Endothelium-derived hyperpolarizing factors and vascular cytochrome P450 metabolites of arachidonic acid in the regulation of tone.

The discovery of prostacyclin and its endothelial origin established the idea of endothelium-derived vasoactive eicosanoids and led to the realization that endothelial cells were a source of autacoids that regulate vascular tone.1,2 In 1980, Furchgott and Zawadzki3 described endothelium-derived relaxing factor (EDRF) and presented evidence that EDRF was a lipoxygenase metabolite of arachidonic acid. Subsequent research indicated that EDRF was nitric oxide.4 Several laboratories have described an endotheliumderived vasodilating factor that is distinct from nitric oxide or prostacyclin.5–10 These laboratories reported that acetylcholine caused endothelium-dependent relaxation and hyperpolarization of vascular smooth muscle. The relaxations and hyperpolarizations were not altered by arginine analogs that inhibit nitric oxide synthase or inhibitors of cyclooxygenase or lipoxygenases. They were blocked by inhibitors of calcium-activated potassium channels such as tetraethylammonium or charybdotoxin but not by inhibitors of ATPsensitive potassium channels such as glibenclamide. Subsequent studies indicated that this factor was released by other agonists including bradykinin and substance P. It was concluded that this vasodilating factor acts by opening calciumactivated potassium channels and hyperpolarizing the smooth muscle membrane. This factor has been termed endotheliumderived hyperpolarizing factor or EDHF. The article by Thollon and coworkers11 in this issue of Circulation Research further emphasizes the importance of EDHF in normal coronary arteries and arteries with regenerated endothelium. Several important insights are provided into the action and nature of EDHF by this work. First, this study demonstrates the importance of measuring membrane potential in defining the contribution of EDHF to the action of agonists. Hyperpolarization of vascular smooth muscle unequivocally defines EDHF activity. Second, the study indicates the importance of the resting membrane potential of the smooth muscle on the magnitude of the response to EDHF. Removal of the endothelial lining of porcine coronary arteries results in depolarization of the underlying vascular smooth muscle cells, suggesting a tonic hyperpolarizing influence of the endothelium. When the endothelium is allowed to regenerate, the hyperpolarizations to serotonin are abolished while the hyperpolarizations to bradykinin are preserved. The arteries with the less negative membrane potentials exhibit the greatest hyperpolarizations to bradykinin. Third, there are fundamental differences in the vascular effects of serotonin and bradykinin. Serotonin induces vascular smooth muscle hyperpolarizations of 3 to 13 mV that are transient in duration. In contrast, bradykinin hyperpolarizes the coronary smooth muscle by 40 mV at the highest concentration of the peptide, and the hyperpolarizations are long lasting. The authors raise the possibility that different EDHFs mediate the responses to these two agonists. Bradykinin clearly acts to hyperpolarize coronary arterial smooth muscle through the release of a transferable endothelial factor that activates calcium-activated potassium channels.12–14 Serotonin has not been as extensively studied. This Editorial will examine the possible identity of EDHF(s), the need to apply chemical, biochemical, electrophysiological, and pharmacological approaches to defining EDHF and the importance of the experimental conditions in interpreting these studies. Many studies define EDHF activity as relaxations to an agonist in the presence of inhibitors of nitric oxide synthase and prostaglandin synthase. However, it should be emphasized that endothelium-dependent hyperpolarization of smooth muscle is the hallmark of EDHF activity. Thollon and coworkers11 emphasize this point by demonstrating that agonist-induced changes in membrane potential may be dissociated from mechanical activity of the vessel. Serotonin produces small, transient hyperpolarizations but sustained relaxations.11,15 The possibility must always be considered that endothelial factors other than a hyperpolarizing factor may mediate relaxations that are resistant to inhibitors of nitric oxide and prostaglandin synthase. Thollon and coworkers11 also emphasize the importance of the resting membrane potential on the magnitude of the response to agonists that release EDHF and the influence of the experimental conditions on these electrical events. The resting membrane potential of arteries pinned to a supporting matrix in the present study is markedly different from arteries under physiological transmural pressures.16 Arterial smooth muscle cells of pressurized coronary arteries exhibiting physiological transmural pressures are depolarized compared with arteries under nonpressurized conditions.17,18 Furthermore, pressurization of arteries activates second messenger systems The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Departments of Pharmacology and Toxicology and Physiology and the Cardiovascular Research Center, Medical College of Wisconsin, Milwaukee, Wis. Correspondence to William B. Campbell, PhD, Department of Pharmacology and Toxicology, Medical College of Wisconsin, 8701 Watertown Plank Rd, Milwaukee, WI 53226. E-mail [email protected] (Circ Res. 1999;84:484-488.) © 1999 American Heart Association, Inc.