Developmental changes in myoendothelial gap junction mediated vasodilator activity in the rat saphenous artery

Abstract

A role for myoendothelial gap junctions (MEGJs) has been proposed in the action of the vasodilator endothelium-derived hyperpolarizing factor (EDHF). EDHF activity varies in disease and during ageing, but little is known of the role of EDHF during development when, in many organ systems, gap junctions are up-regulated. The aims of the present study were therefore to determine whether an up-regulation of heterocellular gap junctional coupling occurs during arterial development and whether this change is reflected functionally through an increased action of EDHF. Results demonstrated that in the saphenous artery of juvenile WKY rats, MEGJs were abundant and application of acetylcholine (ACh) evoked EDHF-mediated hyperpolarization and relaxation in the presence of N ω -nitro-L-arginine methyl ester (L-NAME) and indomethacin to inhibit nitric oxide and prostaglandins, respectively. Responses were blocked by a combination of charybdotoxin plus apamin, or 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) plus apamin, or by blockade of gap junctions with the connexin (Cx)-mimetic peptides,43 Gap26, 40 Gap27 and 37,43 Gap27. On the other hand, we found no evidence for the involvement of the putative chemical mediators of EDHF, eicosanoids, L-NAME-insensitive nitric oxide, hydrogen peroxide or potassium ions, since 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE), hydroxocobalamin, catalase or barium and ouabain were without effect. In contrast, in the adult saphenous artery, MEGJs were rare, EDHF-mediated relaxation was absent and hyperpolarizations were small and unstable. The present study demonstrates that MEGJs and EDHF are up-regulated during arterial development. Furthermore, the data show for the first time that this developmentally regulated EDHF is dependent on direct electrotonic coupling via MEGJs.

Obesity Up-Regulates Intermediate Conductance Calcium-Activated Potassium Channels and Myoendothelial Gap Junctions to Maintain Endothelial Vasodilator Function

“…This leads to the subsequent hyperpolarization of the smooth muscle via myoendothelial gap junction (MEGJ)-mediated transfer of current and/or K ϩ release and activation of inward rectifying potassium channels and/or smooth muscle Na ϩ /K ϩ ATPase (Hill, 2008;Félétou, 2009;Sandow et al, 2009b). The relationship between MEGJs and EDH activity is supported by studies where gap junction "block" with connexin (Cx)-mimetic "Gap" peptides (Chaytor et al, 2001;Sandow et al, 2004; 43 Gap26,40 Gap27, and 37,43 Gap27) or licorice derivatives such as carbenoxolone attenuate EDH-mediated vasodilation (Coleman et al, 2001). In rat mesenteric artery, myoendothelial microdomain IK Ca and MEGJs show close spatial association (Sandow et al, , 2009b that is functionally associated with their activity in EDH (Dora et al, 2008;Félétou, 2009;Sandow et al, 2009b).…”

“…Electron Microscopy. Tissue preparation and analysis for serial section electron microscopy were as described previously (Sandow et al, 2002(Sandow et al, , 2004. In brief, control and obese saphenous artery segments used for functional and Western blot studies were dissected from perfusion fixed rats (1% paraformaldehyde, 3% glutaraldehyde in 0.1 mM sodium cacodylate buffer, with 10 mM betaine, pH 7.4).…”

“…The inability of an EDH to be transferred to the adjacent smooth muscle is caused by the lack of sufficient MEGJs in this bed to facilitate current/ion transfer between the cell layers (Sandow et al, 2002(Sandow et al, , 2004, which contrasts with the saphenous artery of juvenile rat, where MEGJ-dependent EDH is down-regulated after development (Sandow et al, 2004).…”

Reduced EDHF responses and connexin activity in mesenteric arteries from the insulin-resistant obese Zucker rat

“…These peptides contain a motif identical in sequence to a portion of the secondary extracellular loop of the specific connexin it targets, and is thought to interfere with both functional gap junction formation and stability [11]. Gap peptides have been shown to inhibit the hyperpolarisation induced by endothelium-dependent vasodilators in rat mesenteric [39] and saphenous [6] arteries, and inhibit relaxations to acetylcholine in rabbit central ear arteries [40]. Additionally, gap peptides have been shown to abolish gap junctionmediated vasomotion in rat basilar arteries [41].…”

“…Furthermore, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), a specific inhibitor of IK Ca channels, has been used in combination with apamin to confirm that IK Ca and SK Ca channels are involved in EDHFmediated vasodilation. The combination of TRAM-34 and apamin has been used to block EDHF responses in rat caudal [5], saphenous [6], middle cerebral [7] and mesenteric arteries [8] and guinea pig carotid arteries [9].…”