Connexin expression and conducted vasodilation along arteriolar endothelium in mouse skeletal muscle

Intravital microscopy of the inguinal lymph node

Lymph nodes (LN's), located throughout the body, are an integral component of the immune system. They serve as a site for induction of adaptive immune response and therefore, the development of effector cells. As such, LNs are key to fighting invading pathogens and maintaining health. The choice of LN to study is dictated by accessibility and the desired model; the inguinal lymph node is well situated and easily supports studies of biologically relevant models of skin and genital mucosal infection. The inguinal LN, like all LNs, has an extensive microvascular network supplying it with blood. In general, this microvascular network includes the main feed arteriole of the LN that subsequently branches and feeds high endothelial venules (HEVs). HEVs are specialized for facilitating the trafficking of immune cells into the LN during both homeostasis and infection. How HEVs regulate trafficking into the LN under both of these circumstances is an area of intense exploration. The LN feed arteriole, has direct upstream influence on the HEVs and is the main supply of nutrients and cell rich blood into the LN. Furthermore, changes in the feed arteriole are implicated in facilitating induction of adaptive immune response. The LN microvasculature has obvious importance in maintaining an optimal blood supply to the LN and regulating immune cell influx into the LN, which are crucial elements in proper LN function and subsequently immune response. The ability to study the LN microvasculature in vivo is key to elucidating how the immune system and the microvasculature interact and influence one another within the LN. Here, we present a method for in vivo imaging of the inguinal lymph node. We focus on imaging of the microvasculature of the LN, paying particular attention to methods that ensure the study of healthy vessels, the ability to maintain imaging of viable vessels over a number of hours, and quantification of vessel magnitude. Methods for perfusion of the microvasculature with vasoactive drugs as well as the potential to trace and quantify cellular traffic are also presented. Intravital microscopy of the inguinal LN allows direct evaluation of microvascular functionality and real-time interface of the direct interface between immune cells, the LN, and the microcirculation. This technique potential to be combined with many immunological techniques and fluorescent cell labelling as well as manipulated to study vasculature of other LNs.

Non-linear relationship between hyperpolarisation and relaxation enables long distance propagation of vasodilatation

Blood flow is adjusted to tissue demand through rapidly ascending vasodilatations resulting from conduction of hyperpolarisation through vascular gap junctions. We investigated how these dilatations can spread without attenuation if mediated by an electrical signal. Cremaster muscle arterioles were studied in vivo by simultaneously measuring membrane potential and vessel diameter. Focal application of acetylcholine elicited hyperpolarisations which decayed passively with distance from the local site,while dilatation spread upstream without attenuation. Analysis of simultaneous recordings at the local site revealed that hyperpolarisation and dilatation were only linearly related over a restricted voltage range to a threshold potential, beyond which dilatation was maximal. Experimental data could be simulated in a computational model with electrotonic decay of hyperpolarisation but imposition of this threshold. The model was tested by reducing the amplitude of the local hyperpolarisation which led to entry into the linear range closer to the local site and decay of dilatation. Serial section electron microscopy and light dye treatment confirmed that the spread of dilatation occurred through the endothelium and that the two cell layers were tightly coupled. Generality of the mechanism was demonstrated by applying the model to the attenuated propagation of dilatation found in larger arteries.We conclude that long distance spread of locally initiated dilatations is not due to a regenerative electrical phenomenon, but rather a restricted linear relationship between voltage and vessel tone, which minimises the impact of electrotonic decay of voltage. Disease-related alterations in endothelial coupling or ion channel expression could therefore decrease the ability to adjust blood flow to meet metabolic demand.

Role of connexins in microvascular dysfunction during inflammation

In arterioles, a locally initiated diameter change can propagate rapidly along the vessel length (arteriolar conducted response), thus contributing to arteriolar hemodynamic resistance. The response is underpinned by electrical coupling along the arteriolar endothelial layer. Connexins (Cx; constituents of gap junctions) are required for this coupling. This review addresses the effect of acute systemic inflammation (sepsis) on arteriolar conduction and interendothelial electrical coupling. Lipopolysaccharide (LPS; an initiating factor in sepsis) and polymicrobial sepsis (24 h model) attenuate conducted vasoconstriction in mice. In cultured microvascular endothelial cells harvested from rat and mouse skeletal muscle, LPS reduces both conducted hyperpolarization–depolarization along capillary-like structures and electrical coupling along confluent cell monolayers. LPS also tyrosine-phosphorylates Cx43 and serine-dephosphorylates Cx40. Since LPS-reduced coupling is Cx40- but not Cx43-dependent, only Cx40 dephosphorylation may be consequential. Nitric oxide (NO) overproduction is critical in advanced sepsis, since the removal of this overproduction prevents the attenuated conduction. Consistently, (i) exogenous NO in cultured cells reduces coupling in a Cx37-dependent manner, and (ii) the septic microvasculature in vivo shows no Cx40 phenotype. A complex role emerges for endothelial connexins in sepsis. Initially, LPS may reduce interendothelial coupling and arteriolar conduction by targeting Cx40, whereas NO overproduction in advanced sepsis reduces coupling and conduction by targeting Cx37 instead.

Evidence for impaired neurovascular transmission in a murine model of Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a muscle-wasting disease caused by mutations in the dystrophin gene. Little is known about how blood flow control is affected in arteriolar networks supplying dystrophic muscle. We tested the hypothesis that mdx mice, a murine model for DMD, exhibit defects in arteriolar vasomotor control. The cremaster muscle was prepared for intravital microscopy in pentobarbital sodium-anesthetized mdx and C57BL/10 control mice (n ≥ 5 per group). Spontaneous vasomotor tone increased similarly with arteriolar branch order in both mdx and C57BL/10 mice [pooled values: first order (1A), 6%; second order (2A), 56%; and third order (3A), 61%] with no difference in maximal diameters between groups measured during equilibration with topical 10 μM sodium nitroprusside (pooled values: 1A, 70 ± 3 μm; 2A, 31 ± 3 μm; and 3A, 19 ± 3 μm). Concentration-response curves to acetylcholine (ACh) and norepinephrine added to the superfusion solution did not differ between mdx and C57BL/10 mice, nor did constriction to elevated (21%) oxygen. In response to local stimulation from a micropipette, conducted vasodilation to ACh and conducted vasoconstriction to KCl were also not different between groups; however, constriction decayed with distance (P < 0.05) whereas dilation did not. Remarkably, arteriolar constriction to perivascular nerve stimulation (PNS) at 2, 4, and 8 Hz was reduced by ∼25-30% in mdx mice compared with C57BL/10 mice (P < 0.05). With intact arteriolar reactivity to agonists, attenuated constriction to perivascular nerve stimulation indicates impaired neurovascular transmission in arterioles controlling blood flow in mdx mice.

Loss of connexin40 is associated with decreased endothelium-dependent relaxations and eNOS levels in the mouse aorta

Upon agonist stimulation, endothelial cells trigger smooth muscle relaxation through the release of relaxing factors such as nitric oxide (NO). Endothelial cells of mouse aorta are interconnected by gap junctions made of connexin40 (Cx40) and connexin37 (Cx37), allowing the exchange of signaling molecules to coordinate their activity. Wild-type (Cx40(+/+)) and hypertensive Cx40-deficient mice (Cx40(-/-)), which also exhibit a marked decrease of Cx37 in the endothelium, were used to investigate the link between the expression of endothelial connexins (Cx40 and Cx37) and endothelial nitric oxide synthase (eNOS) expression and function in the mouse aorta. With the use of isometric tension measurements in aortic rings precontracted with U-46619, a stable thromboxane A(2) mimetic, we first demonstrate that ACh- and ATP-induced endothelium-dependent relaxations solely depend on NO release in both Cx40(+/+) and Cx40(-/-) mice, but are markedly weaker in Cx40(-/-) mice. Consistently, both basal and ACh- or ATP-induced NO production were decreased in the aorta of Cx40(-/-) mice. Altered relaxations and NO release from aorta of Cx40(-/-) mice were associated with lower expression levels of eNOS in the aortic endothelium of Cx40(-/-) mice. Using immunoprecipitation and in situ ligation assay, we further demonstrate that eNOS, Cx40, and Cx37 tightly interact with each other at intercellular junctions in the aortic endothelium of Cx40(+/+) mice, suggesting that the absence of Cx40 in association with altered Cx37 levels in endothelial cells from Cx40(-/-) mice participate to the decreased levels of eNOS. Altogether, our data suggest that the endothelial connexins may participate in the control of eNOS expression levels and function.

Oxygen sensing and conducted vasomotor responses in mouse cremaster arterioles in situ

This study examines mechanisms by which changes in tissue oxygen tension elicit vasomotor responses and whether localized changes in oxygen tension initiates conducted vasomotor responses in mouse cremaster arterioles. Intravital microscopy was used to visualize the mouse cremaster microcirculation. The cremaster was superfused with Krebs' solution with different oxygen tensions, and a gas exchange chamber was used to induce localized changes in oxygen tension. In arterioles where red blood cells were removed by buffer perfusion, arterioles responded with same magnitudes of vasodilatation (DeltaD = 16.0 +/- 4.9 microm) when changing from high (PO(2) = 242.5 +/- 13.3 mm Hg) to low (PO(2) = 22.5 +/- 4.8 mm Hg) oxygen tension as seen in the intact cremaster circulation (DeltaD = 18.7 +/- 1.0 microm). Blockade of NO synthases by L: -NAME and adenosine receptors by DPCPX had no effects on vasomotor responses to low or high oxygen. Induction of localized low (PO(2) = 23.3 +/- 5.7 mmHg) or high (PO(2) = 300.0 +/- 25.7 mm Hg) oxygen tension caused vasodilatation or -constriction locally and at a site 1,000 microm upstream (distantly). Glibenclamide blocker of ATP-sensitive K(+) channels inhibited vasodilatation and -constriction to low (PO(2) = 16.0 +/- 6.4 mm Hg) and high (PO(2) = 337.4 +/- 12.8 mm Hg) oxygen tension. 1) ATP-sensitive K(+) channels seem to mediate, at least in part, vasodilatation and vasoconstriction to low and high oxygen tension; 2) Red blood cells are not necessary for inducing vasodilatation and vasoconstriction to low or high oxygen tension; 3) localized changes in the oxygen tension cause vasomotor responses, which are conducted upstream along arterioles in mouse cremaster microcirculation.

Connexins and gap junctions in the EDHF phenomenon and conducted vasomotor responses

It is becoming increasingly evident that electrical signaling via gap junctions plays a central role in the physiological control of vascular tone via two related mechanisms (1) the endothelium-derived hyperpolarizing factor (EDHF) phenomenon, in which radial transmission of hyperpolarization from the endothelium to subjacent smooth muscle promotes relaxation, and (2) responses that propagate longitudinally, in which electrical signaling within the intimal and medial layers of the arteriolar wall orchestrates mechanical behavior over biologically large distances. In the EDHF phenomenon, the transmitted endothelial hyperpolarization is initiated by the activation of Ca(2+)-activated K(+) channels channels by InsP(3)-induced Ca(2+) release from the endoplasmic reticulum and/or store-operated Ca(2+) entry triggered by the depletion of such stores. Pharmacological inhibitors of direct cell-cell coupling may thus attenuate EDHF-type smooth muscle hyperpolarizations and relaxations, confirming the participation of electrotonic signaling via myoendothelial and homocellular smooth muscle gap junctions. In contrast to isolated vessels, surprisingly little experimental evidence argues in favor of myoendothelial coupling acting as the EDHF mechanism in arterioles in vivo. However, it now seems established that the endothelium plays the leading role in the spatial propagation of arteriolar responses and that these involve poorly understood regenerative mechanisms. The present review will focus on the complex interactions between the diverse cellular signaling mechanisms that contribute to these phenomena.

Different pathways with distinct properties conduct dilations in the microcirculation in vivo

AIMS

Conduction of vasomotor signals along the vessel coordinates the behaviour of vascular cells and is attributed to the spread of hyperpolarizations through gap junctions. Intriguingly, conducted dilations encompass larger distances than can be expected by passive electrotonic spread. Because distances are quite distinct for different dilators, we hypothesized that separate pathways with distinct properties are involved.

METHODS AND RESULTS

We characterized local and conducted responses elicited by acetylcholine (ACh) and adenosine (Ado) in the murine microcirculation in vivo. Local (and remote) ACh dilations were nearly abrogated by blockade of K(Ca) channels (charybdotoxin), but dilations to Ado were abolished by the K(ATP) blocker glibenclamide. Bupivacaine, a blocker of Na+ and K+ channels, and similarly the blockade of inwardly rectifying K+ channels (barium) revealed different conduction mechanisms, as the remote dilation to Ado, but not ACh, was abrogated. Surprisingly, expression of connexin37 (Cx37) was not detected in Cx40-deficient arterioles, although abundantly expressed in endothelium of wild-type arterioles. In contrast to the wild-type mice, the amplitude of conducted ACh and Ado dilations decreased similarly with distance in Cx40-deficient mice. Recordings of membrane potential in vivo showed endothelial hyperpolarization by approximately 10 mV in response to ACh, whereas Ado did not alter endothelial membrane potential.

CONCLUSION

Distinct pathways conduct responses along the vessel wall which involve dissimilar K+ channels and connexins in initiation and spreading. Most likely, the endothelium is the preferential conduction pathway activated by ACh, whereas in the case of Ado the smooth muscle serves as the signalling pathway. However, in arterioles nearly devoid of Cx40 and Cx37, ACh responses can likewise be conducted along the smooth muscle.

Regulation of Blood Flow in the Microcirculation

The regulation of blood flow has rich history of investigation and is exemplified in exercising skeletal muscle by a concerted interaction between striated muscle fibers and their microvascular supply. This review considers blood flow control in light of the regulation of capillary perfusion by and among terminal arterioles, the distribution of blood flow in arteriolar networks according to metabolic and hemodynamic feedback from active muscle fibers, and the balance between peak muscle blood flow and arterial blood pressure governed by sympathetic nerve activity. As metabolic demand increases,the locus of regulating oxygen delivery to muscle fibers "ascends" from terminal arterioles, through intermediate distributing arterioles, and into the proximal arterioles and feed arteries, which govern total flow into a muscle. At multiple levels, venules are positioned to provide feedback to nearby arterioles regarding the metabolic state of the tissue through the convection, production and diffusion of vasodilator stimuli. Electrical signals initiated on microvascular smooth muscle and endothelial cells can travel rapidly for millimeters through cell-to-cell conduction via gap junction channels, rapidly coordinating vasodilator responses that govern the distribution and magnitude of blood flow to active muscle fibers. Sympathetic constriction of proximal arterioles and feed arteries can restrict functional hyperemia while dilation prevails in distal arterioles to promote oxygen extraction. With vasomotor tone reflecting myogenic contraction of smooth muscle cells modulated by shear stress on the endothelium, the initiation of functional vasodilation and its modulation by sympathetic innervation dictate how and where blood flow is distributed in response to metabolic demand. A remarkable ensemble of signaling pathways underlies the integration of smooth muscle and endothelial cell function in microvascular networks. These pathways are being defined with refreshing new insight as novel approaches are applied to understanding the cellular and molecular mechanisms of blood flow control.

Site-specific connexin phosphorylation is associated with reduced heterocellular communication between smooth muscle and endothelium

BACKGROUND/AIMS

Myoendothelial junctions (MEJs) represent a specialized signaling domain between vascular smooth muscle cells (VSMC) and endothelial cells (EC). The functional consequences of phosphorylation state of the connexins (Cx) at the MEJ have not been explored.

METHODS/RESULTS

Application of adenosine 3',5'-cyclic monophosphate sodium (pCPT) to mouse cremasteric arterioles reduces the detection of connexin 43 (Cx43) phosphorylated at its carboxyl terminal serine 368 site (S368) at the MEJ in vivo. After single-cell microinjection of a VSMC in mouse cremaster arterioles, only in the presence of pCPT was dye transfer to EC observed. We used a vascular cell co-culture (VCCC) and applied the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (PMA) or fibroblast growth factor-2 (FGF-2) to induce phosphorylation of Cx43 S368. This phosphorylation event was associated with a significant reduction in dye transfer and calcium communication. Using a novel method to monitor increases in intracellular calcium across the in vitro MEJ, we noted that PMA and FGF-2 both inhibited movement of inositol 1,4,5-triphosphate (IP(3)), but to a lesser extent Ca(2+).

CONCLUSION

These data indicate that site-specific connexin phosphorylation at the MEJ can potentially regulate the movement of solutes between EC and VSMC in the vessel wall.

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.

Reduction of electrical coupling between microvascular endothelial cells by NO depends on connexin37

We have previously shown that increased nitric oxide (NO) production in sepsis impairs arteriolar-conducted vasoconstriction cGMP independently and that the gap junction protein connexin (Cx) 37 is required for this conducted response. In the present study, we hypothesized that NO impairs interendothelial electrical coupling in sepsis by targeting Cx37. We examined the effect of exogenous NO on coupling in monolayers of cultured microvascular endothelial cells derived from the hindlimb skeletal muscle of wild-type (WT), Cx37 null, Cx40 null, and Cx43(G60S) (nonfunctional mutant) mice. To assess coupling, we measured the spread of electrical current injected in the monolayer and calculated the monolayer intercellular resistance (inverse measure of coupling). The NO donor 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (DETA) rapidly and reversibly reduced coupling in cells from WT mice, cGMP independently. NO scavenger HbO(2) did not affect baseline coupling, but it eliminated DETA-induced reduction in coupling. Reduced coupling in response to DETA was also seen in cells from Cx40 null and Cx43(G60S) mice, but not in cells from Cx37 null mice. DETA did not alter the expression of Cx37, Cx40, and Cx43 in WT cells analyzed by immunoblotting and immunofluorescence. Furthermore, neither the peroxynitrite scavenger 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III), superoxide scavenger Mn(III)tetrakis(4-benzoic acid)porphyrin chloride, nor preloading of WT cells with the antioxidant ascorbate affected this reduction. We conclude that NO-induced reduction of electrical coupling between microvascular endothelial cells depends on Cx37 and propose that NO in sepsis impairs arteriolar-conducted vasoconstriction by targeting Cx37 within the arteriolar wall.

Structural adaptation and heterogeneity of normal and tumor microvascular networks

Relative to normal tissues, tumor microcirculation exhibits high structural and functional heterogeneity leading to hypoxic regions and impairing treatment efficacy. Here, computational simulations of blood vessel structural adaptation are used to explore the hypothesis that abnormal adaptive responses to local hemodynamic and metabolic stimuli contribute to aberrant morphological and hemodynamic characteristics of tumor microcirculation. Topology, vascular diameter, length, and red blood cell velocity of normal mesenteric and tumor vascular networks were recorded by intravital microscopy. Computational models were used to estimate hemodynamics and oxygen distribution and to simulate vascular diameter adaptation in response to hemodynamic, metabolic and conducted stimuli. The assumed sensitivity to hemodynamic and conducted signals, the vascular growth tendency, and the random variability of vascular responses were altered to simulate ‘normal’ and ‘tumor’ adaptation modes. The heterogeneous properties of vascular networks were characterized by diameter mismatch at vascular branch points (d³_var) and deficit of oxygen delivery relative to demand (O_2def). In the tumor, d³_var and O_2def were higher (0.404 and 0.182) than in normal networks (0.278 and 0.099). Simulated remodeling of the tumor network with ‘normal’ parameters gave low values (0.288 and 0.099). Conversely, normal networks attained tumor-like characteristics (0.41 and 0.179) upon adaptation with ‘tumor’ parameters, including low conducted sensitivity, increased growth tendency, and elevated random biological variability. It is concluded that the deviant properties of tumor microcirculation may result largely from defective structural adaptation, including strongly reduced responses to conducted stimuli.

Blood vessels of tumors have abnormal structures, being irregular and tortuous. Oxygen supply to tumors is heterogeneous, with regions of low oxygen that resist radiation treatment and some types of chemotherapy. Blood vessels undergo continual structural change (adaptation) in response to blood flow and metabolite levels. Our hypothesis is that abnormal adaptation of tumor microvessels causes their heterogeneous structure and impaired function. We used computational models to estimate blood flow and oxygen delivery and to simulate diameter adaptation in networks of microvessels, using network structures derived from microscopic observations of living normal and tumor tissues. The simulation of adaptation depends on several parameters that describe vessel sensitivity to fluid shear stress, to blood pressure, to oxygen levels, and to signals propagated along vessel walls (conducted response). We found that structural adaptation of a tumor network using parameters derived from normal tissues could ‘normalize’ the network, giving it properties similar to a normal tissue. Conversely, adaptation of normal networks using parameters derived from the tumor network, including reduced conducted response, gave tumor-like properties. We conclude that the deviant properties of tumor microcirculation may result largely from defective structural adaptation, including reduced conducted responses.

Current perspective on differential communication in small resistance arteries

Blood flow is controlled by an integrated network of resistance arteries that are coupled in series and parallel with one another. To dramatically alter tissue perfusion as required during periods of high metabolic demand, arterial networks must dilate in a coordinated manner. Gap junctions facilitate arterial coordination by enabling electrical stimuli to conduct among endothelial and (or) smooth muscle cells. The goal of this review was to provide an introduction to the field of vascular communication, the process of intercellular conduction, and the manner in which key properties influence charge flow. After a brief historical introduction, we establish the idea that electrical stimuli conduct differentially among neighbouring endothelial and smooth muscle cells. Highlighting recent studies that have synergistically combined computational and experimental approaches, this perspective explores how specific structural, electrical, and gap junctional properties enable electrical phenomenon to conduct differentially. To close, the concept of differential communication is functionally integrated into a mechanistic understanding of blood flow control.

Connexin abundance in resistance vessels from the renal microcirculation in normo- and hypertensive rats

The expression of connexins in renal arterioles is believed to have a profound impact on conducted responses, regulation of arteriolar tonus and renal blood flow. We have previously shown that in renal preglomerular arterioles, conducted vasomotor responses are 40% greater in spontaneously hypertensive rats (SHR) than in normotensive Sprague-Dawley (SD) rats. Because conducted vasomotor responses depend on the cell-cell communication mediated through gap junctions, we hypothesized that the increased magnitude of conducted vasomotor response in SHR is associated with an increased amount of connexins in renal arterioles. To test this hypothesis, the amount of connexin 37 (Cx37), Cx40 and Cx43 was assessed in renal arterioles from normo- and hypertensive rats using quantitative immunofluorescence laser confocal microscopy. To account for differences in genetic background, we included both normotensive Wistar-Kyoto (WKY) and SD rats in the study. In all three strains of rats, and for all three isoforms, the expression of connexins was predominantly confined to the endothelial cells. We found a significantly increased abundance (240 +/- 17.6%, p<0.05) of Cx37 in arterioles from WKY compared with SD and SHR. This high abundance of Cx37 was not related to blood pressure because normotensive SD demonstrated a level of Cx37 similar to that of SHR. Additionally, we found no evidence for an increased abundance of Cx40 and Cx43 in renal arterioles of SHR when compared with normotensive counterparts.

The myoendothelial junction: breaking through the matrix?

Within the vasculature, specialized cellular extensions from endothelium (and sometimes smooth muscle) protrude through the extracellular matrix where they interact with the opposing cell type. These structures, termed myoendothelial junctions, have been cited as a possible key element in the control of several vascular physiologies and pathologies. This review will discuss observations that have led to a focus on the myoendothelial junction as a cellular integration point in the vasculature for both homeostatic and pathological conditions and as a possible independent signaling entity. We will also highlight the need for novel approaches to studying the myoendothelial junction in order to comprehend the cellular biology associated with this structure.

Connexins in vascular physiology and pathology

Cellular interaction in blood vessels is maintained by multiple communication pathways, including gap junctions. They consist of intercellular channels ensuring direct interaction between endothelial and smooth muscle cells and the synchronization of their behavior along the vascular wall. Gap-junction channels arise from the docking of two hemichannels or connexons, formed by the assembly of six connexins, and achieve direct cellular communication by allowing the transport of small metabolites, second messengers, and ions between two adjacent cells. Physiologic variations in connexin expression are observed along the vascular tree, with most common connexins being Cx37, Cx40, and Cx43. Changes in the level of expression of connexins have been correlated to the development of vascular disease, such as hypertension, atherosclerosis, or restenosis. Recent studies on connexin-deficient mice highlighted key roles of these communication pathways in the development of these pathologies and confirmed the need for targeted pharmacologic approaches for their prevention and treatment. The aim of this issue is to review the current knowledge on the implication of gap junctions in vascular function and most common cardiovascular diseases.

Biological and biophysical properties of vascular connexin channels

Intercellular channels formed by connexin proteins play a pivotal role in the direct movement of ions and larger cytoplasmic solutes between vascular endothelial cells, between vascular smooth muscle cells, and between endothelial and smooth muscle cells. Multiple genetic and epigenetic factors modulate connexin expression levels and/or channel function, including cell-type-independent and cell-type-specific transcription factors, posttranslational modifications, and localized membrane targeting. Additionally, differences in protein-protein interactions, including those between connexins, significantly contribute to both vascular homeostasis and disease progression. The biophysical properties of the connexin channels identified in the vasculature, those formed by Cx37, Cx40, Cx43 and/or Cx45 proteins, are discussed in this chapter in the physiological and pathophysiological context of vessel function.

Connexin isoform expression in smooth muscle cells and endothelial cells of hamster cheek pouch arterioles and retractor feed arteries

OBJECTIVE

Gap junction channels formed by connexin (Cx) protein subunits enable cell-to-cell conduction of vasoactive signals. Given the lack of quantitative measurements of Cx expression in microvascular endothelial cells (EC) and smooth muscle cells (SMC), the objective was to determine whether Cx expression differed between EC and SMC of resistance microvessels for which conduction is well-characterized.

METHODS

Cheek pouch arterioles (CPA) and retractor feed arteries (RFA) were hand-dissected and dissociated to obtain SMC or endothelial tubes. In complementary experiments, small intestine was dissociated to obtain SMC. Following reverse transcription, quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) was performed by using specific primers and fluorescent probes for Cx37, Cx40, and Cx43. Smooth muscle alpha-actin (SMAA) and platelet endothelial cell adhesion molecule-1 (PECAM-1) served as respective reference genes.

RESULTS

Transcript copy numbers were similar for each Cx isoform in EC from CPA and RFA (approximately 0.5 Cx/PECAM-1). For SMC, Cx43 transcript in CPA and RFA (< 0.1 Cx/SMAA) was less (p < 0.05) than that in small intestine (approximately 0.4 Cx/SMAA). Transcripts for Cx37 and Cx40 were also detected in SMC. Punctate immunolabeling for each Cx isoform was pronounced at EC borders and that for Cx43 was pronounced in SMC of small intestine. In contrast, Cx immunolabeling was not detected in SMC of CPA or RFA.

CONCLUSIONS

Connexin expression occurs primarily within the endothelium of arterioles and feed arteries, supporting a highly effective pathway for conducting vasoactive signals along resistance networks. The apparent paucity of Cx expression within SMC underscores discrete homocellular coupling and focal localization of myoendothelial gap junctions.

Dissection of two Cx37-independent conducted vasodilator mechanisms by deletion of Cx40: electrotonic versus regenerative conduction

Conduction of changes in diameter plays an important role in the coordination of peripheral vascular resistance and, thereby, in the control of arterial blood pressure. It is thought that conduction of vasomotor signals relies on the electrotonic spread of changes in membrane potential from a site of stimulation through gap junctions connecting the cells of the vessel wall. To explore this idea, we stimulated a short segment of mouse cremasteric arterioles with an application, via micropipette, of ACh, an endothelium-dependent vasodilator, or pinacidil, an ATP-sensitive K+ channel opener. Vasodilations were evaluated at the stimulation site (local) and at 500, 1,000, and 2,000 microm upstream. The vasodilator response evoked by direct arteriolar hyperpolarization induced by pinacidil decayed rapidly with distance, as expected for the passive spread of an electrical signal. Deletion of the gap junction proteins connexin37 or connexin40 did not alter the conduction of pinacidil-induced vasodilation. In contrast to pinacidil, the vasodilator response activated by ACh spread along the entire vessel without decrement. Although the ACh-induced conducted vasodilation was similar in wild-type and connexin37 knockout mice, deletion of connexin40 converted the nondecremental conducted response activated by ACh into one similar to that of pinacidil, with a decline in magnitude along the vessel length. These results suggest that ACh activates a mechanism of regenerative conduction of vasodilator responses. Connexin40 is essential for the ACh-activated regenerative vasodilator mechanism. However, neither connexin40 nor connexin37 is indispensable for the electrotonic spread of hyperpolarizing signals.

Modeling structural adaptation of microcirculation

The functional properties of microcirculation crucially depend on its angioarchitecture, (i.e., vessel arrangement and morphology). The microcirculation is subject to continuous dynamic structural adaptation (i.e., remodeling) controlled by hemodynamic and metabolic stimuli. Due to the complexity of the interactions among stimuli, reactions, and functional properties, an adequate understanding of structural adaptation requires mathematical models in addition to experimental investigations. Mathematical models have been developed that allow the prediction of realistic vascular properties, based on generic patterns of vascular responses. These models can be used to investigate and predict distributions of vessel morphology consistent with certain putative adaptation principles of terminal vascular beds in response to local hemodynamic and metabolic conditions. They have suggested new hypotheses, including the importance of conducted responses in network adaptation, and can explain the mechanisms underlying observed structural and functional network properties. In the future, the value of such models can be enhanced by including the effects of longitudinal stretch and pulsatility, the relationship between acute tone and structural adaptation, and the description of molecular and cellular mechanisms underlying structural responses of microvessels.

Functional role of gap junctions in cytokine-induced leukocyte adhesion to endothelium in vivo

To assess the hypothesis that gap junctions (GJs) participate on leukocyte-endothelium interactions in the inflammatory response, we compared leukocyte adhesion and transmigration elicited by cytokine stimulation in the presence or absence of GJ blockers in the hamster cheek pouch and also in the cremaster muscle of wild-type (WT) and endothelium-specific connexin 43 (Cx43) null mice (Cx43e(-/-)). In the cheek pouch, topical tumor necrosis factor-alpha (TNF-alpha; 150 ng/ml, 15 min) caused a sustained increment in the number of leukocytes adhered to venular endothelium (LAV) and located at perivenular regions (LPV). Superfusion with the GJ blockers 18-alpha-glycyrrhetinic acid (AGA; 75 microM) or 18-beta-glycyrrhetinic acid (50 microM) abolished the TNF-alpha-induced increase in LAV and LPV; carbenoxolone (75 microM) or oleamide (100 microM) reduced LAV by 50 and 75%, respectively, and LPV to a lesser extent. None of these GJ blockers modified venular diameter, blood flow, or leukocyte rolling. In contrast, glycyrrhizin (75 microM), a non-GJ blocker analog of AGA, was devoid of effect. Interestingly, when AGA was removed 90 min after TNF-alpha stimulation, LAV started to rise at a similar rate as in control. Conversely, application of AGA 90 min after TNF-alpha reduced the number of previously adhered cells. In WT mice, intrascrotal injection of TNF-alpha (0.5 microg/0.3 ml) increased LAV (fourfold) and LPV (threefold) compared with saline-injected controls. In contrast to the observations in WT animals, TNF-alpha stimulation did not increase LAV or LPV in Cx43e(-/-) mice. These results demonstrate an important role for GJ communication in leukocyte adhesion and transmigration during acute inflammation in vivo and further suggest that endothelial Cx43 is key in these processes.

Incidence of protein on actin bridges between endothelium and smooth muscle in arterioles demonstrates heterogeneous connexin expression and phosphorylation

Although much physiology in resistance vessels has been attributed to the cytoplasmic connection between endothelial cells (ECs) and vascular smooth muscle cells (VSMCs), little is known of the protein expression between the two cell types. In an attempt to identify the proteins between ECs and VSMCs, mouse cremaster arterioles were stained with phalloidin-Alexa 594 and viewed on a confocal microscope that resolved "actin bridges" within the internal elastic lamina between ECs and VSMCs. To determine the incidence of protein, the pixel intensity from the antibodies on actin bridges were compared with the pixel intensity from antibodies within ECs or VSMCs. N-cadherin, desmin, connexin (Cx)40, and Cx43 and phosphorylated Cx43 at serine-368 were identified on actin bridges, but NG2, CD31, and Cx45 were not evident. Cx37 expression was more variable than the other connexins examined. Using this method on rat mesentery, we confirm the previously published predominance of Cx37 and Cx40 at the myoendothelial junction that was determined using electron microscopy. We conclude that this new method represents an important screening mechanism in which to rapidly test for protein expression between ECs and VSMCs and possibly a first-step in quantifying protein expression at the myoendothelial junction.

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

Propagated endothelial Ca2+ waves and arteriolar dilation in vivo: measurements in Cx40BAC GCaMP2 transgenic mice

To study endothelial cell (EC)- specific Ca(2+) signaling in vivo we engineered transgenic mice in which the Ca(2+) sensor GCaMP2 is placed under control of endogenous connexin40 (Cx40) transcription regulatory elements within a bacterial artificial chromosome (BAC), resulting in high sensor expression in arterial ECs, atrial myocytes, and cardiac Purkinje fibers. High signal/noise Ca(2+) signals were obtained in Cx40(BAC)-GCaMP2 mice within the ventricular Purkinje cell network in vitro and in ECs of cremaster muscle arterioles in vivo. Microiontophoresis of acetylcholine (ACh) onto arterioles triggered a transient increase in EC Ca(2+) fluorescence that propagated along the arteriole with an initial velocity of approximately 116 microm/s (n=28) and decayed over distances up to 974 microm. The local rise in EC Ca(2+) was followed (delay, 830+/-60 ms; n=8) by vasodilation that conducted rapidly (mm/s), bidirectionally, and into branches for distances exceeding 1 mm. At intermediate distances (300 to 600 microm), rapidly-conducted vasodilation occurred without changing EC Ca(2+), and additional dilation occurred after arrival of a Ca(2+) wave. In contrast, focal delivery of sodium nitroprusside evoked similar local dilations without Ca(2+) signaling or conduction. We conclude that in vivo responses to ACh in arterioles consists of 2 phases: (1) a rapidly-conducted vasodilation initiated by a local rise in EC Ca(2+) but independent of EC Ca(2+) signaling at remote sites; and (2) a slower complementary dilation associated with a Ca(2+) wave that propagates along the endothelium.

Connexin45 cannot replace the function of connexin40 in conducting endothelium-dependent dilations along arterioles

Intercellular communication through gap junctions coordinates vascular tone by the conduction of vasomotor responses along the vessel wall. Gap junctions in arterioles are composed of different connexins (Cxs) (Cx40, Cx37, Cx45, Cx43), but it is unknown whether Cxs are interchangeable. We used mice with a targeted replacement of Cx40 by Cx45 (Cx40KI45) to explore whether Cx45 can functionally replace Cx40 in arterioles. Arterioles were locally stimulated using acetylcholine, bradykinin, adenosine, and K(+) in the cremaster of Cx40KI45, Cx40-deficient (Cx40ko), and wild-type mice, and diameter changes were assessed by intravital microscopy. Additionally, arterial pressure was measured by telemetry and Cx expression verified by immunofluorescence. Acetylcholine initiated a local dilation of a similar amplitude in all genotypes ( approximately 50%), which was rapidly conducted to upstream sites (1200 mum distance) without attenuation in wild type. In marked contrast, the remote dilation was significantly reduced in Cx40ko (25+/-3%) and Cx40KI45 (24+/-2%). Likewise, dilations initiated by bradykinin application were conducted without attenuation up to 1200 mum in wild type but not in Cx40ko and Cx40KI45. Adenosine-induced dilations and K(+)-induced constrictions were conducted similarly with decaying amplitude in all genotypes. Arterial pressure was strongly elevated in Cx40ko (161+/-1 versus 116+/-2 mm Hg) but only moderately in Cx40KI45 (133+/-8 mm Hg). This demonstrates that Cx40 function is critical for the conduction of acetylcholine and bradykinin dilations and cannot be substituted by Cx45. Therefore, unique properties of Cx40 are required for endothelial signal conduction, whereas nonspecific restoration of communication maintains additional functions related to blood pressure control.

Associations between connexin37 gene polymorphism and markers of subclinical atherosclerosis: the Cardiovascular Risk in Young Finns study

OBJECTIVE

Connexin37 (cx37) C1019T polymorphism has been shown to associate with coronary artery disease in different populations. We investigated whether this polymorphism associates with carotid artery intima-media thickness (IMT), carotid artery compliance (CAC) and brachial artery flow mediated dilatation (FMD) - i.e., early ultrasound markers of subclinical atherosclerosis - in a clinically healthy population of young Finnish adults.

METHODS AND RESULTS

1440 individuals from the Cardiovascular Risk in Young Finns study were genotyped and studied using cardiovascular risk factor and ultrasound data obtained in 2001. In linear regression models, no significant association between the cx37 polymorphism and carotid IMT, CAC or brachial artery FMD (ANOVA, p=0.159, 0.151 and 0.547), respectively, was found in the whole population or in women and men separately.

CONCLUSIONS

The connexin37 C1019T polymorphism is not related with markers of subclinical atherosclerosis in young adults.

Age-related changes in conducted vasodilation: effects of exercise training and role in functional hyperemia

Conducted vasodilation may coordinate blood flow in microvascular networks during skeletal muscle contraction. We tested the hypotheses that 1) exercise training enhances conducted vasodilation and 2) age-related changes in the capacity for conduction affect muscle perfusion during contractions. To address hypothesis 1, young (4-5 mo), adult (12-14 mo), and old (19-21 mo) C57BL6 male mice were sedentary or given access to running wheels for 8 wk. Voluntary running distances were significantly different (in km/day): young = 5.8 +/- 0.1, adult = 3.9 +/- 0.1, old = 2.2 +/- 0.1 (P < 0.05). In gluteus maximus muscles, conducted vasodilation was greater in adult than in young or old mice (P < 0.05) and greater in young sedentary than in old sedentary mice but was not affected by exercise training. Citrate synthase activity was greater with exercise training at all ages (P < 0.05). mRNA for endothelial nitric oxide synthase did not differ among ages, but endothelial nitric oxide synthase protein expression was greater in adult and old mice with exercise training (P < 0.05). Connexin 37, connexin 40, and connexin 43 mRNA were not affected by exercise training and did not differ by age. To address hypothesis 2, perfusion of the gluteus maximus muscle during light to severe workloads was assessed by Doppler microprobe at 3-26 mo of age. Maximum perfusion decreased linearly across the lifespan. Perfusion at the highest workload, absolute and relative to maximum, decreased across the lifespan, with a steeper decline beyond approximately 20 mo of age. In this model, 1) exercise training does not alter conducted vasodilation and 2) muscle perfusion is maintained up to near maximum workloads despite age-related changes in conducted vasodilation.

Calcium-activated potassium channel and connexin expression in small mesenteric arteries from eNOS-deficient (eNOS-/-) and eNOS-expressing (eNOS+/+) mice

Endothelium-derived hyperpolarizing factor (EDHF), notably in the microcirculation, plays an important role in the regulation of vascular tone. The cellular events that mediate EDHF are critically dependent, in a vessel dependent manner, on small conductance calcium-activated potassium channels (SK) and intermediate conductance calcium-activated potassium channels (IK) as well as the presence of the gap junction connexins 37, 40, and 43. We hypothesized that the expression levels of SK, IK, as well as vascular connexins, notably 37, 40 and 43 but, potentially, connexin 45, would show correlation with the contribution of EDHF to acetylcholine-mediated vasodilatation as well as, in the absence of endothelial-derived NO, higher expression levels in eNOS(-/-) mice. Wire myograph studies were performed to confirm the contribution of EDHF to endothelium-dependent relaxation in 1st, 2nd and 3rd order small mesenteric arteries from C57BL/6J eNOS-expressing (eNOS(+/+)) and eNOS-deficient C57BL/6J (eNOS(-/-)) mice. Small mesenteric arteries, as well as the branch points between 1st and 2nd and 2nd and 3rd order vessels, were analysed for the expression of mRNA for SK1, SK2, SK3, IK and large conductance calcium-activated potassium channels (BK) and comparable studies were performed for connexins 37, 40, 43 and 45. Although the contribution of EDHF to endothelium-dependent relaxation was significantly greater in the 3rd order vessels from the eNOS(+/+) the real-time (RT) polymerase chain reaction (PCR) data showed no differences for the expression levels of mRNA for any of the channel subtypes or the connexins within the small mesenteric arteries from either the eNOS(+/+) or eNOS(-/-) mice, nor, based on RT PCR analysis, were there differences in expression of the potassium channels studied in the branch points versus 1st, 2nd or 3rd order vessels. These data suggest that neither the gene expression of calcium-activated potassium channels nor vascular connexins are modulated by NO; however, their functional contribution to endothelium-dependent relaxation may be modulated by other physiological parameters.

Endogenous and exogenous NO attenuates conduction of vasoconstrictions along arterioles in the microcirculation

Vascular coordination in the microcirculation depends on gap junctional intercellular communication (GJIC), which is reflected by the conduction of locally initiated vasomotor responses. However, little is known about the regulation of GJIC in vivo. We hypothesized that endothelial NO regulates GJIC and therefore studied whether conduction of constrictions and dilations along the vessel wall is modulated by modifying the level of microcirculatory NO. Arterioles were focally stimulated using high K(+) or acetylcholine in the cremaster muscle in situ, and diameter changes were assessed at the local and remote upstream sites by intravital microscopy. Local stimulation with K(+) initiated a constriction that conducted along the arteriole with diminishing amplitude (length constant lambda: 371 +/- 42 mum). After N(omega)-nitro-l-arginine (l-NNA), lambda increased to 507 +/- 30 mum, indicating that GJIC is attenuated by endogenous NO. Exogenous NO, but not adenosine, reduced lambda after l-NNA in a reversible, concentration-dependent, and mainly cGMP-dependent manner as assessed by inhibition of soluble guanylate cyclase. In endothelial NO synthase-deficient mice, lambda was 530 +/- 80 mum and thus similar to that in wild-type mice after l-NNA. Exogenous NO likewise reduced lambda in these mice. The effects of NO were comparable to those of wild-type animals in Cx40-deficient mice, which excludes Cx40 as a specific target of NO. In contrast to constrictions, the amplitude of conducted dilations on acetylcholine did not diminish up to 1,300 mum and were not altered by l-NNA or exogenous NO. We conclude that endogenously released NO attenuates the conduction of vasoconstrictions most likely due to a modulation of gap junctional conductivity. We suggest that this effect is specific for smooth muscle cells, which probably transmit constricting signals, and involves connexins other than Cx40. This mechanism may support the dilatory potency of NO by preventing the conduction of remote vasoconstrictions into areas with basal or activated NO release.

Mechanisms of renal blood flow autoregulation: dynamics and contributions

Autoregulation of renal blood flow (RBF) is caused by the myogenic response (MR), tubuloglomerular feedback (TGF), and a third regulatory mechanism that is independent of TGF but slower than MR. The underlying cause of the third regulatory mechanism remains unclear; possibilities include ATP, ANG II, or a slow component of MR. Other mechanisms, which, however, exert their action through modulation of MR and TGF are pressure-dependent change of proximal tubular reabsorption, resetting of RBF and TGF, as well as modulating influences of ANG II and nitric oxide (NO). MR requires < 10 s for completion in the kidney and normally follows first-order kinetics without rate-sensitive components. TGF takes 30-60 s and shows spontaneous oscillations at 0.025-0.033 Hz. The third regulatory component requires 30-60 s; changes in proximal tubular reabsorption develop over 5 min and more slowly for up to 30 min, while RBF and TGF resetting stretch out over 20-60 min. Due to these kinetic differences, the relative contribution of the autoregulatory mechanisms determines the amount and spectrum of pressure fluctuations reaching glomerular and postglomerular capillaries and thereby potentially impinge on filtration, reabsorption, medullary perfusion, and hypertensive renal damage. Under resting conditions, MR contributes approximately 50% to overall RBF autoregulation, TGF 35-50%, and the third mechanism < 15%. NO attenuates the strength, speed, and contribution of MR, whereas ANG II does not modify the balance of the autoregulatory mechanisms.

Connexin 43 mediates spread of Ca2+-dependent proinflammatory responses in lung capillaries

Acute lung injury (ALI), which is associated with a mortality of 30-40%, is attributable to inflammation that develops rapidly across the lung's vast vascular surface, involving an entire lung or even both lungs. No specific mechanism explains this extensive inflammatory spread, probably because of the lack of approaches for detecting signal conduction in lung capillaries. Here, we addressed this question by applying the photolytic uncaging approach to induce focal increases in Ca2+ levels in targeted endothelial cells of alveolar capillaries. Uncaging caused Ca2+ levels to increase not only in the targeted cell, but also in vascular locations up to 150 microm from the target site, indicating that Ca2+ was conducted from the capillary to adjacent vessels. No such conduction was evident in mouse lungs lacking endothelial connexin 43 (Cx43), or in rat lungs in which we pretreated vessels with peptide inhibitors of Cx43. These findings provide the first direct evidence to our knowledge that interendothelial Ca2+ conduction occurs in the lung capillary bed and that Cx43-containing gap junctions mediate the conduction. A proinflammatory effect was evident in that induction of increases in Ca2+ levels in the capillary activated expression of the leukocyte adherence receptor P-selectin in venules. Further, peptide inhibitors of Cx43 completely blocked thrombin-induced microvascular permeability increases. Together, our findings reveal a novel role for Cx43-mediated gap junctions, namely as conduits for the spread of proinflammatory signals in the lung capillary bed. Gap junctional mechanisms require further consideration in the understanding of ALI.

Connexin40 and connexin43 in mouse aortic endothelium: evidence for coordinated regulation

In the vessel wall, endothelial cells are metabolically and electrically coupled to each other and to the adjacent smooth muscle cells by gap junctions composed of connexins. Gap junctions may be formed from combinations of several different connexin proteins, and deletion of one connexin can lead to modification of the expression of another. To reveal a possible interaction between connexin40 (Cx40) and connexin43 (Cx43) in endothelium, we studied their distribution in vessels from C57Bl/6 and Cx40 knockout mice (Cx40-/-) using immunoblots and immunocytochemistry on aortic cross sections and en face whole mounts. En face preparations from C57Bl/6 mice revealed two distinct pools of Cx43, one pericellular and the other intracellular. Cx40 was largely restricted to the periphery of the cells, and in Cx40-/- mice it was, as expected, undetectable. In the Cx40-/- mice, total Cx43 protein was also modestly reduced (immunoblots), but there was a major redistribution of the protein within the cell. The pericellular component of Cx43 was rendered virtually undetectable, and the intracellular compartments were normal or even slightly elevated. Smooth muscle Cx43 was also reduced in the Cx40-/- animals. These findings indicate that the cellular distribution of Cx43 is dependent on the presence of Cx40, and in view of the profound effects on the pericellular pool of the Cx43, the findings suggest that interactions between Cx40 and Cx43 regulate communication between endothelial cells and perhaps between smooth muscle and endothelial cells as well.

Vascular endothelium and blood flow

Major advances have been made over the last decade towards the elucidation of the molecular mechanisms involved in the endothelium-dependent regulation of vascular tone and blood flow. While the primary endothelium-derived vasodilator autacoid is nitric oxide, it is clear that epoxyeicosatrienoic acids and other endothelium-derived hyperpolarising factors, as well as endothelin-1 and reactive oxygen species, play a significant role in the regulation of vascular tone and gene expression. This review is intended as an overview of the signalling mechanisms that link haemodynamic stimuli (such as shear stress and cyclic stretch) and endothelial cell perturbation to the activation of enzymes generating vasoactive autacoids.

Endothelial mediators and communication through vascular gap junctions

Cellular interaction in vessels is achieved by multiple communication pathways, including gap junctions (GJs). They provide intercellular channels, allowing direct interaction of endothelial and smooth muscle cells and the coordination of cellular behaviour along the vessel. The latter is a prerequisite for large flow increases because an adaptation of resistance along the vessel length is required. Longitudinal communication is studied by confined local stimulation of arterioles and the observation of responses at distant locations. Certain vascular stimuli induce local and concomitant remote responses of a similar type, verifying rapid longitudinal conduction of vasomotor signals, most likely changes in membrane potential. This is achieved for dilatory responses via the endothelium, possibly by an endothelium-derived hyperpolarising factor (EDHF) that induces local hyperpolarisation, which is then transferred to remote sites through GJs. In vessels, GJs are composed of different connexins (Cx), but Cx40 is of special importance because its lack impairs longitudinal conduction of vasodilations. Interestingly, Cx40-deficient mice are hypertensive, suggesting that Cx40-dependent coupling is necessary to regulate vascular behaviour and peripheral resistance. While the role of other connexins is less well established, an abundance of data has proven the necessity of GJ communication to coordinate vascular behaviour during blood flow regulation.

Intact endothelium-dependent dilation and conducted responses in resistance vessels of hypercholesterolemic mice in vivo

Atherosclerosis and hyperlipidemia impair endothelium-dependent nitric oxide (NO)-mediated dilations in conducting arteries. In addition to NO, the endothelium releases an endothelium-derived hyperpolarizing factor (EDHF) in response to acetylcholine (ACh), which is particularly important in microvessels and initiates a dilation that conducts along the vessel through gap junctional communication. The expression of connexins is, however, altered by hypercholesterolemia. Therefore, we studied endothelium-dependent dilations and their conduction in murine hypercholesterolemic models. Dilations were assessed by intravital microscopy in arterioles with a diameter of approximately 35 microm in ApoE and LDL receptor (LDLR(-/-))-deficient mice after superfusion or locally confined application of ACh. ACh induced comparable concentration-dependent dilations in wild-type, LDLR(-/-), and ApoE(-/-) mice fed a normal or high-cholesterol diet, however EC(50) was slightly higher in ApoE(-/-) mice. Furthermore, the NO donor sodium-nitroprusside dilated arterioles to a similar extent (approximately 60%). Locally initiated ACh dilations (approximately 68%) conducted up to a distance of 1,100 microm without significant attenuation even under severe hypercholesterolemic conditions. Since ACh dilation in the arterioles of mice is mainly mediated via EDHF, we conclude that hypercholesterolemia does not alter EDHF release and efficacy. This conclusion is confirmed by an intact conducted response since EDHF is a prerequisite for this response. The intact conduction also suggests that gap-junctional communication is functionally preserved in these models.

Defining electrical communication in skeletal muscle resistance arteries: a computational approach

Vascular cells communicate electrically to coordinate their activity and control tissue blood flow. To foster a quantitative understanding of this fundamental process, we developed a computational model that was structured to mimic a skeletal muscle resistance artery. Each endothelial cell and smooth muscle cell in our virtual artery was treated as the electrical equivalent of a capacitor coupled in parallel with a non-linear resistor representing ionic conductance; intercellular gap junctions were represented by ohmic resistors. Simulations revealed that the vessel wall is not a syncytium in which electrical stimuli spread equally to all constitutive cells. Indeed, electrical signals spread in a differential manner among and between endothelial cells and smooth muscle cells according to the initial stimulus. The predictions of our model agree with physiological data from the feed artery of the hamster retractor muscle. Cell orientation and coupling resistance were the principal factors that enable electrical signals to spread differentially along and between the two cell types. Our computational observations also illustrated how gap junctional coupling enables the vessel wall to filter and transform transient electrical events into sustained voltage responses. Functionally, differential electrical communication would permit discrete regions of smooth muscle activity to locally regulate blood flow and the endothelium to coordinate regional changes in tissue perfusion.

The conduction of dilation along an arteriole is diminished in the cremaster muscle of hypertensive hamsters

Arteriolar vasomotor responses can include a component that conducts along the vessel through gap junction channels. This study examined conducted vasomotor responses in arterioles of the hypertensive hamster. The cremaster muscle of normotensive (CHF-148) and spontaneously hypertensive (CHF-H4) hamsters was exteriorized. Micropipettes containing phenylephrine (0.1 M) or acetylcholine (ACh; 1.0 M) were positioned along second-order arterioles and diameter responses were recorded locally for every 0.4 mm upstream to 1.6 mm. Substantative local constrictions to phenylephrine(PE) were poorly conducted to the 0.4-mm site in normotensive and hypertensive hamsters. Local dilation to ACh decayed by 3 +/- 1 microm/mm as it conducted along arterioles of the normotensive hamster. In contrast, conducted dilation decayed by 7 +/- 1 microm/mm (p < 0.05) in the hypertensive hamster. This hypertension-induced increase in decay was reversed by alpha-adrenergic receptor blockade (phentolamine: 1 microM). However, arteriolar constriction to global alpha(1)- (PE) and alpha(2)- (clonidine) adrenergic agonists was unaffected by hypertension. Rather, sympathetic nervous activity was elevated in the hypertensive hamster as indicated by a greater reduction in arterial pressure upon sympathetic ablation (hexamethonium infusion: 30 mg/kg). This study provides the first evidence that vascular cell-cell communication is altered by the elevated sympathetic nervous activity observed in the hypertensive hamster.