An endothelium‐derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K+ ions, eicosanoids, hydrogen peroxide and C‐type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a ‘universal EDHF’. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist‐induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K+ ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone. British Journal of Pharmacology (2004) 141, 881–903. doi:

The existence of endothelium-derived vascular relaxant factor (EDRF) was postulated by Furchgott and colleagues when they observed that acetylcholine paradoxically relaxed preconstricted aortic strip preparations by an endothelium-dependent mechanism. This phenomenon has since been demonstrated in different blood vessels and mammalian species and it can be elicited by several other agents. EDRF has been thought to be a humoral agent, a lipoxygenase derivative and possibly a free radical. In the study reported here, by using aortic preparations from the rabbit, alone and in cascade experiments with isolated perfused coronary preparations, we demonstrate definitively that EDRF is a humoral agent. It is released from unstimulated aortic preparations containing endothelium, its release can be stimulated for prolonged periods by acetylcholine, and it is not a lipoxygenase derivative or free radical but an unstable compound with a carbonyl group at or near its active site.

Phenylephrine (10 μM) evoked rises in tension in isolated rings of endothelium‐denuded rabbit superior mesenteric artery. These increases consisted of a tonic component with superimposed rhythmic activity, the frequency of which generally remained constant over time but whose amplitude exhibited cycle‐to‐cycle variability. The amplitude, but not the frequency, of the rhythmic activity was affected by a series of short peptides possessing sequence homology with extracellular loops 1 and 2 of connexin 43 (Cx43). Oscillatory behaviour was abolished at concentrations of 100–300 μM (IC50 of 20–30 μM), without change in average tone. No synergy was evident between peptides corresuponding to the extracellular loops, and cytoplasmic loop peptides were biologically inactive. The putative gap junction inhibitor heptanol mimicked the action of the extracellular loop peptides and abolished rhythmic activity at concentrations of 100–300 μM without effects on frequency. However, in marked contrast to the peptides, heptanol completely inhibited the contraction evoked by phenylephrine (IC50, 283 ± 28 μM). The presence of mRNA encoding Cx32, Cx40 and Cx43 was detected in the rabbit superior mesenteric artery by reverse transcriptase‐polymerase chain reaction. Western blot analysis showed that Cx43 was the major connexin in the endothelium‐denuded vessel wall. We conclude that intercellular communication between vascular smooth muscle cells via gap junctions is essential for synchronized rhythmic activity in isolated arterial tissue, whereas tonic force development appears to be independent of cell‐cell coupling. The molecular specificity of the peptide probes employed in the study suggests that the smooth muscle relaxant effects of heptanol may be non‐supecific and unrelated to inhibition of gap junctional communication.

An endothelium-derived hyperpolarizing factor (EDHF) is now widely recognized to mediate endothelium-dependent vascular relaxations that are independent of nitric oxide (NO) and prostanoid synthesis (Mombouli & Vanhoutte, 1997). Although the chemical identity of EDHF remains controversial, there is accumulating evidence that this mediator normally effects relaxation following diffusion from the endothelium to smooth muscle via myoendothelial gap junctions rather than the extracellular space Taylor et al. 1998;Dora et al. 1999;Hutcheson et al. 1999). Anatomically, this hypothesis is supported by the demonstration of myoendothelial gap junction plaques in rabbit conduit arteries (Spagnoli et al. 1982), and functional dye transfer experiments confirm direct chemical coupling between endothelium and subjacent smooth muscle (Little et al. 1995). Gap junctions are membrane structural proteins which consist of two hemichannels or connexons contributed by apposing cells, with each connexon being formed from six protein subunits or connexins arranged around an aqueous central pore that permits intercellular transfer of electrical current and molecules < 1 kDa in size (Yeager & Nicholson, 1996). Connexin 43 (Cx43) is present in both endothelial and vascular smooth muscle cells, and EDHFmediated relaxations and hyperpolarizations of rabbit

We have developed a mathematical model of arterial vasomotion in which irregular rhythmic activity is generated by the nonlinear interaction of intracellular and membrane oscillators that depend on cyclic release of Ca2+ from internal stores and cyclic influx of extracellular Ca2+, respectively. Four key control variables were selected on the basis of the pharmacological characteristics of histamine-induced vasomotion in rabbit ear arteries: Ca2+ concentration in the cytosol, Ca2+ concentration in ryanodine-sensitive stores, cell membrane potential, and the open state probability of Ca2+-activated K+ channels. Although not represented by independent dynamic variables, the model also incorporates Na+/Ca2+exchange, the Na+-K+-ATPase, Cl− fluxes, and Ca2+ efflux via the extrusion ATPase. Simulations reproduce a wide spectrum of experimental observations, including 1) the effects of interventions that modulate the functionality of Ca2+ stores and membrane ion channels, 2) paradoxes such as the apparently unpredictable dual action of Ca2+ antagonists and low extracellular Na+ concentration, which can abolish vasomotion or promote the appearance of large-amplitude oscillations, and 3) period-doubling, quasiperiodic, and intermittent routes to chaos. Nonlinearity is essential to explain these diverse patterns of experimental vascular response.