Identifying endothelium-derived hyperpolarizing factor: Recent approaches to assay the role of epoxyeicosatrienoic acids

Investigation of endothelial regulation of vascular reactivity and tone has led to the discovery of chemical mediators such as nitric oxide (NO) and p rostacyclin (PGI(2)). Evidence has emerged indicating another as yet uniden tified hyperpolarizing agent (endothelium-derived hyperpolarizing factor or EDHF) that is different from NO and PGI(2) and exerts it effects through c alcium-activated potassium channels (K-Ca). Previous studies to identify ED HF have been carried out using inhibitors that block NOS and COX before app lication of K-Ca channel and/or muscarinic receptor antagonists. Such pharm acological manipulation has complicated interpretation of results, clearly pointing to the need for altered approaches to verify previous studies. Evi dence has emerged that potential EDHF candidates vary with vessel size, spe cies and tissue beds, indicating that there may be more than one EDHF. To d ate, the most commonly described and best characterized of them all are a s et of arachidonic acid metabolites, epoxyeicosatrienoic acids (EETs). These compounds are synthesized both intra- and extravascularly. Until recently, methodology to detect EETs in the microvasculature has been tedious and ex pensive, limiting the experimentation that is necessary to confirm EETs as an EDHF. This review describes state-of-the-art methods for assaying EETs i n biological samples, after summarizing evidence for EETs as an EDHF and in troducing emerging concepts of the role of extravascular EETs in linking ne uronal activity to localized blood flow during functional hyperemia.