The life cycle of a capillary: Mechanisms of angiogenesis and rarefaction in microvascular physiology and pathologies

Capillaries are the smallest blood vessels (<10 μm in diameter) in the body and their walls are lined by endothelial cells. These microvessels play a crucial role in nutrient and gas exchange between blood and tissues. Capillary endothelial cells also produce vasoactive molecules and initiate the electrical signals that underlie functional hyperemia and neurovascular coupling. Accordingly, capillary function and density are critical for all cell types to match blood flow to cellular activity. This begins with the process of angiogenesis, when new capillary blood vessels emerge from pre-existing vessels, and ends with rarefaction, the loss of these microvascular structures. This review explores the mechanisms behind these processes, emphasizing their roles in various microvascular diseases and their impact on surrounding cells in health and disease. We discuss recent work on the mechanisms controlling endothelial cell proliferation, migration, and tube formation that underlie angiogenesis under physiological and pathological conditions. The mechanisms underlying functional and anatomical rarefaction and the role of pericytes in this process are also discussed. Based on this work, a model is proposed in which the balance of angiogenic and rarefaction signaling pathways in a particular tissue match microvascular density to the metabolic demands of the surrounding cells. This negative feedback loop becomes disrupted during microvascular rarefaction: angiogenic mechanisms are blunted, reactive oxygen species accumulate, capillary function declines and eventually, capillaries disappear. This, we propose, forms the foundation of the reciprocal relationship between vascular density, blood flow, and metabolic needs and functionality of nearby cells.

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

Higher potassium intake is associated with a lower risk of chronic kidney disease: population-based prospective study

BACKGROUND

High-potassium intake is associated with a lower risk of cardiovascular disease. However, the association between potassium intake and the development of chronic kidney disease (CKD) remains unclear.

OBJECTIVE

The objective of this study was to investigate whether potassium intake is associated with outcomes of incident CKD.

METHODS

This is a population-based prospective observational cohort study from the UK Biobank cohort between 2006 and 2010. We included 317,162 participants without CKD from the UK Biobank cohort. The main predictor was spot urine potassium-to-creatinine ratio (KCR). The primary outcome was incident CKD, which was defined by the International Classification of Disease 10 codes or Operating Procedure Codes Supplement 4 codes.

RESULTS

At baseline, individuals with higher KCR had lower blood pressure, body mass index, and inflammation, and were less likely to have diabetes and hypertension. During a median follow-up of 11.9 y, primary outcome events occurred in 15,246 (4.8%) participants. In the cause-specific model, the adjusted hazard ratio (aHR) per 1-standard deviation increase in KCR for incident CKD was 0.90 [95% confidence interval (CI): 0.89, 0.92]. Compared with quartile 1 of KCR, the aHRs (95% CIs) for quartiles 2-4 were 0.98 (0.94, 1.02), 0.90 (0.86, 0.95), and 0.80 (0.76, 0.84), respectively. In sensitivity analysis with different definitions of CKD, the results were similar. In addition, further analysis with dietary potassium intake also showed a negatively graded association with the primary outcome.

CONCLUSIONS

Higher urinary potassium excretion and intake were associated with a lower risk of incident CKD.

Peristalsis prevents ureteral dilation

PURPOSE

The etiology of ureteral dilation in primary nonrefluxing, nonobstructing megaureters is still not well understood. Impaired ureteral peristalsis has been theorized as one of the contributing factors. However, ureteral peristalsis and its "normal" function is not well defined. In this study, using mathematical modeling techniques, we aim to better understand how ureteral peristalsis works. This is the first model to consider clinically observed, back-and-forth, cyclic wall longitudinal motion during peristalsis. We hypothesize that dysfunctional ureteral peristalsis, caused by insufficient peristaltic amplitudes (e.g., circular muscle dysfunction) and/or lack of ureteral wall longitudinal motion (e.g., longitudinal muscle dysfunction), promotes peristaltic reflux (i.e., retrograde flow of urine during an episode of peristalsis) and may result in urinary stasis, urine accumulation, and consequent dilation.

METHODS

Based on lubrication theory in fluid mechanics, we developed a two-dimensional (planar) model of ureteral peristalsis. In doing so, we treated ureteral peristalsis as an infinite train of sinusoidal waves. We then analyzed antegrade and retrograde flows in the ureter under different bladder-kidney differential pressure and peristalsis conditions.

RESULTS

There is a minimum peristaltic amplitude required to prevent peristaltic reflux. Ureteral wall longitudinal motion decreases this minimum required amplitude, increasing the nonrefluxing range of peristaltic amplitudes. As an example, for a normal bladder-kidney differential pressure of 5 cmH2 O, ureteral wall longitudinal motion increases nonrefluxing range of peristaltic amplitude by 65%. Additionally, ureteral wall longitudinal motion decreases refluxing volumetric flow rates. For a similar normal bladder pressure example of 5 cmH2 O, refluxing volumetric flow rate decreases by a factor of 18. Finally, elevated bladder pressure, not only increases the required peristaltic amplitude for reflux prevention but it increases maximum refluxing volumetric flow rates. For the case without wall longitudinal motion, as bladder-kidney differential pressure increases from 5 to 40 cmH2 O, minimum required peristaltic amplitude to prevent reflux increases by 40% while the maximum refluxing volumetric flow rate increases by approximately 100%.

CONCLUSION

The results presented in this study show how abnormal ureteral peristalsis, caused by the absence of wall longitudinal motion and/or lack of sufficient peristaltic amplitudes, facilitates peristaltic reflux and retrograde flow. We theorize that this retrograde flow can lead to urinary stasis and urine accumulation in the ureters, resulting in ureteral dilation seen on imaging studies and elevated infection risk. Our results also show how chronically elevated bladder pressures are more susceptible to such refluxing conditions that could lead to ureteral dilation.

Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca²⁺-activated K⁺ (K_Ca) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. K_Ca channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by K_Ca channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of K_Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.

Intradialytic hypertension: epidemiology and pathophysiology of a silent killer

The term intradialytic hypertension (IDH) describes a paradoxical rise in blood pressure (BP) during or immediately after the hemodialysis session. Although it was formerly considered a phenomenon without clinical implications, current evidence suggests that IDH may affect up to 15% of hemodialysis patients and exhibit independent associations with future cardiovascular events and all-cause mortality. Furthermore, during the last decade, several studies have tried to elucidate the complex pathophysiological mechanisms responsible for this phenomenon. Volume overload, intradialytic sodium gain, overactivity of the sympathetic-nervous-system and renin-angiotensin-aldosterone system, endothelial dysfunction and dialysis-related electrolyte disturbances have been proposed to be involved in the pathogenesis of the BP increase during hemodialysis. This review attempts to summarize existing evidence on the epidemiology, pathophysiology and clinical characteristics of the distinct phenomenon of IDH.

Functionally linked potassium channel activity in cerebral endothelial and smooth muscle cells is compromised in Alzheimer's disease

Patients with Alzheimer’s disease show hypoperfusion of the brain and this may contribute to disease progression. To elucidate underlying mechanisms, we studied pial arteries from 18-mo-old mice with Alzheimer’s disease due to overexpression of amyloid precursor protein. We found enhanced pressure-induced constriction of arteries because of reduction in ryanodine receptor-mediated, local calcium-release events (“Ca²⁺ sparks”) in arterial smooth muscle cells and a consequent decrease in the activity of large-conductance Ca²⁺-activated K⁺ (BK) channels. This phenotype was partially recapitulated by application of an amyloid-β peptide to healthy arteries. Our results will direct further research to restore cerebrovascular function, which is damaged in Alzheimer’s disease, leading to potentially new treatment options.

The brain microcirculation is increasingly viewed as a potential target for disease-modifying drugs in the treatment of Alzheimer’s disease patients, reflecting a growing appreciation of evidence that cerebral blood flow is compromised in such patients. However, the pathogenic mechanisms in brain resistance arteries underlying blood flow defects have not yet been elucidated. Here we probed the roles of principal vasodilatory pathways in cerebral arteries using the APP23 mouse model of Alzheimer’s disease, in which amyloid precursor protein is increased approximately sevenfold, leading to neuritic plaques and cerebrovascular accumulation of amyloid-β similar to those in patients with Alzheimer’s disease. Pial arteries from APP23 mice (18 mo old) exhibited enhanced pressure-induced (myogenic) constriction because of a profound reduction in ryanodine receptor-mediated, local calcium-release events (“Ca²⁺ sparks”) in arterial smooth muscle cells and a consequent decrease in the activity of large-conductance Ca²⁺-activated K⁺ (BK) channels. The ability of the endothelial cell inward rectifier K⁺ (Kir2.1) channel to cause dilation was also compromised. Acute application of amyloid-β 1-40 peptide to cerebral arteries from wild-type mice partially recapitulated the BK dysfunction seen in APP23 mice but had no effect on Kir2.1 function. If mirrored in human Alzheimer’s disease, these tandem defects in K⁺ channel-mediated vasodilation could account for the clinical cerebrovascular presentation seen in patients: reduced blood flow and crippled functional hyperemia. These data direct future research toward approaches that reverse this dual vascular channel dysfunction, with the ultimate aim of restoring healthy cerebral blood flow and improving clinical outcomes.

Large‐Conductance Calcium‐Activated Potassium Channel Opener, NS1619, Protects Against Mesenteric Artery Remodeling Induced by Agonistic Autoantibodies Against the Angiotensin II Type 1 Receptor

Background

Agonistic autoantibodies against the angiotensin II type 1 receptor (AT1‐AAs) extensively exist in patients with hypertensive diseases and have been demonstrated to play crucial roles in the pathophysiological process of vascular remodeling. However, the treatment options are limited. The large‐conductance calcium‐activated potassium (BK) channel is a critical regulator and potential therapeutic target of vascular tone and architecture. We have previously observed that AT1‐AAs have an inhibitory effect on BK channels. However, whether BK channel dysfunction is involved in AT1‐AAs‐induced vascular remodeling and the therapeutic effect of BK channel opener is unclear.

Methods and Results

In our study, mesenteric arteries from AT1‐AAs‐positive rats exhibited increased wall thickness, narrowing of the arteriolar lumen, and increased collagen accumulation. Patch clamp test results showed that the voltage sensitivity of BK channel declined in mesenteric arteriolar smooth muscle cells from AT1‐AAs‐positive rats. Experiments with freshly isolated mesenteric arteriolar smooth muscle cells showed that AT1‐AAs reduced the opening probability, open levels, open dwell time, and calcium sensitivity of BK channel. Experiments with HEK293T cells transfected with GFP‐ZERO‐BK α‐subunit plasmids suggested a BK channel α‐subunit‐dependent mechanism. BK channel α‐subunit deficient, namely KCNMA1^−/− rats showed a phenotype of mesenteric artery remodeling. The administration of NS1619, a specific BK channel opener targeting the α‐subunit, reversed the phenotypic transition and migration induced by AT1‐AAs in cultured mesenteric arteriolar smooth muscle cells. Finally, perfusion of NS1619 significantly relieved the pathological effects induced by AT1‐AAs in vivo.

Conclusions

In summary, we provide compelling evidence that BK channel α‐subunit dysfunction mediates AT1‐AAs‐induced mesenteric artery remodeling. Preservation of BK channel activity may serve as a potential strategy for the treatment of AT1‐AAs‐induced maladaptive resistance artery remodeling.

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.

Cardiovascular co-morbidities, inflammation and cerebral small vessel disease

Cerebral small vessel disease (cSVD) is the most common cause of vascular cognitive impairment and affects all levels of the brain's vasculature. Features include diverse structural and functional changes affecting small arteries and capillaries that lead to a decline in cerebral perfusion. Due to an aging population, incidence of cerebral small vessel disease (cSVD) is continually rising. Despite its prevalence and its ability to cause multiple debilitating illnesses, such as stroke and dementia, there are currently no therapeutic strategies for the treatment of cSVD. In the healthy brain, interactions between neuronal, vascular and inflammatory cells are required for normal functioning. When these interactions are disturbed, chronic pathological inflammation can ensue. The interplay between cSVD and inflammation has attracted much recent interest and this review discusses chronic cardiovascular diseases, particularly hypertension, and explores how the associated inflammation may impact on the structure and function of the small arteries of the brain in cSVD. Molecular approaches in animal studies are linked to clinical outcomes in patients and novel hypotheses regarding inflammation and cSVD are proposed that will hopefully stimulate further discussion and study in this important area.

Remodeling of Arterial Tone Regulation in Postnatal Development: Focus on Smooth Muscle Cell Potassium Channels

Maturation of the cardiovascular system is associated with crucial structural and functional remodeling. Thickening of the arterial wall, maturation of the sympathetic innervation, and switching of the mechanisms of arterial contraction from calcium-independent to calcium-dependent occur during postnatal development. All these processes promote an almost doubling of blood pressure from the moment of birth to reaching adulthood. This review focuses on the developmental alterations of potassium channels functioning as key smooth muscle membrane potential determinants and, consequently, vascular tone regulators. We present evidence that the pattern of potassium channel contribution to vascular control changes from K_ir2, K_v1, K_v7 and TASK-1 channels to BK_Ca channels with maturation. The differences in the contribution of potassium channels to vasomotor tone at different stages of postnatal life should be considered in treatment strategies of cardiovascular diseases associated with potassium channel malfunction.

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.

Antenatal Dexamethasone Exposure Impairs the High-Conductance Ca2+-Activated K+ Channels via Epigenetic Alteration at Gene Promoter in Male Offspring

OBJECTIVE

Antenatal exposure to glucocorticoids increases cardiovascular risks related to vascular dysfunctions in offspring, although underlying mechanisms are still unknown. As an important vascular mediator, high-conductance Ca²⁺-activated K⁺ channels (BK) plays an essential role in determining vascular tone. Long-term effects of antenatal glucocorticoids on BK in offspring are largely unknown. This study examined the effects and mechanisms of antenatal exposure to clinically relevant doses of glucocorticoids on vascular BK in offspring. Approach and Results: Pregnant Sprague-Dawley rats received synthetic glucocorticoids dexamethasone or vehicle during the last week of pregnancy. Vascular functions, cellular electrophysiology, target gene expression, and promoter methylation were examined in mesenteric arteries of male offspring (gestational day 21 [fetus] and postnatal day 120 [adult offspring]). Antenatal dexamethasone exposure impaired BK activators-mediated relaxation and reduced whole-cell BK currents in mesenteric arteries. Antenatal dexamethasone exposure did not alter Ca²⁺/voltage-sensitivity of BK but downregulated the expressions of BK α and β1 subunits in both fetal and adult mesenteric arteries. In addition, increased promoter methylations within BKα and BKβ1 were compatible with reduced expressions of the 2 genes.

CONCLUSIONS

Our findings showed a profound and long-term impact of antenatal dexamethasone exposure on vascular BK via an altered epigenetic pattern from fetal stage to adulthood, advancing understanding of prolonged adverse effects and mechanisms of antenatal glucocorticoids exposure on vascular health in offspring.

Inhibition of big-conductance Ca2+-activated K+ channels in cerebral artery (vascular) smooth muscle cells is a major novel mechanism for tacrolimus-induced hypertension

Tacrolimus (TAC, also called FK506), a common immunosuppressive drug used to prevent allograft rejection in transplant patients, is well known to alter the functions of blood vessels. In this study, we sought to determine whether chronic treatment of TAC could inhibit the activity of big-conductance Ca²⁺-activated K⁺ (BK) channels in vascular smooth muscle cells (SMCs), leading to hypertension. Our data reveal that the activity of BK channels was inhibited in cerebral artery SMCs (CASMCs) from mice after intraperitoneal injection of TAC once a day for 4 weeks. The voltage sensitivity, Ca²⁺ sensitivity, and open time of single BK channels were all decreased. In support, BK channel β1-, but not α-subunit protein expression was significantly decreased in cerebral arteries. In TAC-treated mice, application of norepinephrine induced stronger vasoconstriction in both cerebral and mesenteric arteries as well as a larger [Ca²⁺]_i in CASMCs. Chronic treatment of TAC, similar to BK channel β1-subunit knockout (KO), resulted in hypertension in mice, but did not cause a further increase in blood pressure in BK channel β1-subunit KO mice. Moreover, BK channel activity in CASMCs was negatively correlated with blood pressure. Our findings provide novel evidence that TAC inhibits BK channels by reducing the channel β1-subunit expression and functions in vascular SMCs, leading to enhanced vasoconstriction and hypertension.

Clinical Importance of the Human Umbilical Artery Potassium Channels

Potassium (K⁺) channels are usually predominant in the membranes of vascular smooth muscle cells (SMCs). These channels play an important role in regulating the membrane potential and vessel contractility-a role that depends on the vascular bed. Thus, the activity of K⁺ channels represents one of the main mechanisms regulating the vascular tone in physiological and pathophysiological conditions. Briefly, the activation of K⁺ channels in SMC leads to hyperpolarization and vasorelaxation, while its inhibition induces depolarization and consequent vascular contraction. Currently, there are four different types of K⁺ channels described in SMCs: voltage-dependent K⁺ (K_V) channels, calcium-activated K⁺ (K_Ca) channels, inward rectifier K⁺ (Kir) channels, and 2-pore domain K⁺ (K_2P) channels. Due to the fundamental role of K⁺ channels in excitable cells, these channels are promising therapeutic targets in clinical practice. Therefore, this review discusses the basic properties of the various types of K⁺ channels, including structure, cellular mechanisms that regulate their activity, and new advances in the development of activators and blockers of these channels. The vascular functions of these channels will be discussed with a focus on vascular SMCs of the human umbilical artery. Then, the clinical importance of K⁺ channels in the treatment and prevention of cardiovascular diseases during pregnancy, such as gestational hypertension and preeclampsia, will be explored.

TRPML1 channels initiate Ca2+ sparks in vascular smooth muscle cells

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel localized to the membranes of endosomes and lysosomes and is not present or functional on the plasma membrane. Ca²⁺ released from endosomes and lysosomes into the cytosol through TRPML1 channels is vital for trafficking, acidification, and other basic functions of these organelles. Here, we investigated the function of TRPML1 channels in fully differentiated contractile vascular smooth muscle cells (SMCs). In live-cell confocal imaging studies, we found that most endosomes and lysosomes in freshly isolated SMCs from cerebral arteries were essentially immobile. Using nanoscale super-resolution microscopy, we found that TRPML1 channels present in late endosomes and lysosomes formed stable complexes with type 2 ryanodine receptors (RyR2) on the sarcoplasmic reticulum (SR). Spontaneous Ca²⁺ signals resulting from the release of SR Ca²⁺ through RyR2s ("Ca²⁺ sparks") and corresponding Ca²⁺-activated K⁺ channel activity are critically important for balancing vasoconstriction. We found that these signals were essentially absent in SMCs from TRPML1-knockout (Mcoln1^-/- ) mice. Using ex vivo pressure myography, we found that loss of this critical signaling cascade exaggerated the vasoconstrictor responses of cerebral and mesenteric resistance arteries. In vivo radiotelemetry studies showed that Mcoln1^-/- mice were spontaneously hypertensive. We conclude that TRPML1 is crucial for the initiation of Ca²⁺ sparks in SMCs and the regulation of vascular contractility and blood pressure.

Disruption of Pressure-Induced Ca2+ Spark Vasoregulation of Resistance Arteries, Rather Than Endothelial Dysfunction, Underlies Obesity-Related Hypertension

Supplemental Digital Content is available in the text.

Obesity-related hypertension is one of the world’s leading causes of death and yet little is understood as to how it develops. As a result, effective targeted therapies are lacking and pharmacological treatment is unfocused. To investigate underlying microvascular mechanisms, we studied small artery dysfunction in a high fat–fed mouse model of obesity. Pressure-induced constriction and responses to endothelial and vascular smooth muscle agonists were studied using myography; the corresponding intracellular Ca²⁺ signaling pathways were examined using confocal microscopy. Principally, we observed that the enhanced basal tone of mesenteric resistance arteries was due to failure of intraluminal pressure-induced Ca²⁺ spark activation of the large conductance Ca²⁺ activated K⁺ potassium channel (BK) within vascular smooth muscle cells. Specifically, the uncoupling site of this mechanotransduction pathway was at the sarcoplasmic reticulum, distal to intraluminal pressure-induced oxidation of Protein Kinase G. In contrast, the vasodilatory function of the endothelium and the underlying endothelial IP-3 and TRPV4 (vanilloid 4 transient receptor potential ion channel) Ca²⁺ signaling pathways were not affected by the high-fat diet or the elevated blood pressure. There were no structural alterations of the arterial wall. Our work emphasizes the importance of the intricate cellular pathway by which intraluminal pressure maintains Ca²⁺ spark vasoregulation in the origin of obesity-related hypertension and suggests previously unsuspected avenues for pharmacological intervention.

Pregnancy Increases Ca2+ Sparks/Spontaneous Transient Outward Currents and Reduces Uterine Arterial Myogenic Tone

Spontaneous transient outward currents (STOCs) at physiological membrane potentials of vascular smooth muscle cells fundamentally regulate vascular myogenic tone and blood flow in an organ. We hypothesize that heightened STOCs play a key role in uterine vascular adaptation to pregnancy. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Ca²⁺ sparks were measured by confocal microscopy, and STOCs were determined by electrophysiological recording in smooth muscle cells. Percentage of Ca²⁺ spark firing myocytes increased dramatically at the resting condition in uterine arterial smooth muscle of pregnant animals, as compared with nonpregnant animals. Pregnancy upregulated the expression of RyRs (ryanodine receptors) and significantly boosted Ca²⁺ spark frequency. Ex vivo treatment of uterine arteries of nonpregnant sheep with estrogen and progesterone imitated pregnancy-induced RyR upregulation. STOCs occurred at much more negative membrane potentials in uterine arterial myocytes of pregnant animals. STOCs in uterine arterial myocytes were diminished by inhibiting large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels and RyRs, thus functionally linking Ca²⁺ sparks and BK_Ca channel activity to STOCs. Pregnancy and steroid hormone treatment significantly increased STOCs frequency and amplitude in uterine arteries. Of importance, inhibition of STOCs with RyR inhibitor ryanodine eliminated pregnancy- and steroid hormone-induced attenuation of uterine arterial myogenic tone. Thus, the present study demonstrates a novel role of Ca²⁺ sparks and STOCs in the regulation of uterine vascular tone and provides new insights into the mechanisms underlying uterine vascular adaptation to pregnancy.

Impaired Trafficking of β1 Subunits Inhibits BK Channels in Cerebral Arteries of Hypertensive Rats

Hypertension is a risk factor for cerebrovascular diseases, including stroke and dementia. During hypertension, arteries become constricted and are less responsive to vasodilators, including nitric oxide (NO). The regulation of arterial contractility by smooth muscle cell (myocyte) large-conductance calcium (Ca²⁺)-activated potassium (BK) channels is altered during hypertension, although mechanisms involved are unclear. We tested the hypothesis that dysfunctional trafficking of pore-forming BK channel (BKα) and auxiliary β1 subunits contributes to changes in cerebral artery contractility of stroke-prone spontaneously hypertensive rats (SP-SHRs). Our data indicate that the amounts of total and surface BKα and β1 proteins are similar in unstimulated arteries of age-matched SP-SHRs and normotensive Wistar-Kyoto rats. In contrast, stimulated surface-trafficking of β1 subunits by NO or membrane depolarization is inhibited in SP-SHR myocytes. PKCα (protein kinase C α) and PKCβII total protein and activity were both higher in SP-SHR than in Wistar-Kyoto rat arteries. NO or depolarization robustly activated Rab11, a small trafficking GTPase, in Wistar-Kyoto rat arteries but weakly activated Rab11 in SP-SHRs. Bisindolylmaleimide, a PKC inhibitor, and overexpression of a PKC phosphorylation-deficient Rab11A mutant (Rab11A S177A) restored stimulated β1 subunit surface-trafficking in SP-SHR myocytes. BK channel activation by NO was inhibited in SP-SHR myocytes and restored by Rab11A S177A expression. Vasodilation to NO and lithocholate, a BKα/β1 channel activator, was inhibited in pressurized SP-SHR arteries and reestablished by bisindolylmaleimide. In summary, data indicate that spontaneously active PKC inhibits Rab11A-mediated β1 subunit trafficking in arterial myocytes of SP-SHRs, leading to dysfunctional NO-induced BK channel activation and vasodilation.

Potassium channels in vascular smooth muscle: a pathophysiological and pharmacological perspective

Potassium (K⁺ ) ion channel activity is an important determinant of vascular tone by regulating cell membrane potential (MP). Activation of K⁺ channels leads to membrane hyperpolarization and subsequently vasodilatation, while inhibition of the channels causes membrane depolarization and then vasoconstriction. So far five distinct types of K⁺ channels have been identified in vascular smooth muscle cells (VSMCs): Ca⁺² -activated K⁺ channels (BK_{C a} ), voltage-dependent K⁺ channels (K_V ), ATP-sensitive K⁺ channels (K_ATP ), inward rectifier K⁺ channels (K_ir ), and tandem two-pore K⁺ channels (K₂ P). The activity and expression of vascular K⁺ channels are changed during major vascular diseases such as hypertension, pulmonary hypertension, hypercholesterolemia, atherosclerosis, and diabetes mellitus. The defective function of K⁺ channels is commonly associated with impaired vascular responses and is likely to become as a result of changes in K⁺ channels during vascular diseases. Increased K⁺ channel function and expression may also help to compensate for increased abnormal vascular tone. There are many pharmacological and genotypic studies which were carried out on the subtypes of K⁺ channels expressed in variable amounts in different vascular beds. Modulation of K⁺ channel activity by molecular approaches and selective drug development may be a novel treatment modality for vascular dysfunction in the future. This review presents the basic properties, physiological functions, pathophysiological, and pharmacological roles of the five major classes of K⁺ channels that have been determined in VSMCs.

Mechanisms of vascular dysfunction evoked by ionizing radiation and possible targets for its pharmacological correction

Ionizing radiation (IR) leads to a variety of the cardiovascular diseases, including the arterial hypertension. A number of studies have demonstrated that blood vessels represent important target for IR, and the endothelium is one of the most vulnerable components of the vascular wall. IR causes an inhibition of nitric oxide (NO)-mediated endothelium-dependent vasodilatation and generation of reactive oxygen (ROS) and nitrogen (RNS) species trigger this process. Inhibition of NO-mediated vasodilatation could be due to endothelial NO synthase (eNOS) down-regulation, inactivation of endothelium-derived NO, and abnormalities in diffusion of NO from the endothelial cells (ECs) leading to a decrease in NO bioavailability. Beside this, IR suppresses endothelial large conductance Ca²⁺-activated K⁺ channels (BK_Ca) activity, which control NO synthesis. IR also leads to inhibition of the BK_Ca current in vascular smooth muscle cells (SMCs) which is mediated by protein kinase C (PKC). On the other hand, IR-evoked enhanced vascular contractility may result from PKC-mediated increase in SMCs myofilament Ca²⁺ sensitivity. Also, IR evokes vascular wall inflammation and atherosclerosis development. Vascular function damaged by IR can be effectively restored by quercetin-filled phosphatidylcholine liposomes and mesenchymal stem cells injection. Using RNA-interference technique targeted to different PKC isoforms can also be a perspective approach for pharmacological treatment of IR-induced vascular dysfunction.

Biochemical and Functional Interplay Between Ion Channels and the Components of the Dystrophin-Associated Glycoprotein Complex

Dystrophin is a cytoskeleton-linked membrane protein that binds to a larger multiprotein assembly called the dystrophin-associated glycoprotein complex (DGC). The deficiency of dystrophin or the components of the DGC results in the loss of connection between the cytoskeleton and the extracellular matrix with significant pathophysiological implications in skeletal and cardiac muscle as well as in the nervous system. Although the DGC plays an important role in maintaining membrane stability, it can also be considered as a versatile and flexible molecular complex that contribute to the cellular organization and dynamics of a variety of proteins at specific locations in the plasma membrane. This review deals with the role of the DGC in transmembrane signaling by forming supramolecular assemblies for regulating ion channel localization and activity. These interactions are relevant for cell homeostasis, and its alterations may play a significant role in the etiology and pathogenesis of various disorders affecting muscle and nerve function.

Differential participation of calcium-activated potassium channel in endothelium-dependent hyperpolarization-type relaxation in superior mesenteric arteries of spontaneously hypertensive rats

The purpose of this study was to determine the relationship of K_Ca channels to endothelium-dependent hyperpolarizing factor (EDHF)-mediated relaxation induced by acetylcholine (ACh) in the superior mesenteric arteries of 7-month-old spontaneously hypertensive rats (SHR). Upon inhibition of nitric oxide synthase and cyclooxygenase, ACh-induced EDHF-mediated relaxation was found to be weaker in SHR than in age-matched Wistar Kyoto rats (WKY). These relaxations in both group were attenuated by combined treatment with small-conductance and intermediate-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) inhibitors, with the exception of relaxation resistant to inhibition of these channels in SHR (vs. WKY). Treatment with large-conductance Ca²⁺-activated K⁺ channels (BK_Ca) inhibitor specifically attenuated relaxation in SHR, but not in WKY. Protein expression of IK_Ca and SK_Ca in the arteries did not differ between the 2 groups, whereas ratio of sloβ1 subunit to α subunit of BK_Ca was increased in SHR (vs. WKY). These results suggest that EDHF-mediated relaxations in superior mesenteric arteries are impaired in SHR, and utilize components of BK_Ca in addition to SK_Ca/IK_Ca channel activities, that the increased participation of BK_Ca may be attributable to alterations in α and sloβ1 subunit ratio, and that components unrelated to K_Ca activity may also contribute to the difference between SHR and WKY arteries.

Negative regulation of cellular Ca2+ mobilization by ryanodine receptor type 3 in mouse mesenteric artery smooth muscle

Physiological functions of type 3 ryanodine receptors (RyR3) in smooth muscle (SM) tissues are not well understood, in spite of their wide expression. However, the short isoform of RyR3 is known to be a dominant-negative variant (DN-RyR3), which may negatively regulate functions of both RyR2 and full-length (FL) RyR3 by forming hetero-tetramers. Here, functional roles of RyR3 in the regulation of Ca²⁺ signaling in mesenteric artery SM cells (MASMCs) were examined using RyR3 homozygous knockout mice (RyR3^-/-). Quantitative PCR analyses suggested that the predominant RyR3 subtype in MASMs from wild-type mice (RyR3^+/+) was DN-RyR3. In single MASMCs freshly isolated from RyR3^-/-, the EC₅₀ of caffeine to induce Ca²⁺ release was lower than that in RyR3^+/+ myocytes. The amplitude and frequency of Ca²⁺ sparks and spontaneous transient outward currents in MASMCs from RyR3^-/- were all larger than those from RyR3^+/+. Importantly, mRNA and functional expressions of voltage-dependent Ca²⁺ channel and large-conductance Ca²⁺-activated K⁺ (BK) channel in MASMCs from RyR3^-/- were identical to those from RyR3^+/+. However, in the presence of BK channel inhibitor, paxilline, the pressure rises induced by BayK8644 in MA vascular beds of RyR3^-/- were significantly larger than in those of RyR3^+/+. This indicates that the negative feedback effects of BK channel activity on intracellular Ca²⁺ signaling was enhanced in RyR3^-/-. Thus, RyR3, and, in fact, mainly DN-RyR3, via a complex with RyR2 suppresses Ca²⁺ release and indirectly regulated membrane potential by reducing BK channel activity in MASMCs and presumably can affect the regulation of intrinsic vascular tone.

Impaired BKCa channel function in native vascular smooth muscle from humans with type 2 diabetes

Large-conductance Ca2+-activated potassium (BKCa) channels are key determinants of vascular smooth muscle excitability. Impaired BKCa channel function through remodeling of BKCa β1 expression and function contributes to vascular complications in animal models of diabetes. Yet, whether similar alterations occur in native vascular smooth muscle from humans with type 2 diabetes is unclear. In this study, we evaluated BKCa function in vascular smooth muscle from small resistance adipose arteries of non-diabetic and clinically diagnosed type 2 diabetic patients. We found that BKCa channel activity opposes pressure-induced constriction in human small resistance adipose arteries, and this is compromised in arteries from diabetic patients. Consistent with impairment of BKCa channel function, the amplitude and frequency of spontaneous BKCa currents, but not Ca2+ sparks were lower in cells from diabetic patients. BKCa channels in diabetic cells exhibited reduced Ca2+ sensitivity, single-channel open probability and tamoxifen sensitivity. These effects were associated with decreased functional coupling between BKCa α and β1 subunits, but no change in total protein abundance. Overall, results suggest impairment in BKCa channel function in vascular smooth muscle from diabetic patients through unique mechanisms, which may contribute to vascular complications in humans with type 2 diabetes.

Allosteric-activation mechanism of BK channel gating ring triggered by calcium ions

Calcium ions bind at the gating ring which triggers the gating of BK channels. However, the allosteric mechanism by which Ca²⁺ regulates the gating of BK channels remains obscure. Here, we applied Molecular Dynamics (MD) and Targeted MD to the integrated gating ring of BK channels, and achieved the transition from the closed state to a half-open state. Our date show that the distances of the diagonal subunits increase from 41.0 Å at closed state to 45.7Å or 46.4 Å at a half-open state. It is the rotatory motion and flower-opening like motion of the gating rings which are thought to pull the bundle crossing gate to open ultimately. Compared with the ‘Ca²⁺ bowl’ at RCK2, the RCK1 Ca²⁺ sites make more contribution to opening the channel. The allosteric motions of the gating ring are regulated by three group of interactions. The first weakened group is thought to stabilize the close state; the second strengthened group is thought to stabilize the open state; the third group thought to lead AC region forming the CTD pore to coordinated motion, which exquisitely regulates the conformational changes during the opening of BK channels by Ca²⁺.

Microtubule structures underlying the sarcoplasmic reticulum support peripheral coupling sites to regulate smooth muscle contractility

Junctional membrane complexes facilitate excitation-contraction coupling in skeletal and cardiac muscle cells by forming subcellular invaginations that maintain close (≤20 nm) proximity of ryanodine receptors (RyRs) on the sarcoplasmic reticulum (SR) with voltage-dependent Ca2+ channels in the plasma membrane. In fully differentiated smooth muscle cells, junctional membrane complexes occur as distributed sites of peripheral coupling. We investigated the role of the cytoskeleton in maintaining peripheral coupling and associated Ca2+ signaling networks within native smooth muscle cells of mouse and rat cerebral arteries. Using live-cell confocal and superresolution microscopy, we found that the tight interactions between the SR and the plasma membrane in these cells relied on arching microtubule structures present at the periphery of smooth muscle cells and were independent of the actin cytoskeleton. Loss of peripheral coupling associated with microtubule depolymerization altered the spatiotemporal properties of localized Ca2+ sparks generated by the release of Ca2+ through type 2 RyRs (RyR2s) on the SR and decreased the number of sites of colocalization between RyR2s and large-conductance Ca2+-activated K+ (BK) channels. The reduced BK channel activity associated with the loss of SR-plasma membrane interactions was accompanied by increased pressure-induced constriction of cerebral resistance arteries. We conclude that microtubule structures maintain peripheral coupling in contractile smooth muscle cells, which is crucial for the regulation of contractility and cerebral vascular tone.

Role of DNA methylation in perinatal nicotine-induced development of heart ischemia-sensitive phenotype in rat offspring

Background and purpose

Maternal cigarette smoking increases the risk of cardiovascular disease in offspring. Recently, we have demonstrated that perinatal nicotine exposure alters heart development and increases heart susceptibility to ischemia/reperfusion (I/R) injury in rat offspring. The present study tested the hypothesis that DNA methylation plays a key role in the nicotine-induced development of heart ischemia-sensitive phenotype in offspring.

Experimental approach

Nicotine was administered to pregnant rats via subcutaneous osmotic minipumps from gestational day 4 until postnatal day 10. After birth, the postnatal offspring were treated with the DNA methylation inhibitor, 5-aza-2'-deoxycytidine (5-Aza) or saline from postnatal day 3 to day 10. Experiments were conducted in 1 month old offspring.

Key results

Perinatal nicotine increased I/R-induced left ventricular (LV) injury, and decreased post-ischemic recovery of the LV function and coronary flow rate in both male and female offspring. Nicotine differentially increased DNMT3a expression and global DNA methylation levels in LV tissues. Treatment with 5-Aza inhibited nicotine-induced an increase in DNMT3a and global DNA methylation, and blocked the nicotine-induced increase in I/R injury and dysfunction in the heart. In addition, nicotine attenuated protein kinases C_ε and large-conductance Ca(2+)-activated K(+) (BKca) channel β1 subunit protein abundances in the heart, which were reversed by 5-Aza treatment.

Conclusions and implications

The present findings provide novel evidence that the increased DNA methylation plays a causal role in nicotine-induced development of heart ischemic sensitive phenotype, and suggest a potential therapeutic target of DNA demethylation for the fetal programming of heart ischemic disease later in life.

Distance constraints on activation of TRPV4 channels by AKAP150-bound PKCα in arterial myocytes

Vascular smooth muscle tone can be regulated by angiotensin II, which enhances TRPV4 channel activity via AKAP150-bound protein kinase C. Tajada et al. show that the effect of AKAP150 on TRPV4 channels is inversely proportional to the distance between them, which varies with sex and arterial bed.

TRPV4 (transient receptor potential vanilloid 4) channels are Ca²⁺-permeable channels that play a key role in regulating vascular tone. In arterial myocytes, opening of TRPV4 channels creates local increases in Ca²⁺ influx, detectable optically as “TRPV4 sparklets.” TRPV4 sparklet activity can be enhanced by the action of the vasoconstrictor angiotensin II (AngII). This modulation depends on the activation of subcellular signaling domains that comprise protein kinase C α (PKCα) bound to the anchoring protein AKAP150. Here, we used super-resolution nanoscopy, patch-clamp electrophysiology, Ca²⁺ imaging, and mathematical modeling approaches to test the hypothesis that AKAP150-dependent modulation of TRPV4 channels is critically dependent on the distance between these two proteins in the sarcolemma of arterial myocytes. Our data show that the distance between AKAP150 and TRPV4 channel clusters varies with sex and arterial bed. Consistent with our hypothesis, we further find that basal and AngII-induced TRPV4 channel activity decays exponentially as the distance between TRPV4 and AKAP150 increases. Our data suggest a maximum radius of action of ∼200 nm for local modulation of TRPV4 channels by AKAP150-associated PKCα.

Aerobic Exercise of Low to Moderate Intensity Corrects Unequal Changes in BKCa Subunit Expression in the Mesenteric Arteries of Spontaneously Hypertensive Rats

Accumulating evidence indicates that hypertension is associated with “ion channel remodeling” of vascular smooth muscle cells (VSMCs). The objective of this study was to determine the effects of exercise intensity/volume on hypertension-associated changes in large-conductance Ca2+-activated K+ (BKCa) channels in mesenteric arteries (MAs) from spontaneously hypertensive rats (SHR). Male SHRs were randomly assigned to three groups: a low-intensity aerobic exercise group (SHR-L: 14 m/min), a moderate-intensity aerobic exercise group (SHR-M: 20 m/min), and a sedentary group (SHR). Age-matched Wistar-Kyoto rats (WKYs) were used as normotensive controls. Exercise groups completed an 8-week exercise program. Elevation of the α and β1 proteins was unequal in MA myocytes from SHRs, with the β1 subunit increasing more than the α subunit. BKCa contribution to vascular tone regulation was higher in the myocytes and arteries of SHRs compared to WKYs. SHR BKCa channel subunit protein expression, β1/α ratio, whole cell current density and single-channel open probability was also increased compared with WKYs. Aerobic exercise lowered systemic blood pressure and normalized hypertension-associated BKCa alterations to normotensive control levels in the SHRs. These effects were more pronounced in the moderate-intensity group than in the low-intensity group. There is a dose-effect for aerobic exercise training in the range of low to moderate-intensity and accompanying volume for the correction of the pathological adaptation of BKCa channels in myocytes of MAs from SHR.

Pregnancy Reprograms Large-Conductance Ca2+-Activated K+ Channel in Uterine Arteries: Roles of Ten-Eleven Translocation Methylcytosine Dioxygenase 1-Mediated Active Demethylation

The large-conductance Ca²⁺-activated K⁺ (BK_Ca) channel is of critical importance in pregnancy-mediated increase in uterine artery vasodilation and blood flow. The present study tested the hypothesis that active DNA demethylation plays a key role in pregnancy-induced reprogramming and upregulation of BK_Ca channel β1 subunit (BKβ1) in uterine arteries. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Pregnancy significantly increased the expression of ten-eleven translocation methylcytosine dioxygenase 1 (TET1) in uterine arteries. A half-palindromic estrogen response element was identified at the TET1 promoter, and estrogen treatment increased TET1 promoter activity and TET1 expression in uterine arteries. In accordance, pregnancy and steroid hormone treatment resulted in demethylation of BKβ1 promoter by increasing 5-hydroxymethylcytosine and decreasing 5-methylcytosine at the CpG in the Sp1_-380 binding site that is of critical importance in the regulation of the promoter activity and BKβ1 expression. Inhibition of TET1 with fumarate significantly decreased BKβ1 expression in uterine arteries of pregnant animals and blocked steroid hormone-induced upregulation of BKβ1. Functionally, fumarate treatment inhibited pregnancy and steroid hormone-induced increases in BK_Ca channel current density and BK_Ca channel-mediated relaxations. In addition, fumarate blocked pregnancy and steroid hormone-induced decrease in pressure-dependent myogenic tone of the uterine artery. The results demonstrate a novel mechanism of estrogen-mediated active DNA demethylation in reprogramming of BK_Ca channel expression and function in the adaption of uterine circulation during pregnancy.

Unravelling the complexities of vascular smooth muscle ion channels: Fine tuning of activity by ancillary subunits

Which ion channel is the most important for regulating vascular tone? Which one is responsible for controlling the resting membrane potential or repolarization? Which channels are recruited by different intracellular signalling pathways or change in certain vascular diseases? Many different ion channels have been identified in the vasculature over the years and claimed as future therapeutic targets. Unfortunately, several of these ion channels are not just found in the vasculature, with many of them also found to have prominent functional roles in different organs of the body, which then leads to off-target effects. As cardiovascular diseases are expected to increase worldwide to epidemic proportions, ion channel research and the hunt for the next major therapeutic target to treat different vascular diseases has never been more important. However, I believe that the question we should now be asking is: which ancillary subunits are involved in regulating specific ion channels in the vasculature and do they have the potential to be new therapeutic targets?

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.

Differential distribution and functional impact of BK channel beta1 subunits across mesenteric, coronary, and different cerebral arteries of the rat

Large conductance, Ca²⁺_i- and voltage-gated K⁺ (BK) channels regulate myogenic tone and, thus, arterial diameter. In smooth muscle (SM), BK channels include channel-forming α and auxiliary β1 subunits. BK β1 increases the channel's Ca²⁺ sensitivity, allowing BK channels to negatively feedback on depolarization-induced Ca²⁺ entry, oppose SM contraction and favor vasodilation. Thus, endothelial-independent vasodilation can be evoked though targeting of SM BK β1 by endogenous ligands, including lithocholate (LCA). Here, we investigated the expression of BK β1 across arteries of the cerebral and peripheral circulations, and the contribution of such expression to channel function and BK β1-mediated vasodilation. Data demonstrate that endothelium-independent, BK β1-mediated vasodilation by LCA is larger in coronary (CA) and basilar (BA) arteries than in anterior cerebral (ACA), middle cerebral (MCA), posterior cerebral (PCA), and mesenteric (MA) arteries, all arterial segments having a similar diameter. Thus, differential dilation occurs in extracranial arteries which are subjected to similar vascular pressure (CA vs. MA) and in arteries that irrigate different brain regions (BA vs. ACA, MCA, and PCA). SM BK channels from BA and CA displayed increased basal activity and LCA responses, indicating increased BK β1 functional presence. Indeed, in the absence of detectable changes in BK α, BA and CA myocytes showed an increased location of BK β1 in the plasmalemma/subplasmalemma. Moreover, these myocytes distinctly showed increased BK β1 messenger RNA (mRNA) levels. Supporting a major role of enhanced BK β1 transcripts in artery dilation, LCA-induced dilation of MCA transfected with BK β1 complementary DNA (cDNA) was as high as LCA-induced dilation of untransfected BA or CA.

Hydrogen Peroxide as a Paracrine Vascular Mediator: Regulation and Signaling Leading to Dysfunction

Numerous studies have demonstrated the ability of a variety of vascular cells, including endothelial cells, smooth muscle cells, and fibroblasts, to produce reactive oxygen species (ROS). Until recently, major emphasis was placed on the production of superoxide anion (O2–) in the vasculature as a result of its ability to directly attenuate the biological activity of endothelium-derived nitric oxide (NO). The short half-life and radius of diffusion of O2– drastically limit the role of this ROS as an important paracrine hormone in vascular biology. On the contrary, in recent years, the O2– metabolite hydrogen peroxide (H2O2) has increasingly been viewed as an important cellular signaling agent in its own right, capable of modulating both contractile and growth-promoting pathways with more far-reaching effects. In this review, we will assess the vascular production of H2O2, its regulation by endogenous scavenger systems, and its ability to activate a variety of vascular signaling pathways, thereby leading to vascular contraction and growth. This discussion will include the ability of H2O2 to (i) Initiate calcium flux as well as (ii) stimulate pathways leading to sensitization of contractile elements to calcium. The latter involves a variety of protein kinases that have also been strongly implicated in vascular hypertrophy. Previous Intensive study has emphasized the ability of NADPH oxidase-derived O2– and H2O2 to activate these pathways in cultured smooth muscle cells. However, growing evidence indicates a considerably more complex array of unique oxidase systems in the endothelium, media, and adventitia that appear to participate in these deleterious effects in a sequential and temporal manner. Taken together, these findings seem consistent with a paracrine effect of H2O2 across the vascular wall.

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.

Pressure-induced oxidative activation of PKG enables vasoregulation by Ca2+ sparks and BK channels

Activation of Ca²⁺-sensitive, large-conductance potassium (BK) channels in vascular smooth muscle cells (VSMCs) by local, ryanodine receptor-mediated Ca²⁺ signals (Ca²⁺ sparks) acts as a brake on pressure-induced (myogenic) vasoconstriction-a fundamental mechanism that regulates blood flow in small resistance arteries. We report that physiological intraluminal pressure within resistance arteries activated cGMP-dependent protein kinase (PKG) in VSMCs through oxidant-induced formation of an intermolecular disulfide bond between cysteine residues. Oxidant-activated PKG was required to trigger Ca²⁺ sparks, BK channel activity, and vasodilation in response to pressure. VSMCs from arteries from mice expressing a form of PKG that could not be activated by oxidants showed reduced Ca²⁺ spark frequency, and arterial preparations from these mice had decreased pressure-induced activation of BK channels. Thus, the absence of oxidative activation of PKG disabled the BK channel-mediated negative feedback regulation of vasoconstriction. Our results support the concept of a negative feedback control mechanism that regulates arterial diameter through mechanosensitive production of oxidants to activate PKG and enhance Ca²⁺ sparks.

Different Effects of Hypertension and Age on the Function of Large Conductance Calcium- and Voltage-Activated Potassium Channels in Human Mesentery Artery Smooth Muscle Cells

BACKGROUND

Large-conductance calcium- and voltage-activated potassium channels (BKC a channels) play important roles in the maintenance of vascular tone, and their dysregulation is associated with abnormal vascular relaxation and contraction. We tested the changes in BKC a channel properties in patients at different ages to assess the effects of hypertension and aging on the functional changes of BKC a channels.

METHODS AND RESULTS

Patch clamp was performed to detect the activities of BKC a channels in freshly isolated human mesenteric artery smooth muscle cells from younger patients (aged ≤45 years) without hypertension, older patients (aged ≥65 years) without hypertension, and older patients with hypertension. The expression of mRNA and protein from BKC a channels was evaluated by reverse transcription polymerase chain reaction and Western blot analysis, respectively. Results showed that the whole-cell current density, spontaneous transient outward current, and Ca(2+) sensitivity of the artery smooth muscle cells were significantly decreased in the older patients with hypertension; the decreases were insignificant in the older patients without hypertension, although a clear tendency to have spontaneous transient outward current was detected in these patients. The expression of both mRNA and protein of BKC a subunits α and β1 was significantly decreased in the older patients with hypertension but not in the older patients without hypertension compared with the younger patients without hypertension.

CONCLUSIONS

Our findings demonstrate for the first time that hypertension is an important factor for the pathological alteration of the properties of BKC a channels in human mesenteric artery smooth muscle cells, and aging itself may also be a factor in these changes in the cells.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Exercise Prevents Upregulation of RyRs-BKCa Coupling in Cerebral Arterial Smooth Muscle Cells From Spontaneously Hypertensive Rats

OBJECTIVE

Regular exercise is an effective nonpharmacological means of preventing and controlling hypertension. However, the molecular mechanisms underlying its effects remain undetermined. The hypothesis that hypertension increases the functional coupling of large-conductance Ca(2+)-activated K(+) (BKCa) channels with ryanodine receptors in spontaneously hypertensive rats (SHR) as a compensatory response to an increase in intracellular Ca(2+) concentration in cerebral artery smooth muscle cells was assessed here. It was further hypothesized that exercise training would prevent this increase in functional coupling.

APPROACH AND RESULTS

SHR and Wistar-Kyoto (WKY) rats were randomly assigned to sedentary groups (SHR-SED and WKY-SED) and exercise training groups (SHR-EX and WKY-EX). Cerebral artery smooth muscle cells displayed spontaneous transient outward currents at membrane potentials more positive than -40 mV. The amplitude of spontaneous transient outward currents together with the spontaneous Ca(2+) sparks in isolated cerebral artery smooth muscle cells was significantly higher in SHR-SED than in WKY-SED. Moreover, hypertension displayed increased whole-cell BKCa, voltage-gated Ca(2+) channel, but decreased KV currents in cerebral artery smooth muscle cells. In SHRs, the activity of the single BKCa channel increased markedly, and the protein expression of BKCa (β1, but not α-subunit) also increased, but KV1.2 decreased significantly. Exercise training ameliorated all of these functional and molecular alterations in hypertensive rats.

CONCLUSIONS

These data indicate that hypertension leads to enhanced functional coupling of ryanodine receptors-BKCa to buffer pressure-induced constriction of cerebral arteries, which attributes not only to an upregulation of BKCa β1-subunit function but also to an increase of Ca(2+) release from ryanodine receptors. However, regular aerobic exercise efficiently prevents augmented coupling and so alleviates the pathological compensation and restores cerebral arterial function.

Chronic intermittent hypobaric hypoxia antagonizes renal vascular hypertension by enhancement of vasorelaxation via activating BKCa

AIM

The purpose of the present study was to explore anti-hypertensive effect of chronic intermittent hypobaric hypoxia (CIHH) in renovascular hypertension (RVH) rats, as well as the role of large-conductance calcium-activated potassium channel (BKCa) in anti-hypertensive effect of CIHH.

MAIN METHODS

Male adult age- and body weight-matched Sprague-Dawley rats were divided into SHAM, CIHH, RVH and RVH+CIHH groups. Hypertension was induced by two-kidney-1-clip method (2K1C) in RVH rats. CIHH rats were exposed to 28-days hypobaric hypoxia simulating 5000m altitude, 6h daily. SHAM rats got an operation without 2K1C, and RVH+CIHH rats received CIHH treatment after 2K1C. The endothelium-dependent vasorelaxation induced by acetylcholine (ACh), BKCa currents in smooth muscle cells (VSMCs) of mesenteric arteries and the protein expression of BKCa in mesenteric arteries was examined.

KEY FINDINGS

The systolic arterial blood pressure (SAP) in RVH rats was higher than that in SHAM rats and CIHH treatment significantly decreased SAP in RVH rats. The enhanced vasorelaxation of mesenteric artery in CIHH-treated RVH rats was cancelled by BKCa blocker IBTX. The vasorelaxation induced by BKCa activator was reduced in RVH rats and the decreased vasorelaxation was improved by CIHH treatment. The β1 subunit of BKCa in mesenteric artery was upregulated and BKCa current in VSMCs was increased in CIHH-treated RVH rats compared with RVH rats.

SIGNIFICANCE

In conclusion, CIHH treatment enhances the relaxation of mesenteric artery through activation of BKCavia up-regulating β1 subunit of BKCa, which might be one of mechanisms for anti-hypertensive effect of CIHH in RVH rats.

Big-conductance Ca2+-activated K+ channels in physiological and pathophysiological urinary bladder smooth muscle cells

Contraction and relaxation of urinary bladder smooth muscle cells (UBSMCs) represent the important physiological functions of the bladder. Contractile responses in UBSMCs are regulated by a number of ion channels including big-conductance Ca²⁺- activated K⁺ (BK) channels. Great progress has been made in studies of BK channels in UBSMCs. The intent of this review is to summarize recent exciting findings with respect to the functional interactions of BK channels with muscarinic receptors, ryanodine receptors (RyRs) and inositol triphosphate receptors (IP₃Rs) as well as their functional importance under normal and pathophysiological conditions. BK channels are highly expressed in UBSMCs. Activation of muscarinic M₃ receptors inhibits the BK channel activity, facilitates opening of voltage-dependent Ca²⁺ (Ca_V) channels, and thereby enhances excitability and contractility of UBSMCs. Signaling molecules and regulatory mechanisms involving RyRs and IP₃Rs have a significant effect on functions of BK channels and thereby regulate cellular responses in UBSMCs under normal and pathophysiological conditions including overactive bladders. Moreover, BK channels may represent a novel target for the treatment of bladder dysfunctions.

BK Channels in the Vascular System

Autoregulation of blood flow is essential for the preservation of organ function to ensure continuous supply of oxygen and essential nutrients and removal of metabolic waste. This is achieved by controlling the diameter of muscular arteries and arterioles that exhibit a myogenic response to changes in arterial blood pressure, nerve activity and tissue metabolism. Large-conductance voltage and Ca(2+)-dependent K(+) channels (BK channels), expressed exclusively in smooth muscle cells (SMCs) in the vascular wall of healthy arteries, play a critical role in regulating the myogenic response. Activation of BK channels by intracellular, local, and transient ryanodine receptor-mediated "Ca(2+) sparks," provides a hyperpolarizing influence on the SMC membrane potential thereby decreasing the activity of voltage-dependent Ca(2+) channels and limiting Ca(2+) influx to promote SMC relaxation and vasodilation. The BK channel α subunit, a large tetrameric protein with each monomer consisting of seven-transmembrane domains, a long intracellular C-terminal tail and an extracellular N-terminus, associates with the β1 and γ subunits in vascular SMCs. The BK channel is regulated by factors originating within the SMC or from the endothelium, perivascular nerves and circulating blood, that significantly alter channel gating properties, Ca(2+) sensitivity and expression of the α and/or β1 subunit. The BK channel thus serves as a central receiving dock that relays the effects of the changes in several such concomitant autocrine and paracrine factors and influences cardiovascular health. This chapter describes the primary mechanism of regulation of myogenic response by BK channels and the alterations to this mechanism wrought by different vasoactive mediators.

Modulation of BK Channel Function by Auxiliary Beta and Gamma Subunits

The large-conductance, Ca(2+)- and voltage-activated K(+) (BK) channel is ubiquitously expressed in mammalian tissues and displays diverse biophysical or pharmacological characteristics. This diversity is in part conferred by channel modulation with different regulatory auxiliary subunits. To date, two distinct classes of BK channel auxiliary subunits have been identified: β subunits and γ subunits. Modulation of BK channels by the four auxiliary β (β1-β4) subunits has been well established and intensively investigated over the past two decades. The auxiliary γ subunits, however, were identified only very recently, which adds a new dimension to BK channel regulation and improves our understanding of the physiological functions of BK channels in various tissues and cell types. This chapter will review the current understanding of BK channel modulation by auxiliary β and γ subunits, especially the latest findings.