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

Abstract

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 µM) 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 Ca²⁺-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 µM), the relaxation to CPA could be almost completely inhibited with the putative gap junction uncoupler, carbenoxolone (100 µM). Inhibition of the synthesis of prostaglandins or metabolites of arachidonic acid had no effect under the same conditions, and small rises in exogenous K⁺ 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.

Chronic diet-induced hyperhomocysteinemia impairs eNOS regulation in mouse mesenteric arteries

“…The mesenteric arteries used in the present study rely on both nitric oxide and EDHF for agonist-induced endotheliumdependent dilation (12,15,30). Studies in which nitric oxide has been blocked pharmacologically or by genetic knockout of eNOS have shown a general compensation for the loss of nitric oxide by upregulation of EDHF or prostacyclin (4,6,16,27).…”

“…Thus, it provides a model for examining the correlation between nitric oxidedependent function and eNOS expression and regulation, and the relative contribution of the various mechanisms of endothelium-dependent dilation with HHcy. The non-nitric oxide component of endothelium-dependent dilation, attributed to endothelium-dependent hyperpolarizing factor (EDHF) in these arteries (12), has recently been shown to be responsible for early transient peak dilation, while nitric oxide is responsible for long-lasting dilation in both rat and mouse mesenteric arteries (15). For this reason, we measured the time course of ACh-induced dilation to examine the various components of endothelium-dependent dilation.…”

Control and Coordination of Vasomotor Tone in the Microcirculation

“…As mentioned above, these vasodilator responses are typically paralleled by hyperpolarization of the underlying smooth muscle cells (Emerson & Segal, 2000;Goto et al, 2002;Griffith, 2004), which has been attributed to the release of an EDHF (Vanhoutte, 2004;Feletou & Vanhoutte, 2009). However, the direct electrotonic transmission of a hyperpolarizing current from the endothelial cells to the smooth muscle cells via myoendothelial gap junctions may explain the EDHF pathway (Busse et al, 2002;Dora et al, 2003;Griffith, 2004). In this perspective, the increase in endothelial cell intracellular Ca 2+ concentration activates SK Ca and IK Ca channels leading to the endothelium-dependent hyperpolarization of smooth muscle cells via gap junctions located at the MEJ (Busse et al, 2002;Crane et al, 2003;Eichler et al, 2003;Feletou et al, 2003) (Figure 3).…”

Theoretical Investigations of Communication in the Microcirculation: Conducted Responses, Myoendothelial Projections and Endothelium Derived Hyperpolarizing Factor

“…Many studies have supported MEGJs as being EDHF [88][89][90][91]116] although in some arterial beds, hyperpolarization is transferred from EC to SMC even in the presence of gap junction uncouplers like 18GA [93] . MEGJs provide a low resistance pathway for ionic communication between the two cell types.…”

“…Electrical change measurements and dye transfer studies have established the ability of these junctions to transfer electrical and ionic changes between the two cell types along with the transport of small second messenger molecules such as IP 3 [84][85][86]. There is now a sizeable amount of evidence that EDHF is simply the electrotonic spread of hyperpolarization from EC to SMC via gap junctions [88][89][90][91]116]. MEGJ expression has been shown to increase with decrease in vessel size which is coincidental with regards to EDHF action [89].…”