10^-7 M was as potent an inhibitor of AVP's activity as was 10^-7 M PGE₂. AVP's hydroosmotic effect is mediated through its intracellular second messenger, cAMP. 8-p-Chlorophenylthio-cAMP (CcAMP) at 10^-4 M induced a peak L(p) of 189.6 ± 11.0, n = 8; pretreatment with 10^-6 M 14,15-DHET reduced CcAMP-peak L(p) to 132.0 ± 13.4, n = 5, P < 0.01, demonstrating a post-cAMP effect. Gas chromatography/mass spectroscopy suggests that EETs are present in extracts purified from CCDs. We conclude that cytochrome P450 epoxygenase eicosanoids are potent inhibitors of the hydroosmotic effect of vasopressin and are endogenous constituents of normal CCDs, the major target tissue for AVP.