Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

Gap-junction-mediated bioelectric signaling required for slow muscle development and function in zebrafish

Bioelectric signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the connexins. The connexin gene family is large and complex, creating challenges in identifying specific connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for slow muscle development and function. Through mutant analysis and in vivo imaging, we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging, we find that spinal-cord-generated neural activity is transmitted to developing slow muscle cells, and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization and that disruption leads to defects in behavior. Our work reveals a molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role of GJ communication in coordinating bioelectric signaling during development.

Identification of Distinct, Quantitative Pattern Classes from Emergent Tissue-Scale hiPSC Bioelectric Properties

Bioelectric signals possess the ability to robustly control and manipulate patterning during embryogenesis and tissue-level regeneration. Endogenous local and global electric fields function as a spatial 'pre-pattern', controlling cell fates and tissue-scale anatomical boundaries; however, the mechanisms facilitating these robust multiscale outcomes are poorly characterized. Computational modeling addresses the need to predict in vitro patterning behavior and further elucidate the roles of cellular bioelectric signaling components in patterning outcomes. Here, we modified a previously designed image pattern recognition algorithm to distinguish unique spatial features of simulated non-excitable bioelectric patterns under distinct cell culture conditions. This algorithm was applied to comparisons between simulated patterns and experimental microscopy images of membrane potential (Vmem) across cultured human iPSC colonies. Furthermore, we extended the prediction to a novel co-culture condition in which cell sub-populations possessing different ionic fluxes were simulated; the defining spatial features were recapitulated in vitro with genetically modified colonies. These results collectively inform strategies for modeling multiscale spatial characteristics that emerge in multicellular systems, characterizing the molecular contributions to heterogeneity of membrane potential in non-excitable cells, and enabling downstream engineered bioelectrical tissue design.

Gap junction mediated bioelectric coordination is required for slow muscle development, organization, and function

Bioelectrical signaling, intercellular communication facilitated by membrane potential and electrochemical coupling, is emerging as a key regulator of animal development. Gap junction (GJ) channels can mediate bioelectric signaling by creating a fast, direct pathway between cells for the movement of ions and other small molecules. In vertebrates, GJ channels are formed by a highly conserved transmembrane protein family called the Connexins. The connexin gene family is large and complex, presenting a challenge in identifying the specific Connexins that create channels within developing and mature tissues. Using the embryonic zebrafish neuromuscular system as a model, we identify a connexin conserved across vertebrate lineages, gjd4, which encodes the Cx46.8 protein, that mediates bioelectric signaling required for appropriate slow muscle development and function. Through a combination of mutant analysis and in vivo imaging we show that gjd4/Cx46.8 creates GJ channels specifically in developing slow muscle cells. Using genetics, pharmacology, and calcium imaging we find that spinal cord generated neural activity is transmitted to developing slow muscle cells and synchronized activity spreads via gjd4/Cx46.8 GJ channels. Finally, we show that bioelectrical signal propagation within the developing neuromuscular system is required for appropriate myofiber organization, and that disruption leads to defects in behavior. Our work reveals the molecular basis for GJ communication among developing muscle cells and reveals how perturbations to bioelectric signaling in the neuromuscular system_may contribute to developmental myopathies. Moreover, this work underscores a critical motif of signal propagation between organ systems and highlights the pivotal role played by GJ communication in coordinating bioelectric signaling during development.

The vascular Na,K-ATPase: clinical implications in stroke, migraine, and hypertension

In the vascular wall, the Na,K-ATPase plays an important role in the control of arterial tone. Through cSrc signaling, it contributes to the modulation of Ca2+ sensitivity in vascular smooth muscle cells. This review focuses on the potential implication of Na,K-ATPase-dependent intracellular signaling pathways in severe vascular disorders; ischemic stroke, familial migraine, and arterial hypertension. We propose similarity in the detrimental Na,K-ATPase-dependent signaling seen in these pathological conditions. The review includes a retrospective proteomics analysis investigating temporal changes after ischemic stroke. The analysis revealed that the expression of Na,K-ATPase α isoforms is down-regulated in the days and weeks following reperfusion, while downstream Na,K-ATPase-dependent cSrc kinase is up-regulated. These results are important since previous studies have linked the Na,K-ATPase-dependent cSrc signaling to futile recanalization and vasospasm after stroke. The review also explores a link between the Na,K-ATPase and migraine with aura, as reduced expression or pharmacological inhibition of the Na,K-ATPase leads to cSrc kinase signaling up-regulation and cerebral hypoperfusion. The review discusses the role of an endogenous cardiotonic steroid-like compound, ouabain, which binds to the Na,K-ATPase and initiates the intracellular cSrc signaling, in the pathophysiology of arterial hypertension. Currently, our understanding of the precise control mechanisms governing the Na,K-ATPase/cSrc kinase regulation in the vascular wall is limited. Understanding the role of vascular Na,K-ATPase signaling is essential for developing targeted treatments for cerebrovascular disorders and hypertension, as the Na,K-ATPase is implicated in the pathogenesis of these conditions and may contribute to their comorbidity.

Targeting Endothelial Connexin37 Reduces Angiogenesis and Decreases Tumor Growth

Connexin37 (Cx37) and Cx40 form intercellular channels between endothelial cells (EC), which contribute to the regulation of the functions of vessels. We previously documented the participation of both Cx in developmental angiogenesis and have further shown that loss of Cx40 decreases the growth of different tumors. Here, we report that loss of Cx37 reduces (1) the in vitro proliferation of primary human EC; (2) the vascularization of subcutaneously implanted matrigel plugs in Cx37−/− mice or in WT using matrigel plugs supplemented with a peptide targeting Cx37 channels; (3) tumor angiogenesis; and (4) the growth of TC-1 and B16 tumors, resulting in a longer mice survival. We further document that Cx37 and Cx40 function in a collaborative manner to promote tumor growth, inasmuch as the injection of a peptide targeting Cx40 into Cx37−/− mice decreased the growth of TC-1 tumors to a larger extent than after loss of Cx37. This loss did not alter vessel perfusion, mural cells coverage and tumor hypoxia compared to tumors grown in WT mice. The data show that Cx37 is relevant for the control of EC proliferation and growth in different tumor models, suggesting that it may be a target, alone or in combination with Cx40, in the development of anti-tumoral treatments.

Carbenoxolone and 18β-glycyrrhetinic acid inhibit inositol 1,4,5-trisphosphate-mediated endothelial cell calcium signalling and depolarise mitochondria

BACKGROUND AND PURPOSE

Coordinated endothelial control of cardiovascular function is proposed to occur by endothelial cell communication via gap junctions and connexins. To study intercellular communication, the pharmacological agents carbenoxolone (CBX) and 18β-glycyrrhetinic acid (18βGA) are used widely as connexin inhibitors and gap junction blockers.

EXPERIMENTAL APPROACH

We investigated the effects of CBX and 18βGA on intercellular Ca²⁺ waves, evoked by inositol 1,4,5-trisphosphate (IP₃ ) in the endothelium of intact mesenteric resistance arteries.

KEY RESULTS

Acetycholine-evoked IP₃ -mediated Ca²⁺ release and propagated waves were inhibited by CBX (100 μM) and 18βGA (40 μM). Unexpectedly, the Ca²⁺ signals were inhibited uniformly in all cells, suggesting that CBX and 18βGA reduced Ca²⁺ release. Localised photolysis of caged IP₃ (cIP₃ ) was used to provide precise spatiotemporal control of site of cell activation. Local cIP₃ photolysis generated reproducible Ca²⁺ increases and Ca²⁺ waves that propagated across cells distant to the photolysis site. CBX and 18βGA each blocked Ca²⁺ waves in a time-dependent manner by inhibiting the initiating IP₃ -evoked Ca²⁺ release event rather than block of gap junctions. This effect was reversed on drug washout and was unaffected by small or intermediate K⁺ -channel blockers. Furthermore, CBX and 18βGA each rapidly and reversibly collapsed the mitochondrial membrane potential.

CONCLUSION AND IMPLICATIONS

CBX and 18βGA inhibit IP₃ -mediated Ca²⁺ release and depolarise the mitochondrial membrane potential. These results suggest that CBX and 18βGA may block cell-cell communication by acting at sites that are unrelated to gap junctions.

Neuronal regulation of the blood-brain barrier and neurovascular coupling

To continuously process neural activity underlying sensation, movement and cognition, the CNS requires a homeostatic microenvironment that is not only enriched in nutrients to meet its high metabolic demands but that is also devoid of toxins that might harm the sensitive neural tissues. This highly regulated microenvironment is made possible by two unique features of CNS vasculature absent in the peripheral organs. First, the blood-blood barrier, which partitions the circulating blood from the CNS, acts as a gatekeeper to facilitate the selective trafficking of substances between the blood and the parenchyma. Second, neurovascular coupling ensures that, following local neural activation, regional blood flow is increased to quickly supply more nutrients and remove metabolic waste. Here, we review how neural and vascular activity act on one another with regard to these two properties.

Novel findings of 18β-glycyrrhetinic acid on sRAGE secretion through inhibition of transient receptor potential canonical channels in high-glucose environment

Enhancing soluble receptor for advanced glycation endproducts (sRAGE) is considered as a potent strategy for diabetes therapy. sRAGE secretion is regulated by calcium and transient receptor potential canonical (TRPC) channels. However, the role of TRPC channels in diabetes remains unknown. 18β-Glycyrrhetinic acid (18β-GA), produced from liquorice, has shown antidiabetic properties. This study was aimed to investigate the effect of 18β-GA on sRAGE secretion via TRPC channels in high glucose (HG)-induced THP-1 cells. HG treatment enhanced TRPC3 and TRPC6 expression and consequently caused reactive oxygen species (ROS) accumulation mediated through p47 nicotinamide-adenine dinucleotide phosphate oxidase and inducible nitric oxide synthase (iNOS) associated with uncoupling protein 2 (UCP2) decline and lower sRAGE secretion. Interestingly, 18β-GA showed the dramatic effects similar to Pyr3 or 2-aminoethyl diphenyl borinate inhibitors and effectively reversed HG-elicited mechanisms including that blocking TRPC3 and TRPC6 protein expressions, suppressing intracellular [Ca2+] concentration, decreasing expressions of ROS, p47s, and iNOS, but increasing UCP2 level and promoting sRAGE secretion. Therefore, 18β-GA provides a potential implication to diabetes mellitus and its complications.

Sustained conduction of vasomotor responses in rat mesenteric arteries in a two-compartment in vitro set-up

Aim

Conduction of vasomotor responses may contribute to long‐term regulation of resistance artery function and structure. Most previous studies have addressed conduction of vasoactivity only during very brief stimulations. We developed a novel set‐up that allows the local pharmacological stimulation of arteries in vitro for extended periods of time and studied the conduction of vasomotor responses in rat mesenteric arteries under those conditions.

Methods

The new in vitro set‐up was based on the pressure myograph. The superfusion chamber was divided halfway along the vessel into two compartments, allowing an independent superfusion of the arterial segment in each compartment. Local and remote cumulative concentration‐response curves were obtained for a range of vasoactive agents. Additional experiments were performed with the gap junction inhibitor 18β‐glycyrrhetinic acid and in absence of the endothelium.

Results

Phenylephrine‐induced constriction and acetylcholine‐induced dilation were conducted over a measured distance up to 2.84 mm, and this conduction was maintained for 5 minutes. Conduction of acetylcholine‐induced dilation was inhibited by 18β‐glycyrrhetinic acid, and conduction of phenylephrine‐induced constriction was abolished in absence of the endothelium. Constriction in response to high K⁺ was not conducted. Absence of remote stimulation dampened the local response to phenylephrine.

Conclusion

This study demonstrates maintained conduction of vasoactive responses to physiological agonists in rat mesenteric small arteries likely via gap junctions and endothelial cells, providing a possible mechanism for the sustained functional and structural control of arterial networks.

Calcium movement in smooth muscle and evaluation of graded functional intercellular coupling

Spontaneous activity of vascular smooth muscle is present in small arteries and some venous tissues like the hepatic portal vein. Whereas the ability to generate rhythmic membrane potential changes is expressed in a high number of primary oscillators, the generation of physiological tone and phasic activity requires synchronization of specialized pacemaker activity (Interstitial Cajal-like cells) by intercellular propagation and regeneration of excitation or a strong coupling mechanism of smooth muscle cells. The aim of this study was to deduce oscillator coupling by analyzing the spatiotemporal homogeneity of calcium oscillations within a native tissue preparation. Portal vein tissue was loaded with a calcium-sensitive dye (Fluo-3). By combining confocal microscopy and computation of spatial auto- and cross-correlation of the calcium signals, temporal and spatial coupling between cells was characterized. Spontaneous oscillations of calcium signals were measured at different predefined regions of interest. Cross-correlation analysis of these signals revealed that their damping was very similar in all directions of the investigated z-plane. In single experiments, improved cell-to-cell coupling was seen when noradrenaline (1-10 μM) was added to the bath-solution. With the chosen parameters of frame refresh, the velocity of signal propagation was faster than the maximum detectable velocity, but it could be estimated to exceed 0.1 mm/s. Correlative Network Analysis is a new and very useful tool to determine the functional coupling parameters of quasi-homogenous biological networks and their temporal changes. The action and significance of pharmacological modulators can be well studied on cellular and functional aspects with this newly introduced technique in biological sciences.

Deletion of endothelial arginase 1 does not improve vasomotor function in diabetic mice

Endothelial arginase 1 was ablated to assess whether this prevents hyperglycemia-induced endothelial dysfunction by improving arginine availability for nitric oxide production. Endothelial Arg1-deficient mice (Arg1-KOTie2 ) were generated by crossing Arg1fl/fl (controls) with Tie2Cretg/- mice and analyzed by immunohistochemistry, measurements of hemodynamics, and wire myography. Ablation was confirmed by immunohistochemistry. Mean arterial blood pressure was similar in conscious male control and Arg1-KOTie2 mice. Depletion of circulating arginine by intravenous infusion of arginase 1 or inhibition of nitric oxide synthase activity with L-NG -nitro-arginine methyl ester increased mean arterial pressure similarly in control (9 ± 2 and 34 ± 2 mmHg, respectively) and Arg1-KOTie2 mice (11 ± 3 and 38 ± 4 mmHg, respectively). Vasomotor responses were studied in isolated saphenous arteries of 12- and 34-week-old Arg1-KOTie2 and control animals by wire myography. Diabetes was induced in 10-week-old control and Arg1-KOTie2 mice with streptozotocin, and vasomotor responses were studied 10 weeks later. Optimal arterial diameter, contractile responses to phenylephrine, and relaxing responses to acetylcholine and sodium nitroprusside were similar in normoglycemic control and Arg1-KOTie2 mice. The relaxing response to acetylcholine was dependent on the availability of extracellular l-arginine. In the diabetic mice, arterial relaxation responses to endothelium-dependent hyperpolarization and to exogenous nitric oxide were impaired. The data show that endothelial ablation of arginase 1 in mice does not markedly modify smooth muscle and endothelial functions of a resistance artery under normo- and hyperglycemic conditions.

Glycyrrhetinic Acid Antagonizes Pressure-Induced Venous Remodeling in Mice

Development of spider veins is caused by the remodeling of veins located in the upper dermis and promoted by risk factors such as obesity or pregnancy that chronically increase venous pressure. We have repeatedly shown that the pressure-induced increase in biomechanical wall stress is sufficient to evoke the formation of enlarged corkscrew-like superficial veins in mice. Subsequent experimental approaches revealed that interference with endothelial- and/or smooth muscle cell (SMC) activation counteracts this remodeling process. Here, we investigate whether the herbal agent glycyrrhetinic acid (GA) is a suitable candidate for that purpose given its anti-proliferative as well as anti-oxidative properties. While basic abilities of cultured venous SMCs such as migration and proliferation were not influenced by GA, it inhibited proliferation but not angiogenic sprouting of human venous endothelial cells (ECs). Further analyses of biomechanically stimulated ECs revealed that GA inhibits the DNA binding capacity of the mechanosensitive transcription factor activator protein-1 (AP-1) which, however, had only a minor impact on the endothelial transcriptome. Nevertheless, by decreasing gelatinase activity in ECs or mouse veins exposed to biomechanical stress, GA diminished a crucial cellular response in the context of venous remodeling. In line with the observed inhibitory effects, local transdermal application of GA attenuated pressure-mediated enlargement of veins in the mouse auricle. In summary, our data identifies GA as an inhibitor of EC proliferation, gelatinase activity and venous remodeling. It may thus have the capacity to attenuate spider vein formation and remodeling in humans.

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc kinase-dependent connexin43 phosphorylation

Communication between vascular smooth muscle cells (VSMCs) is dependent on gap junctions and is regulated by the Na-K-ATPase. The Na-K-ATPase is therefore important for synchronized VSMC oscillatory activity, i.e., vasomotion. The signaling between the Na-K-ATPase and gap junctions is unknown. We tested here the hypothesis that this signaling involves cSrc kinase. Intercellular communication was assessed by membrane capacitance measurements of electrically coupled VSMCs. Vasomotion in isometric myograph, input resistance, and synchronized [Ca2+]i transients were used as readout for intercellular coupling in rat mesenteric small arteries in vitro. Phosphorylation of cSrc kinase and connexin43 (Cx43) were semiquantified by Western blotting. Micromole concentration of ouabain reduced the amplitude of norepinephrine-induced vasomotion and desynchronized Ca2+ transients in VSMC in the arterial wall. Ouabain also increased input resistance in the arterial wall. These effects of ouabain were antagonized by inhibition of tyrosine phosphorylation with genistein, PP2, and by an inhibitor of the Na-K-ATPase-dependent cSrc activation, pNaKtide. Moreover, inhibition of cSrc phosphorylation increased vasomotion amplitude and decreased the resistance between cells in the vascular wall. Ouabain inhibited the electrical coupling between A7r5 cells, but pNaKtide restored the electrical coupling. Ouabain increased cSrc autophosphorylation of tyrosine 418 (Y418) required for full catalytic activity whereas pNaKtide antagonized it. This cSrc activation was associated with Cx43 phosphorylation of tyrosine 265 (Y265). Our findings demonstrate that Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc-dependent Cx43 tyrosine phosphorylation.

Enhanced expression of Cx43 and gap junction communication in vascular smooth muscle cells of spontaneously hypertensive rats

Niflumic acid (NFA) is a novel gap junction (GJ) inhibitor. The aim of the present study was to investigate the effect of NFA on GJ communication and the expression of connexin (Cx) in vascular smooth muscle cells (VSMCs) of mesenteric arterioles of spontaneously hypertensive rats (SHR). Whole-cell patch clamp recording demonstrated that NFA at 1×10–4 M significantly inhibited the inward current and its effect was reversible. The time for charging and discharging of cell membrane capacitance (Cinput) reduced from 9.73 to 0.48 ms (P<0.05; n=6). Pressure myograph measurement showed that NFA at 3×10-4 M fully neutralized the contraction caused by phenylephrine. The relaxation responses of normotensive control Wistar Kyoto (WKY) rats were significantly higher, compared with those of the SHRs (P<0.05; n=6). Western blot and reverse transcription-quantitative polymerase chain reaction analyses showed that the mRNA and protein expression levels of Cx43 of the third-level branch of mesenteric arterioles of the SHRs and WKY rats were higher, compared with those of the first-level branch. The mRNA and protein expression levels of Cx43 of the primary and third-level branches of the mesenteric arterioles in the SHRs were higher, compared with those in the WKY rats (P<0.05; n=6). The mRNA levels of Cx43 in the mesenteric arterioles were significantly downregulated by NFA in a concentration-dependent manner (P<0.01; n=6). The protein levels of Cx43 in primary cultured VSMCs isolated from the mesenteric arterioles were also significantly downregulated by NFA in a concentration-dependent manner (P<0.01; n=6). These results showed that the vasorelaxatory effects of GJ inhibitors were reduced in the SHRs, which was associated with a higher protein expression level of Cx43 in the mesenteric arterioles of the SHRs. NFA also relaxed the mesenteric arterioles by reducing the expression of Cx43, which decreased blood pressure. Therefore, regulation of the expression of GJs may be a therapeutic target for the treatment of hypertension.

Central Role of P2Y6 UDP Receptor in Arteriolar Myogenic Tone

OBJECTIVE

Myogenic tone (MT) of resistance arteries ensures autoregulation of blood flow in organs and relies on the intrinsic property of smooth muscle to contract in response to stretch. Nucleotides released by mechanical strain on cells are responsible for pleiotropic vascular effects, including vasoconstriction. Here, we evaluated the contribution of extracellular nucleotides to MT.

APPROACH AND RESULTS

We measured MT and the associated pathway in mouse mesenteric resistance arteries using arteriography for small arteries and molecular biology. Of the P2 receptors in mouse mesenteric resistance arteries, mRNA expression of P2X1 and P2Y6 was dominant. P2Y6 fully sustained UDP/UTP-induced contraction (abrogated in P2ry6(-/-) arteries). Preventing nucleotide hydrolysis with the ectonucleotidase inhibitor ARL67156 enhanced pressure-induced MT by 20%, whereas P2Y6 receptor blockade blunted MT in mouse mesenteric resistance arteries and human subcutaneous arteries. Despite normal hemodynamic parameters, P2ry6(-/-) mice were protected against MT elevation in myocardial infarction-induced heart failure. Although both P2Y6 and P2Y2 receptors contributed to calcium mobilization, P2Y6 activation was mandatory for RhoA-GTP binding, myosin light chain, P42-P44, and c-Jun N-terminal kinase phosphorylation in arterial smooth muscle cells. In accordance with the opening of a nucleotide conduit in pressurized arteries, MT was altered by hemichannel pharmacological inhibitors and impaired in Cx43(+/-) and P2rx7(-/-) mesenteric resistance arteries.

CONCLUSIONS

Signaling through P2 nucleotide receptors contributes to MT. This mechanism encompasses the release of nucleotides coupled to specific autocrine/paracrine activation of the uracil nucleotide P2Y6 receptor and may contribute to impaired tissue perfusion in cardiovascular diseases.

Communication Through Gap Junctions in the Endothelium

A swarm of fish displays a collective behavior (swarm behavior) and moves "en masse" despite the huge number of individual animals. In analogy, organ function is supported by a huge number of cells that act in an orchestrated fashion and this applies also to vascular cells along the vessel length. It is obvious that communication is required to achieve this vital goal. Gap junctions with their modular bricks, connexins (Cxs), provide channels that interlink the cytosol of adjacent cells by a pore sealed against the extracellular space. This allows the transfer of ions and charge and thereby the travel of membrane potential changes along the vascular wall. The endothelium provides a low-resistance pathway that depends crucially on connexin40 which is required for long-distance conduction of dilator signals in the microcirculation. The experimental evidence for membrane potential changes synchronizing vascular behavior is manifold but the functional verification of a physiologic role is still open. Other molecules may also be exchanged that possibly contribute to the synchronization (eg, Ca(2+)). Recent data suggest that vascular Cxs have more functions than just facilitating communication. As pharmacological tools to modulate gap junctions are lacking, Cx-deficient mice provide currently the standard to unravel their vascular functions. These include arteriolar dilation during functional hyperemia, hypoxic pulmonary vasoconstriction, vascular collateralization after ischemia, and feedback inhibition on renin secretion in the kidney.

The licorice pentacyclic triterpenoid component 18β-glycyrrhetinic acid enhances the activity of antibiotics against strains of methicillin-resistant Staphylococcus aureus

This study aimed to identify compounds that enhance the activity of current antibiotics against multidrug-resistant bacteria. Screening of a 350+ compound proprietary small molecules library revealed that the Glycyrrhiza glabra (licorice)-derived triterpenoid 18β-glycyrrhetinic acid (18β-GA) potentiated the antibacterial activity of certain antibiotics against Staphylococcus aureus. Here, we evaluated the ability of pentacyclic triterpenoids to potentiate the activity of antibiotics against strains of methicillin-resistant S. aureus (MRSA). Checkerboard assays were used to assess the minimum inhibitory concentration (MIC) of tobramycin and ten pentacyclic triterpenoids against S. aureus. The effect of 18β-GA on the MIC of different antibiotics against MRSA was also determined in an in vitro airway MRSA infection model. 18β-GA enhanced the bactericidal activity of the aminoglycosides tobramycin, gentamicin and amikacin, and of polymyxin B against two MRSA strains, reducing the MIC of these antibiotics 32-64-fold [fractional inhibitory concentration index (FICI) of 0.12-0.13]. Other β-amyrin triterpenoids and α-amyrin triterpenoids did not exert such synergistic effects. 18β-GA did not enhance the activity of antibiotics from other structural classes against the MRSA strains. In an air-exposed airway epithelial cell culture, 18β-GA enhanced the bactericidal activity of tobramycin and polymyxin B against the MRSA strain. These data demonstrate the potential of 18β-GA to synergise with certain types of antibiotics to eliminate strains of MRSA.

Endothelial arginine resynthesis contributes to the maintenance of vasomotor function in male diabetic mice

Aim

Argininosuccinate synthetase (ASS) is essential for recycling L-citrulline, the by-product of NO synthase (NOS), to the NOS substrate L-arginine. Here, we assessed whether disturbed arginine resynthesis modulates endothelium-dependent vasodilatation in normal and diabetic male mice.

Methods and Results

Endothelium-selective Ass-deficient mice (Ass^fl/fl/Tie2Cre^tg/− = Ass-KO^Tie2) were generated by crossing Ass^fl/fl mice ( = control) with Tie2Cre mice. Gene ablation in endothelial cells was confirmed by immunohistochemistry. Blood pressure (MAP) was recorded in 34-week-old male mice. Vasomotor responses were studied in isolated saphenous arteries of 12- and 34-week-old Ass-KO^Tie2 and control animals. At the age of 10 weeks, diabetes was induced in control and Ass-KO^Tie2 mice by streptozotocin injections. Vasomotor responses of diabetic animals were studied 10 weeks later. MAP was similar in control and Ass-KO^Tie2 mice. Depletion of circulating L-arginine by arginase 1 infusion or inhibition of NOS activity with L-NAME resulted in an increased MAP (10 and 30 mmHg, respectively) in control and Ass-KO^Tie2 mice. Optimal arterial diameter, contractile responses to phenylephrine, and relaxing responses to acetylcholine and sodium nitroprusside were similar in healthy control and Ass-KO^Tie2 mice. However, in diabetic Ass-KO^Tie2 mice, relaxation responses to acetylcholine and endothelium-derived NO (EDNO) were significantly reduced when compared to diabetic control mice.

Conclusions

Absence of endothelial citrulline recycling to arginine did not affect blood pressure and systemic arterial vasomotor responses in healthy mice. EDNO-mediated vasodilatation was significantly more impaired in diabetic Ass-KO^Tie2 than in control mice demonstrating that endothelial arginine recycling becomes a limiting endothelial function in diabetes.

Role of gap junctions modulating hepatic vascular tone in cirrhosis

BACKGROUND & AIMS

Gap junctions are formed by connexins (Cx), a family of proteins that couple endothelial and smooth muscle cells in systemic vessels. In this context, Cx allow the transmission of signals modulating vascular tone. Recently, vascular Cx have been observed in liver cells implicated in liver blood flow regulation. Here, we investigated the role of Cx in the regulation of intrahepatic vascular tone in cirrhosis.

METHODS

Livers of Sprague-Dawley control and cirrhotic (common bile duct ligation-CBDL and CCl4 ) rats were perfused, and concentration-effect curves in response to acetylcholine (ACh) precontracted with methoxamine were obtained in the presence of the specific Cx inhibitor 18-alpha-glycyrrhetinic acid or vehicle. Cx expression was assessed by immunofluorescence, western blot and reverse-transcription polymerase chain reaction in liver tissue, hepatic stellate cells, sinusoidal endothelial cells and hepatocytes isolated from control and cirrhotic rat livers. Cx protein expression was also determined in cirrhotic human tissue.

RESULTS

Gap junction blockade markedly attenuated relaxation of hepatic vasculature in response to ACh in control (maximal relaxation, -55 ± 10.5% vs. -95.3 ± 10% with vehicle; P < 0.01) and CBDL rats (50.9 ± 18.5% vs. -18.7 ± 5.5% with vehicle; P = 0.01). Livers from CBDL rats and patients with cirrhosis exhibited Cx overexpression. By contrast, CCl4 -cirrhotic rats did not show attenuated relaxation of hepatic vasculature after blockade and Cx expression was significantly lower than in controls.

CONCLUSIONS

Gap junctions may contribute to modulating portal pressure and intrahepatic vascular relaxation. Liver gap junctions may represent a new therapeutic target in cirrhotic portal hypertension.

Downregulation of L-type Ca2+ channel in rat mesenteric arteries leads to loss of smooth muscle contractile phenotype and inward hypertrophic remodeling

L-type Ca2+ channels (LTCCs) are important for vascular smooth muscle cell (VSMC) contraction, as well as VSMC differentiation, as indicated by loss of LTCCs during VSMC dedifferentiation. However, it is not clear whether loss of LTCCs is a primary event underlying phenotypic modulation or whether loss of LTCCs has significance for vascular structure. We used small interference RNA (siRNA) transfection in vivo to investigate the role of LTCCs in VSMC phenotypic expression and structure of rat mesenteric arteries. siRNA reduced LTCC mRNA and protein expression in rat mesenteric arteries 3 days after siRNA transfection to 12.7 ± 0.7% and 47.3 ± 13%, respectively: this was associated with an increased resting intracellular Ca2+ concentration ([Ca2+]i). Despite the high [Ca2+]i, the contractility was reduced (tension development to norepinephrine was 3.5 ± 0.2 N/m and 0.8 ± 0.2 N/m for sham-transfected and downregulated arteries respectively; P < 0.05). Expression of contractile phenotype marker genes was reduced in arteries downregulated for LTCCs. Phenotypic changes were associated with a 45% increase in number of VSMCs and a consequent increase of media thickness and media area. Ten days after siRNA transfection arterial structure was again normalized. The contractile responses of LTCC-siRNA transfected arteries were elevated in comparison with matched controls 10 days after transfection. The study provides strong evidence for causal relationships between LTCC expression and VSMC contractile phenotype, as well as novel data addressing the complex relationship between VSMC contractility, phenotype, and vascular structure. These findings are relevant for understanding diseases, associated with phenotype changes of VSMC and vascular remodeling, such as atherosclerosis and hypertension.

A microdevice for studying intercellular electromechanical transduction in adult cardiac myocytes

Intercellular electromechanical transduction in adult cardiac myocytes plays an important role in regulating heart function. The efficiency of intercellular electromechanical transduction has so far been investigated only to a limited extent, which is largely due to the lack of appropriate tools that can quantitatively assess the contractile performance of interconnected adult cardiac myocytes. In this paper we report a microengineered device that is capable of applying electrical stimulation to the selected adult cardiac myocyte in a longitudinally connected cell doublet and quantifying the intercellular electromechanical transduction by measuring the contractile performance of stimulated and un-stimulated cells in the same doublet. The capability of applying selective electrical stimulation on only one cell in the doublet is validated by examining cell contractile performance while blocking the intercellular communication. Quantitative assessment of cell contractile performance in isolated adult cardiac myocytes is performed by measuring the change in cell length. The proof-of-concept assessment of gap junction performance shows that the device is useful in studying the efficiency of gap junctions in adult cardiac myocytes, which is most relevant to the synchronized pumping performance of native myocardium. Collectively, this work provides a quantitative tool for studying intercellular electromechanical transduction and is expected to develop a comprehensive understanding of the role of intercellular communication in various heart diseases.

Gap junctions support the sustained phase of hypoxic pulmonary vasoconstriction by facilitating calcium sensitization

AIMS

To determine the role of gap junctions (GJs) in hypoxic pulmonary vasoconstriction (HPV).

METHODS AND RESULTS

Studies were performed in rat isolated intrapulmonary arteries (IPAs) mounted on a myograph and in anaesthetized rats. Hypoxia induced a biphasic HPV response in IPAs preconstricted with prostaglandin F2α (PGF2α, 3 µM) or 20 mM K⁺. The GJ inhibitors 18β-glycyrrhetinic acid (18β-GA, 30 µM), heptanol (3.5 mM), or 2-aminoethoxydiphenyl borate (2-APB) (75 µM) had little effect on the transient Phase 1 of HPV, but abolished the sustained Phase 2 which is associated with Ca²⁺ sensitization. The voltage-dependent Ca²⁺ channel blocker diltiazem (10 µM) had no effect on HPV, and did not alter the inhibitory action of 18β-GA. Sustained HPV is enhanced by high glucose (15 mM) via potentiation of Ca²⁺ sensitization, in the presence of high glucose 18β-GA still abolished sustained HPV. Simultaneous measurement of tension and intracellular Ca²⁺ using Fura PE-3 demonstrated that whilst 18β-GA abolished tension development during sustained HPV, it did not affect the elevation of intracellular Ca²⁺. Consistent with this, 18β-GA abolished hypoxia-induced phosphorylation of the Rho kinase target MYPT-1. In anaesthetized rats hypoxia caused a biphasic increase in systolic right ventricular pressure. Treatment with oral 18β-GA (25 mg/kg) abolished the sustained component of the hypoxic pressor response.

CONCLUSION

These results imply that GJs are critically involved in the signalling pathways leading to Rho kinase-dependent Ca²⁺ sensitization during sustained HPV, but not elevation of intracellular Ca²⁺, and may explain the dependence of the former on an intact endothelium.

Gap junctions are essential for generating the correlated spike activity of neighboring retinal ganglion cells

Neurons throughout the brain show spike activity that is temporally correlated to that expressed by their neighbors, yet the generating mechanism(s) remains unclear. In the retina, ganglion cells (GCs) show robust, concerted spiking that shapes the information transmitted to central targets. Here we report the synaptic circuits responsible for generating the different types of concerted spiking of GC neighbors in the mouse retina. The most precise concerted spiking was generated by reciprocal electrical coupling of GC neighbors via gap junctions, whereas indirect electrical coupling to a common cohort of amacrine cells generated the correlated activity with medium precision. In contrast, the correlated spiking with the lowest temporal precision was produced by shared synaptic inputs carrying photoreceptor noise. Overall, our results demonstrate that different synaptic circuits generate the discrete types of GC correlated activity. Moreover, our findings expand our understanding of the roles of gap junctions in the retina, showing that they are essential for generating all forms of concerted GC activity transmitted to central brain targets.

18β-Glycyrrhetinic acid potently inhibits Kv1.3 potassium channels and T cell activation in human Jurkat T cells

ETHNOPHARMACOLOGICAL RELEVANCE

Licorice has been extensively used in traditional medicines for treatment of many diseases, including inflammations and immunological disorders. Recent studies have shown that the anti-inflammatory and immunomodulation activities of licorice have been attributed to its active component, glycyrretinic acid (GA). GA consists of two isoforms, 18α- and 18β-. However, its mechanism remains poorly understood.

AIM OF THE STUDY

We compared the effects of two isoforms on Kv1.3 channels in Jurkat T cells and further characterized the inhibition of Kv1.3 channels by 18β-GA in CHO cells. In addition, we examined the effects of 18β-GA on Kv1.3 gene expression, Ca(2+) influx, proliferation, as well as IL-2 production in Jurkat T cells.

MATERIALS AND METHODS

Whole-cell patch-clamp technique was applied to record Kv1.3 currents in Jurkat T or CHO cells. Real-time PCR and Western blotting were used to detect gene expression. Fluo-4, CCK-8 kit and ELISA kit were used to measure Ca(2+) influx, proliferation, and IL-2 secretion in Jurkat T cells, respectively.

RESULTS

Superfusion of 18β-GA (10-100 µM) blocked Kv1.3 currents in Jurkat T cells, while 18α-GA at the same concentration had no effect. The 18β-GA induced inhibition had a voltage- and concentration-dependent manner with an IC50 of 23.9±1.5 µM at +40 mV in CHO cells. Furthermore, 18β-GA significantly inhibited Kv1.3 gene expression. In addition, paralleling Kv1.3 inhibition, 18β-GA also inhibited Ca(2+) influx, proliferation as well as IL-2 production in Jurkat T cells.

CONCLUSION

18β-GA blocks Kv1.3 channels, which probably involves its anti-inflammatory and immunomodulation effects.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.

Endothelial alkalinisation inhibits gap junction communication and endothelium-derived hyperpolarisations in mouse mesenteric arteries

Abstract  Gap junctions mediate intercellular signalling in arteries and contribute to endothelium-dependent vasorelaxation, conducted vascular responses and vasomotion. Considering its putative role in vascular dysfunction, mechanistic insights regarding the control of gap junction conductivity are required. Here, we investigated the consequences of endothelial alkalinisation for gap junction communication and endothelium-dependent vasorelaxation in resistance arteries. We studied mesenteric arteries from NMRI mice by myography, confocal fluorescence microscopy and electrophysiological techniques. Removing CO2/HCO3(-), reducing extracellular [Cl(-)] or adding 4,4-diisothiocyanatostilbene-2,2-disulphonic acid inhibited or reversed Cl(-)/HCO3(-) exchange, alkalinised the endothelium by 0.2-0.3 pH units and inhibited acetylcholine-induced vasorelaxation. NO-synthase-dependent vasorelaxation was unaffected by endothelial alkalinisation whereas vasorelaxation dependent on small- and intermediate-conductance Ca(2+)-activated K(+) channels was attenuated by ∼75%. The difference in vasorelaxation between arteries with normal and elevated endothelial intracellular pH (pHi) was abolished by the gap junction inhibitors 18β-glycyrrhetinic acid and carbenoxolone while other putative modulators of endothelium-derived hyperpolarisations - Ba(2+), ouabain, iberiotoxin, 8Br-cAMP and polyethylene glycol catalase - had no effect. In the absence of CO2/HCO3(-), addition of the Na(+)/H(+)-exchange inhibitor cariporide normalised endothelial pHi and restored vasorelaxation to acetylcholine. Endothelial hyperpolarisations and Ca(2+) responses to acetylcholine were unaffected by omission of CO2/HCO3(-). By contrast, dye transfer between endothelial cells and endothelium-derived hyperpolarisations of vascular smooth muscle cells stimulated by acetylcholine or the proteinase-activated receptor 2 agonist SLIGRL-amide were inhibited in the absence of CO2/HCO3(-). We conclude that intracellular alkalinisation of endothelial cells attenuates endothelium-derived hyperpolarisations in resistance arteries due to inhibition of gap junction communication. These findings highlight the role of pHi in modulating vascular function.

Electrical conduction along endothelial cell tubes from mouse feed arteries: confounding actions of glycyrrhetinic acid derivatives

BACKGROUND AND PURPOSE

Electrical conduction along endothelium of resistance vessels has not been determined independently of the influence of smooth muscle, surrounding tissue or blood. Two interrelated hypotheses were tested: (i) Intercellular conduction of electrical signals is manifest in endothelial cell (EC) tubes; and (ii) Inhibitors of gap junction channels (GJCs) have confounding actions on EC electrical and Ca(2+) signalling.

EXPERIMENTAL APPROACH

Intact EC tubes were isolated from abdominal muscle feed (superior epigastric) arteries of C57BL/6 mice. Hyperpolarization was initiated with indirect (ACh) and direct (NS309) stimulation of intermediate- and small-conductance Ca(2+) -activated K(+) channels (IK(Ca) /SK(Ca) ). Remote membrane potential (V(m) ) responses to intracellular current injection defined the length constant (λ) for electrical conduction. Dye coupling was evaluated following intracellular microinjection of propidium iodide. Intracellular Ca(2+) dynamics were determined using Fura-2 photometry. Carbenoxolone (CBX) or β-glycyrrhetinic acid (βGA) was used to investigate the role of GJCs.

KEY RESULTS

Steady-state V(m) of ECs was -25 mV. ACh and NS309 hyperpolarized ECs by -40 and -60 mV respectively. Electrical conduction decayed monoexponentially with distance (λ∼1.4 mm). Propidium iodide injected into one EC spread into surrounding ECs. CBX or βGA inhibited dye transfer, electrical conduction and EC hyperpolarization reversibly. Both agents elevated resting Ca(2+) while βGA inhibited responses to ACh.

CONCLUSIONS AND IMPLICATIONS

Individual cells were effectively coupled to each other within EC tubes. Inhibiting GJCs with glycyrrhetinic acid derivatives blocked hyperpolarization mediated by IK(Ca) /SK(Ca) channels, regardless of Ca(2+) signalling, obviating use of these agents in distinguishing key determinants of electrical conduction along the endothelium.

Multiple factors influence calcium synchronization in arterial vasomotion

The intercellular synchronization of spontaneous calcium (Ca(2+)) oscillations in individual smooth muscle cells is a prerequisite for vasomotion. A detailed mathematical model of Ca(2+) dynamics in rat mesenteric arteries shows that a number of synchronizing and desynchronizing pathways may be involved. In particular, Ca(2+)-dependent phospholipase C, the intercellular diffusion of inositol trisphosphate (IP(3), and to a lesser extent Ca(2+)), IP(3) receptors, diacylglycerol-activated nonselective cation channels, and Ca(2+)-activated chloride channels can contribute to synchronization, whereas large-conductance Ca(2+)-activated potassium channels have a desynchronizing effect. Depending on the contractile state and agonist concentrations, different pathways become predominant, and can be revealed by carefully inhibiting the oscillatory component of their total activity. The phase shift between the Ca(2+) and membrane potential oscillations can change, and thus electrical coupling through gap junctions can mediate either synchronization or desynchronization. The effect of the endothelium is highly variable because it can simultaneously enhance the intercellular coupling and affect multiple smooth muscle cell components. Here, we outline a system of increased complexity and propose potential synchronization mechanisms that need to be experimentally tested.

Large conductance Ca2+-activated K+ channels modulate endothelial cell outward currents and nitric oxide release in the intact rat superior mesenteric artery

Endothelial cells (EC) control vascular smooth muscle cell (VSMC) tone by release of paracrine factors. VSMC may also influence the EC layer, and therefore, the present study hypothesized that the opening of large-conductance Ca(2+) activated K(+) (BK(Ca)) channels may indirectly modulate EC hyperpolarization and nitric oxide (NO) release via myoendothelial gap junctions (MEGJ). To address this hypothesis 'in situ' EC ion current recordings, isolated VSMC patch clamp recordings, and simultaneous measurements of NO concentration and relaxation were conducted using segments of the rat superior mesenteric artery. In arteries constricted by α(1)-adrenoceptor activation, ACh (1 μM) evoked EC outward currents, vasorelaxation, and NO release. In contrast to preincubation with iberiotoxin (IbTx, 100nM) application of IbTx after ACh decreased EC outward currents, NO release and vasorelaxation. Furthermore, in phenylephrine (Phe)-contracted arteries treated with a gap junction uncoupler, cabenoxolone (CBX), IbTx failed to decrease ACh-evoked EC outward currents. In addition, CBX decreased EC outward currents, time constant of the capacitative transients, input capacitance, and increased input resistance. In isolated VSMC CBX did not affect BK(Ca) currents. Immunohistochemistry revealed only BK(Ca) channel positive staining in the VSMC layer. Therefore, the present results suggest that BK(Ca) channels are expressed in the VSMC, and that Phe by activation of VSMC BK(Ca) channels modulates ACh-evoked EC outward currents, NO release and vasorelaxation via MEGJ in rat superior mesenteric artery.

Low-intensity pulsed ultrasound (LIPUS) and cell-to-cell communication in bone marrow stromal cells

Low-intensity pulsed ultrasound (LIPUS) is an established therapy for fracture repair and has been used widely in the clinics, but its underlying mechanism of action remains unclear. The aim of the current research was to determine the effect of LIPUS on gap junctional cell-to-cell intercellular communication in rat bone marrow stromal cells (BMSC) in vitro and to determine whether the ability of BMSCs to communicate by gap junctions would affect their response to LIPUS. Single or daily-multiple LIPUS treatment at 1.5MHz, 30mW/cm(2), for 20min was applied to BMSC. We demonstrated that BMSC form functional gap junctions and single LIPUS treatment significantly increased the intracellular dye transfer between BMSC. In addition, activated phosphorylation of ERK1/2 and p38 by LIPUS stimulation was diminished when cells were treated with a gap junction inhibitor 18β-glycyrrhetinic acid (18β). We further demonstrated that 18β diminished the significant increase in alkaline phosphatase activity following LIPUS stimulation. These results suggest a potential role of gap junctional cell-to-cell intercellular communication on the effects of LIPUS in BMSC.

Vasomotion - what is currently thought?

This minireview discusses vasomotion, which is the oscillation in tone of blood vessels leading to flowmotion. We will briefly discuss the prevalence of vasomotion and its potential physiological and pathophysiological relevance. We will also discuss the models that have been suggested to explain how a coordinated oscillatory activity of the smooth muscle tone can occur and emphasize the role of the endothelium, the handling of intracellular Ca(2+) and the role of smooth muscle cell ion conductances. It is concluded that vasomotion is likely to enhance tissue dialysis, although this concept still requires more experimental verification, and that an understanding at the molecular level for the pathways leading to vasomotion is beginning to emerge.

Intact rat superior mesenteric artery endothelium is an electrical syncytium and expresses strong inward rectifier K+ conductance

BACKGROUND AND PURPOSE

Vascular endothelial and smooth muscle cell phenotypes may change dramatically after isolation and in cell cultures. This study was designed to investigate gap junctions coupling in an integrated intact preparation and to test if K(IR) channels modulate resting membrane conductance in "in situ" endothelial cells (EC), and acetylcholine (ACh)-evoked relaxation of the rat superior mesenteric artery.

EXPERIMENTAL APPROACH

Whole cell blind patch recordings of ionic currents from in situ EC, dye-coupling experiments, and functional studies were performed in rat superior mesenteric artery.

KEY RESULTS

EC were dye-coupled through gap junctions. 18β-glycyrretinic acid (25 μM) decreased outward and inward currents, the 80% decay of time and time constant of the capacitative transients, capacitance, and increased input resistance. Barium chloride (30 μM) decreased resting and ACh-evoked inward currents, the sensitivity of ACh-evoked relaxation, and decreased both the sensitivity and the maximal relaxation to S-nitroso-N-acetyl penicillamine in arteries with, but not in arteries without endothelium.

CONCLUSIONS

The present results suggest that the EC layer of this large artery is electrically coupled, and that K(IR) channels regulate resting inward conductance, hence suggesting that they are of importance for resting membrane potential in in situ EC. Moreover, EC K(IR) channels are involved in ACh-evoked relaxation.

Effects of connexin-mimetic peptides on perfusion pressure in response to phenylephrine in isolated, perfused rat kidneys

BACKGROUND

Gap junction intercellular communication plays a fundamental role in various tissues and organs. Gap junctions transfer ions and molecules between adjacent cells and are formed by connexins (Cx). It is supposed that vascular conducted responses, which most likely spread through gap junctions in vascular beds, regulate microcirculatory blood flow and maintain vascular resistance. This study provides functional evidence supporting the critical role of gap junctions in a physiological setting and in phenylephrine (PE)-induced vasoconstriction using an ex vivo kidney perfusion technique.

METHODS

Using the isolated, perfused kidney model, infusion of gap junction inhibitors and PE, we examined the local effect of gap junction communication. Additionally, gap junction proteins Cx37, Cx40 and Cx43 were detected by immunofluorescence.

RESULTS

First, changes in the perfusion pressure were analyzed by infusing the nonselective gap junction uncoupler, 18α-glycyrrhetinic acid (18α-GA), and specific connexin-mimetic peptide inhibitors, (37,43)Gap27, (40)Gap27 and (43)Gap26. Administration of 18α-GA and (43)Gap26 significantly elevated perfusion pressure while infusion of (40)Gap27 and (37,43)Gap27 had no effect. Second, we examined the effect of infusing gap junction inhibitors on PE-induced vasoconstriction. Infusion of 18α-GA and (40)Gap27 significantly suppressed the increase in perfusion pressure induced by PE, while (43)Gap26 and (37,43)Gap27 had no effect. Third, we confirmed by immunofluorescence that Cx37, Cx40 and Cx43 were found in the endothelial cells of interstitial microvessels and that Cx40 was localized in glomerular mesangial cells as well as in smooth muscle cells of the juxtaglomerular area.

CONCLUSIONS

This study showed that Cx43 plays a pivotal role in regulating renal vascular resistance and that Cx40 attenuates PE-induced vasoconstriction. These results provide new evidence that gap junctions may control renal circulation and vascular responses.

Heptanol triggers cardioprotection via mitochondrial mechanisms and mitochondrial potassium channel opening in rat hearts

AIM

To investigate mechanisms behind heptanol (Hp)-induced infarct size reduction and in particular if protection by pre-treatment with Hp is triggered through mitochondrial mechanisms.

METHODS

Langendorff perfused rat hearts, isolated mitochondria and isolated myocytes were used. Infarct size, mitochondrial respiration, time to mitochondrial permeability transition pore (MPTP) opening and AKT and glycogen synthase kinase 3 beta (GSK-3β) phosphorylation were examined.

RESULTS

Pre-treatment with Hp reduced infarct size from 29.7 ± 3.4% to 12.6 ± 2.1%. Mitochondrial potassium channel blockers 5-hydroxy decanoic acid (5HD) blocking mitoK(ATP) and paxilline (PAX) blocking mitoK(Ca) abolished cardioprotective effect of Hp (Hp + 5HD 36.7 ± 2.9% and Hp + PAX 40.2 ± 2.8%). Hp significantly reduced respiratory control ratio in both subsarcolemmal and interfibrillar mitochondria in a dose-dependent manner (0.5-5.0 mm). The ADP oxygen ratio was also significantly reduced by Hp (2 mm). Laser scanning confocal microscopy of tetramethylrhodamine-loaded isolated rat myocytes using line scan mode showed that Hp increased time to MPTP opening. Western blot analysis showed that pre-treatment with Hp increased phosphorylation of AKT and GSK-3β before ischaemia and after 30 min of global ischaemia.

CONCLUSION

Pre-treatment with Hp protects the heart against ischaemia-reperfusion injury. This protection is most likely mediated via mitochondrial mechanisms which initiate a signalling cascade that converges on inhibition of opening of MPTP.

Informational dynamics of vasomotion in microvascular networks: a review

Vasomotion refers to spontaneous oscillation of small vessels observed in many microvascular beds. It is an intrinsic phenomenon unrelated to cardiac rhythm or neural and hormonal regulation. Vasomotion is found to be particularly prominent under conditions of metabolic stress. In spite of a significant existent literature on vasomotion, its physiological and pathophysiological roles are not clear. It is thought that modulation of vasomotion by vasoactive substances released by metabolizing tissue plays a role in ensuring optimal delivery of nutrients to the tissue. Vasomotion rhythms exhibit a great variety of temporal patterns from regular oscillations to chaos. The nature of vasomotion rhythm is believed to be significant to its function, with chaotic vasomotion offering several physiological advantages over regular, periodic vasomotion. In this article, we emphasize that vasomotion is best understood as a network phenomenon. When there is a local metabolic demand in tissue, an ideal vascular response should extend beyond local microvasculature, with coordinated changes over multiple vascular segments. Mechanisms of information transfer over a vessel network have been discussed in the literature. The microvascular system may be regarded as a network of dynamic elements, interacting, either over the vascular anatomical network via gap junctions, or physiologically by exchange of vasoactive substances. Drawing analogies with spatiotemporal patterns in neuronal networks of central nervous system, we ask if properties like synchronization/desynchronization of vasomotors have special significance to microcirculation. Thus the contemporary literature throws up a novel view of microcirculation as a network that exhibits complex, spatiotemporal and informational dynamics.

Diet-induced obesity impairs endothelium-derived hyperpolarization via altered potassium channel signaling mechanisms

Background

The vascular endothelium plays a critical role in the control of blood flow. Altered endothelium-mediated vasodilator and vasoconstrictor mechanisms underlie key aspects of cardiovascular disease, including those in obesity. Whilst the mechanism of nitric oxide (NO)-mediated vasodilation has been extensively studied in obesity, little is known about the impact of obesity on vasodilation to the endothelium-derived hyperpolarization (EDH) mechanism; which predominates in smaller resistance vessels and is characterized in this study.

Methodology/Principal Findings

Membrane potential, vessel diameter and luminal pressure were recorded in 4^th order mesenteric arteries with pressure-induced myogenic tone, in control and diet-induced obese rats. Obesity, reflecting that of human dietary etiology, was induced with a cafeteria-style diet (∼30 kJ, fat) over 16–20 weeks. Age and sexed matched controls received standard chow (∼12 kJ, fat). Channel protein distribution, expression and vessel morphology were determined using immunohistochemistry, Western blotting and ultrastructural techniques. In control and obese rat vessels, acetylcholine-mediated EDH was abolished by small and intermediate conductance calcium-activated potassium channel (SK_Ca/IK_Ca) inhibition; with such activity being impaired in obesity. SK_Ca-IK_Ca activation with cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine (CyPPA) and 1-ethyl-2-benzimidazolinone (1-EBIO), respectively, hyperpolarized and relaxed vessels from control and obese rats. IK_Ca-mediated EDH contribution was increased in obesity, and associated with altered IK_Ca distribution and elevated expression. In contrast, the SK_Ca-dependent-EDH component was reduced in obesity. Inward-rectifying potassium channel (K_ir) and Na⁺/K⁺-ATPase inhibition by barium/ouabain, respectively, attenuated and abolished EDH in arteries from control and obese rats, respectively; reflecting differential K_ir expression and distribution. Although changes in medial properties occurred, obesity had no effect on myoendothelial gap junction density.

Conclusion/Significance

In obese rats, vasodilation to EDH is impaired due to changes in the underlying potassium channel signaling mechanisms. Whilst myoendothelial gap junction density is unchanged in arteries of obese compared to control, increased IK_Ca and Na⁺/K⁺-ATPase, and decreased K_ir underlie changes in the EDH mechanism.

Evidence both L-type and non-L-type voltage-dependent calcium channels contribute to cerebral artery vasospasm following loss of NO in the rat

We recently found block of NO synthase in rat middle cerebral artery caused spasm, associated with depolarizing oscillations in membrane potential (E_m) similar in form but faster in frequency (circa 1 Hz) to vasomotion. T-type voltage-gated Ca²⁺ channels contribute to cerebral myogenic tone and vasomotion, so we investigated the significance of T-type and other ion channels for membrane potential oscillations underlying arterial spasm. Smooth muscle cell membrane potential (E_m) and tension were measured simultaneously in rat middle cerebral artery. NO synthase blockade caused temporally coupled depolarizing oscillations in cerebrovascular E_m with associated vasoconstriction. Both events were accentuated by block of smooth muscle BK_Ca. Block of T-type channels or inhibition of Na⁺/K⁺-ATPase abolished the oscillations in E_m and reduced vasoconstriction. Oscillations in E_m were either attenuated or accentuated by reducing [Ca²⁺]_o or block of K_V, respectively. TRAM-34 attenuated oscillations in both E_m and tone, apparently independent of effects against K_Ca3.1. Thus, rapid depolarizing oscillations in E_m and tone observed after endothelial function has been disrupted reflect input from T-type calcium channels in addition to L-type channels, while other depolarizing currents appear to be unimportant. These data suggest that combined block of T and L-type channels may represent an effective approach to reverse cerebral vasospasm.

Replacement of connexin 43 by connexin 32 in a knock-in mice model attenuates aortic endothelium-derived hyperpolarizing factor-mediated relaxation

Previous studies demonstrated that intercellular communication through endothelial, smooth muscle or myoendothelial connexin channels contributes to the control of vascular tone. At least four connexin types are present in the arterial wall. The aim of the present work was to assess the role played by connexin 43 (Cx43)-formed gap junctions on vessel function. Aortic reactivity to noradrenaline, acetylcholine and sodium nitroprusside, and endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxations, were analysed in a Cx43KI32 mouse model in which the coding region of Cx43 was replaced by that of connexin 32 (Cx32). Aortic rings were placed in organ baths containing a Krebs solution oxygenated at 37 degrees C (pH 7.4). Confocal images of aortic rings confirmed connexin substitution in mutant mice. In control conditions, replacement of Cx43 by Cx32 in homozygous mutant mice did not modify endothelium-independent contractile responses to noradrenaline, or relaxations in response to sodium nitroprusside (endothelium independent) or acetylcholine (endothelium dependent). However, residual endothelium-dependent relaxations in response to acetylcholine after nitric oxide synthase and cyclooxygenase inhibition (EDHF type) were significantly reduced in homozygous Cx43KI32 mice (maximal effect values: 4.86 +/- 0.37% of noradrenaline precontraction versus 7.06 +/- 0.31% in wild-type, n = 8, P < 0.05). This attenuation was mimicked by treatment of rings from wild-type animals with the connexin-mimetic peptide (37,43)Gap27 (5 x 10(-6)m). In conclusion, replacement of Cx43 by Cx32 attenuates EDHF-mediated relaxations in mice aortic rings, suggesting that they are, at least in part, dependent on Cx43-formed gap junctions. In contrast, aortic responses to tested endothelium-independent agonists were not modified in knock-in animals.

GABA blockade unmasks an OFF response in ON direction selective ganglion cells in the mammalian retina

One unique subtype of retinal ganglion cell is the direction selective (DS) cell, which responds vigorously to stimulus movement in a preferred direction, but weakly to movement in the opposite or null direction. Here we show that the application of the GABA receptor blocker picrotoxin unmasks a robust excitatory OFF response in ON DS ganglion cells. Similar to the characteristic ON response of ON DS cells, the masked OFF response is also direction selective, but its preferred direction is opposite to that of the ON component. Given that the OFF response is unmasked with picrotoxin, its direction selectivity cannot be generated by a GABAergic mechanism. Alternatively, we find that the direction selectivity of the OFF response is blocked by cholinergic drugs, suggesting that acetylcholine release from presynaptic starburst amacrine cells is crucial for its generation. Finally, we find that the OFF response is abolished by application of a gap junction blocker, suggesting that it arises from electrical synapses between ON DS and polyaxonal amacrine cells. Our results suggest a novel role for gap junctions in mixing excitatory ON and OFF signals at the ganglion cell level. We propose that OFF inputs to ON DS cells are normally masked by a GABAergic inhibition, but are unmasked under certain stimulus conditions to mediate optokinetic signals in the brain.

Glycyrretinic acid blocks cardiac sodium channels expressed in Xenopus oocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Licorice has been used to treat many ailments including cardiovascular disorders in China for long time. Recent studies have shown that the cardiac actions of licorice have been attributed to its active component, glycyrretinic acid (GA). However, its mechanism remains poorly understood.

AIM OF THE STUDY

The effects of GA on the cardiac sodium currents (I(Na)), L-type calcium currents (I(Ca,L)) and hyperpolarization-activated inward currents (I(f)) were investigated.

MATERIALS AND METHODS

Human isoforms of wild-type and DeltaKPQ-mutant type sodium channels were expressed in Xenopus oocytes, and the resulting currents (peak and late I(Na)) were recorded using a two-microelectrode voltage-clamp technique. A perforated patch clamp technique was employed to record I(Ca,L) and I(f) from isolated rabbit sinoatrial node pacemaker cells.

RESULTS

GA inhibited peak I(Na) (33% at 90 microM) and late I(Na) (72% at 90 microM), but caused no significant effects on I(Ca,L) and I(f).

CONCLUSION

GA blocked cardiac sodium currents, particularly late I(Na.) Our findings might help to understand the traditional use of licorice in the treatment of cardiovascular disorders, because reduction of sodium currents (particularly late I(Na)) would be expected to provide protection from Na(+)-induced Ca(2+) overload and cell damage.

GAP-134 ([2S,4R]-1-[2-Aminoacetyl]4-Benzamidopyrrolidine-2-Carboxylic Acid) Prevents Spontaneous Ventricular Arrhythmias and Reduces Infarct Size During Myocardial Ischemia/Reperfusion Injury in Open-Chest Dogs

The antiarrhythmic dipeptide, GAP-134, ([2S,4R]-1[2-aminoacetyl]-4-benzamido-pyrrolidine-2-carboxylic acid) was evaluated in canine ischemia/reperfusion model. In dogs subjected to 60-minute ischemia and 4-hour reperfusion, GAP-134 was administered 10 minutes before reperfusion as a bolus + intravenous (IV) infusion. The doses administered were 0.25 µg/kg bolus + 0.19 µg/kg per hour infusion; 2.5 µg/kg + 1.9 µg/kg per hour; 25 mg/kg + 19 mg/kg per hour; 75 mg/kg + 57 mg/kg per hour. Ventricular ectopy was quantified during reperfusion, including premature ventricular contractions (PVC) and ventricular tachycardia (VT). Total incidence of VT was reduced significantly with the 2 highest doses of GAP-134 (1.7 + 0.8; 2.2 + 1.4 events; P < .05) compared to controls (23.0 + 6.1). Total PVCs were reduced significantly from 11.1 + 1.6% in control animals to 2.0% + 0.7% and 1.8% + 0.8% after the 2 highest doses of GAP-134. Infarct size, expressed as percentage of left ventricle, was reduced significantly from 19.0% + 3.5% in controls to 7.9% + 1.5% and 7.1% + 0.8% (P < .05) at the 2 highest doses of GAP-134. GAP-134 is an effective antiarrhythmic agent with potential to reduce ischemia/reperfusion injury.

Prejunctional and postjunctional actions of heptanol and 18 beta-glycyrretinic acid in the rodent vas deferens

Heptanol and 18β-glycyrrhetinic acid (18βGA) block gap junctions, but have other actions on transmitter release that have not been characterised. This study investigates the prejunctional and postjunctional effects of these compounds in guinea pig and mouse vas deferens using intracellular electrophysiological recording and confocal Ca²⁺ imaging of sympathetic nerve terminals. In mice, heptanol (2 mM) reversibly decreased the amplitude of purinergic excitatory junction potentials (EJPs; 52 ± 5%, P < 0.05) while having little effect on spontaneous excitatory junction potentials (sEJPs). Heptanol (2 mM) reversibly abolished the nerve terminal Ca²⁺ transient in 52% of terminals. 18βGA (10 μM) decreased the mean EJP amplitude, and increased input resistance in both mouse (137 ± 17%, P < 0.05) and guinea pig (354 ± 50%, P < 0.001) vas deferens indicating gap junction blockade. Further, 18βGA increased the sEJP frequency significantly in guinea pigs (by 71 ± 25%, P < 0.05) and in 5 out of 6 tissues in mice (19 ± 3%, P < 0.05). Moreover, 18βGA depolarised cells from both mice (11 ± 1%, P < 0.01) and guinea pigs (8 ± 1%, P < 0.005). Therefore, we conclude that heptanol (2 mM) decreases neurotransmitter release (given the decrease in EJP amplitude) by abolishing the nerve terminal action potential in a proportion of nerve terminals. 18βGA (10 μM) effectively blocks the gap junctions, but the increase in sEJP frequency suggests an additional prejunctional effect, which might involve the induction of spontaneous nerve terminal action potentials.

Myoendothelial coupling in the mesenteric arterial bed; segmental differences and interplay between nitric oxide and endothelin-1

BACKGROUND AND PURPOSE

We tested the hypothesis that activated arterial smooth muscle (ASM) stimulates endothelial vasomotor influences via gap junctions and that the significance of this myoendothelial coupling increases with decreasing arterial diameter.

EXPERIMENTAL APPROACH

From WKY rats, first-, second-, third- and fourth-order branches of the superior mesenteric artery (MA1, MA2, MA3 and MA4 respectively) were isolated and mounted in wire-myographs to record vasomotor responses to 0.16-20 micromol x L(-1) phenylephrine.

KEY RESULTS

Removal of endothelium increased the sensitivity (pEC(50)) to phenylephrine in all arteries. The nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) (100 micromol x L(-1)) did not modify pEC(50) to phenylephrine in all denuded arteries, and increased it in intact MA1, MA2 and MA3 to the same extent as denudation. However, in intact MA4, the effect of L-NAME was significantly larger (DeltapEC(50) 0.57 +/- 0.02) than the effect of endothelium removal (DeltapEC(50) 0.20 +/- 0.06). This endothelium-dependent effect of L-NAME in MA4 was inhibited by (i) steroidal and peptidergic uncouplers of gap junctions; (ii) a low concentration of the NO donor sodium nitroprusside; and (iii) by the endothelin-receptor antagonist bosentan. It was also observed during contractions induced by (i) calcium channel activation (BayK 8644, 0.001-1 micromol x L(-1)); (ii) depolarization (10-40 mmol x L(-1) K(+)); and (iii) sympathetic nerve stimulation (0.25-32 Hz).

CONCLUSIONS AND IMPLICATIONS

These pharmacological observations indicated feedback control by endothelium of ASM reactivity involving gap junctions and a balance between endothelium-derived NO and endothelin-1. This myoendothelial coupling was most prominent in distal resistance arteries.