Vasodilation by hyperpolarization: beyond NO.

Clinical assessment of human vascular endothelial function in vivo involves use of specific antagonists such as L-N monomethyl arginine (L-NMMA) and cyclooxygenase inhibition to establish the contribution of NO and prostaglandins, respectively, to either resting vascular tone, agonist-stimulated vasodilation, or physiological and metabolic vasodilation.1–4 Even after complete inhibition of NO and prostaglandin (PG) synthesis, endotheliumdependent vasodilation persists, revealing the existence of a substantial NOand PG-independent component that has been attributed to endothelium-derived hyperpolarizing factor (EDHF) release. Prime candidate EDHFs that often differ by species and circulatory beds have been extensively reviewed (Figure).5,6 In the human vasculature, endothelium-dependent hyperpolarization is at least partly caused by the release of epoxyeicosatrienoic acids (EETs) from the cytochrome P450 (CYP450)-dependent metabolism of arachidonic acid that promote vasodilation by stimulation of small and large calcium-dependent potassium channels (KCa) on endothelial cells (Figure).7 Agonists such as bradykinin stimulate endothelial G-protein-coupled receptors, provoking an increase in endothelial cellular calcium [Ca2 ]i, causing opening endothelial KCa channels and triggering processes that explain the EDHF phenomena, including1: synthesis of EETs,2 transmission of endothelial cell hyperpolarization to the vascular smooth muscle via gap junctions,3 and release of K from the endothelial cells via KCa channels that, in turn, induces smooth muscle hyperpolarization by activating several other K channels (Figure).5,6 The hallmark of the EDHF-mediated responses is its abolition by the combination of apamin (a specific inhibitor of KCa channels of small conductance) plus charybdotoxin (a nonselective inhibitor of large-conductance and intermediate-conductance KCa channels), and of some voltage-dependent K channels, but not ATP-dependent K channels. Hydrogen peroxide can also activate KCa channels and remains a contender as another EDHF (Figure).8 Various endothelial oxidases, including NO synthase, lipoxygenases, P-450 epoxygenases, NAD(P)H oxidases, and xanthine oxidase generate superoxide anions that are degraded to hydrogen peroxide spontaneously or through superoxide dismutase– dependent dismutation.9 Gap junctions couple endothelial cells to other endothelial cells and smooth muscle cells provide a low-resistance electrical pathway between these 2 cell layers. Gap junctions are formed by the docking of 2 connexons present in adjacent cells that creates an aqueous pore permitting the transfer of ions and electrical continuity that establishes a uniform membrane potential across cells (Figure).10 Their number increases with diminution in the size of the artery, paralleling the importance of EDHF to vessel size. Finally, a moderate increase in the myoendothelial K concentration can in some species induce hyperpolarization of vascular smooth muscle cells by activating the inwardly rectifying K (KIR) channels and the Na –K -ATPase, but is an unlikely candidate as EDHF in people. The absence of a consensus regarding the precise identity of EDHFs and a consequent lack of specific inhibitors has long hampered clinical translation of this phenomenon. Recently, with improved understanding of the major signaling mechanisms underlying vascular hyperpolarization, the role of EDHF in the human circulation in vivo has begun to be dissected, but experimental pitfalls remain: the nature of the antagonists that is often used is nonspecific, the concentrations and duration of action of these blockers are variable, and complete blockade cannot be achieved in vivo, even with high doses, because of the competitive nature of the antagonism. Nevertheless, an impressive body of knowledge has already emerged regarding the role of EDHF in the human circulation, including the article in this issue of Hypertension.13 The role of EETs as potential EDHFs can be studied using azoles such as miconazole that selectively inhibit epoxidation (EET generation) of arachidonic acid and are partly responsible for non-NO and non–prostacyclin-mediated, endothelium-dependent vasodilation in the human microcirculation.7,11,12 In vivo, CYP450 inhibition does not alter conductance vessel diameter or resting blood flow,11–14 but after inhibition of NO and prostacyclin, inhibition of EET synthesis further decreases radial arterial blood flow and diameter as shown by Bellien.13 Thus, although it appears that under resting conditions in the healthy human forearm conductance and resistance vessel tone is not modulated by tonic activity of CYP450-derived epoxides, their role becomes evident after inhibition of NO and prostacyclin synthesis, illustrating the compensatory role of NO and EETs on maintenance of basal tone. Future studies need to investigate whether this contribution is altered in patients with endothelial dysfunction. Because KCa channel activation on the endothelial or smooth cells is a prerequisite for hyperpolarization and their The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Cardiology, Emory University School of Medicine, Atlanta, Ga. Correspondence to Arshed A. Quyyumi, MD, Professor of Medicine, Division of Cardiology, 1364 Clifton Rd, Ste D403C, Atlanta, GA 30322. E-mail [email protected] (Hypertension. 2006;48:1023-1025.) © 2006 American Heart Association, Inc.