© 2000 American Heart Association, Inc.
Editorials |
Myoendothelial Gap Junctions
The Gap Is There, but Does EDHF Go Through It?
From the Institut für Kardiovaskuläre Physiologie, Klinikum der J.W.G-Universität, Frankfurt am Main, Germany.
Correspondence to Ingrid Fleming, Institut für Kardiovaskuläre Physiologie, Klinikum der J.W.G-Universität, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany.
Key Words: endothelium • gap junctions • smooth muscle
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Introduction |
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 Top Introduction References |
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The endothelium-derived autacoids, nitric oxide (NO) and prostacyclin (PGI2), play a crucial role in the regulation of local vascular tone. However, these two factors alone cannot account for all of the endothelium-dependent dilator responses observed in a number of arteries, most notably in coronary, mesenteric, carotid, and renal arteries. Because endothelium-dependent, but NO synthase and cyclo-oxygenase inhibitor-insensitive dilator responses are associated with vascular smooth muscle hyperpolarization, the existence of a third dilator autacoid, an endothelium-derived hyperpolarizing factor (EDHF), has been proposed.
By monitoring the membrane potential of detector vascular smooth muscle cells situated downstream from donor endothelial cells, it is possible to monitor the production of an EDHF.1 On the other hand, there has been no convincing demonstration of the transfer of an NO/PGI2-independent hyperpolarizing and relaxing factor from one artery to another in classical bioassay experiments. For such a bioassay system to detect EDHF, the hyperpolarizing factor, like NO or PGI2, must be able to permeate the endothelial cell membrane and diffuse down its concentration gradient to stimulate a target (in this case Ca2+-dependent K+ channels) on smooth muscle cells. There is however no reason to assume that the transfer of an EDHF from endothelial to smooth muscle cells in vivo involves the generation of a membrane-permeable hyperpolarizing compound, as the transfer of EDHF may take place via a direct intercellular pathway.
Not all arteries generate an EDHF or exhibit an NO/PGI2-independent relaxation upon agonist stimulation, and the general rule of thumb is that the importance of EDHF as a vasodilator principle increases with decreasing vessel size.2 3 At least three alternative explanations have been proposed to explain the apparent predominance of EDHF-mediated relaxation in small arteries. The first is that the endothelium of one vessel or vascular bed is not identical with that of another. For example, the intracellular localization of the endothelial NO synthase is not identical in all arteries within the coronary system,4 and expression of the cytochrome P450 (CYP) enzymes thought to represent the coronary EDHF synthase is greater in smaller than in larger arteries.5 6 The second proposal is that EDHF-mediated relaxation is predominant in vessels in which contraction depends on the entry of Ca2+ into vascular smooth muscle cells through L-type Ca2+ channels. Indeed, a decrease in the open probability of L-type Ca2+ channels appears to be the major mechanism of EDHF-mediated relaxation.7 The third explanation is that a discrete pathway for electrical and/or chemical communication exists between endothelial and smooth muscle cells of the medium- to small-sized arteries in which the EDHF component of agonist-induced vasodilatation is greatest.
In this issue of Circulation Research, Sandow and Hill8 describe and quantify physical junctions between the endothelium and smooth muscle cells of first- and third-order branches of the rat superior mesenteric artery. These heterocellular junctions exhibit a pentalaminar structure that is typical for gap junctions. Moreover, just as the contribution of EDHF to vasodilatation is greater in third-order than in first-order mesenteric arteries from the rat,2 more myoendothelial gap junctions were detected in the smaller arteries.8
Despite the name "myoendothelial gap junction," these junctional structures are described to derive from endothelial cells and transverse a break in the continuity of the basal lamina to make contact with the smooth muscle membrane. This observation in itself suggests that there may be significant differences between the myoendothelial gap junction and homocellular gap junctions, where a connexon (6 connexins arranged around a central pore) in the membrane of one cell couples with a connexon in the adjacent cell. Estimates made by Sandow and Hill8 imply that two myoendothelial gap junctions border on each smooth muscle cell in smaller arteries, a situation that might explain the heightened sensitivity of these smooth muscle cells to hyperpolarizing stimuli. Not all of the junctional structures described to emanate from the endothelium actually made physical contact with smooth muscle cells, an observation that, by analogy with other cell types, was taken to indicate the dynamic nature of gap junctions. Although it is tempting to suggest that such structures represent the "off" configuration of inactive gap junctions, such speculation should be taken together with the usual pinch of salt required when overinterpreting histological data.
Although Sandow and Hill8 have concentrated purely on
morphology, convincing evidence for a functional role for gap junctions
in EDHF-mediated responses has come from Griffith and coworkers in
Cardiff. These investigators demonstrated that peptides based on the
sequence of one of the extracellular loops of connexin 43 were able to
attenuate EDHF-mediated relaxation in the rabbit mesenteric
artery9 10 11 to much the same extent as gap junction
uncoupling agents, such as heptanol and 18-
glycyrrhetinic
acid.12 Similar experimental approaches have since been
repeated by many others using different vascular preparations, with the
result being that gap junctional communication appears to be involved
in EDHF-mediated responses in some, but not all,
arteries.13 At this point, it is necessary to stress that
none of the pharmacological gap junction uncoupling agents studied to
date can differentiate between an interendothelial,
intermyocyte, or myoendothelial gap junction. This is
especially relevant when considering the importance of electrical
coupling between endothelial cells and, to a lesser
extent, between smooth muscle cells, in propagating the
hyperpolarization initiated as a consequence of
endothelial stimulation.14 Thus, the use
of uncoupling agents does not allow the differentiation between a
factor that directly activates endothelial or
smooth muscle cells and one that is transmitted from the
endothelium to smooth muscle cells via
myoendothelial gap junctions.
In the field of EDHF, it is currently not possible to present a unifying hypothesis or "state of the art," given that, clearly, more than one EDHF exists. In some arteries, especially in the coronary and renal vasculature, a CYP epoxygenase product, such as an epoxyeicosatrienoic acid (EET), is an essential component of EDHF-mediated responses.5 In other arteries, the rat mesenteric artery included, this is obviously not the case.15 Although the transfer of an electrical or chemical signal via gap junctions may account for some of the CYP-independent EDHF-mediated responses, "hybrid" arteries exist in which EDHF-mediated relaxation is dependent on both a CYP epoxygenase and gap junctional communication.16
Assuming that myoendothelial gap junctions do play a central role in the EDHF-mediated hyperpolarization and relaxation of some smooth muscle cells, a number of points must be addressed. The first is what can pass through these channels? EETs are highly lipophilic, and it is unlikely that they would be able to pass through what is essentially an aqueous pore. Therefore, EETs may rather modulate the permeability of these channels to facilitate the transfer of a soluble hyperpolarizing factor. How the gating of myoendothelial gap junctions is controlled will perhaps be the most difficult problem to answer, given that it is possible that the junctions described by Sandow and Hill,8 which are smaller than the homocellular gap junctions between endothelial cells, also have distinct structural and functional properties.
Although it is good to know that a physical structure does connect endothelial and smooth muscle cells, it is not possible to make a link between the incidence of myoendothelial gap junctions and EDHF-mediated responses until we know that these junctions are functional and that they conduct hyperpolarizing signals in the right direction, ie, from the endothelium to smooth muscle cells.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionReferences |
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2. Shimokawa H, Yasuaake H, Fujii K, Owada MK, Nakaike R, Fukumoto Y, Takayanagi T, Nagao T, Egashira K, Fujishima M, Takeshita A. The importance of the hyperpolarizing mechanism increases as the vessel size decreases in endothelium-dependent relaxations in rat mesenteric circulation. J Cardiovasc Pharmacol. 1996;28:703–711.[Medline] [Order article via Infotrieve]
3. Tomioka H, Hattori Y, Fukao M, Sato A, Liu M-Y, Sakuma I, Kitabatake A, Kanno M. Relaxation in different-sized rat blood vessels mediated by endothelium-derived hyperpolarizing factor: importance of processes mediating precontractions. J Vasc Res. 1999;36:311–320.[Medline] [Order article via Infotrieve]
4.
Andries LJ, Brutsaert DL, Sys SU. Nonuniformity of
endothelial constitutive nitric oxide synthase
distribution in cardiac endothelium. Circ
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5. Fisslthaler B, Popp R, Kiss L, Potente M, Harder DR, Fleming I, Busse R. Cytochrome P450 2C is an EDHF synthase in coronary arteries. Nature. 1999;401:493–497.[Medline] [Order article via Infotrieve]
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8.
Sandow SL, Hill CE. Incidence of
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endothelium-derived hyperpolarizing factor–mediated
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Chaytor AT, Evans WH, Griffith TM. Central role of
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Chaytor AT, Evans WH, Griffith TM. Peptides homologous
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Taylor HJ, Chaytor AT, Evans WH, Griffith TM.
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13. Edwards G, Félétou M, Gardener MJ, Thollon C, Vanhoutte PM, Weston AH. Role of gap junctions in the responses to EDHF in rat and guinea-pig small arteries. Br J Pharmacol. 1999;128:1788–1794.[Medline] [Order article via Infotrieve]
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Emerson GG, Segal SS. Endothelial cell
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16. Bolz SS, Derwand R, Pieperhoff S, de Wit C, Pohl U. NO- and EDHF-mediated dilations in resistance arteries are reduced after specific inhibition of gap junctional communication. Circulation. 1999;100(suppl I):I-486. Abstract.
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