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

Signalling switches maintain intercellular communication in the vascular endothelium

BACKGROUND AND PURPOSE

The single layer of cells lining all blood vessels, the endothelium, is a sophisticated signal co-ordination centre that controls a wide range of vascular functions including the regulation of blood pressure and blood flow. To co-ordinate activities, communication among cells is required for tissue level responses to emerge. While a significant form of communication occurs by the propagation of signals between cells, the mechanism of propagation in the intact endothelium is unresolved.

EXPERIMENTAL APPROACH

Precision signal generation and targeted cellular manipulation was used in conjunction with high spatiotemporal mesoscale Ca2+ imaging in the endothelium of intact blood vessels.

KEY RESULTS

Multiple mechanisms maintain communication so that Ca2+ wave propagation occurs irrespective of the status of connectivity among cells. Between adjoining cells, regenerative IP3-induced IP3 production transmits Ca2+ signals and explains the propagated vasodilation that underlies the increased blood flow accompanying tissue activity. The inositide is itself sufficient to evoke regenerative phospholipase C-dependent Ca2+ waves across coupled cells. None of gap junctions, Ca2+ diffusion or the release of extracellular messengers is required to support this type of intercellular Ca2+ signalling. In contrast, when discontinuities exist between cells, ATP released as a diffusible extracellular messenger transmits Ca2+ signals across the discontinuity and drives propagated vasodilation.

CONCLUSION AND IMPLICATIONS

These results show that signalling switches underlie endothelial cell-to-cell signal transmission and reveal how communication is maintained in the face of endothelial damage. The findings provide a new framework for understanding wave propagation and cell signalling in the endothelium.

The role of pannexin/purinergic signaling in intervascular communication from capillaries during skeletal muscle contraction in male Golden hamsters

We sought to determine the physiological relevance of pannexin/purinergic-dependent signaling in mediating conducted vasodilation elicited by capillary stimulation through skeletal muscle contraction. Using hamster cremaster muscle and intravital microscopy we stimulated capillaries through local muscle contraction while observing the associated upstream arteriole. Capillaries were stimulated with muscle contraction at low and high contraction (6 and 60CPM) and stimulus frequencies (4 and 40 Hz) in the absence and presence of pannexin blocker mefloquine (MEF; 10-5 M), purinergic receptor antagonist suramin (SUR 10-5 M) and gap-junction uncoupler halothane (HALO, 0.07%) applied between the capillary stimulation site and the upstream arteriolar observation site. Conducted vasodilations elicited at 6CPM were inhibited by HALO while vasodilations at 60CPM were inhibited by MEF and SUR. The conducted response elicited at 4 Hz was inhibited by MEF while the vasodilation at 40 Hz was unaffected by any blocker. Therefore, upstream vasodilations resulting from capillary stimulation via muscle contraction are dependent upon a pannexin/purinergic-dependent pathway that appears to be stimulation parameter-dependent. Our data highlight a physiological importance of the pannexin/purinergic pathway in facilitating communication between capillaries and upstream arteriolar microvasculature and, consequently, indicating that this pathway may play a crucial role in regulating blood flow in response to skeletal muscle contraction.

Complementary and Inducible creERT2 Mouse Models for Functional Evaluation of Endothelial Cell Subtypes in the Bone Marrow

In the adult bone marrow (BM), endothelial cells (ECs) are an integral component of the hematopoietic stem cell (HSC)-supportive niche, which modulates HSC activity by producing secreted and membrane-bound paracrine signals. Within the BM, distinct vascular arteriole, transitional, and sinusoidal EC subtypes display unique paracrine expression profiles and create anatomically-discrete microenvironments. However, the relative contributions of vascular endothelial subtypes in supporting hematopoiesis is unclear. Moreover, constitutive expression and off-target activity of currently available endothelial-specific and endothelial-subtype-specific murine cre lines potentially confound data analysis and interpretation. To address this, we describe two tamoxifen-inducible cre-expressing lines, Vegfr3-creERT2 and Cx40-creERT2, that efficiently label sinusoidal/transitional and arteriole endothelium respectively in adult marrow, without off-target activity in hematopoietic or perivascular cells. Utilizing an established mouse model in which cre-dependent recombination constitutively-activates MAPK signaling within adult endothelium, we identify arteriole ECs as the driver of MAPK-mediated hematopoietic dysfunction. These results define complementary tamoxifen-inducible creERT2-expressing mouse lines that label functionally-discrete and non-overlapping sinusoidal/transitional and arteriole EC populations in the adult BM, providing a robust toolset to investigate the differential contributions of vascular subtypes in maintaining hematopoietic homeostasis.

Connexin 40-Mediated Regulation of Systemic Circulation and Arterial Blood Pressure

Vascular system is a complex network in which different cell types and vascular segments must work in concert to regulate blood flow distribution and arterial blood pressure. Although paracrine/autocrine signaling is involved in the regulation of vasomotor tone, direct intercellular communication via gap junctions plays a central role in the control and coordination of vascular function in the microvascular network. Gap junctions are made up by connexin (Cx) proteins, and among the four Cxs expressed in the cardiovascular system (Cx37, Cx40, Cx43, and Cx45), Cx40 has emerged as a critical signaling pathway in the vessel wall. This Cx is predominantly found in the endothelium, but it is involved in the development of the cardiovascular system and in the coordination of endothelial and smooth muscle cell function along the length of the vessels. In addition, Cx40 participates in the control of vasomotor tone through the transmission of electrical signals from the endothelium to the underlying smooth muscle and in the regulation of arterial blood pressure by renin-angiotensin system in afferent arterioles. In this review, we discuss the participation of Cx40-formed channels in the development of cardiovascular system, control and coordination of vascular function, and regulation of arterial blood pressure.

Capillary communication: the role of capillaries in sensing the tissue environment, coordinating the microvascular, and controlling blood flow

Historically, capillaries have been viewed as the microvascular site for flux of nutrients to cells and removal of waste products. Capillaries are the most numerous blood vessel segment within the tissue, whose vascular wall consists of only a single layer of endothelial cells and are situated within microns of each cell of the tissue, all of which optimizes capillaries for the exchange of nutrients between the blood compartment and the interstitial space of tissues. There is, however, a growing body of evidence to support that capillaries play an important role in sensing the tissue environment, coordinating microvascular network responses, and controlling blood flow. Much of our growing understanding of capillaries stems from work in skeletal muscle and more recent work in the brain, where capillaries can be stimulated by products released from cells of the tissue during increased activity and are able to communicate with upstream and downstream vascular segments, enabling capillaries to sense the activity levels of the tissue and send signals to the microvascular network to coordinate the blood flow response. This review will focus on the emerging role that capillaries play in communication between cells of the tissue and the vascular network required to direct blood flow to active cells in skeletal muscle and the brain. We will also highlight the emerging central role that disruptions in capillary communication may play in blood flow dysregulation, pathophysiology, and disease.

Connexin37 Regulates Cell Cycle in the Vasculature

Control of vascular cell growth responses is critical for development and maintenance of a healthy vasculature. Connexins - the proteins comprising gap junction channels - are key regulators of cell growth in diseases such as cancer, but their involvement in controlling cell growth in the vasculature is less well appreciated. Connexin37 (Cx37) is one of four connexin isotypes expressed in the vessel wall. Its primary role in blood vessels relies on its unique ability to transduce flow-sensitive signals into changes in cell cycle status of endothelial (and perhaps, mural) cells. Here, we review available evidence for Cx37's role in the regulation of vascular growth, vessel organization, and vascular tone in healthy and diseased vasculature. We propose a novel mechanism whereby Cx37 accomplishes this with a phosphorylation-dependent transition between closed (growth-suppressive) and multiple open (growth-permissive) channel conformations that result from interactions of the C-terminus with cell-cycle regulators to limit or support cell cycle progression. Lastly, we discuss Cx37 and its downstream signaling as a novel potential target in the treatment of cardiovascular disease, and we address outstanding research questions that still challenge the development of such therapies.

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Connexin and Pannexin Large-Pore Channels in Microcirculation and Neurovascular Coupling Function

Microcirculation homeostasis depends on several channels permeable to ions and/or small molecules that facilitate the regulation of the vasomotor tone, hyperpermeability, the blood–brain barrier, and the neurovascular coupling function. Connexin (Cxs) and Pannexin (Panxs) large-pore channel proteins are implicated in several aspects of vascular physiology. The permeation of ions (i.e., Ca²⁺) and key metabolites (ATP, prostaglandins, D-serine, etc.) through Cxs (i.e., gap junction channels or hemichannels) and Panxs proteins plays a vital role in intercellular communication and maintaining vascular homeostasis. Therefore, dysregulation or genetic pathologies associated with these channels promote deleterious tissue consequences. This review provides an overview of current knowledge concerning the physiological role of these large-pore molecule channels in microcirculation (arterioles, capillaries, venules) and in the neurovascular coupling function.

Endothelium-Derived Hyperpolarizing Factor and Myoendothelial Coupling: The in vivo Perspective

The endothelium controls vascular tone adopting blood flow to tissue needs. It releases chemical mediators [e.g., nitric oxide (NO), prostaglandins (PG)] and exerts appreciable dilation through smooth muscle hyperpolarization, thus termed endothelium-dependent hyperpolarization (EDH). Initially, EDH was attributed to release of a factor, but later it was suggested that smooth muscle hyperpolarization might be derived from radial spread of an initial endothelial hyperpolarization through heterocellular channels coupling these vascular cells. The channels are indeed present and formed by connexins that enrich in gap junctions (GJ). In vitro data suggest that myoendothelial coupling underlies EDH-type dilations as evidenced by blocking experiments as well as simultaneous, merely identical membrane potential changes in endothelial and smooth muscle cells (SMCs), which is indicative of coupling through ohmic resistors. However, connexin-deficient animals do not display any attenuation of EDH-type dilations in vivo, and endothelial and SMCs exhibit distinct and barely superimposable membrane potential changes exerted by different means in vivo. Even if studied in the exact same artery EDH-type dilation exhibits distinct features in vitro and in vivo: in isometrically mounted vessels, it is rather weak and depends on myoendothelial coupling through connexin40 (Cx40), whereas in vivo as well as in vitro under isobaric conditions it is powerful and independent of myoendothelial coupling through Cx40. It is concluded that EDH-type dilations are distinct and a significant dependence on myoendothelial coupling in vitro does not reflect the situation under physiologic conditions in vivo. Myoendothelial coupling may act as a backup mechanism that is uncovered in the absence of the powerful EDH-type response and possibly reflects a situation in a pathophysiologic environment.

Conceptualizing conduction as a pliant electrical response: impact of gap junctions and ion channels

Vasomotor responses conduct among resistance arteries to coordinate blood flow delivery pursuant to energetic demand. Conduction is set by the electrical and mechanical properties of vascular cells, the former tied to how gap junctions and ion channels distribute and dissipate charge, respectively. These membrane proteins are subject to modulation; thus, conduction could be viewed as "pliant" to the current regulatory state. This study used in silico approaches to conceptualize electrical pliancy and to illustrate how gap junctional and ion channel properties distinctly impact conduction along a single skeletal muscle artery or a branching cerebrovascular network. Initial simulations revealed how vascular cells encoded with electrotonic properties best reproduced spreading behavior; the endothelium's importance as a charge source and a longitudinal conduit was readily observed. Alterations in gap junctional conductance produced unique electrical fingerprints: 1) decreased endothelial coupling impaired longitudinal but enhanced radial spread, and 2) reduced myoendothelial coupling limited radial but enhanced longitudinal spread. Subsequent simulations illustrated how tuning ion channel activity, e.g., inward rectifying- and voltage-gated K⁺ channels, modified charge dissipation, resting membrane potential, and the spread of the electrical phenomenon. Restricting ion channel tuning to a network subregion then revealed how electrical spread could be locally shaped in accordance with the aggregate changes in membrane resistance. In summary, our analysis frames and reimagines electrical conduction as a pliable process, with subtle regulatory changes to membrane proteins shaping network spread and tissue perfusion.NEW & NOTEWORTHY Conducted vasomotor responses depend on initiation and spread of electrical phenomena along arterial walls and their translation into contractile responses. Using computational approaches, we show how subtle but widespread regulation of gap junctions and ion channels can modulate the range and amplitude of electrical spread. Ion channels are regulated by endocrine and mechanical signals and may differ regionally in networks. Subregional electrical changes are not spatially confined but may affect electrical conduction in neighboring regions.

Conceptualizing Conduction as a Pliant Vasomotor response: Impact of Ca2+ fluxes and Ca2+ Sensitization

Coordinating blood flow to active tissue requires vasomotor responses to conduct among resistance arteries. Vasomotor spread is governed by the electrical and mechanical properties of vessels; the latter being linked to the sigmoid relations between membrane potential (VM), [Ca2+], and smooth muscle contractility. Proteins guiding electrical-to-tone translation are subject to regulation; thus, vasomotor conduction could be viewed as "pliant" to the current regulatory state. Using simple in silico approaches, we explored vasomotor pliancy and how the regulation of contractility impacts conduction along a skeletal muscle artery and a branching cerebrovascular network. Initial simulations revealed how limited electromechanical linearity affects the translation of electrical spread into arterial tone. Subtle changes to the VM-[Ca2+] or [Ca2+]-diameter relationship, akin to regulatory alterations in Ca2+ influx and Ca2+ sensitivity, modified the distance and amplitude of the conducted vasomotor response. Simultaneous changes to both relationships, consistent with agonist stimulation, augmented conduction although the effect varied with stimulus strength and polarity (depolarization vs hyperpolarization). Final simulations using our cerebrovascular network revealed how localized changes to the VM-[Ca2+] or [Ca2+]-diameter relationships could regionally shape conduction without interfering with the electrical spread. We conclude that regulatory changes to key effector proteins (e.g. L-type Ca2+ channels, myosin light chain phosphatase), integral to voltage translation, not only impact conducted vasomotor tone but likely blood flow delivery to active tissues.

Connexins: Key Players in the Control of Vascular Plasticity and Function

Of the 21 members of the connexin family, 4 (Cx37, Cx40, Cx43, and Cx45) are expressed in the endothelium and/or smooth muscle of intact blood vessels to a variable and dynamically regulated degree. Full-length connexins oligomerize and form channel structures connecting the cytosol of adjacent cells (gap junctions) or the cytosol with the extracellular space (hemichannels). The different connexins vary mainly with regard to length and sequence of their cytosolic COOH-terminal tails. These COOH-terminal parts, which in the case of Cx43 are also translated as independent short isoforms, are involved in various cellular signaling cascades and regulate cell functions. This review focuses on channel-dependent and -independent effects of connexins in vascular cells. Channels play an essential role in coordinating and synchronizing endothelial and smooth muscle activity and in their interplay, in the control of vasomotor actions of blood vessels including endothelial cell reactivity to agonist stimulation, nitric oxide-dependent dilation, and endothelial-derived hyperpolarizing factor-type responses. Further channel-dependent and -independent roles of connexins in blood vessel function range from basic processes of vascular remodeling and angiogenesis to vascular permeability and interactions with leukocytes with the vessel wall. Together, these connexin functions constitute an often underestimated basis for the enormous plasticity of vascular morphology and function enabling the required dynamic adaptation of the vascular system to varying tissue demands.

Endothelial connexins in vascular function

Gap junctions are essential for intercellular crosstalk in blood and lymphatic vasculature. These clusters of intercellular channels ensure direct communication among endothelial cells and between endothelial and smooth muscle cells, and the synchronization of their behavior along the vascular tree. Gap junction channels are formed by connexins; six connexins form a connexon or hemichannel and the docking of two connexons result in a full gap junction channel allowing for the exchange of ions and small metabolites between neighboring cells. Recent evidence indicates that the intracellular domains of connexins may also function as an interaction platform (interactome) for other proteins, thereby regulating their function. Interestingly, fragments of Cx proteins generated by alternative internal translation were recently described, although their functions in the vascular wall remain to be uncovered. Variations in connexin expression are observed along different types of blood and lymphatic vessels; the most commonly found endothelial connexins are Cx37, Cx40, Cx43 and Cx47. Physiological studies on connexin-knockout mice demonstrated the essential roles of these channel-forming proteins in the coordination of vasomotor activity, endothelial permeability and inflammation, angiogenesis and in the maintenance of fluid balance in the body.

CGRP signalling inhibits NO production through pannexin-1 channel activation in endothelial cells

Blood flow distribution relies on precise coordinated control of vasomotor tone of resistance arteries by complex signalling interactions between perivascular nerves and endothelial cells. Sympathetic nerves are vasoconstrictors, whereas endothelium-dependent NO production provides a vasodilator component. In addition, resistance vessels are also innervated by sensory nerves, which are activated during inflammation and cause vasodilation by the release of calcitonin gene-related peptide (CGRP). Inflammation leads to superoxide anion (O₂^{• -}) formation and endothelial dysfunction, but the involvement of CGRP in this process has not been evaluated. Here we show a novel mechanistic relation between perivascular sensory nerve-derived CGRP and the development of endothelial dysfunction. CGRP receptor stimulation leads to pannexin-1-formed channel opening and the subsequent O₂^{• -}-dependent connexin-based hemichannel activation in endothelial cells. The prolonged opening of these channels results in a progressive inhibition of NO production. These findings provide new therapeutic targets for the treatment of the inflammation-initiated endothelial dysfunction.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

Endothelial-smooth muscle cell interactions in the regulation of vascular tone in skeletal muscle

The SMCs of skeletal muscle arterioles are intricately sensitive to changes in membrane potential. Upon increasing luminal pressure, the SMCs depolarize, thereby opening VDCCs, which leads to contraction. Mechanisms that oppose this myogenic tone can involve voltage-dependent and independent dilator pathways, and can be endothelium-dependent or independent. Of particular interest are the pathways leading to hyperpolarization of SMCs, as these can potentially evoke both local and conducted dilation. This review focuses on three agonists that cause local and conducted dilation in skeletal muscle: ACh, ATP, and KCl. The mechanisms for the release of these agonists during motor nerve stimulation and/or hypoxia, and their actions to open either Ca²⁺ -activated K⁺ channels (K_Ca ) or inwardly rectifying K⁺ channels (K_IR ) are described. By causing local and conducted dilation, each agonist has the ability to improve skeletal muscle blood flow during exercise and ischemia.

Integrative model of coronary flow in anatomically based vasculature under myogenic, shear, and metabolic regulation

Coronary blood flow is regulated to match the oxygen demand of myocytes in the heart wall. Flow regulation is essential to meet the wide range of cardiac workload. The blood flows through a complex coronary vasculature of elastic vessels having nonlinear wall properties, under transmural heterogeneous myocardial extravascular loading. To date, there is no fully integrative flow analysis that incorporates global and local passive and flow control determinants. Here, we provide an integrative model of coronary flow regulation that considers the realistic asymmetric morphology of the coronary network, the dynamic myocardial loading on the vessels embedded in it, and the combined effects of local myogenic effect, local shear regulation, and conducted metabolic control driven by venous O₂ saturation level. The model predicts autoregulation (approximately constant flow over a wide range of coronary perfusion pressures), reduced heterogeneity of regulated flow, and presence of flow reserve, in agreement with experimental observations. Furthermore, the model shows that the metabolic and myogenic regulations play a primary role, whereas shear has a secondary one. Regulation was found to have a significant effect on the flow except under extreme (high and low) inlet pressures and metabolic demand. Novel outcomes of the model are that cyclic myocardial loading on coronary vessels enhances the coronary flow reserve except under low inlet perfusion pressure, increases the pressure range of effective autoregulation, and reduces the network flow in the absence of metabolic regulation. Collectively, these findings demonstrate the utility of the present biophysical model, which can be used to unravel the underlying mechanisms of coronary physiopathology.

Skeletal muscle contraction-induced vasodilation in the microcirculation

Maximal whole body exercise leads skeletal muscle blood flow to markedly increase to match metabolic demands, a phenomenon termed exercise hyperaemia that is accomplished by increasing vasodilation. However, local vasodilatory mechanisms in response to skeletal muscle contraction remain uncertain. This review highlights metabolic vasodilators released from contracting skeletal muscle, endothelium, or blood cells. As a considerable skeletal muscle vasodilation potentially results in hypotension, sympathetic nerve activity needs to be augmented to elevate cardiac output and blood pressure during dynamic exercise. However, since the enhanced sympathetic vasoconstriction restrains skeletal muscle blood flow, intramuscular arteries have an indispensable ability to blunt sympathetic activity for exercise hyperaemia. In addition, we discuss that mechanical compression of the intramuscular vasculature contributes to causing the initial phase of increasing vasodilation following a single muscle contraction. We have also chosen to focus on conducted (or ascending) electrical signals that evoke vasodilation of proximal feed arteries to elevate blood flow in the microcirculation of skeletal muscle. Endothelial hyperpolarization originating within distal arterioles ascends into the proximal feed arteries, thereby increasing total blood flow in contracting skeletal muscle. This brief review summarizes molecular mechanisms underlying the regulation of skeletal muscle blood flow to a single or sustained muscle contraction.

The Conducted Vasomotor Response: Function, Biophysical Basis, and Pharmacological Control

Arterial tone is coordinated among vessel segments to optimize nutrient transport and organ function. Coordinated vasomotor activity is remarkable to observe and depends on stimuli, sparsely generated in tissue, eliciting electrical responses that conduct lengthwise among electrically coupled vascular cells. The conducted response is the focus of this topical review, and in this regard, the authors highlight literature that advances an appreciation of functional significance, cellular mechanisms, and biophysical principles. Of particular note, this review stresses that conduction is enabled by a defined pattern of charge movement along the arterial wall as set by three key parameters (tissue structure, gap junctional resistivity, and ion channel activity). The impact of disease on conduction is carefully discussed, as are potential strategies to restore this key biological response and, along with it, the match of blood flow delivery with tissue energetic demand.

Connexin 50 Mutation Lowers Blood Pressure in Spontaneously Hypertensive Rat

We assessed the effect of the previously uncovered gap junction protein alpha 8 (Gja8) mutation present in spontaneously hypertensive rat – dominant cataract (SHR-Dca) strain on blood pressure, metabolic profile, and heart and renal transcriptomes. Adult, standard chow-fed male rats of SHR and SHR-Dca strains were used. We found a significant, consistent 10-15 mmHg decrease in both systolic and diastolic blood pressures in SHR-Dca compared with SHR (P<0.01 and P<0.05, respectively; repeated measures analysis of variance (ANOVA)). With immunohistochemistry, we were able to localize Gja8 in heart, kidney, aorta, liver, and lungs, mostly in endothelium; with no differences in expression between strains. SHR-Dca rats showed decreased body weight, high-density lipoprotein cholesterol concentrations and basal insulin sensitivity in muscle. There were 21 transcripts common to the sets of 303 transcripts in kidney and 487 in heart showing >1.2-fold difference in expression between SHR and SHR-Dca. Tumor necrosis factor was the most significant upstream regulator and glial cell-derived neurotrophic factor family ligand-receptor interactions was the common enriched and downregulated canonical pathway both in heart and kidney of SHR-Dca. The connexin 50 mutation L7Q lowers blood pressure in the SHR-Dca strain, decreases high-density lipoprotein cholesterol, and leads to substantial transcriptome changes in heart and kidney.

Pannexin channel and connexin hemichannel expression in vascular function and inflammation

Control of blood flow distribution and tissue homeostasis depend on the tight regulation of and coordination between the microvascular network and circulating blood cells. Channels formed by connexins or pannexins that connect the intra- and extracellular compartments allow the release of paracrine signals, such as ATP and prostaglandins, and thus play a central role in achieving fine regulation and coordination of vascular function. This review focuses on vascular connexin hemichannels and pannexin channels. We review their expression pattern within the arterial and venous system with a special emphasis on how post-translational modifications by phosphorylation and S-nitrosylation of these channels modulate their function and contribute to vascular homeostasis. Furthermore, we highlight the contribution of these channels in smooth muscle cells and endothelial cells in the regulation of vasomotor tone as well as how these channels in endothelial cells regulate inflammatory responses such as during ischemic and hypoxic conditions. In addition, this review will touch on recent evidence implicating a role for these proteins in regulating red blood cell and platelet function.

Conducted dilatation to ATP and K+ in rat skeletal muscle arterioles

AIM

During exercise in humans, circulating levels of ATP and K+ increase at a time when blood flow increases to satisfy metabolic demand. Both molecules can activate arteriolar K+ channels to stimulate vasodilatation; here, it is established whether conducted dilatation is observed in a skeletal muscle bed.

METHODS

Isolated and cannulated rat cremaster arterioles were used to assess both local and conducted responses. Agents were either added to the bath, focally pulse-ejected to the downstream end of arterioles, or in triple-cannulated arterioles, luminally perfused into the downstream branches to assess both local and conducted responses.

RESULTS

The endothelium-dependent agonist ACh and the KATP channel opener levcromakalim each stimulated both local and conducted vasodilatation. Focal, bolus delivery of ATP (10 μm) or KCl (33 mm) to the outside of arterioles stimulated a biphasic vasomotor response: rapid vasoconstriction followed by dilatation as each washed away. At lower concentrations of KCl (19 mm), constriction was avoided, and instead, Ba2+ -sensitive local dilatation and conducted dilatation were both observed. Luminal perfusion of ATP avoided constriction and activated P2Y1 receptors stimulating vasodilatation secondary to opening of KCa channels. In triple-cannulated arterioles, either ATP (10 μm) or K+ (15 mm) luminally perfused into daughter branches of a bifurcation stimulated local dilatation which conducted into the parent arteriole.

CONCLUSION

The recognized physiological autocrine and paracrine mediators ATP and K+ each act to evoke both local and conducted vasodilatation in rat cremaster arterioles. Therefore, in situations when circulating levels are raised, such as during exercise, these agents can act as important regulators of blood flow.

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.

Local control of blood flow during active hyperaemia: what kinds of integration are important?

The focus of this review is on local mechanisms modifying arteriolar resistance to match blood flow to metabolism. In skeletal muscle many local mediators are known, including K(+) , nitric oxide (NO), purines and prostaglandins. Each accounts for about 30% of the response; it is widely held that these act redundantly: this concept awaits systematic testing. Understanding signal integration also requires consideration of microvascular network morphology in relation to local communication pathways between endothelial and smooth muscle cells (which are critical for many local responses, including dilatation to skeletal muscle contraction) and in relation to the spread of vasodilator signals up- and downstream throughout the network. Mechanisms mediating the spread of dilatation from local to remote sites have been well studied using acetylcholine (ACh), but remote dilatations to contraction of skeletal muscle fibres also occur. Importantly, these mechanisms clearly differ from those initiated by ACh, but much remains undefined. Furthermore, capillaries contribute to metabolic dilatation as they dilate arterioles directly upstream in response to vasoactive agents or contraction of adjacent muscle fibres. Given the dispersed arrangement of motor units, precise matching of flow to metabolism is not attainable unless signals are initiated only by 'active' capillaries. As motor units are recruited, signals that direct blood flow towards these active fibres will eventually be supported by local and spreading responses in the arterioles associated with those fibres. Thus, mechanisms of integration of vasodilator signalling across elements of the microvasculature remain an important area of focus for new studies.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

P2Y₂ receptor activation decreases blood pressure via intermediate conductance potassium channels and connexin 37

AIMS

Nucleotides are important paracrine regulators of vascular tone. We previously demonstrated that activation of P2Y₂ receptors causes an acute, NO-independent decrease in blood pressure, indicating this signalling pathway requires an endothelial-derived hyperpolarization (EDH) response. To define the mechanisms by which activation of P2Y₂ receptors initiates EDH and vasodilation, we studied intermediate-conductance (KCa3.1, expressed in endothelial cells) and big-conductance potassium channels (KCa1.1, expressed in smooth muscle cells) as well as components of the myoendothelial gap junction, connexins 37 and 40 (Cx37, Cx40), all hypothesized to be part of the EDH response.

METHODS

We compared the effects of a P2Y₂/₄ receptor agonist in wild-type (WT) mice and in mice lacking KCa3.1, KCa1.1, Cx37 or Cx40 under anaesthesia, while monitoring intra-arterial blood pressure and heart rate.

RESULTS

Acute activation of P2Y₂/₄ receptors (0.01-3 mg kg(-1) body weight i.v.) caused a biphasic blood pressure response characterized by a dose-dependent and rapid decrease in blood pressure in WT (maximal response % of baseline at 3 mg kg(-1) : -38 ± 1%) followed by a consecutive increase in blood pressure (+44 ± 11%). The maximal responses in KCa3.1(-/-) and Cx37(-/-) were impaired (-13 ± 5, +17 ± 7 and -27 ± 1, +13 ± 3% respectively), whereas the maximal blood pressure decrease in response to acetylcholine at 3 μg kg(-1) was not significantly different (WT: -53 ± 3%; KCa3.1(-/-) : -52 ± 3; Cx37(-/-) : -53 ± 3%). KCa1.1(-/-) and Cx40(-/-) showed an identical biphasic response to P2Y2/4 receptor activation compared to WT.

CONCLUSIONS

The data suggest that the P2Y2/4 receptor activation elicits blood pressure responses via distinct mechanisms involving KCa3.1 and Cx37.

A critical role for the vascular endothelium in functional neurovascular coupling in the brain

Background

The functional modulation of blood flow in the brain is critical for brain health and is the basis of contrast in functional magnetic resonance imaging. There is evident coupling between increases in neuronal activity and increases in local blood flow; however, many aspects of this neurovascular coupling remain unexplained by current models. Based on the rapid dilation of distant pial arteries during cortical functional hyperemia, we hypothesized that endothelial signaling may play a key role in the long‐range propagation of vasodilation during functional hyperemia in the brain. Although well characterized in the peripheral vasculature, endothelial involvement in functional neurovascular coupling has not been demonstrated.

Methods and Results

We combined in vivo exposed‐cortex multispectral optical intrinsic signal imaging (MS‐OISI) with a novel in vivo implementation of the light‐dye technique to record the cortical hemodynamic response to somatosensory stimulus in rats before and after spatially selective endothelial disruption. We demonstrate that discrete interruption of endothelial signaling halts propagation of stimulus‐evoked vasodilation in pial arteries, and that wide‐field endothelial disruption in pial arteries significantly attenuates the hemodynamic response to stimulus, particularly the early, rapid increase and peak in hyperemia.

Conclusions

Involvement of endothelial pathways in functional neurovascular coupling provides new explanations for the spatial and temporal features of the hemodynamic response to stimulus and could explain previous results that were interpreted as evidence for astrocyte‐mediated control of functional hyperemia. Our results unify many aspects of blood flow regulation in the brain and body and prompt new investigation of direct links between systemic cardiovascular disease and neural deficits.

Multiple roles of connexins in atherosclerosis- and restenosis-induced vascular remodelling

Endothelial dysfunction is the initial step in atherosclerotic plaque development in large- and medium-sized arteries. This progressive disease, which starts during childhood, is characterized by the accumulation of lipids, macrophages, neutrophils, T lymphocytes and smooth muscle cells in the intima of the vessels. Erosion and rupture of the atherosclerotic plaque may induce myocardial infarction and cerebrovascular accidents, which are responsible for a large percentage of sudden deaths. The most common treatment for atherosclerosis is angioplasty and stent implantation, but these surgical interventions favour a vascular reaction called restenosis and the associated de-endothelialization increases the risk of thrombosis. This review provides an overview of the role of connexins, a large family of transmembrane proteins, in vascular remodelling associated with atherosclerosis and restenosis. The connexins expressed in the vascular wall are Cx37, Cx40, Cx43 and Cx45; their expressions vary with vascular territory and species. Connexins form hemichannels or gap junction channels, allowing the exchange of ions and small metabolites between the cytosol and extracellular space or between neighbouring cells, respectively. Connexins have important roles in vascular physiology; they support radial and longitudinal cell-to-cell communication in the vascular wall, and significant changes in their expression patterns have been described during atherosclerosis and restenosis.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Aging impairs electrical conduction along endothelium of resistance arteries through enhanced Ca2+-activated K+ channel activation

OBJECTIVE

Intercellular conduction of electrical signals underlies spreading vasodilation of resistance arteries. Small- and intermediate-conductance Ca(2+)-activated K(+) channels of endothelial cells serve a dual function by initiating hyperpolarization and modulating electrical conduction. We tested the hypothesis that regulation of electrical signaling by small- and intermediate-conductance Ca(2+)-activated K(+) channels is altered with advancing age.

APPROACH AND RESULTS

Intact endothelial tubes (60 µm wide; 1-3 mm long) were freshly isolated from male C57BL/6 mouse (Young: 4-6 months; Intermediate: 12-14 months; Old: 24-26 months) superior epigastric arteries. Using dual intracellular microelectrodes, current was injected (± 0.1-3 nA) at site 1 while recording membrane potential (Vm) at site 2 (separation distance: 50-2000 µm). Across age groups, greatest differences were observed between Young and Old. Resting Vm in Old (-38 ± 1 mV) was more negative (P<0.05) than Young (-30 ± 1 mV). Maximal hyperpolarization to both direct (NS309) and indirect (acetylcholine) activation of small- and intermediate-conductance Ca(2+)-activated K(+) channels was sustained (ΔVm ≈-40 mV) with age. The length constant (λ) for electrical conduction was reduced (P<0.05) from 1630 ± 80 µm (Young) to 1320 ± 80 µm (Old). Inhibiting small- and intermediate-conductance Ca(2+)-activated K(+) channels with apamin+charybdotoxin or scavenging hydrogen peroxide (H2O2) with catalase improved electrical conduction (P<0.05) in Old. Exogenous H2O2 (200 µmol/L) in Young evoked hyperpolarization and impaired electrical conduction; these effects were blocked by apamin+charybdotoxin.

CONCLUSIONS

Enhanced current loss through Ca2+-activated K+ channel activation impairs electrical conduction along the endothelium of resistance arteries with aging. Attenuating the spatial domain of electrical signaling will restrict the spread of vasodilation and thereby contribute to blood flow limitations associated with advanced age.

The vascular conducted response in cerebral blood flow regulation

Despite recent advances in our understanding of the molecular and cellular mechanisms behind vascular conducted responses (VCRs) in systemic arterioles, we still know very little about their potential physiological and pathophysiological role in brain penetrating arterioles controlling blood flow to the deeper areas of the brain. The scope of the present review is to present an overview of the conceptual, mechanistic, and physiological role of VCRs in resistance vessels, and to discuss in detail the recent advances in our knowledge of VCRs in brain arterioles controlling cerebral blood flow. We provide a schematic view of the ion channels and intercellular communication pathways necessary for conduction of an electrical and mechanical response in the arteriolar wall, and discuss the local signaling mechanisms and cellular pathway involved in the responses to different local stimuli and in different vascular beds. Physiological modulation of VCRs, which is a rather new finding in this field, is discussed in the light of changes in plasma membrane ion channel conductance as a function of health status or disease. Finally, we discuss the possible role of VCRs in cerebrovascular function and disease as well as suggest future directions for studying VCRs in the cerebral circulation.

Polymorphism in endothelial connexin40 enhances sensitivity to intraluminal pressure and increases arterial stiffness

OBJECTIVE

To determine whether impairment of endothelial connexin40 (Cx40), an effect that can occur in hypertension and aging, contributes to the arterial dysfunction and stiffening in these conditions.

APPROACH AND RESULTS

A new transgenic mouse strain, expressing a mutant Cx40, (Cx40T202S), specifically in the vascular endothelium, has been developed and characterized. This mutation produces nonfunctional hemichannels, whereas gap junctions containing the mutant are electrically, but not chemically, patent. Mesenteric resistance arteries from Cx40T202S mice showed increased sensitivity of the myogenic response to intraluminal pressure in vitro, compared with wild-type mice, whereas transgenic mice overexpressing native Cx40 (Cx40Tg) showed reduced sensitivity. In control and Cx40Tg mice, the sensitivity to pressure of myogenic constriction was modulated by both NO and endothelium-derived hyperpolarization; however, the endothelium-derived hyperpolarization component was absent in Cx40T202S arteries. Analysis of passive mechanical properties revealed that arterial stiffness was enhanced in vessels from Cx40T202S mice, but not in wild-type or Cx40Tg mice.

CONCLUSIONS

Introduction of a mutant form of Cx40 in the endogenous endothelial Cx40 population prevents endothelium-derived hyperpolarization activation during myogenic constriction, enhancing sensitivity to intraluminal pressure and increasing arterial stiffness. We conclude that genetic polymorphisms in endothelial Cx40 can contribute to the pathogenesis of arterial disease.

Diffusion of nitric oxide across cell membranes of the vascular wall requires specific connexin-based channels

NO is generated within cells and frequently must be transferred to responsive neighboring cells, as occurs in the endothelium-dependent relaxation of smooth muscle cells observed in blood vessels. It is thought that NO diffuses freely across cell membranes, but it may also permeate through low resistant membrane pathways. Here, we describe the participation of connexin (Cx)-formed channels in the NO transport across cell membranes and between endothelial and smooth muscle cells. We used a water-soluble NO donor of high molecular weight (S-nitrosylated albumin, BSA-NO) that does not permeate through cell membranes or Cx-based channels and the NO-sensitive dye 4,5-diaminofluorescein diacetate to detect changes of intracellular NO concentration. We found that NO generated in the extracellular space was not detected intracellularly in Cx-deficient HeLa cells, suggesting that cell membrane represents a significant diffusion barrier for NO transfer. However, Cx-based channels provide efficient pathways for NO signaling because NO opened and permeated hemichannels expressed in HeLa cells transfected with Cx43, Cx40, or Cx37. In contrast, NO closed hemichannels of HeLa-Cx32 cells, which otherwise are permeable to NO if are opened by a divalent cation-free extracellular solution. Consistent with this, blockade of Cx-based channels abolished the myoendothelial NO transfer and associated NO-dependent vasodilation induced by acethylcholine. These results indicate that Cx-based channels play a key role in the NO-dependent tonic control of vascular function and may direct the NO signal to specific targets, which provides a novel mechanistic basis for the critical role of Cxs in cell-cell communication in the vessel wall. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Defective Cx40 maintains Cx37 expression but intact Cx40 is crucial for conducted dilations irrespective of hypertension

The gap junction channel protein connexin40 (Cx40) is crucial in vascular and renal physiology, because Cx40-deficient mice exhibit impaired conduction of endothelium-dependent dilations and pronounced hypertension. The latter precludes mechanistic insights into the role of endothelial Cx40, because long-lasting hypertension itself may affect conduction and Cx expression. We aimed to identify endothelial Cx40 functions, their dependency on the conductive capability, and to separate these from hypertension-related alterations. We assessed conduction and Cx expression in mice with cell type-specific deletion of Cx40 and in mice expressing a defective Cx40 (Cx40A96S) identified in humans, which forms nonconducting gap junction channels. Confined arteriolar stimulation with acetylcholine or bradykinin elicited local dilations that conducted upstream without attenuation of the amplitude for distances up to 1.2-mm in controls with a floxed Cx40 gene (Cx40(fl/fl)). Conducted responses in hypertensive animals devoid of Cx40 in renin-producing cells were unaltered but remote dilations were reduced in normotensive animals deficient for Cx40 in endothelial cells (Cx40(fl/fl):Tie2-Cre). Surprisingly, Cx37 expression was undetectable by immunostaining in arteriolar endothelium only in Cx40(fl/fl):Tie2-Cre; however, transcriptional activity of Cx37 in the cremaster was comparable with Cx40(fl/fl) controls. Cx40A96S mice were hypertensive with preserved expression of Cx40 and Cx37. Nevertheless, conducted responses were blunted. We conclude that endothelial Cx40 is necessary to support conducted dilations initiated by endothelial agonists and to locate Cx37 into the plasma membrane. These functions are unaltered by long-lasting hypertension. In the presence of a nonconducting Cx40, Cx37 is present but cannot support the conduction highlighting the importance of endothelial Cx40.

Functional coordination of the spread of vasodilations through skeletal muscle microvasculature: implications for blood flow control

AIM

We sought to understand the integrated vascular response to muscle contraction by determining how different branch orders of the terminal microvascular network respond to stimulation using a K(ATP) channel opener pinacidil (PIN) as a muscle contraction mimetic.

METHODS

Using the blood perfused, hamster cremaster preparation in situ, we locally micropipette-applied 10(-5) M PIN on the capillaries, Branch arteriole (third order, two branch orders up from the capillaries) and transverse arterioles (TA) (second order, three branch orders up from the capillaries) and observed different branch orders of the microvasculature to determine where the localized vasodilation spread throughout the terminal microvascular network.

RESULTS

We observed that PIN stimulation of capillaries caused associated upstream vasodilation of the module inflow arteriole (MI) (fourth order, the terminal arteriole) (2.1 ± 0.4 μm), the associate Branch (1.4 ± 0.5 μm) and in the upstream direction on the TA (2.1 ± 0.5 μm). Vasodilation did not occur in all MIs (-0.2 ± 0.2 μm) from the vasodilated branch and did not go downstream on the TA (0.7 ± 0.4 μm). Branch stimulation caused upstream TA (3.3 ± 1.0 μm) and upstream Branch (1.7 ± 0.3 μm) vasodilation but not downstream TA (1.5 ± 0.6 μm) or downstream Branch (0.2 ± 0.3 μm) vasodilation. TA stimulation caused conducted responses in both directions and into all associated arteriolar Branches and MIs.

CONCLUSIONS

The spread of the conducted response is dependent on the vascular branch order stimulated: capillary stimulation was most specific in its direction and TA stimulation was the least specific. Our data indicate that vascular branch order is important in determining the vascular response needed to direct blood flow to contracting skeletal muscle cells.

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.

Renal connexins and blood pressure

The kidneys are centrally involved in the regulation of blood pressure. Kidney function requires the coordinated actions of a number of different vascular and tubular cell types in the renal vasculature and in the renal tubular system. The intrarenal coordination of these actions is not well understood. Since gap junctions have been identified in the kidneys, possible pathways involved in this context could be direct intercellular communication via gap junctions or via connexin hemichannels. In this context nine different connexins have been found to be expressed in the kidney, either localized to the vasculature or to the tubular system. Evidence is arising that malfunctions of certain connexins have an impact on the capability of the kidney to maintain blood pressure homeostasis. Findings reported in this context will be outlined and discussed in this review. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Connexin45 is expressed in vascular smooth muscle but its function remains elusive

Connexins (Cx) form gap junctions and allow the coordination of cellular behaviour. In vessels, expression of Cx40, Cx37, and Cx43 is well established and specifically Cx40 serves important functions in endothelial cells. In contrast, expression and physiological functions of Cx45 is unclear although its expression has been suggested in vascular smooth muscle (VSM). Therefore, we studied expression and function of Cx45 in vessels using different mice models allowing to identify and delete Cx45. Smooth muscle cell (SMC)-specific deletion was achieved by the Cre/loxP system using Cre-recombinase driven by a Nestin promoter. Deletion of Cx45 leads concomitantly to the expression of enhanced green fluorescence protein (EGFP) in these mice. Conduction of vasomotor responses was studied in cremasteric arterioles using intravital microscopy and arterial pressure was measured telemetrically. Cx45 is transcriptionally expressed in VSM as detected by EGFP expression in SMC-specific Cx45-deficient mice (Cx45fl/fl:Nestin-Cre) but not in endothelial cells (Cx45fl/fl:TIE2-Cre). Moreover, EGFP was located at VSM cell borders in arterioles of transgenic mice carrying an EGFP-tagged Cx45. Expectedly, arteriolar conduction of dilations evoked by the endothelium-dependent agonist acetylcholine were not different between Cx45fl/fl:Nestin-Cre mice and controls carrying homozygously a floxed Cx45 gene (Cx45fl/fl). Surprisingly, the amplitude of locally initiated endothelium-independent constrictions (K⁺) and dilations (adenosine) declined similarly with distance in both genotypes indicating an intact VSM conduction pathway also in mice being deficient for Cx45 in VSM. Arterial pressure was not different between freely moving Cx45fl/fl and Cx45fl/fl:Nestin-Cre mice during day or night. We conclude that Cx45 is physiologically expressed in VSM, but not in EC in murine arterioles. However, Cx45 is dispensable for the conduction of vasomotor responses along these arterioles. Possibly, other Cx functionally replace the lack of Cx45 in VSM. The reported role of Cx45 in renin secretion does not seem to alter arterial pressure in freely moving mice.

Tuning electrical conduction along endothelial tubes of resistance arteries through Ca(2+)-activated K(+) channels

RATIONALE

Electrical conduction through gap junction channels between endothelial cells of resistance vessels is integral to blood flow control. Small and intermediate-conductance Ca(2+)-activated K(+) channels (SK(Ca)/IK(Ca)) initiate electrical signals in endothelial cells, but it is unknown whether SK(Ca)/IK(Ca) activation alters signal transmission along the endothelium.

OBJECTIVE

We tested the hypothesis that SK(Ca)/IK(Ca) activity regulates electrical conduction along the endothelium of resistance vessels.

METHODS AND RESULTS

Freshly isolated endothelial cell tubes (60 μm wide; 1-3 mm long; cell length, ≈35 μm) from mouse skeletal muscle feed (superior epigastric) arteries were studied using dual intracellular microelectrodes. Current was injected (±0.1-3 nA) at site 1 while recording membrane potential (V(m)) at site 2 (separation distance=50-2000 μm). SK(Ca)/IK(Ca) activation (NS309, 1 μmol/L) reduced the change in V(m) along endothelial cell tubes by ≥50% and shortened the electrical length constant (λ) from 1380 to 850 μm (P<0.05) while intercellular dye transfer (propidium iodide) was maintained. Activating SK(Ca)/IK(Ca) with acetylcholine or SKA-31 also reduced electrical conduction. These effects of SK(Ca)/IK(Ca) activation persisted when hyperpolarization (>30 mV) was prevented with 60 mmol/L [K(+)](o). Conversely, blocking SK(Ca)/IK(Ca) (apamin+charybdotoxin) depolarized cells by ≈10 mV and enhanced electrical conduction (ie, changes in V(m)) by ≈30% (P<0.05).

CONCLUSIONS

These findings illustrate a novel role for SK(Ca)/IK(Ca) activity in tuning electrical conduction along the endothelium of resistance vessels by governing signal dissipation through changes in membrane resistance. Voltage-insensitive ion channels can thereby tune intercellular electrical signaling independent from gap junction channels.

Modeling Ca2+ signaling in the microcirculation: intercellular communication and vasoreactivity

A network of intracellular signaling pathways and complex intercellular interactions regulate calcium mobilization in vascular cells, arteriolar tone, and blood flow. Different endothelium-derived vasoreactive factors have been identified and the importance of myoendothelial communication in vasoreactivity is now well appreciated. The ability of many vascular networks to conduct signals upstream also is established. This phenomenon is critical for both short-term changes in blood perfusion as well as long-term adaptations of a vascular network. In addition, in a phenomenon termed vasomotion, arterioles often exhibit spontaneous oscillations in diameter. This is thought to improve tissue oxygenation and enhance blood flow. Experimentation has begun to reveal important aspects of the regulatory machinery and the significance of these phenomena for the regulation of local perfusion and oxygenation. Mathematical modeling can assist in elucidating the complex signaling mechanisms that participate in these phenomena. This review highlights some of the important experimental studies and relevant mathematical models that provide the current understanding of these mechanisms in vasoreactivity.

Applicability of cable theory to vascular conducted responses

Conduction processes in the vasculature have traditionally been described using cable theory, i.e., locally induced signals decaying passively along the arteriolar wall. The decay is typically quantified using the steady-state length-constant, λ, derived from cable theory. However, the applicability of cable theory to blood vessels depends on assumptions that are not necessarily fulfilled in small arteries and arterioles. We have employed a morphologically and electrophysiologically detailed mathematical model of a rat mesenteric arteriole to investigate if the assumptions hold and whether λ adequately describes simulated conduction profiles. We find that several important cable theory assumptions are violated when applied to small blood vessels. However, the phenomenological use of a length-constant from a single exponential function is a good measure of conduction length. Hence, λ should be interpreted as a descriptive measure and not in light of cable theory. Determination of λ using cable theory assumes steady-state conditions. In contrast, using the model it is possible to probe how conduction behaves before steady state is achieved. As ion channels have time-dependent activation and inactivation, the conduction profile changes considerably during this dynamic period with an initially longer spread of current. This may have implications in relation to explaining why different agonists have different conduction properties. Also, it illustrates the necessity of using and developing models that handle the nonlinearity of ion channels.

Rapid and slow nitric oxide responses during conducted vasodilation in the in vivo intestine and brain cortex microvasculatures

Conduction of arteriolar vasodilation is initiated by activation of nitric oxide (NO) mechanisms, but dependent on conduction of hyperpolarization. Most studies have used brief (<1 second) activation of the initial vasodilation to evaluate the fast conduction processes. However, most arteriolar mechanisms involving NO production persist for minutes. In this study, fast and slower components of arteriolar conduction in the in vivo rat brain and small intestine were compared using three-minute stimulation of NO-dependent vasodilation and measurement of [NO] at the distal sites. Within 10-15 seconds, both vasculatures had a rapidly conducted vasodilation and dilation at distance had a fast but small [NO] component. A slower but larger distal vasodilation occurred after 60-90 seconds in the intestine, but not the brain, and was associated with a substantial increase in [NO]. This slowly developed dilation appeared to be caused by flow mediated responses of larger arterioles as smaller arterioles dilated to lower downstream resistance. These results indicate while the intestinal and cerebral arterioles have a fast conducted vasodilation system, the intestinal arterioles also have a slower but larger dilation of major arterioles that is NO related and dependent on the conduction of vasodilation between small arterioles.

Regulation of endothelial connexin40 expression by shear stress

Endothelial connexin (Cx)40 plays an important role in signal propagation along blood vessel walls, modulating vessel diameter and thereby blood flow. Blood flow, in turn, has been shown to alter endothelial Cx40 expression. However, the timing and shear stress dependence of this relationship have remained unclear, as have the signal transduction pathways involved and the functional implications. Therefore, the aim of this study was to quantify the effects of shear stress on endothelial Cx40 expression, to analyze the role of phosphoinositide 3-kinase (PI3K)/Akt signaling involved, and to assess the possible functional consequences for the adaptation of microvascular networks. First-passage human umbilical vein endothelial cells were exposed to defined shear stress conditions and analyzed for Cx40 using real-time RT-PCR and immunoblot analysis. Shear stress caused long-term induction of Cx40 protein expression, with two short-term mRNA peaks at 4 and 16 h, indicating the dynamic nature of the adaptation process. Maximum shear stress-dependent induction was observed at shear levels between 6 and 10 dyn/cm(2). Simulation of this pattern of shear-dependent Cx expression in a vascular adaptation model of a microvascular network led to an improved fit for the simulated results to experimental measurements. Cx40 expression was greatly reduced by inhibiting PI3K or Akt, with PI3K activity being required for basal Cx40 expression and Akt activity taking part in its shear stress-dependent induction.

Cx37 deletion enhances vascular growth and facilitates ischemic limb recovery

The unique contributions of connexin (Cx)37 and Cx40, gap junction-forming proteins that are coexpressed in vascular endothelium, to the recovery of tissues from ischemic injury are unknown. We recently reported that Cx37-deficient (Cx37(-/-)) animals recovered ischemic hindlimb function more quickly and to a greater extent than wild-type (WT) or Cx40(-/-) animals, suggesting that Cx37 limits recovery in the WT animal. Here, we tested the hypothesis that enhanced angiogenesis, arteriogenesis, and vasculogenesis contribute to improved postischemic hindlimb recovery in Cx37(-/-) animals. Ischemia was induced unilaterally in the hindlimbs of WT or Cx37(-/-) mice (isoflurane anesthesia). Postsurgical limb appearance, use, and perfusion were documented during recovery, and the number (and size) of large and small vessels was determined. Native collateral number, predominantly established during embryonic development (vasculogenesis), was also determined in the pial circulation. Both microvascular density in the gastrocnemius of the ischemic limb (an angiogenic field) and the number and tortuosity of larger vessels in the gracilis vasculature (an arteriogenic field) were increased in Cx37(-/-) animals compared with WT animals. Cx37(-/-) mice also had an increased (vs. WT) number of collateral vessels in the pial circulation. These findings suggest that in Cx37(-/-) animals, improved recovery of the ischemic hindlimb involves enhanced vasculogenesis, resulting in increased numbers of collaterals in the hindlimb (and pial circulations) and more extensive collateral remodeling and angiogenesis. These results are consistent with Cx37 exerting a growth-suppressive effect in the vasculature that limits embryonic vasculogenesis as well as arteriogenic and angiogenic responses to ischemic injury in the adult animal.

Calcium and electrical signalling along endothelium of the resistance vasculature

This MiniReview is focused on the nature of intercellular signalling along the endothelium that helps to co-ordinate blood flow control in vascular resistance networks. Vasodilation initiated by contracting skeletal muscle ascends from arterioles within the tissue to encompass resistance arteries upstream and thereby increase blood flow during exercise. In resistance vessels, acetylcholine microiontophoresis or intracellular current injection initiates hyperpolarization that conducts through gap junction channels (GJCs) along the vessel wall resulting in conducted vasodilation (CVD). Both ascending vasodilation and CVD are eliminated with endothelial cell (EC) disruption, pointing to common signalling events and mutual dependence upon EC integrity. As demonstrated by electrical coupling and dye transfer during intracellular recording, their longitudinal orientation and robust expression of GJCs enable ECs to play a predominant role in CVD. Once conduction is initiated, a major interest centres on whether CVD is purely passive or involves additional 'active' signalling events. Here, we discuss components for Ca²⁺ and electrical signalling with an emphasis on intercellular coupling through endothelial GJCs. We stress the importance of understanding relationships between intracellular Ca²⁺ dynamics, EC hyperpolarization and CVD while integrating findings from isolated ECs into more complex interactions in vivo. Whereas endothelial dysfunction accompanies cardiovascular disease and the components of intra- and inter-cellular signalling are increasingly well defined, little is known of how Ca²⁺ signalling and electrical conduction along microvascular endothelium are altered in diseased states. Thus, greater insight into how these relationships are governed and interact is a key goal for continued research efforts.