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      Myoendothelial Gap Junctions May Provide the Pathway for EDHF in Mouse Mesenteric Artery


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          Endothelium-dependent hyperpolarization of vascular smooth muscle provides a major pathway for relaxation in resistance arteries. This can occur due to direct electrical coupling via myoendothelial gap junctions (MEGJs) and/or the release of factors (EDHF). Here we provide evidence for the existence of functional MEGJs in the same, defined branches of BALB/C mouse mesenteric arteries which show robust EDHF-mediated smooth muscle relaxation. Cyclopiazonic acid (CPA, 10 µ M) was used to stimulate EDHF in arteries mounted under isometric conditions and constricted with phenylephrine. Simultaneous measurement of smooth muscle membrane potential and tension demonstrated that CPA caused a hyperpolarization of around 10 mV, reversing the depolarization to phenylephrine by 94% and the associated constriction by 66%. The relaxation to CPA was endothelium dependent, associated with the opening of Ca<sup>2+</sup>-activated K channels, and only in part due to the release of nitric oxide (NO). In the presence of the NO synthase inhibitor, L-NAME (100 µ M), the relaxation to CPA could be almost completely inhibited with the putative gap junction uncoupler, carbenoxolone (100 µ M). Inhibition of the synthesis of prostaglandins or metabolites of arachidonic acid had no effect under the same conditions, and small rises in exogenous K<sup>+</sup> failed to evoke consistent or marked smooth muscle relaxation, arguing against a role for these molecules and ions as EDHF. Serial section electron microscopy revealed a high incidence of MEGJs, which was correlated with heterocellular dye coupling. Taken together, these functional and morphological data from a defined mouse resistance artery suggest that the EDHF response in this vessel may be explained by extensive heterocellular coupling through MEGJs, enabling spread of hyperpolarizing current.

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          Most cited references 14

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          K+ is an endothelium-derived hyperpolarizing factor in rat arteries.

          In arteries, muscarinic agonists such as acetylcholine release an unidentified, endothelium-derived hyperpolarizing factor (EDHF) which is neither prostacyclin nor nitric oxide. Here we show that EDHF-induced hyperpolarization of smooth muscle and relaxation of small resistance arteries are inhibited by ouabain plus Ba2+; ouabain is a blocker of Na+/K+ ATPase and Ba2+ blocks inwardly rectifying K+ channels. Small increases in the amount of extracellular K+ mimic these effects of EDHF in a ouabain- and Ba2+-sensitive, but endothelium-independent, manner. Acetylcholine hyperpolarizes endothelial cells and increases the K+ concentration in the myoendothelial space; these effects are abolished by charbdotoxin plus apamin. Hyperpolarization of smooth muscle by EDHF is also abolished by this toxin combination, but these toxins do not affect the hyperpolarizaiton of smooth muscle by added K+. These data show that EDHF is K+ that effluxes through charybdotoxin- and apamin-sensitive K+ channels on endothelial cells. The resulting increase in myoendothelial K+ concentration hyperpolarizes and relaxes adjacent smooth-muscle cells by activating Ba2+-sensitive K+ channels and Na+/K+ ATPase. These results show that fluctuations in K+ levels originating within the blood vessel itself are important in regulating mammalian blood pressure and flow.
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            The importance of the hyperpolarizing mechanism increases as the vessel size decreases in endothelium-dependent relaxations in rat mesenteric circulation.

            Endothelium-dependent relaxations are achieved by a combination of endothelium-derived prostacyclin (PGI2), nitric oxide (NO), and endothelium-derived hyperpolarizing factor (EDHF). However, it remains to be fully clarified whether the relative contribution of these three mechanisms to endothelium-dependent relaxations varies as a function of the vessel size. This study was designed to clarify this point. Acetylcholine (ACh)-induced endothelium-dependent relaxations were examined in isolated blood vessels taken from the aorta and the proximal and distal mesenteric arteries of the rat. The contributions of PGI2, NO, and EDHF were evaluated by the inhibitory effects of indomethacin, N omega-nitro-L-arginine methyl ester (L-NAME) in the presence of indomethacin, and KCl in the presence of indomethacin and L-NAME, respectively. The membrane potentials were recorded with microelectrodes. The expression of endothelial No synthase (eNOS) was examined by both immunostaining and immunoblotting. The contribution of PGI2 was negligible in three different-sized blood vessels. The contribution of NO was most prominent in the aorta, whereas that of EDHF was most prominent in the distal mesenteric arteries. The resting membrane potential was significantly deeper and the ACh-induced hyperpolarization was greater in the distal mesenteric arteries than those in the aorta. The expression of eNOS was the highest in the aorta and the lowest in the distal mesenteric arteries. These results indicate that the importance of EDHF increases as the vessel size decreases in endothelium-dependent relaxations in the rat mesenteric circulation.
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              Altered expression of small-conductance Ca2+-activated K+ (SK3) channels modulates arterial tone and blood pressure.

              The endothelium is a critical regulator of vascular tone, and dysfunction of the endothelium contributes to numerous cardiovascular pathologies. Recent studies suggest that apamin-sensitive, small-conductance, Ca2+-activated K+ channels may play an important role in active endothelium-dependent vasodilations, and expression of these channels may be altered in disease states characterized by vascular dysfunction. Here, we used a transgenic mouse (SK3T/T) in which SK3 expression levels can be manipulated with dietary doxycycline (DOX) to test the hypothesis that the level of expression of the SK subunit, SK3, in endothelial cells alters arterial function and blood pressure. SK3 protein was elevated in small mesenteric arteries from SK3T/T mice compared with wild-type mice and was greatly suppressed by dietary DOX. SK3 was detected in the endothelium and not in the smooth muscle by immunohistochemistry. In whole-cell patch-clamp experiments, SK currents in endothelial cells from SK3T/T mice were almost completely suppressed by dietary DOX. In intact arteries, SK3 channels contributed to sustained hyperpolarization of the endothelial membrane potential, which was communicated to the arterial smooth muscle. Pressure- and phenylephrine-induced constrictions of SK3T/T arteries were substantially enhanced by treatment with apamin, suppression of SK3 expression with DOX, or removal of the endothelium. In addition, suppression of SK3 expression caused a pronounced and reversible elevation of blood pressure. These results indicate that endothelial SK3 channels exert a profound, tonic, hyperpolarizing influence in resistance arteries and suggest that the level of SK3 channel expression in endothelial cells is a fundamental determinant of vascular tone and blood pressure.

                Author and article information

                J Vasc Res
                Journal of Vascular Research
                S. Karger AG
                October 2003
                03 October 2003
                : 40
                : 5
                : 480-490
                aDepartment of Pharmacy and Pharmacology, University of Bath, Bath, UK; bDivision of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, Australia
                74549 J Vasc Res 2003;40:480–490
                © 2003 S. Karger AG, Basel

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                Page count
                Figures: 5, Tables: 1, References: 55, Pages: 11
                Research Paper


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