Intravascular pressure augments cerebral arterial constriction by inducing voltage-insensitive Ca2+ waves

Electro-metabolic signaling

Precise matching of energy substrate delivery to local metabolic needs is essential for the health and function of all tissues. Here, we outline a mechanistic framework for understanding this critical process, which we refer to as electro-metabolic signaling (EMS). All tissues exhibit changes in metabolism over varying spatiotemporal scales and have widely varying energetic needs and reserves. We propose that across tissues, common signatures of elevated metabolism or increases in energy substrate usage that exceed key local thresholds rapidly engage mechanisms that generate hyperpolarizing electrical signals in capillaries that then relax contractile elements throughout the vasculature to quickly adjust blood flow to meet changing needs. The attendant increase in energy substrate delivery serves to meet local metabolic requirements and thus avoids a mismatch in supply and demand and prevents metabolic stress. We discuss in detail key examples of EMS that our laboratories have discovered in the brain and the heart, and we outline potential further EMS mechanisms operating in tissues such as skeletal muscle, pancreas, and kidney. We suggest that the energy imbalance evoked by EMS uncoupling may be central to cellular dysfunction from which the hallmarks of aging and metabolic diseases emerge and may lead to generalized organ failure states-such as diverse flavors of heart failure and dementia. Understanding and manipulating EMS may be key to preventing or reversing these dysfunctions.

A new model for evaluating pressure-induced vascular tone in small cerebral arteries

The capacity of small cerebral arteries (SCAs) to adapt to pressure fluctuations has a fundamental physiological role and appears to be relevant in different pathological conditions. Here, we present a new computational model for quantifying the link, and its contributors, between luminal pressure and vascular tone generation in SCAs. This is assembled by combining a chemical sub-model, representing pressure-induced smooth muscle cell (SMC) signalling, with a mechanical sub-model for the tone generation and its transduction at tissue level. The devised model can accurately reproduce the impact of luminal pressure on different cytoplasmic components involved in myogenic signalling, both in the control case and when combined with some specific pharmacological interventions. Furthermore, the model is also able to capture and predict experimentally recorded pressure-outer diameter relationships obtained for vessels under control conditions, both in a Ca2 + -free bath and under drug inhibition. The modularity of the proposed framework allows the integration of new components for the study of a broad range of processes involved in the vascular function.

CaV3.1 channels facilitate calcium wave generation and myogenic tone development in mouse mesenteric arteries

The arterial myogenic response to intraluminal pressure elicits constriction to maintain tissue perfusion. Smooth muscle [Ca2+] is a key determinant of constriction, tied to L-type (CaV1.2) Ca2+ channels. While important, other Ca2+ channels, particularly T-type could contribute to pressure regulation within defined voltage ranges. This study examined the role of one T-type Ca2+ channel (CaV3.1) using C57BL/6 wild type and CaV3.1-/- mice. Patch-clamp electrophysiology, pressure myography, blood pressure and Ca2+ imaging defined the CaV3.1-/- phenotype relative to C57BL/6. CaV3.1-/- mice had absent CaV3.1 expression and whole-cell current, coinciding with lower blood pressure and reduced mesenteric artery myogenic tone, particularly at lower pressures (20-60 mmHg) where membrane potential is hyperpolarized. This reduction coincided with diminished Ca2+ wave generation, asynchronous events of Ca2+ release from the sarcoplasmic reticulum, insensitive to L-type Ca2+ channel blockade (Nifedipine, 0.3 µM). Proximity ligation assay (PLA) confirmed IP3R1/CaV3.1 close physical association. IP3R blockade (2-APB, 50 µM or xestospongin C, 3 µM) in nifedipine-treated C57BL/6 arteries rendered a CaV3.1-/- contractile phenotype. Findings indicate that Ca2+ influx through CaV3.1 contributes to myogenic tone at hyperpolarized voltages through Ca2+-induced Ca2+ release tied to the sarcoplasmic reticulum. This study helps establish CaV3.1 as a potential therapeutic target to control blood pressure.

IP3 receptor orchestrates maladaptive vascular responses in heart failure

Patients with heart failure (HF) have augmented vascular tone, which increases cardiac workload, impairing ventricular output and promoting further myocardial dysfunction. The molecular mechanisms underlying the maladaptive vascular responses observed in HF are not fully understood. Vascular smooth muscle cells (VSMCs) control vasoconstriction via a Ca²⁺-dependent process, in which the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1) on the sarcoplasmic reticulum (SR) plays a major role. To dissect the mechanistic contribution of intracellular Ca²⁺ release to the increased vascular tone observed in HF, we analyzed the remodeling of IP3R1 in aortic tissues from patients with HF and from controls. VSMC IP3R1 channels from patients with HF and HF mice were hyperphosphorylated by both serine and tyrosine kinases. VSMCs isolated from IP3R1^VSMC–/– mice exhibited blunted Ca²⁺ responses to angiotensin II (ATII) and norepinephrine compared with control VSMCs. IP3R1^VSMC–/– mice displayed significantly reduced responses to ATII, both in vivo and ex vivo. HF IP3R^1VSMC–/– mice developed significantly less afterload compared with HF IP3R1^fl/fl mice and exhibited significantly attenuated progression toward decompensated HF and reduced interstitial fibrosis. Ca²⁺-dependent phosphorylation of the MLC by MLCK activated VSMC contraction. MLC phosphorylation was markedly increased in VSMCs from patients with HF and HF mice but reduced in VSMCs from HF IP3R1^VSMC–/– mice and HF WT mice treated with ML-7. Taken together, our data indicate that VSMC IP3R1 is a major effector of increased vascular tone, which contributes to increased cardiac afterload and decompensation in HF.

Long-Term Hypoxia Negatively Influences Ca2+ Signaling in Basilar Arterial Myocytes of Fetal and Adult Sheep

Cerebral arterial vasoreactivity is vital to the regulation of cerebral blood flow. Depolarization of arterial myocytes elicits whole-cell Ca²⁺ oscillations as well as subcellular Ca²⁺ sparks due to activation of ryanodine receptors on the sarcoplasmic reticulum. Previous evidence illustrates that contraction of cerebral arteries from sheep and underlying Ca²⁺ signaling pathways are modified by age and that long-term hypoxia (LTH) causes aberrations in Ca²⁺ signaling pathways and downstream effectors impacting vasoregulation. We hypothesize that age and LTH affect the influence of membrane depolarization on whole-cell intracellular Ca²⁺ oscillations and sub-cellular Ca²⁺ spark activity in cerebral arteries. To test this hypothesis, we examined Ca²⁺ oscillatory and spark activities using confocal fluorescence imaging techniques of Fluo-4 loaded basilar arterial myocytes of low- and high-altitude term fetal (∼145 days of gestation) and adult sheep, where high-altitude pregnant and non-pregnant sheep were placed at 3,801 m for >100 days. Ca²⁺ oscillations and sparks were recorded using an in situ preparation evaluated in the absence or presence of 30 mM K⁺ (30K) to depolarize myocytes. Myocytes from adult animals tended to have a lower basal rate of whole-cell Ca²⁺ oscillatory activity and 30K increased the activity within cells. LTH decreased the ability of myocytes to respond to depolarization independent of age. These observations illustrate that both altitude and age play a role in affecting whole-cell and localized Ca²⁺ signaling, which are important to arterial vasoreactivity and cerebral blood flow.

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

EPAC in Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are major components of blood vessels. They regulate physiological functions, such as vascular tone and blood flow. Under pathological conditions, VSMCs undergo a remodeling process known as phenotypic switching. During this process, VSMCs lose their contractility and acquire a synthetic phenotype, where they over-proliferate and migrate from the tunica media to the tunica interna, contributing to the occlusion of blood vessels. Since their discovery as effector proteins of cyclic adenosine 3′,5′-monophosphate (cAMP), exchange proteins activated by cAMP (EPACs) have been shown to play vital roles in a plethora of pathways in different cell systems. While extensive research to identify the role of EPAC in the vasculature has been conducted, much remains to be explored to resolve the reported discordance in EPAC’s effects. In this paper, we review the role of EPAC in VSMCs, namely its regulation of the vascular tone and phenotypic switching, with the likely involvement of reactive oxygen species (ROS) in the interplay between EPAC and its targets/effectors.

Inositol 1,4,5-Trisphosphate Receptors in Hypertension

Chronic hypertension remains a major cause of global mortality and morbidity. It is a complex disease that is the clinical manifestation of multiple genetic, environmental, nutritional, hormonal, and aging-related disorders. Evidence supports a role for vascular aging in the development of hypertension involving an impairment in endothelial function together with an alteration in vascular smooth muscle cells (VSMCs) calcium homeostasis leading to increased myogenic tone. Changes in free intracellular calcium levels ([Ca²⁺] _i ) are mediated either by the influx of Ca²⁺ from the extracellular space or release of Ca²⁺ from intracellular stores, mainly the sarcoplasmic reticulum (SR). The influx of extracellular Ca²⁺ occurs primarily through voltage-gated Ca²⁺ channels (VGCCs), store-operated Ca²⁺ channels (SOC), and Ca²⁺ release-activated channels (CRAC), whereas SR-Ca²⁺ release occurs through inositol trisphosphate receptor (IP₃R) and ryanodine receptors (RyRs). IP₃R-mediated SR-Ca²⁺ release, in the form of Ca²⁺ waves, not only contributes to VSMC contraction and regulates VGCC function but is also intimately involved in structural remodeling of resistance arteries in hypertension. This involves a phenotypic switch of VSMCs as well as an alteration of cytoplasmic Ca²⁺ signaling machinery, a phenomena tightly related to the aging process. Several lines of evidence implicate changes in expression/function levels of IP₃R isoforms in the development of hypertension, VSMC phenotypic switch, and vascular aging. The present review discusses the current knowledge of these mechanisms in an integrative approach and further suggests potential new targets for hypertension management and treatment.

Voltage-gated Ca2+ channel activity modulates smooth muscle cell calcium waves in hamster cremaster arterioles

Cremaster muscle arteriolar smooth muscle cells (SMCs) display inositol 1,4,5-trisphosphate receptor-dependent Ca²⁺ waves that contribute to global myoplasmic Ca²⁺ concentration and myogenic tone. However, the contribution made by voltage-gated Ca²⁺ channels (VGCCs) to arteriolar SMC Ca²⁺ waves is unknown. We tested the hypothesis that VGCC activity modulates SMC Ca²⁺ waves in pressurized (80 cmH₂O/59 mmHg, 34°C) hamster cremaster muscle arterioles loaded with Fluo-4 and imaged by confocal microscopy. Removal of extracellular Ca²⁺ dilated arterioles (32 ± 3 to 45 ± 3 μm, n = 15, P < 0.05) and inhibited the occurrence, amplitude, and frequency of Ca²⁺ waves ( n = 15, P < 0.05), indicating dependence of Ca²⁺ waves on Ca²⁺ influx. Blockade of VGCCs with nifedipine (1 μM) or diltiazem (10 μM) or deactivation of VGCCs by hyperpolarization of smooth muscle with the K⁺ channel agonist cromakalim (10 μM) produced similar inhibition of Ca²⁺ waves ( P < 0.05). Conversely, depolarization of SMCs with the K⁺ channel blocker tetraethylammonium (1 mM) constricted arterioles from 26 ± 3 to 14 ± 2 μm ( n = 11, P < 0.05) and increased wave occurrence (9 ± 3 to 16 ± 3 waves/SMC), amplitude (1.6 ± 0.07 to 1.9 ± 0.1), and frequency (0.5 ± 0.1 to 0.9 ± 0.2 Hz, n = 10, P < 0.05), effects that were blocked by nifedipine (1 μM, P < 0.05). Similarly, the VGCC agonist Bay K8644 (5 nM) constricted arterioles from 14 ± 1 to 8 ± 1 μm and increased wave occurrence (3 ± 1 to 10 ± 1 waves/SMC) and frequency (0.2 ± 0.1 to 0.6 ± 0.1 Hz, n = 6, P < 0.05), effects that were unaltered by ryanodine (50 μM, n = 6, P > 0.05). These data support the hypothesis that Ca²⁺ waves in arteriolar SMCs depend, in part, on the activity of VGCCs. NEW & NOTEWORTHY Arterioles that control blood flow to and within skeletal muscle depend on Ca²⁺ influx through voltage-gated Ca²⁺ channels and release of Ca²⁺ from internal stores through inositol 1,4,5-trisphosphate receptors in the form of Ca²⁺ waves to maintain pressure-induced smooth muscle tone.

Vasoconstrictor stimulus determines the functional contribution of myoendothelial feedback to mesenteric arterial tone

KEY POINTS

In isolated resistance arteries, endothelial modulation of vasoconstrictor responses to α₁ -adrenoceptor agonists occurs via a process termed myoendothelial feedback: localized inositol trisphosphate (InsP₃ )-dependent Ca²⁺ transients activate intermediate conductance Ca²⁺ -activated K⁺ (IK_Ca ) channels, hyperpolarizing the endothelial membrane potential to limit further reductions in vessel diameter. We demonstrate that IK_Ca channel-mediated myoendothelial feedback limits responses of isolated mesenteric arteries to noradrenaline and nerve stimulation, but not to the thromboxane A₂ mimetic U46619 or to increases in intravascular pressure. In contrast, in the intact mesenteric bed, although responses to exogenous noradrenaline were limited by IK_Ca channel-mediated myoendothelial feedback, release of NO and activation of endothelial small conductance Ca²⁺ -activated K⁺ (SK_Ca ) channels in response to increases in shear stress appeared to be the primary mediators of endothelial modulation of vasoconstriction. We propose that (1) the functional contribution of myoendothelial feedback to arterial tone is determined by the nature of the vasoconstrictor stimulus, and (2) although IK_Ca channel-mediated myoendothelial feedback may contribute to local control of arterial diameter, in the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

ABSTRACT

Constriction of isolated resistance arteries in response to α₁ -adrenoceptor agonists is limited by reciprocal engagement of inhibitory endothelial mechanisms via myoendothelial feedback. In the current model of feedback, agonist stimulation of smooth muscle cells results in localized InsP₃ -dependent Ca²⁺ transients that activate endothelial IK_Ca channels. The subsequent hyperpolarization of the endothelial membrane potential then feeds back to the smooth muscle to limit further reductions in vessel diameter. We hypothesized that the functional contribution of InsP₃ -IK_Ca channel-mediated myoendothelial feedback to limiting arterial diameter may be influenced by the nature of the vasoconstrictor stimulus. To test this hypothesis, we investigated the functional role of myoendothelial feedback in modulating responses of rat mesenteric resistance arteries to the adrenoceptor agonist noradrenaline, the thromboxane A₂ mimetic U46619, increases in intravascular pressure and stimulation of perivascular sympathetic nerves. In isolated arteries, responses to noradrenaline and stimulation of sympathetic nerves, but not to U46619 and increases in intravascular pressure, were modulated by IK_Ca channel-dependent myoendothelial feedback. In the intact mesenteric bed perfused under conditions of constant flow, responses to exogenous noradrenaline were modulated by myoendothelial feedback, but shear stress-induced release of NO and activation of endothelial SK_Ca channels appeared to be the primary mediators of endothelial modulation of vasoconstriction to agonists and nerve stimulation. Thus, we propose that myoendothelial feedback may contribute to local control of diameter within arterial segments, but at the level of the intact vascular bed, increases in shear stress may be the major stimulus for engagement of the endothelium during vasoconstriction.

Pericytes of the neurovascular unit: key functions and signaling pathways

Pericytes are vascular mural cells embedded in the basement membrane of blood microvessels. They extend their processes along capillaries, pre-capillary arterioles and post-capillary venules. CNS pericytes are uniquely positioned in the neurovascular unit between endothelial cells, astrocytes and neurons. They integrate, coordinate and process signals from their neighboring cells to generate diverse functional responses that are critical for CNS functions in health and disease, including regulation of the blood-brain barrier permeability, angiogenesis, clearance of toxic metabolites, capillary hemodynamic responses, neuroinflammation and stem cell activity. Here we examine the key signaling pathways between pericytes and their neighboring endothelial cells, astrocytes and neurons that control neurovascular functions. We also review the role of pericytes in CNS disorders including rare monogenic diseases and complex neurological disorders such as Alzheimer's disease and brain tumors. Finally, we discuss directions for future studies.

Detection of TRPV4 channel current-like activity in Fawn Hooded hypertensive (FHH) rat cerebral arterial muscle cells

The transient receptor potential vallinoid type 4 (TRPV4) is a calcium entry channel known to modulate vascular function by mediating endothelium–dependent vasodilation. The present study investigated if isolated cerebral arterial myocytes of the Fawn Hooded hypertensive (FHH) rat, known to display exaggerated K_Ca channel current activity and impaired myogenic tone, express TRPV4 channels at the transcript and protein level and exhibit TRPV4-like single-channel cationic current activity. Reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunostaining analysis detected the expression of mRNA transcript and translated protein of TRPV4 channel in FHH rat cerebral arterial myocytes. Patch clamp recording of single-channel current activity identified the presence of a single-channel cationic current with unitary conductance of ~85 pS and ~96 pS at hyperpolarizing and depolarizing potentials, respectively, that was inhibited by the TRPV4 channel antagonist RN 1734 or HC 067074 and activated by the potent TRPV4 channel agonist GSK1016790A. Application of negative pressure via the interior of the patch pipette increased the NPo of the TRPV4-like single-channel cationic current recorded in cell-attached patches at a patch potential of 60 mV that was inhibited by prior application of the TRPV4 channel antagonist RN 1734 or HC 067047. Treatment with the TRPV4 channel agonist GSK1016790A caused concentration-dependent increase in the NPo of K_Ca single-channel current recorded in cell-attached patches of cerebral arterial myocytes at a patch potential of 40 mV, which was not influenced by pretreatment with the voltage-gated L-type Ca²⁺ channel blocker nifedipine or the T-type Ca²⁺ channel blocker Ni²⁺. These findings demonstrate that FHH rat cerebral arterial myocytes express mRNA transcript and translated protein for TRPV4 channel and display TRPV4-like single-channel cationic current activity that was stretch-sensitive and activation of which increased the open state probability of K_Ca single-channel current in these arterial myocytes.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Calcium Channels in Vascular Smooth Muscle

Calcium (Ca²⁺) plays a central role in excitation, contraction, transcription, and proliferation of vascular smooth muscle cells (VSMs). Precise regulation of intracellular Ca²⁺ concentration ([Ca²⁺]_i) is crucial for proper physiological VSM function. Studies over the last several decades have revealed that VSMs express a variety of Ca²⁺-permeable channels that orchestrate a dynamic, yet finely tuned regulation of [Ca²⁺]_i. In this review, we discuss the major Ca²⁺-permeable channels expressed in VSM and their contribution to vascular physiology and pathology.

Ion channel networks in the control of cerebral blood flow

One hundred and twenty five years ago, Roy and Sherrington made the seminal observation that neuronal stimulation evokes an increase in cerebral blood flow.(1) Since this discovery, researchers have attempted to uncover how the cells of the neurovascular unit-neurons, astrocytes, vascular smooth muscle cells, vascular endothelial cells and pericytes-coordinate their activity to control this phenomenon. Recent work has revealed that ionic fluxes through a diverse array of ion channel species allow the cells of the neurovascular unit to engage in multicellular signaling processes that dictate local hemodynamics.In this review we center our discussion on two major themes: (1) the roles of ion channels in the dynamic modulation of parenchymal arteriole smooth muscle membrane potential, which is central to the control of arteriolar diameter and therefore must be harnessed to permit changes in downstream cerebral blood flow, and (2) the striking similarities in the ion channel complements employed in astrocytic endfeet and endothelial cells, enabling dual control of smooth muscle from either side of the blood-brain barrier. We conclude with a discussion of the emerging roles of pericyte and capillary endothelial cell ion channels in neurovascular coupling, which will provide fertile ground for future breakthroughs in the field.

Are they in or out? The elusive interaction between Qtracker®800 vascular labels and brain endothelial cells

Aim: Qtracker®800 Vascular labels (Qtracker®800) are promising biomedical tools for high-resolution vasculature imaging; their effects on mouse and human endothelia, however, are still unknown. Materials & methods: Qtracker®800 were injected in Balb/c mice, and brain endothelium uptake was investigated by transmission electron microscopy 3-h post injection. We then investigated, in vitro, the effects of Qtracker®800 exposure on mouse and human endothelial cells by calcium imaging. Results: Transmission electron microscopy images showed nanoparticle accumulation in mouse brain endothelia. A subset of mouse and human endothelial cells generated intracellular calcium transients in response to Qtracker®800. Conclusion: Qtracker®800 nanoparticles elicit endothelial functional responses, which prompts biomedical safety evaluations and may bias the interpretation of experimental studies involving vascular imaging.

Implications of αvβ3 Integrin Signaling in the Regulation of Ca2+ Waves and Myogenic Tone in Cerebral Arteries

OBJECTIVE

The myogenic response is central to blood flow regulation in the brain. Its induction is tied to elevated cytosolic [Ca(2+)], a response primarily driven by voltage-gated Ca(2+) channels and secondarily by Ca(2+) wave production. Although the signaling events leading to the former are well studied, those driving Ca(2+) waves remain uncertain.

APPROACH AND RESULTS

We postulated that αvβ3 integrin signaling is integral to the generation of pressure-induced Ca(2+) waves and cerebral arterial tone. This hypothesis was tested in rat cerebral arteries using the synergistic strengths of pressure myography, rapid Ca(2+) imaging, and Western blot analysis. GRGDSP, a peptide that preferentially blocks αvβ3 integrin, attenuated myogenic tone, indicating the modest role for sarcoplasmic reticulum Ca(2+) release in myogenic tone generation. The RGD peptide was subsequently shown to impair Ca(2+) wave generation and myosin light chain 20 (MLC20) phosphorylation, the latter of which was attributed to the modulation of MLC kinase and MLC phosphatase via MYPT1-T855 phosphorylation. Subsequent experiments revealed that elevated pressure enhanced phospholipase Cγ1 phosphorylation in an RGD-dependent manner and that phospholipase C inhibition attenuated Ca(2+) wave generation. Direct inhibition of inositol 1, 4, 5-triphosphate receptors also impaired Ca(2+) wave generation, myogenic tone, and MLC20 phosphorylation, partly through the T-855 phosphorylation site of MYPT1.

CONCLUSIONS

Our investigation reveals a hitherto unknown role for αvβ3 integrin as a cerebral arterial pressure sensor. The membrane receptor facilitates Ca(2+) wave generation through a signaling cascade, involving phospholipase Cγ1, inositol 1,3,4 triphosphate production, and inositol 1, 4, 5-triphosphate receptor activation. These discrete asynchronous Ca(2+) events facilitate MLC20 phosphorylation and, in part, myogenic tone by influencing both MLC kinase and MLC phosphatase activity.

α5-Integrin-mediated cellular signaling contributes to the myogenic response of cerebral resistance arteries

The myogenic response of resistance arterioles and small arteries involving constriction in response to intraluminal pressure elevation and dilation on pressure reduction is fundamental to local blood flow regulation in the microcirculation. Integrins have garnered considerable attention in the context of initiating the myogenic response, but evidence indicative of mechanotransduction by integrin adhesions, for example established changes in tyrosine phosphorylation of key adhesion proteins, has not been obtained to substantiate this interpretation. Here, we evaluated the role of integrin adhesions and associated cellular signaling in the rat cerebral arterial myogenic response using function-blocking antibodies against α5β1-integrins, pharmacological inhibitors of focal adhesion kinase (FAK) and Src family kinase (SFK), an ultra-high-sensitivity western blotting technique, site-specific phosphoprotein antibodies to quantify adhesion and contractile filament protein phosphorylation, and differential centrifugation to determine G-actin levels in rat cerebral arteries at varied intraluminal pressures. Pressure-dependent increases in the levels of phosphorylation of FAK (FAK-Y397, Y576/Y577), SFK (SFK-Y416; Y527 phosphorylation was reduced), vinculin-Y1065, paxillin-Y118 and phosphoinositide-specific phospholipase C-γ1 (PLCγ1)-Y783 were detected. Treatment with α5-integrin function-blocking antibodies, FAK inhibitor FI-14 or SFK inhibitor SU6656 suppressed the changes in adhesion protein phosphorylation, and prevented pressure-dependent phosphorylation of the myosin targeting subunit of myosin light chain phosphatase (MYPT1) at T855 and 20kDa myosin regulatory light chains (LC20) at S19, as well as actin polymerization that are necessary for myogenic constriction. We conclude that mechanotransduction by integrin adhesions and subsequent cellular signaling play a fundamental role in the cerebral arterial myogenic response.

Abnormal Rho-associated kinase activity contributes to the dysfunctional myogenic response of cerebral arteries in type 2 diabetes

The structural and functional integrity of the brain, and therefore, cognition, are critically dependent on the appropriate control of blood flow within the cerebral circulation. Inadequate flow leads to ischemia, whereas excessive flow causes small vessel rupture and (or) blood-brain-barrier disruption. Cerebral blood flow is controlled through the interplay of several physiological mechanisms that regulate the contractile state of vascular smooth muscle cells (VSMCs) within the walls of cerebral resistance arteries and arterioles. The myogenic response of cerebral VSMCs is a key mechanism that is responsible for maintaining constant blood flow during variations in systemic pressure, i.e., flow autoregulation. Inappropriate myogenic control of cerebral blood flow is associated with, and prognostic of, neurological deterioration and poor outcome in patients with several conditions, including type 2 diabetes. Here, we review recent advances in our understanding of the role of inappropriate Rho-associated kinase activity as a cause of impaired myogenic regulation of cerebral arterial diameter in type 2 diabetes.

Decoding dynamic Ca(2+) signaling in the vascular endothelium

Although acute and chronic vasoregulation is inherently driven by endothelial Ca(2+), control and targeting of Ca(2+)-dependent signals are poorly understood. Recent studies have revealed localized and dynamic endothelial Ca(2+) events comprising an intricate signaling network along the vascular intima. Discrete Ca(2+) transients emerging from both internal stores and plasmalemmal cation channels couple to specific membrane K(+) channels, promoting endothelial hyperpolarization and vasodilation. The spatiotemporal tuning of these signals, rather than global Ca(2+) elevation, appear to direct endothelial functions under physiologic conditions. In fact, altered patterns of dynamic Ca(2+) signaling may underlie essential endothelial dysfunction in a variety of cardiovascular diseases. Advances in imaging approaches and analyses in recent years have allowed for detailed detection, quantification, and evaluation of Ca(2+) dynamics in intact endothelium. Here, we discuss recent insights into these signals, including their sources of origination and their functional encoding. We also address key aspects of data acquisition and interpretation, including broad applications of automated high-content analysis.

Ca(2+) signaling in arterioles and small arteries of conscious, restrained, optical biosensor mice

Two-photon fluorescence microscopy and conscious, restrained optical biosensor mice were used to study smooth muscle Ca²⁺ signaling in ear arterioles. Conscious mice were used in order to preserve normal mean arterial blood pressure (MAP) and sympathetic nerve activity (SNA). ExMLCK mice, which express a genetically-encoded smooth muscle-specific FRET-based Ca²⁺ indicator, were equipped with blood pressure telemetry and immobilized for imaging. MAP was 101 ± 4 mmHg in conscious restrained mice, similar to the freely mobile state (107 ± 3 mmHg). Oscillatory vasomotion or irregular contractions were observed in most arterioles (71%), with the greatest oscillatory frequency observed at 0.25 s⁻¹. In a typical arteriole with an average diameter of ~35 μm, oscillatory vasomotion of a 5–6 μm magnitude was accompanied by nearly uniform [Ca²⁺] oscillations from ~0.1 to 0.5 μM, with maximum [Ca²⁺] occurring immediately before the rapid decrease in diameter. Very rapid, spatially uniform “Ca²⁺ flashes” were also observed but not asynchronous propagating Ca²⁺ waves. In contrast, vasomotion and dynamic Ca²⁺ signals were rarely observed in ear arterioles of anesthetized exMLCK biosensor mice. Hexamethonium (30 μg/g BW, i.p.) caused a fall in MAP to 74 ± 4 mmHg, arteriolar vasodilation, and abolition of vasomotion and synchronous Ca²⁺ transients.

Summary: MAP and heart rate (HR) were normal during high-resolution Ca²⁺ imaging of conscious, restrained mice. SNA induced continuous vasomotion and irregular vasoconstrictions via spatially uniform Ca²⁺ signaling within the arterial wall. FRET-based biosensor mice and two-photon imaging provided the first measurements of [Ca²⁺] in vascular smooth muscle cells in arterioles of conscious animals.

Cytoskeletal reorganization evoked by Rho-associated kinase- and protein kinase C-catalyzed phosphorylation of cofilin and heat shock protein 27, respectively, contributes to myogenic constriction of rat cerebral arteries

Our understanding of the molecular events contributing to myogenic control of diameter in cerebral resistance arteries in response to changes in intravascular pressure, a fundamental mechanism regulating blood flow to the brain, is incomplete. Myosin light chain kinase and phosphatase activities are known to be increased and decreased, respectively, to augment phosphorylation of the 20-kDa regulatory light chain subunits (LC20) of myosin II, which permits cross-bridge cycling and force development. Here, we assessed the contribution of dynamic reorganization of the actin cytoskeleton and thin filament regulation to the myogenic response and serotonin-evoked constriction of pressurized rat middle cerebral arteries. Arterial diameter and the levels of phosphorylated LC(20), calponin, caldesmon, cofilin, and HSP27, as well as G-actin content, were determined. A decline in G-actin content was observed following pressurization from 10 mm Hg to between 40 and 120 mm Hg and in three conditions in which myogenic or agonist-evoked constriction occurred in the absence of a detectable change in LC20 phosphorylation. No changes in thin filament protein phosphorylation were evident. Pressurization reduced G-actin content and elevated the levels of cofilin and HSP27 phosphorylation. Inhibitors of Rho-associated kinase and PKC prevented the decline in G-actin; reduced cofilin and HSP27 phosphoprotein content, respectively; and blocked the myogenic response. Furthermore, phosphorylation modulators of HSP27 and cofilin induced significant changes in arterial diameter and G-actin content of myogenically active arteries. Taken together, our findings suggest that dynamic reorganization of the cytoskeleton involving increased actin polymerization in response to Rho-associated kinase and PKC signaling contributes significantly to force generation in myogenic constriction of cerebral resistance arteries.

A PLCγ1-dependent, force-sensitive signaling network in the myogenic constriction of cerebral arteries

Maintaining constant blood flow in the face of fluctuations in blood pressure is a critical autoregulatory feature of cerebral arteries. An increase in pressure within the artery lumen causes the vessel to constrict through depolarization and contraction of the encircling smooth muscle cells. This pressure-sensing mechanism involves activation of two types of transient receptor potential (TRP) channels: TRPC6 and TRPM4. We provide evidence that the activation of the γ1 isoform of phospholipase C (PLCγ1) is critical for pressure sensing in cerebral arteries. Inositol 1,4,5-trisphosphate (IP3), generated by PLCγ1 in response to pressure, sensitized IP3 receptors (IP3Rs) to Ca(2+) influx mediated by the mechanosensitive TRPC6 channel, synergistically increasing IP3R-mediated Ca(2+) release to activate TRPM4 currents, leading to smooth muscle depolarization and constriction of isolated cerebral arteries. Proximity ligation assays demonstrated colocalization of PLCγ1 and TRPC6 with TRPM4, suggesting the presence of a force-sensitive, local signaling network comprising PLCγ1, TRPC6, TRPM4, and IP3Rs. Src tyrosine kinase activity was necessary for stretch-induced TRPM4 activation and myogenic constriction, consistent with the ability of Src to activate PLCγ isoforms. We conclude that contraction of cerebral artery smooth muscle cells requires the integration of pressure-sensing signaling pathways and their convergence on IP3Rs, which mediate localized Ca(2+)-dependent depolarization through the activation of TRPM4.

Tissue oxygen demand in regulation of the behavior of the cells in the vasculature

The control of arteriolar diameters in microvasculature has been in the focus of studies on mechanisms matching oxygen demand and supply at the tissue level. Functionally, important vascular elements include EC, VSMC, and RBC. Integration of these different cell types into functional units aimed at matching tissue oxygen supply with tissue oxygen demand is only achieved when all these cells can respond to the signals of tissue oxygen demand. Many vasoactive agents that serve as signals of tissue oxygen demand have their receptors on all these types of cells (VSMC, EC, and RBC) implying that there can be a coordinated regulation of their behavior by the tissue oxygen demand. Such functions of RBC as oxygen carrying by Hb, rheology, and release of vasoactive agents are considered. Several common extra- and intracellular signaling pathways that link tissue oxygen demand with control of VSMC contractility, EC permeability, and RBC functioning are discussed.

Regulation of cerebral artery smooth muscle membrane potential by Ca²⁺-activated cation channels

Arterial tone is dependent on the depolarizing and hyperpolarizing currents regulating membrane potential and governing the influx of Ca²⁺ needed for smooth muscle contraction. Several ion channels have been proposed to contribute to membrane depolarization, but the underlying molecular mechanisms are not fully understood. In this review, we will discuss the historical and physiological significance of the Ca²⁺-activated cation channel, TRPM4, in regulating membrane potential of cerebral artery smooth muscle cells. As a member of the recently described transient receptor potential super family of ion channels, TRPM4 possesses the biophysical properties and upstream cellular signaling and regulatory pathways that establish it as a major physiological player in smooth muscle membrane depolarization.

Ca(2+) sparks promote myogenic tone in retinal arterioles

BACKGROUND AND PURPOSE

Ca(2+) imaging reveals subcellular Ca(2+) sparks and global Ca(2+) waves/oscillations in vascular smooth muscle. It is well established that Ca(2+) sparks can relax arteries, but we have previously reported that sparks can summate to generate Ca(2+) waves/oscillations in unpressurized retinal arterioles, leading to constriction. We have extended these studies to test the functional significance of Ca(2+) sparks in the generation of myogenic tone in pressurized arterioles.

EXPERIMENTAL APPROACH

Isolated retinal arterioles (25-40 μm external diameter) were pressurized to 70 mmHg, leading to active constriction. Ca(2+) signals were imaged from arteriolar smooth muscle in the same vessels using Fluo4 and confocal laser microscopy.

KEY RESULTS

Tone development was associated with an increased frequency of Ca(2+) sparks and oscillations. Vasomotion was observed in 40% of arterioles and was associated with synchronization of Ca(2+) oscillations, quantifiable as an increased cross-correlation coefficient. Inhibition of Ca(2+) sparks with ryanodine, tetracaine, cyclopiazonic acid or nimodipine, or following removal of extracellular Ca(2+) , resulted in arteriolar relaxation. Cyclopiazonic acid-induced dilatation was associated with decreased Ca(2+) sparks and oscillations but with a sustained rise in the mean global cytoplasmic [Ca(2+) ] ([Ca(2+) ]c ), as measured using Fura2 and microfluorimetry.

CONCLUSIONS AND IMPLICATIONS

This study provides direct evidence that Ca(2+) sparks can play an excitatory role in pressurized arterioles, promoting myogenic tone. This contrasts with the generally accepted model in which sparks promote relaxation of vascular smooth muscle. Changes in vessel tone in the presence of cyclopiazonic acid correlated more closely with changes in spark and oscillation frequency than global [Ca(2+) ]c , underlining the importance of frequency-modulated signalling in vascular smooth muscle.

Vascular tone and Ca(2+) signaling in murine cremaster muscle arterioles in vivo

OBJECTIVES

We sought to determine some of the molecular requirements for basal state "tone" of skeletal muscle arterioles in vivo, and whether asynchronous Ca(2+) waves are involved or not.

METHODS

Cremaster muscles of anesthetized exMLCK and smGCaMP2 biosensor mice were exteriorized, and the fluorescent arterioles were visualized with wide-field, confocal or multiphoton microscopy to observe Ca(2+) signaling and arteriolar diameter.

RESULTS

Basal state tone of the arterioles was ~50%. Local block of Ang-II receptors (AT1 ) or α1 -adrenoceptors (α1 -AR) had no effect on diameter, nor did complete block of sympathetic nerve activity (SNA). Inhibition of phospholipase C caused dilation nearly to the Ca(2+) -free (passive) diameter, as did exposure to nifedipine or 2-APB. Arterioles were also dilated when treated with SKF96365. High-resolution imaging of exMLCK fluorescence (ratio) or GCaMP2 fluorescence in smooth muscle cells failed to reveal Ca(2+) waves (although Ca(2+) waves/transients were readily detected by both biosensors in small arteries, ex vivo).

CONCLUSIONS

Arterioles of cremaster muscle have vascular tone of ~ 50%, which is not due to α1 -AR, AT1 R, or SNA. PLC activity, L-type Ca(2+) channels, 2-APB- and SKF96365-sensitive channels are required. Propagating Ca(2+) waves are not present. A key role for PLC and InsP3 R in vascular tone in vivo, other than producing Ca(2+) waves, is suggested.

Enhanced contractility in pregnancy is associated with augmented TRPC3, L-type, and T-type voltage-dependent calcium channel function in rat uterine radial artery

In pregnancy, α-adrenoceptor-mediated vasoconstriction is augmented in uterine radial arteries and is accompanied by underlying changes in smooth muscle (SM) Ca2+ activity. This study aims to determine the Ca2+ entry channels associated with altered vasoconstriction in pregnancy, with the hypothesis that augmented vasoconstriction involves transient receptor potential canonical type-3 (TRPC3) and L- and T-type voltage-dependent Ca2+ channels. Immunohistochemistry showed TRPC3, L-type Cav1.2 (as the α1C subunit), T-type Cav3.1 (α1G), and Cav3.2 (α1H) localization to the uterine radial artery SM. Fluorescence intensity of TRPC3, Cav1.2, and Cav3.2 was increased, and Cav3.1 decreased in radial artery SM from pregnant rats. Western blot analysis confirmed increased TRPC3 protein expression in the radial artery from pregnant rats. Pressure myography incorporating pharmacological intervention to examine the role of these channels in uterine radial arteries showed an attenuation of phenylephrine (PE)-induced constriction with Pyr3 {1-[4-[(2,3,3-trichloro-1-oxo-2-propen-1-yl)amino]phenyl]-5-(trifluoromethyl)-1 H-pyrazole-4-carboxylic acid}-mediated TRPC3 inhibition or with nifedipine-mediated L-type channel block alone in vessels from pregnant rats; both effects of which were diminished in radial arteries from nonpregnant rats. Combined TRPC3 and L-type inhibition attenuated PE-induced constriction in radial arteries, and the residual vasoconstriction was reduced and abolished with T-type channel block with NNC 55-0396 in arteries from nonpregnant and pregnant rats, respectively. With SM Ca2+ stores depleted and in the presence of PE, nifedipine, and NNC 55-0396, blockade of TRPC3 reversed PE-induced constriction. These data suggest that TRPC3 channels act synergistically with L- and T-type channels to modulate radial artery vasoconstriction, with the mechanism being augmented in pregnancy.

Recruitment of dynamic endothelial Ca2+ signals by the TRPA1 channel activator AITC in rat cerebral arteries

OBJECTIVE

Stimulation of endothelial TRP channels, specifically TRPA1, promotes vasodilation of cerebral arteries through activation of Ca2+ -dependent effectors along the myoendothelial interface. However, presumed TRPA1-triggered endothelial Ca2+ signals have not been described. We investigated whether TRPA1 activation induces specific spatial and temporal changes in Ca2+ signals along the intima that correlates with incremental vasodilation.

METHODS

Confocal imaging, immunofluorescence staining, and custom image analysis were employed.

RESULTS

We found that endothelial cells of rat cerebral arteries exhibit widespread basal Ca2+ dynamics (44 ± 6 events/minute from 26 ± 3 distinct sites in a 3.6 × 10(4) μm2 field). The TRPA1 activator AITC increased Ca2+ signals in a concentration-dependent manner, soliciting new events at distinct sites. Origination of these new events corresponded spatially with TRPA1 densities in IEL holes, and the events were prevented by the TRPA1 inhibitor HC-030031. Concentration-dependent expansion of Ca2+ events in response to AITC correlated precisely with dilation of pressurized cerebral arteries (p = 0.93 by F-test). Correspondingly, AITC caused rapid endothelium-dependent suppression of asynchronous Ca2+ waves in subintimal smooth muscle.

CONCLUSIONS

Our findings indicate that factors that stimulate TRPA1 channels expand Ca2+ signal-effector coupling at discrete sites along the endothelium to evoke graded cerebral artery vasodilation.

Ryanodine receptors, calcium signaling, and regulation of vascular tone in the cerebral parenchymal microcirculation

The cerebral blood supply is delivered by a surface network of pial arteries and arterioles from which arise (parenchymal) arterioles that penetrate into the cortex and terminate in a rich capillary bed. The critical regulation of CBF, locally and globally, requires precise vasomotor regulation of the intracerebral microvasculature. This vascular region is anatomically unique as illustrated by the presence of astrocytic processes that envelope almost the entire basolateral surface of PAs. There are, moreover, notable functional differences between pial arteries and PAs. For example, in pial VSMCs, local calcium release events ("calcium sparks") through ryanodine receptor (RyR) channels in SR membrane activate large conductance, calcium-sensitive potassium channels to modulate vascular diameter. In contrast, VSMCs in PAs express functional RyR and BK channels, but under physiological conditions, these channels do not oppose pressure-induced vasoconstriction. Here, we summarize the roles of ryanodine receptors in the parenchymal microvasculature under physiologic and pathologic conditions, and discuss their importance in the control of CBF.

Spreading vasodilatation in the murine microcirculation: attenuation by oxidative stress-induced change in electromechanical coupling

Regulation of blood flow in microcirculatory networks depends on spread of local vasodilatation to encompass upstream arteries; a process mediated by endothelial conduction of hyperpolarization. Given that endothelial coupling is reduced in hypertension, we used hypertensive Cx40ko mice, in which endothelial coupling is attenuated, to investigate the contribution of the renin-angiotensin system and reduced endothelial cell coupling to conducted vasodilatation of cremaster arterioles in vivo. When the endothelium was disrupted by light dye treatment, conducted vasodilatation, following ionophoresis of acetylcholine, was abolished beyond the site of endothelial damage. In the absence of Cx40, sparse immunohistochemical staining was found for Cx37 in the endothelium, and endothelial, myoendothelial and smooth muscle gap junctions were identified by electron microscopy. Hyperpolarization decayed more rapidly in arterioles from Cx40ko than wild-type mice. This was accompanied by a shift in the threshold potential defining the linear relationship between voltage and diameter, increased T-type calcium channel expression and increased contribution of T-type (3 μmol l(-1) NNC 55-0396), relative to L-type (1 μmol l(-1) nifedipine), channels to vascular tone. The change in electromechanical coupling was reversed by inhibition of the renin-angiotensin system (candesartan, 1.0 mg kg(-1) day(-1) for 2 weeks) or by acute treatment with the superoxide scavenger tempol (1 mmol l(-1)). Candesartan and tempol treatments also significantly improved conducted vasodilatation. We conclude that conducted vasodilatation in Cx40ko mice requires the endothelium, and attenuation results from both a reduction in endothelial coupling and an angiotensin II-induced increase in oxidative stress. We suggest that during cardiovascular disease, the ability of microvascular networks to maintain tissue integrity may be compromised due to oxidative stress-induced changes in electromechanical coupling.

New insights into the contribution of arterial NCX to the regulation of myogenic tone and blood pressure

Plasma membrane protein Na(+)/Ca(2+) exchanger (NCX) in vascular smooth muscle (VSM) cells plays an important role in intracellular Ca(2+) homeostasis, Ca(2+) signaling, and arterial contractility. Recent evidence in intact animals reveals that VSM NCX type 1 (NCX1) is importantly involved in the control of arterial blood pressure (BP) in the normal state and in hypertension. Increased expression of vascular NCX1 has been implicated in human primary pulmonary hypertension and several salt-dependent hypertensive animal models. Our aim is to determine the molecular and physiological mechanisms by which vascular NCX influences vasoconstriction and BP normally and in salt-dependent hypertension. Here, we describe the relative contribution of VSM NCX1 to Ca(2+) signaling and arterial contraction, including recent data from transgenic mice (NCX1(smTg/Tg), overexpressors; NCX1(sm-/-), knockouts) that has begun to elucidate the specific contributions of NCX to BP regulation. Arterial contraction and BP correlate with the level of NCX1 expression in smooth muscle: NCX1(sm-/-) mice have decreased arterial myogenic tone (MT), vasoconstriction, and low BP. NCX1(smTg/Tg) mice have high BP and are more sensitive to salt; their arteries exhibit upregulated transient receptor potential canonical channel 6 (TRPC6) protein, increased MT, and vasoconstriction. These observations suggest that NCX is a key component of certain distinct signaling pathways that activate VSM contraction in response to stretch (i.e., myogenic response) and to activation of certain G-protein-coupled receptors. Arterial NCX expression and mechanisms that control the local (sub-plasma membrane) Na(+) gradient, including cation-selective receptor-operated channels containing TRPC6, regulate arterial Ca(2+) and constriction, and thus BP.

The role of actin filament dynamics in the myogenic response of cerebral resistance arteries

The myogenic response has a critical role in regulation of blood flow to the brain. Increased intraluminal pressure elicits vasoconstriction, whereas decreased intraluminal pressure induces vasodilatation, thereby maintaining flow constant over the normal physiologic blood pressure range. Improved understanding of the molecular mechanisms underlying the myogenic response is crucial to identify deficiencies with pathologic consequences, such as cerebral vasospasm, hypertension, and stroke, and to identify potential therapeutic targets. Three mechanisms have been suggested to be involved in the myogenic response: (1) membrane depolarization, which induces Ca(2+) entry, activation of myosin light chain kinase, phosphorylation of the myosin regulatory light chains (LC(20)), increased actomyosin MgATPase activity, cross-bridge cycling, and vasoconstriction; (2) activation of the RhoA/Rho-associated kinase (ROCK) pathway, leading to inhibition of myosin light chain phosphatase by phosphorylation of MYPT1, the myosin targeting regulatory subunit of the phosphatase, and increased LC(20) phosphorylation; and (3) activation of the ROCK and protein kinase C pathways, leading to actin polymerization and the formation of enhanced connections between the actin cytoskeleton, plasma membrane, and extracellular matrix to augment force transmission. This review describes these three mechanisms, emphasizing recent developments regarding the importance of dynamic actin polymerization in the myogenic response of the cerebral vasculature.

Dynamic Ca(2+) signal modalities in the vascular endothelium

The endothelium is vital to normal vasoregulation. Although acute vasodilation associated with broad endothelial Ca(2+) elevation is well known, the control and targeting of Ca(2+) -dependent signals in the endothelium are poorly understood. Recent studies have revealed localized IP(3) -motivated Ca(2+) events occurring basally along the intima that may provide the fundamental basis for various endothelial influences. Here, we provide an overview of dynamic endothelial Ca(2+) signals and discuss the potential role of these signals in constant endothelial control of arterial tone and the titration of functional responses in vivo. In particular, we focus on the functional architecture contributing to the properties and ultimate impact of these signals, and explore new avenues in evaluating their prevalence and specific modalities in intact tissue. Finally, we discuss spatial and temporal effector recruitment through modification of these inherent signals. It is suggested that endothelial Ca(2+) signaling is a continuum in which the specific framework of store-release components and cellular targets along the endothelium allows for differential modes of Ca(2+) signal expansion and distinctive profiles of effector recruitment. The precise composition and distribution of these inherent components may underlie dynamic endothelial control and specialized functions of different vascular beds.

Identification of L- and T-type Ca2+ channels in rat cerebral arteries: role in myogenic tone development

L-type Ca(2+) channels are broadly expressed in arterial smooth muscle cells, and their voltage-dependent properties are important in tone development. Recent studies have noted that these Ca(2+) channels are not singularly expressed in vascular tissue and that other subtypes are likely present. In this study, we ascertained which voltage-gated Ca(2+) channels are expressed in rat cerebral arterial smooth muscle and determined their contribution to the myogenic response. mRNA analysis revealed that the α(1)-subunit of L-type (Ca(v)1.2) and T-type (Ca(v)3.1 and Ca(v)3.2) Ca(2+) channels are present in isolated smooth muscle cells. Western blot analysis subsequently confirmed protein expression in whole arteries. With the use of patch clamp electrophysiology, nifedipine-sensitive and -insensitive Ba(2+) currents were isolated and each were shown to retain electrical characteristics consistent with L- and T-type Ca(2+) channels. The nifedipine-insensitive Ba(2+) current was blocked by mibefradil, kurtoxin, and efonidpine, T-type Ca(2+) channel inhibitors. Pressure myography revealed that L-type Ca(2+) channel inhibition reduced tone at 20 and 80 mmHg, with the greatest effect at high pressure when the vessel is depolarized. In comparison, the effect of T-type Ca(2+) channel blockade on myogenic tone was more limited, with their greatest effect at low pressure where vessels are hyperpolarized. Blood flow modeling revealed that the vasomotor responses induced by T-type Ca(2+) blockade could alter arterial flow by ∼20-50%. Overall, our findings indicate that L- and T-type Ca(2+) channels are expressed in cerebral arterial smooth muscle and can be electrically isolated from one another. Both conductances contribute to myogenic tone, although their overall contribution is unequal.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Function and expression of ryanodine receptors and inositol 1,4,5-trisphosphate receptors in smooth muscle cells of murine feed arteries and arterioles

We tested the hypothesis that vasomotor control is differentially regulated between feed arteries and downstream arterioles from the cremaster muscle of C57BL/6 mice. In isolated pressurized arteries, confocal Ca(2+) imaging of smooth muscle cells (SMCs) revealed Ca(2+) sparks and Ca(2+) waves. Ryanodine receptor (RyR) antagonists (ryanodine and tetracaine) inhibited both sparks and waves but increased global Ca(2+) and myogenic tone. In arterioles, SMCs exhibited only Ca(2+) waves that were insensitive to ryanodine or tetracaine. Pharmacological interventions indicated that RyRs are functionally coupled to large-conductance, Ca(2+)-activated K(+) channels (BK(Ca)) in SMCs of arteries, whereas BK(Ca) appear functionally coupled to voltage-gated Ca2+ channels in SMCs of arterioles. Inositol 1,4,5-trisphosphate receptor (IP3R) antagonists (xestospongin D or 2-aminoethoxydiphenyl borate) or a phospholipase C inhibitor (U73122) attenuated Ca(2+) waves, global Ca(2+) and myogenic tone in arteries and arterioles but had no effect on arterial sparks. Real-time PCR of isolated SMCs revealed RyR2 as the most abundant isoform transcript; arteries expressed twice the RyR2 but only 65% the RyR3 of arterioles and neither vessel expressed RyR1. Immunofluorescent localisation of RyR protein indicated bright, clustered staining of arterial SMCs in contrast to diffuse staining in arteriolar SMCs. Expression of IP(3)R transcripts and protein immunofluorescence were similar in SMCs of both vessels with IP(3)R1>>IP(3)R2>IP(3)R3. Despite similar expression of IP(3)Rs and dependence of Ca(2+) waves on IP(3)Rs, these data illustrate pronounced regional heterogeneity in function and expression of RyRs between SMCs of the same vascular resistance network. We conclude that vasomotor control is differentially regulated in feed arteries vs. downstream arterioles.

Calcium signaling in smooth muscle

Changes in intracellular Ca(2+) are central to the function of smooth muscle, which lines the walls of all hollow organs. These changes take a variety of forms, from sustained, cell-wide increases to temporally varying, localized changes. The nature of the Ca(2+) signal is a reflection of the source of Ca(2+) (extracellular or intracellular) and the molecular entity responsible for generating it. Depending on the specific channel involved and the detection technology employed, extracellular Ca(2+) entry may be detected optically as graded elevations in intracellular Ca(2+), junctional Ca(2+) transients, Ca(2+) flashes, or Ca(2+) sparklets, whereas release of Ca(2+) from intracellular stores may manifest as Ca(2+) sparks, Ca(2+) puffs, or Ca(2+) waves. These diverse Ca(2+) signals collectively regulate a variety of functions. Some functions, such as contractility, are unique to smooth muscle; others are common to other excitable cells (e.g., modulation of membrane potential) and nonexcitable cells (e.g., regulation of gene expression).

Regulation of blood flow in the microcirculation: role of conducted vasodilation

This review is concerned with understanding how vasodilation initiated from local sites in the tissue can spread to encompass multiple branches of the resistance vasculature. Within tissues, arteriolar networks control the distribution and magnitude of capillary perfusion. Vasodilation arising from the microcirculation can 'ascend' into feed arteries that control blood flow into arteriolar networks. Thus distal segments of the resistance network signal proximal segments to dilate and thereby increase total oxygen supply to parenchymal cells. August Krogh proposed that innervation of capillaries provided the mechanism for a spreading vasodilatory response. With greater understanding of the ultrastructural organization of resistance networks, an alternative explanation has emerged: Electrical signalling from cell to cell along the vessel wall through gap junctions. Hyperpolarization originates from ion channel activation at the site of stimulation with the endothelium serving as the predominant cellular pathway for signal conduction along the vessel wall. As hyperpolarization travels, it is transmitted into surrounding smooth muscle cells through myoendothelial coupling to promote relaxation. Conducted vasodilation (CVD) encompasses greater distances than can be explained by passive decay and understanding such behaviour is the focus of current research efforts. In the context of athletic performance, the ability of vasodilation to ascend into feed arteries is essential to achieving peak levels of muscle blood flow. CVD is tempered by sympathetic neuroeffector signalling when governing muscle blood flow at rest and during exercise. Impairment of conduction during ageing and in diseased states can limit physical work capacity by restricting muscle blood flow.

Heterogeneous function of ryanodine receptors, but not IP3 receptors, in hamster cremaster muscle feed arteries and arterioles

The roles played by ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP₃Rs) in vascular smooth muscle in the microcirculation remain unclear. Therefore, the function of both RyRs and IP₃Rs in Ca(²+) signals and myogenic tone in hamster cremaster muscle feed arteries and downstream arterioles were assessed using confocal imaging and pressure myography. Feed artery vascular smooth muscle displayed Ca(²+) sparks and Ca(²+) waves, which were inhibited by the RyR antagonists ryanodine (10 μM) or tetracaine (100 μM). Despite the inhibition of sparks and waves, ryanodine or tetracaine increased global intracellular Ca(²+) and constricted the arteries. The blockade of IP₃Rs with xestospongin D (5 μM) or 2-aminoethoxydiphenyl borate (100 μM) or the inhibition of phospholipase C using U-73122 (10 μM) also attenuated Ca(2+) waves without affecting Ca(²+) sparks. Importantly, the IP₃Rs and phospholipase C antagonists decreased global intracellular Ca(2+) and dilated the arteries. In contrast, cremaster arterioles displayed only Ca(²+) waves: Ca(²+) sparks were not observed, and neither ryanodine (10-50 μM) nor tetracaine (100 μM) affected either Ca(²+) signals or arteriolar tone despite the presence of functional RyRs as assessed by responses to the RyR agonist caffeine (10 mM). As in feed arteries, arteriolar Ca(²+) waves were attenuated by xestospongin D (5 μM), 2-aminoethoxydiphenyl borate (100 μM), and U-73122 (10 μM), accompanied by decreased global intracellular Ca(²+) and vasodilation. These findings highlight the contrasting roles played by RyRs and IP₃Rs in Ca(²+) signals and myogenic tone in feed arteries and demonstrate important differences in the function of RyRs between feed arteries and downstream arterioles.