Diacylglycerol and protein kinase C activate cation channels involved in myogenic tone

Nephrectomy and high-salt diet inducing pulmonary hypertension and kidney damage by increasing Ang II concentration in rats

BACKGROUND

Chronic kidney disease (CKD) is a significant risk factor for pulmonary hypertension (PH), a complication that adversely affects patient prognosis. However, the mechanisms underlying this association remain poorly understood. A major obstacle to progress in this field is the lack of a reliable animal model replicating CKD-PH.

METHODS

This study aimed to establish a stable rat model of CKD-PH. We employed a combined approach, inducing CKD through a 5/6 nephrectomy and concurrently exposing the rats to a high-salt diet. The model's hemodynamics were evaluated dynamically, alongside a comprehensive assessment of pathological changes in multiple organs. Lung tissues and serum samples were collected from the CKD-PH rats to analyze the expression of angiotensin-converting enzyme 2 (ACE2), evaluate the activity of key vascular components within the renin-angiotensin-aldosterone system (RAAS), and characterize alterations in the serum metabolic profile.

RESULTS

At 14 weeks post-surgery, the CKD-PH rats displayed significant changes in hemodynamic parameters indicative of pulmonary arterial hypertension. Additionally, right ventricular hypertrophy was observed. Notably, no evidence of pulmonary vascular remodeling was found. Further analysis revealed RAAS dysregulation and downregulated ACE2 expression within the pulmonary vascular endothelium of CKD-PH rats. Moreover, the serum metabolic profile of these animals differed markedly from the sham surgery group.

CONCLUSIONS

Our findings suggest that the development of pulmonary arterial hypertension in CKD-PH rats is likely a consequence of a combined effect: RAAS dysregulation, decreased ACE2 expression in pulmonary vascular endothelial cells, and metabolic disturbances.

Levosimendan Relaxes Thoracic Aortic Smooth Muscle in Mice by Inhibiting PKC and Activating Inwardly Rectifying Potassium Channels

ABSTRACT

Studies have examined the therapeutic effect of levosimendan on cardiovascular diseases such as heart failure, perioperative cardiac surgery, and septic shock, but the specific mechanism in mice remains largely unknown. This study aimed to investigate the relaxation mechanism of levosimendan in the thoracic aorta smooth muscle of mice. Levosimendan-induced relaxation of isolated thoracic aortic rings that were precontracted with norepinephrine or KCl was recorded in an endothelium-independent manner. Vasodilatation by levosimendan was not associated with the production of the endothelial relaxation factors nitric oxide and prostaglandins. The voltage-dependent K + channel (K V ) blocker (4-aminopyridine) and selective K Ca blocker (tetraethylammonium) had no effect on thoracic aortas treated with levosimendan, indicating that K V and K Ca channels may not be involved in the levosimendan-induced relaxation mechanism. Although the inwardly rectifying K + channel (K ir ) blocker (barium chloride) and the K ATP channel blocker (glibenclamide) significantly inhibited levosimendan-induced vasodilation in the isolated thoracic aorta, barium chloride had a much stronger inhibitory effect on levosimendan-induced vasodilation than glibenclamide, suggesting that levosimendan-induced vasodilation may be mediated by K ir channels. The vasodilation effect and expression of K ir 2.1 induced by levosimendan were further enhanced by the PKC inhibitor staurosporine. Extracellular calcium influx was inhibited by levosimendan without affecting intracellular Ca 2+ levels in the isolated thoracic aorta. These results suggest that K ir channels play a more important role than K ATP channels in regulating vascular tone in larger arteries and that the activity of the K ir channel is enhanced by the PKC pathway.

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.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Effects of losartan on vasomotor function and canonical transient receptor potential channels in the aortas of sinoaortic denervation rats

Increased short-term blood pressure variability (BPV) is strongly correlated with target organ damage. However, the molecular mechanisms underlying abnormal BPV-induced organ damage and effective therapeutic targets are poorly understood. The purpose of this study was to investigate the effects of losartan on vasomotor function and canonical transient receptor potential (TRPC) channels in the aortas of rats with arterial pressure lability induced by sinoaortic denervation (SAD). SAD was performed in male Sprague-Dawley rats at the age of 10 weeks. The experiment included sham-operated (Sham), SAD, and losartan-treated SAD (SAD+Los) groups. After 8 weeks of treatment, hemodynamic parameters were measured via catheterization, thoracic aortic vasomotor functions were evaluated using a physiological vascular ring tension recording system, and TRPC1 and 6 mRNA and protein expression levels in the endothelial cells (ECs) and smooth muscle cells (SMCs) of the thoracic aorta were determined via reverse transcription polymerase chain reaction (RT-PCR) and Western-blotting, respectively. Compared with Sham rats, SAD rats exhibited significantly increased BPV, enhanced norepinephrine-induced aortic contraction, and attenuated acetylcholine-induced aortic relaxation. Both the mRNA and the protein expression levels of TRPC1 and 6 were significantly downregulated in the ECs and upregulated in the SMCs of the thoracic aortas of SAD rats. Losartan treatment prevented these SAD-induced changes. In conclusion, losartan efficiently prevented vasomotor function impairment in SAD rats by reducing BPV and regulating TRPC1 and 6 expression levels.

Regional heterogeneity in the mechanisms of myogenic tone in hamster arterioles

Myogenic tone is an important feature of arterioles and resistance arteries, but the mechanisms responsible for this hallmark characteristic remain unclear. We used pharmacological inhibitors to compare the roles played by phospholipase C (PLC; 10 μM U73122), inositol 1,4,5-trisphosphate receptors (IP₃Rs; 100 μM 2-aminoethoxydiphenylborane), protein kinase C (10 μM bisindolylmaleimide I), angiotensin II type 1 receptors (1 μM losartan), Rho kinase (10 nM-30 μM Y27632 or 300 nM H1152), stretch-activated ion channels (10 nM-1 μM Gd³⁺ or 5 μM spider venom toxin GsMTx-4) and L-type voltage-gated Ca²⁺ channels (0.3-100 μM diltiazem) in myogenic tone of cannulated, pressurized (80 cmH₂O), second-order hamster cremaster or cheek pouch arterioles. Effective inhibition of either PLC or IP₃Rs dilated cremaster arterioles, inhibited Ca²⁺ waves, and reduced global Ca²⁺ levels. In contrast, cheek pouch arterioles did not display Ca²⁺ waves and inhibition of PLC or IP₃Rs had no effect on myogenic tone or intracellular Ca²⁺ levels. Inhibition of Rho kinase dilated both cheek pouch and cremaster arterioles with equal efficacy and potency but also reduced intracellular Ca²⁺ signals in both arterioles. Similarly, inhibition of mechanosensitive ion channels with Gd²⁺ or GsMTx-4 produced comparable dilation in both arterioles. Inhibition of L-type Ca²⁺ channels with diltiazem was more effective in dilating cremaster (86 ± 5% dilation, n = 4) than cheek pouch arterioles (54 ± 4% dilation, n = 6, P < 0.05). Thus, there are substantial differences in the mechanisms underlying myogenic tone in hamster cremaster and cheek pouch arterioles. Regional heterogeneity in myogenic mechanisms could provide new targets for drug development to improve regional blood flow in a tissue-specific manner.NEW & NOTEWORTHY Regional heterogeneity in the mechanisms of pressure-induced myogenic tone implies that resistance vessels may be able to alter myogenic signaling pathways to adapt to their environment. A better understanding of the spectrum of myogenic mechanisms could provide new targets to treat diseases that affect resistance artery and arteriolar function.

Stretch-activated TRPV2 channels: Role in mediating cardiopathies

Transient receptor potential vanilloid type 2, TRPV2, is a calcium-permeable cation channel belonging to the TRPV channel family. Although this channel has been first characterized as a noxious heat sensor, its mechanosensor property recently gained importance in various physiological functions. TRPV2 has been described as a stretch-mediated channel and a regulator of calcium homeostasis in several cell types and has been shown to be involved in the stretch-dependent responses in cardiomyocytes. Hence, several studies in the last years support the idea that TRPV2 play a key role in the function and structure of the heart, being involved in the cardiac compensatory mechanisms in response to pathologic or exercise-induced stress. We present here an overview of the current literature and concepts of TRPV2 channels involvement (i) in the mechanical coupling mechanisms in heart and (ii) in the mechanisms that lead to cardiomyopathies. All these studies lead us to think that TRPV2 may also be an important cardiac drug target based on its major physiological roles in heart.

Mechanical activation of angiotensin II type 1 receptors causes actin remodelling and myogenic responsiveness in skeletal muscle arterioles

KEY POINTS

Candesartan, an inverse agonist of the type 1 angiotensin II receptor (AT1 R), causes a concentration-dependent inhibition of pressure-dependent myogenic tone consistent with previous reports of mechanosensitivity of this G protein-coupled receptor. Mechanoactivation of the AT1 R occurs independently of local angiotensin II production and the type 2 angiotensin receptor. Mechanoactivation of the AT1 R stimulates actin polymerization by a protein kinase C-dependent mechanism, but independently of a change in intracellular Ca2+ . Using atomic force microscopy, changes in single vascular smooth muscle cell cortical actin are observed to remodel following mechanoactivation of the AT1 R.

ABSTRACT

The Gq/11 protein-coupled angiotensin II type 1 receptor (AT1 R) has been shown to be activated by mechanical stimuli. In the vascular system, evidence supports the AT1 R being a mechanosensor that contributes to arteriolar myogenic constriction. The aim of this study was to determine if AT1 R mechanoactivation affects myogenic constriction in skeletal muscle arterioles and to determine underlying cellular mechanisms. Using pressure myography to study rat isolated first-order cremaster muscle arterioles the AT1 R inhibitor candesartan (10-7 -10-5  m) showed partial but concentration-dependent inhibition of myogenic reactivity. Inhibition was demonstrated by a rightward shift in the pressure-diameter relationship over the intraluminal pressure range, 30-110 mmHg. Pressure-induced changes in global vascular smooth muscle intracellular Ca2+ (using Fura-2) were similar in the absence or presence of candesartan, indicating that AT1 R-mediated myogenic constriction relies on Ca2+ -independent downstream signalling. The diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) reversed the inhibitory effect of candesartan, while this rescue effect was prevented by the protein kinase C (PKC) inhibitor GF 109203X. Both candesartan and PKC inhibition caused increased G-actin levels, as determined by Western blotting of vessel lysates, supporting involvement of cytoskeletal remodelling. At the single vascular smooth muscle cell level, atomic force microscopy showed that cell swelling (stretch) with hypotonic buffer also caused thickening of cortical actin fibres and this was blocked by candesartan. Collectively, the present studies support growing evidence for novel modes of activation of the AT1 R in arterioles and suggest that mechanically activated AT1 R generates diacylglycerol, which in turn activates PKC which induces the actin cytoskeleton reorganization that is required for pressure-induced vasoconstriction.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

Myogenic and metabolic feedback in cerebral autoregulation: Putative involvement of arachidonic acid-dependent pathways

The present paper presents a mechanistic model of cerebral autoregulation, in which the dual effects of the arachidonic acid metabolites 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) on vascular smooth muscle mediate the cerebrovascular adjustments to a change in cerebral perfusion pressure (CPP). 20-HETE signalling in vascular smooth muscle mediates myogenic feedback to changes in vessel wall stretch, which may be modulated by metabolic feedback through EETs released from astrocytes and endothelial cells in response to changes in brain tissue oxygen tension. The metabolic feedback pathway is much faster than 20-HETE-dependent myogenic feedback, and the former thus initiates the cerebral autoregulatory response, while myogenic feedback comprises a relatively slower mechanism that functions to set the basal cerebrovascular tone. Therefore, assessments of dynamic cerebral autoregulation, which may provide information on the response time of the cerebrovasculature, may specifically be used to yield information on metabolic feedback mechanisms, while data based on assessments of static cerebral autoregulation represent the integrated functionality of myogenic and metabolic feedback.

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.

Renal autoregulation in health and disease

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

TRPM4 channels couple purinergic receptor mechanoactivation and myogenic tone development in cerebral parenchymal arterioles

Cerebral parenchymal arterioles (PAs) have a critical role in assuring appropriate blood flow and perfusion pressure within the brain. They are unique in contrast to upstream pial arteries, as defined by their critical roles in neurovascular coupling, distinct sensitivities to chemical stimulants, and enhanced myogenic tone development. The objective of the present study was to reveal some of the unique mechanisms of myogenic tone regulation in the cerebral microcirculation. Here, we report that in vivo suppression of TRPM4 (transient receptor potential) channel expression, or inhibition of TRPM4 channels with 9-phenanthrol substantially reduced myogenic tone of isolated PAs, supporting a key role of TRPM4 channels in PA myogenic tone development. Further, downregulation of TRPM4 channels inhibited vasoconstriction induced by the specific P2Y4 and P2Y6 receptor ligands (UTPγS and UDP) by 37% and 42%, respectively. In addition, 9-phenanthrol substantially attenuated purinergic ligand-induced membrane depolarization and constriction of PAs, and inhibited ligand-evoked TRPM4 channel activation in isolated PA myocytes. In concert with our previous work showing the essential contributions of P2Y4 and P2Y6 receptors to myogenic regulation of PAs, the current results point to TRPM4 channels as an important link between mechanosensitive P2Y receptor activation and myogenic constriction of cerebral PAs.

Molecular mechanisms of pulmonary arterial remodeling

Pulmonary arterial hypertension (PAH) is characterized by a persistent elevation of pulmonary arterial pressure and pulmonary arterial remodeling with unknown etiology. Current therapeutics available for PAH are primarily directed at reducing the pulmonary blood pressure through their effects on the endothelium. It is well accepted that pulmonary arterial remodeling is primarily due to excessive pulmonary arterial smooth muscle cell (PASMC) proliferation that leads to narrowing or occlusion of the pulmonary vessels. Future effective therapeutics will be successful in reversing the vascular remodeling in the pulmonary arteries and arterioles. The purpose of this review is to provide updated information on molecular mechanisms involved in pulmonary arterial remodeling with a focus on growth factors, transcription factors, and epigenetic pathways in PASMC proliferation. In addition, this review will highlight novel therapeutic strategies for potentially reversing PASMC proliferation.

Brain ischemia in patients with intracranial hemorrhage: pathophysiological reasoning for aggressive diagnostic management

Patients with intracranial hemorrhage have to be managed aggressively to avoid or minimize secondary brain damage due to ischemia, which contributes to high morbidity and mortality. The risk of brain ischemia, however, is not the same in every patient. The risk of complications associated with an aggressive prophylactic therapy in patients with a low risk of brain ischemia can outweigh the benefits of therapy. Accurate and timely identification of patients at highest risk is a diagnostic challenge. Despite the availability of many diagnostic tools, stroke is common in this population, mostly because the pathogenesis of stroke is frequently multifactorial whereas diagnosticians tend to focus on one or two risk factors. The pathophysiological mechanisms of brain ischemia in patients with intracranial hemorrhage are not yet fully elucidated and there are several important areas of ongoing research. Therefore, this review describes physiological and pathophysiological aspects associated with the development of brain ischemia such as the mechanism of oxygen and carbon dioxide effects on the cerebrovascular system, neurovascular coupling and respiratory and cardiovascular factors influencing cerebral hemodynamics. Consequently, we review investigations of cerebral blood flow disturbances relevant to various hemodynamic states associated with high intracranial pressure, cerebral embolism, and cerebral vasospasm along with current treatment options.

Purinergic receptors regulate myogenic tone in cerebral parenchymal arterioles

Myogenic tone is a fundamental aspect of vascular behavior in resistance arteries. This contractile response to changes in intravascular pressure is critically involved in blood flow autoregulation in tissues such as the brain, kidneys, and heart. Myogenic tone also helps regulate precapillary pressure and provides a level of background tone upon which vasodilator stimuli act to increase tissue perfusion when appropriate. Despite the importance of these processes in the brain, little is known about the mechanisms involved in control of myogenic tone in the cerebral microcirculation. Here, we report that pharmacological inhibition of P2Y4 and P2Y6 pyrimidine receptors nearly abolished myogenic tone in cerebral parenchymal arterioles (PAs). Molecular suppression of either P2Y4 or P2Y6 receptors using antisense oligodeoxynucleotides reduced myogenic tone by 44%±8% and 45%±7%, respectively. These results indicate that both receptor isoforms are activated by increased intravascular pressure, which enhances the activity of voltage-dependent calcium channels and increases myogenic tone in PAs. Enhancement or inhibition of ectonucleotidase activity had no effect on parenchymal arteriolar myogenic tone, indicating that this response is not mediated by local release of nucleotides, but rather may involve direct mechanical activation of P2Y receptors in the smooth muscle cells.

Do TRPC-like currents and G protein-coupled receptors interact to facilitate myogenic tone development?

The objective of this study was to determine whether Gq/11-coupled receptor activation can enhance the mechanosensitivity of a canonical transient receptor potential (TRPC)-like current and consequently the myogenic responsiveness of rat anterior cerebral arteries. Initial patch-clamp experiments revealed the presence of a basal cation current in isolated smooth muscle cells that displayed evidence of double rectification, which was blocked by trivalent cations (Gd3+ and La3+). PCR analysis identified the expression of TRPC1, 3, 6 and 7 mRNA and, characteristic of TRPC-like current, the whole-cell conductance was insensitive to a Na+-dependent transport (amiloride), TRP vanilloid (ruthenium red), and chloride channel (DIDS, niflumic acid, and flufenamate) inhibitors. One notable exception was tamoxifen, which elicited a dual effect, blocking or activating the TRPC-like current at 1 and 10 μM, respectively. This TRPC-like current was augmented by constrictor agonists (uridine 5′-triphosphate and U46619) or hyposmotic challenge (303 to 223 mOsm/l), a mechanical stimulus. Although each stimulus was effective alone, smooth muscle cells pretreated with agonist did not augment the whole-cell response to hyposmotic challenge. Consistent with these electrophysiological recordings, functional experiments revealed that neither UTP nor U46619 enhanced the sensitivity of intact cerebral arteries to hyposmotic challenge or elevated intravascular pressure. In summary, this study found no evidence that Gq/11-coupled receptor activation augments the mechanosensitivity of a TRPC-like current and consequently the myogenic responsiveness of anterior cerebral arteries.

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

This study examined whether elevated intravascular pressure stimulates asynchronous Ca(2+) waves in cerebral arterial smooth muscle cells and if their generation contributes to myogenic tone development. The endothelium was removed from rat cerebral arteries, which were then mounted in an arteriograph, pressurized (20-100 mmHg) and examined under a variety of experimental conditions. Diameter and membrane potential (V(M)) were monitored using conventional techniques; Ca(2+) wave generation and myosin light chain (MLC(20))/MYPT1 (myosin phosphatase targeting subunit) phosphorylation were assessed by confocal microscopy and Western blot analysis, respectively. Elevating intravascular pressure increased the proportion of smooth muscle cells firing asynchronous Ca(2+) waves as well as event frequency. Ca(2+) wave augmentation occurred primarily at lower intravascular pressures (<60 mmHg) and ryanodine, a plant alkaloid that depletes the sarcoplasmic reticulum (SR) of Ca(2+), eliminated these events. Ca(2+) wave generation was voltage insensitive as Ca(2+) channel blockade and perturbations in extracellular [K(+)] had little effect on measured parameters. Ryanodine-induced inhibition of Ca(2+) waves attenuated myogenic tone and MLC(20) phosphorylation without altering arterial V(M). Thapsigargin, an SR Ca(2+)-ATPase inhibitor also attenuated Ca(2+) waves, pressure-induced constriction and MLC(20) phosphorylation. The SR-driven component of the myogenic response was proportionally greater at lower intravascular pressures and subsequent MYPT1 phosphorylation measures revealed that SR Ca(2+) waves facilitated pressure-induced MLC(20) phosphorylation through mechanisms that include myosin light chain phosphatase inhibition. Cumulatively, our findings show that mechanical stimuli augment Ca(2+) wave generation in arterial smooth muscle and that these transient events facilitate tone development particularly at lower intravascular pressures by providing a proportion of the Ca(2+) required to directly control MLC(20) phosphorylation.

Isoform-selective physical coupling of TRPC3 channels to IP3 receptors in smooth muscle cells regulates arterial contractility

RATIONALE

Inositol 1,4,5-trisphosphate (IP(3))-induced vasoconstriction can occur independently of intracellular Ca(2+) release and via IP(3) receptor (IP(3)R) and canonical transient receptor potential (TRPC) channel activation, but functional signaling mechanisms mediating this effect are unclear.

OBJECTIVES

Study mechanisms by which IP(3)Rs stimulate TRPC channels in myocytes of resistance-size cerebral arteries.

METHODS AND RESULTS

Immunofluorescence resonance energy transfer (immuno-FRET) microscopy using isoform-selective antibodies indicated that endogenous type 1 IP(3)Rs (IP(3)R1) are in close spatial proximity to TRPC3, but distant from TRPC6 or TRPM4 channels in arterial myocytes. Endothelin-1 (ET-1), a phospholipase C-coupled receptor agonist, elevated immuno-FRET between IP(3)R1 and TRPC3, but not between IP(3)R1 and TRPC6 or TRPM4. TRPC3, but not TRPC6, coimmunoprecipitated with IP(3)R1. TRPC3 and TRPC6 antibodies selectively inhibited recombinant channels, but only the TRPC3 antibody blocked IP(3)-induced nonselective cation current (I(Cat)) in myocytes. TRPC3 knockdown attenuated immuno-FRET between IP(3)R1 and TRPC3, IP(3)-induced I(Cat) activation, and ET-1 and IP(3)-induced vasoconstriction, whereas TRPC6 channel knockdown had no effect. ET-1 did not alter total or plasma membrane-localized TRPC3, as determined using surface biotinylation. RT-PCR demonstrated that C-terminal calmodulin and IP(3)R binding (CIRB) domains are present in myocyte TRPC3 and TRPC6 channels. A peptide corresponding to the IP(3)R N-terminal region that can interact with TRPC channels activated I(Cat). A TRPC3 CIRB domain peptide attenuated IP(3)- and ET-1-induced I(Cat) activation and vasoconstriction.

CONCLUSIONS

IP(3) stimulates direct coupling between IP(3)R1 and membrane-resident TRPC3 channels in arterial myocytes, leading to I(Cat) activation and vasoconstriction. Close spatial proximity between IP(3)R1 and TRPC3 establishes this isoform-selective functional interaction.

Vascular control in humans: focus on the coronary microcirculation

Myocardial perfusion is regulated by a variety of factors that influence arteriolar vasomotor tone. An understanding of the physiological and pathophysiological factors that modulate coronary blood flow provides the basis for the judicious use of medications for the treatment of patients with coronary artery disease. Vasomotor properties of the coronary circulation vary among species. This review highlights the results of recent studies that examine the mechanisms by which the human coronary microcirculation is regulated in normal and disease states, focusing on diabetes. Multiple pathways responsible for myogenic constriction and flow-mediated dilation in human coronary arterioles are addressed. The important role of endothelium-derived hyperpolarizing factors, their interactions in mediating dilation, as well as speculation regarding the clinical significance are emphasized. Unique properties of coronary arterioles in human vs. other species are discussed.

Type 1 inositol 1,4,5-trisphosphate receptors mediate UTP-induced cation currents, Ca2+ signals, and vasoconstriction in cerebral arteries

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) regulate diverse physiological functions, including contraction and proliferation. There are three IP(3)R isoforms, but their functional significance in arterial smooth muscle cells is unclear. Here, we investigated relative expression and physiological functions of IP(3)R isoforms in cerebral artery smooth muscle cells. We show that 2-aminoethoxydiphenyl borate and xestospongin C, membrane-permeant IP(3)R blockers, reduced Ca(2+) wave activation and global intracellular Ca(2+) ([Ca(2+)](i)) elevation stimulated by UTP, a phospholipase C-coupled purinergic receptor agonist. Quantitative PCR, Western blotting, and immunofluorescence indicated that all three IP(3)R isoforms were expressed in acutely isolated cerebral artery smooth muscle cells, with IP(3)R1 being the most abundant isoform at 82% of total IP(3)R message. IP(3)R1 knockdown with short hairpin RNA (shRNA) did not alter baseline Ca(2+) wave frequency and global [Ca(2+)](i) but abolished UTP-induced Ca(2+) wave activation and reduced the UTP-induced global [Ca(2+)](i) elevation by approximately 61%. Antibodies targeting IP(3)R1 and IP(3)R1 knockdown reduced UTP-induced nonselective cation current (I(cat)) activation. IP(3)R1 knockdown also reduced UTP-induced vasoconstriction in pressurized arteries with both intact and depleted sarcoplasmic reticulum (SR) Ca(2+) by approximately 45%. These data indicate that IP(3)R1 is the predominant IP(3)R isoform expressed in rat cerebral artery smooth muscle cells. IP(3)R1 stimulation contributes to UTP-induced I(cat) activation, Ca(2+) wave generation, global [Ca(2+)](i) elevation, and vasoconstriction. In addition, IP(3)R1 activation constricts cerebral arteries in the absence of SR Ca(2+) release by stimulating plasma membrane I(cat).

IP3 constricts cerebral arteries via IP3 receptor-mediated TRPC3 channel activation and independently of sarcoplasmic reticulum Ca2+ release

Vasoconstrictors that bind to phospholipase C-coupled receptors elevate inositol-1,4,5-trisphosphate (IP(3)). IP(3) is generally considered to elevate intracellular Ca(2+) concentration ([Ca(2+)](i)) in arterial myocytes and induce vasoconstriction via a single mechanism: by activating sarcoplasmic reticulum (SR)-localized IP(3) receptors, leading to intracellular Ca(2+) release. We show that IP(3) also stimulates vasoconstriction via a SR Ca(2+) release-independent mechanism. In isolated cerebral artery myocytes and arteries in which SR Ca(2+) was depleted to abolish Ca(2+) release (measured using D1ER, a fluorescence resonance energy transfer-based SR Ca(2+) indicator), IP(3) activated 15 pS sarcolemmal cation channels, generated a whole-cell cation current (I(Cat)) caused by Na(+) influx, induced membrane depolarization, elevated [Ca(2+)](i), and stimulated vasoconstriction. The IP(3)-induced I(Cat) and [Ca(2+)](i) elevation were attenuated by cation channel (Gd(3+), 2-APB) and IP(3) receptor (xestospongin C, heparin, 2-APB) blockers. TRPC3 (canonical transient receptor potential 3) channel knockdown with short hairpin RNA and diltiazem and nimodipine, voltage-dependent Ca(2+) channel blockers, reduced the SR Ca(2+) release-independent, IP(3)-induced [Ca(2+)](i) elevation and vasoconstriction. In pressurized arteries, SR Ca(2+) depletion did not alter IP(3)-induced constriction at 20 mm Hg but reduced IP(3)-induced constriction by approximately 39% at 60 mm Hg. [Ca(2+)](i) elevations and constrictions induced by endothelin-1, a phospholipase C-coupled receptor agonist, were both attenuated by TRPC3 knockdown and xestospongin C in SR Ca(2+)-depleted arteries. In summary, we describe a novel mechanism of IP(3)-induced vasoconstriction that does not occur as a result of SR Ca(2+) release but because of IP(3) receptor-dependent I(Cat) activation that requires TRPC3 channels. The resulting membrane depolarization activates voltage-dependent Ca(2+) channels, leading to a myocyte [Ca(2+)](i) elevation, and vasoconstriction.

Decreased activity of the smooth muscle Na+/Ca2+ exchanger impairs arteriolar myogenic reactivity

Arteriolar myogenic vasoconstriction occurs when stretch or increased membrane tension leads to smooth muscle cell (SMC) depolarization and opening of voltage-gated Ca(2+) channels. While the mechanism underlying the depolarization is uncertain a role for non-selective cation channels has been demonstrated. As such channels may be expected to pass Na(+), we hypothesized that reverse mode Na(+)/Ca(2+) exchange (NCX) may act to remove Na(+) and in addition play a role in myogenic signalling through coupled Ca(2+) entry. Further, reverse (Ca(2+) entry) mode function of the NCX is favoured by the membrane potential found in myogenically active arterioles. All experiments were performed on isolated rat cremaster muscle first order arterioles (passive diameter approximately 150 mum) which were pressurized in the absence of intraluminal flow. Reduction of extracellular Na(+) to promote reverse-mode NCX activity caused significant, concentration-dependent vasoconstriction and increased intracellular Ca(2+). This vasoconstriction was attenuated by the NCX inhibitors KB-R7943 and SEA 04000. Western blotting confirmed the existence of NCX protein while real-time PCR studies demonstrated that the major isoform expressed in the arteriolar wall was NCX1. Oligonucleotide knockdown (24 and 36 h) of NCX inhibited the vasoconstrictor response to reduced extracellular Na(+) while also impairing both steady-state myogenic responses (as shown by pressure-diameter relationships) and acute reactivity to a 50 to 120 mmHg pressure step. The data are consistent with reverse mode activity of the NCX in arterioles and a contribution of this exchanger to myogenic vasoconstriction.

TRP channels in hypertension

Pulmonary and systemic arterial hypertension are associated with profound alterations in Ca(2+) homeostasis and smooth muscle cell proliferation. A novel class of non-selective cation channels, the transient receptor potential (TRP) channels, have emerged at the forefront of research into hypertensive disease states. TRP channels are identified as molecular correlates for receptor-operated and store-operated cation channels in the vasculature. Over 10 TRP isoforms are identified at the mRNA and protein expression levels in the vasculature. Current research implicates upregulation of specific TRP isoforms to be associated with increased Ca(2+) influx, characteristic of vasoconstriction and vascular smooth muscle cell proliferation. TRP channels are implicated as Ca(2+) entry pathways in pulmonary hypertension and essential hypertension. Caveolae have recently emerged as membrane microdomains in which TRP channels may be co-localized with the endoplasmic reticulum in both smooth muscle and endothelial cells. Such enhanced expression and function of TRP channels and their localization in caveolae in pathophysiological hypertensive disease states highlights their importance as potential targets for pharmacological intervention.

TRP channels in endothelial function and dysfunction

Endothelial cells produce various factors that regulate vascular tone, vascular permeability, angiogenesis, and inflammatory responses. The dysfunction of endothelial cells is believed to be the major culprit in various cardiovascular diseases, including hypertension, atherosclerosis, heart and renal failure, coronary syndrome, thrombosis, and diabetes. Endothelial cells express multiple transient receptor potential (TRP) channel isoforms, the activity of which serves to modulate cytosolic Ca(2+) levels ([Ca(2+)](i)) and regulate membrane potential, both of which affect various physiological processes. The malfunction and dysregulation of TRP channels is associated with endothelial dysfunction, which is reflected by decreased nitric oxide (NO) bioavailability, inappropriate regulation of vascular smooth muscle tonicity, endothelial barrier dysfunction, increased oxidative damage, impaired anti-thrombogenic properties, and perturbed angiogenic competence. Evidence suggests that dysregulation of TRPC4 and -C1 results in vascular endothelial barrier dysfunction; malfunction of TRPP1 and -P2 impairs endothelial NO synthase; the reduced expression or activity of TRPC4 and -V1 impairs agonist-induced vascular relaxation; the decreased activity of TRPV4 reduces flow-induced vascular responses; and the activity of TRPC3 and -C4 is associated with oxidative stress-induced endothelial damage. In this review, we present a comprehensive summary of the literature on the role of TRP channels in endothelial cells, with an emphasis on endothelial dysfunction.

Protein kinase C regulates vascular myogenic tone through activation of TRPM4

Myogenic vasoconstriction results from pressure-induced vascular smooth muscle cell depolarization and Ca(2+) influx via voltage-dependent Ca(2+) channels, a process that is significantly attenuated by inhibition of protein kinase C (PKC). It was recently reported that the melastatin transient receptor potential (TRP) channel TRPM4 is a critical mediator of pressure-induced smooth muscle depolarization and constriction in cerebral arteries. Interestingly, PKC activity enhances the activation of cloned TRPM4 channels expressed in cultured cells by increasing sensitivity of the channel to intracellular Ca(2+). Thus we postulated that PKC-dependent activation of TRPM4 might be a critical mediator of vascular myogenic tone. We report here that PKC inhibition attenuated pressure-induced constriction of cerebral vessels and that stimulation of PKC activity with phorbol 12-myristate 13-acetate (PMA) enhanced the development of myogenic tone. In freshly isolated cerebral artery myocytes, we identified a Ca(2+)-dependent, rapidly inactivating, outwardly rectifying, iberiotoxin-insensitive cation current with properties similar to those of expressed TRPM4 channels. Stimulation of PKC activity with PMA increased the intracellular Ca(2+) sensitivity of this current in vascular smooth muscle cells. To validate TRPM4 as a target of PKC regulation, antisense technology was used to suppress TRPM4 expression in isolated cerebral arteries. Under these conditions, the magnitude of TRPM4-like currents was diminished in cells from arteries treated with antisense oligonucleotides compared with controls, identifying TRPM4 as the molecular entity responsible for the PKC-activated current. Furthermore, the extent of PKC-induced smooth muscle cell depolarization and vasoconstriction was significantly decreased in arteries treated with TRPM4 antisense oligonucleotides compared with controls. We conclude that PKC-dependent regulation of TRPM4 activity contributes to the control of cerebral artery myogenic tone.

Role of L-type calcium channels and PKC in active tone development in rabbit coronary artery

The present study investigated active tone development in isolated ring segments of rabbit epicardial coronary artery. Endothelium-denuded (E-) or endothelium-intact (E+) vessels treated with the NO synthase inhibitor N(omega)-nitro-L-arginine (100 microM) developed active tone, which was enhanced by stretch and reversed by the NO donor sodium nitroprusside (SNP; IC(50)=9 nM). Nifedipine abolished active tone and the contractile response to phorbol dibutyrate (PDBu; 10 nM) with the same potency (IC(50)=8 nM), whereas 300 nM PDBu responses were only partially blocked by nifedipine. The classical and novel PKC inhibitors GF-109203X (IC(50)=1-2 microM) and chelerythrine (IC(50)=4-5 microM) and the classical PKC inhibitor Gö-6976 (IC(50)=0.3-0.4 microM) blocked both active tone and 10 nM PDBu responses with similar potency. Active tone development was associated with depolarization of membrane potential (E(m)) and a shift to the left of the E(m)-vs.-contraction relationship determined by varying extracellular potassium. The depolarization and leftward shift were reversed by either chelerythrine (10 microM) or SNP (30 nM). PDBu (100-300 nM) increased peak L-type calcium channel (Ca(v)) currents in isolated coronary myocytes, and this effect was reversed by chelerythrine (1 microM) or Gö-6976 (200 nM). SNP (500 nM) reduced Ca(v) currents only in the presence of the PKA blocker 8-bromo-2'-O-monobutyryl-cAMPS, Rp isomer (10 microM). In conclusion, active tone development in coronary artery is suppressed by basal NO release and is dependent on both enhanced Ca(v) activity and classical PKC activity. Both E(m)-dependent and -independent processes contribute to contraction. Our results suggest that one E(m)-independent process is direct enhancement of Ca(v) current by PKC.

Characteristics of myogenic tone in the rat ophthalmic artery

The pressure-induced constriction in the rat ophthalmic artery was characterized. Ophthalmic arteries were isolated, cannulated in an arteriograph, and pressurized. Arteries developed 25% constriction at 70 mmHg of intraluminal pressure. Arteries maintained almost similar diameter over the range of pressures 50–210 mmHg, and forced dilatation was observed at pressures >210 mmHg. Denudation of endothelium increased the sensitivity of arteries to pressure-induced constriction, and significantly higher myogenic tone was observed in the pressure range of 10–100 mmHg. Indomethacin and cyclooxygenase-2 inhibition by SC-236 decreased myogenic tone, whereas cyclooxygenase-1 inhibition by SC-560 potentiated myogenic tone in a lower concentration range and decreased at a higher concentration. Pressure-induced constriction was completely blocked by 1 μM nifedipine. Phospholipase C inhibition by 6 μM U-73122 decreased myogenic tone by 39%, whereas PKC inhibitor GF-109203X (3 μM) had no effect. Constriction to phenylephrine was significantly decreased by U-73122 (1 μM) and GF-109203X (3 μM) at an intraluminal pressure of 10 mmHg. Rho-kinase inhibition by Y-27632 (30 μM) and HA-1077 (30 μM) decreased myogenic tone by 75% and 73%, respectively, and 1 μM Y-27632 significantly decreased myogenic tone developed in response to graded increases in pressure. These results suggest that rat ophthalmic artery has an efficient pressure-dependent autoregulatory function that is modulated by endothelium. Contribution of phospholipase C-activation to myogenic tone is minimal, whereas Rho-kinase activation plays a predominant role in the myogenic reactivity in this artery.

Calcium sparklets regulate local and global calcium in murine arterial smooth muscle

In arterial smooth muscle, protein kinase Calpha (PKCalpha) coerces discrete clusters of L-type Ca2+ channels to operate in a high open probability mode, resulting in subcellular domains of nearly continual Ca2+ influx called 'persistent Ca2+ sparklets'. Our previous work suggested that steady-state Ca2+ entry into arterial myocytes, and thus global [Ca2+]i, is regulated by Ca2+ influx through clusters of L-type Ca2+ channels operating in this persistently active mode in addition to openings of solitary channels functioning in a low-activity mode. Here, we provide the first direct evidence supporting this 'Ca2+ sparklet' model of Ca2+ influx at a physiological membrane potential and external Ca2+ concentration. In support of this model, we found that persistent Ca2+ sparklets produced local and global elevations in [Ca2+]i. Membrane depolarization increased Ca2+ influx via low-activity and high-activity persistent Ca2+ sparklets. Our data indicate that Ca2+ entering arterial smooth muscle through persistent Ca2+ sparklets accounts for approximately 50% of the total dihydropyridine-sensitive (i.e. L-type Ca2+ channel) Ca2+ influx at a physiologically relevant membrane potential (-40 mV) and external Ca2+ concentration (2 mm). Consistent with this, inhibition of basal PKCalpha-dependent persistent Ca2+ sparklets decreased [Ca2+]i by about 50% in isolated arterial myocytes and intact pressurized arteries. Taken together, these data support the conclusion that in arterial smooth muscle steady-state Ca2+ entry and global [Ca2+]i are regulated by low-activity and PKCalpha-dependent high-activity persistent Ca(2+) sparklets.

Hyposmotic challenge inhibits inward rectifying K+ channels in cerebral arterial smooth muscle cells

This study sought to define whether inward rectifying K(+) (K(IR)) channels were modulated by vasoactive stimuli known to depolarize and constrict intact cerebral arteries. Using pressure myography and patch-clamp electrophysiology, initial experiments revealed a Ba(2+)-sensitive K(IR) current in cerebral arterial smooth muscle cells that was active over a physiological range of membrane potentials and whose inhibition led to arterial depolarization and constriction. Real-time PCR, Western blot, and immunohistochemical analyses established the expression of both K(IR)2.1 and K(IR)2.2 in cerebral arterial smooth muscle cells. Vasoconstrictor agonists known to depolarize and constrict rat cerebral arteries, including uridine triphosphate, U46619, and 5-HT, had no discernable effect on whole cell K(IR) activity. Control experiments confirmed that vasoconstrictor agonists could inhibit the voltage-dependent delayed rectifier K(+) (K(DR)) current. In contrast to these observations, a hyposmotic challenge that activates mechanosensitive ion channels elicited a rapid and sustained inhibition of the K(IR) but not the K(DR) current. The hyposmotic-induced inhibition of K(IR) was 1) mimicked by phorbol-12-myristate-13-acetate, a PKC agonist; and 2) inhibited by calphostin C, a PKC inhibitor. These findings suggest that, by modulating PKC, mechanical stimuli can regulate K(IR) activity and consequently the electrical and mechanical state of intact cerebral arteries. We propose that the mechanoregulation of K(IR) channels plays a role in the development of myogenic tone.

Cation channels of the transient receptor potential superfamily: their role in physiological and pathophysiological processes of smooth muscle cells

Smooth muscle cells (SMC) are essential components of many tissues of the body. Ion channels regulate their membrane potential, the intracellular Ca(2+) concentration ([Ca(2+)](i)) and their contractility. Among the ion channels expressed in SMC cation channels of the transient receptor potential (TRP) superfamily allow the entry of Na(+), Ca(2+) and Mg(2+). Members of the TRP superfamily are essential constituents of tonically active channels (TAC), receptor-operated channels (ROC), store-operated channels (SOC) and stretch-activated channels (SAC). This review focusses on TRP channels (TRPC1, TRPC3, TRPC4, TRPC5, TRPC6, TRPC7, TRPV2, TRPV4, TRPM4, TRPM7, TRPP2) whose physiological functions in SMC were dissected by downregulating channel activity in isolated tissues or by the analysis of gene-deficient mouse models. Their possible functional role and physiological regulation as homomeric or heteromeric channels in SMC are discussed. Moreover, TRP channels may also be responsible for pathophysiological processes involving SMC-like airway hyperresponsiveness and pulmonary hypertension. Therefore, they present important drug targets for future pharmacological interventions.

Heteromultimeric TRPC6-TRPC7 channels contribute to arginine vasopressin-induced cation current of A7r5 vascular smooth muscle cells

The molecular identity of receptor-operated, nonselective cation channels (ROCs) of vascular smooth muscle (VSM) cells is not known for certain. Mammalian homologues of the Drosophila canonical transient receptor potential channels (TRPCs) are possible candidates. This study tested the hypothesis that heteromultimeric TRPC channels contribute to ROC current of A7r5 VSM cells activated by [Arg(8)]-vasopressin. A7r5 cells expressed transcripts encoding TRPC1, TRPC4beta, TRPC6, and TRPC7. TRPC4, TRPC6, and TRPC7 protein expression was confirmed by immunoblotting and association of TRPC6 with TRPC7, but not TRPC4beta, was detected by coimmunoprecipitation. The amplitude of arginine vasopressin (AVP)-induced ROC current was suppressed by dominant-negative mutant TRPC6 (TRPC6(DN)) but not TRPC5 (TRPC5(DN)) mutant subunit expression. These data indicate a role for TRPC6- and/or TRPC7-containing channels and rule a more complex subunit composition including TRPC1 and TRPC4. Increasing extracellular Ca(2+) concentration ([Ca(2+)](o)) from 0.05 to 1 mmol/L suppressed currents owing to native, TRPC7, and heteromultimeric TRPC6-TRPC7 channels, but not TRPC6 current, which was slightly enhanced. The relative changes in native and heteromultimeric TRPC6-TRPC7 current amplitudes for [Ca(2+)](o) between approximately 0.01 and 1 mmol/L were identical, but the changes in homomultimeric TRPC6 and TRPC7 currents were significantly less and greater, respectively, compared with the native channels. Taken together, the data provide biochemical and functional evidence supporting the view that heteromultimeric TRPC6-TRPC7 channels contribute to receptor-activated, nonselective cation channels of A7r5 VSM cells.

Recent developments in vascular endothelial cell transient receptor potential channels

Among the 28 identified and unique mammalian TRP (transient receptor potential) channel isoforms, at least 19 are expressed in vascular endothelial cells. These channels appear to participate in a diverse range of vascular functions, including control of vascular tone, regulation of vascular permeability, mechanosensing, secretion, angiogenesis, endothelial cell proliferation, and endothelial cell apoptosis and death. Malfunction of these channels may result in disorders of the human cardiovascular system. All TRP channels, except for TRPM4 and TRPM5, are cation channels that allow Ca2+ influx. However, there is a daunting diversity in the mode of activation and regulation in each case. Specific TRP channels may be activated by different stimuli such as vasoactive agents, oxidative stress, mechanical stimuli, and heat. TRP channels may then transform these stimuli into changes in the cytosolic Ca2+, which are eventually coupled to various vascular responses. Evidence has been provided to suggest the involvement of at least the following TRP channels in vascular function: TRPC1, TRPC4, TRPC6, and TRPV1 in the control of vascular permeability; TRPC4, TRPV1, and TRPV4 in the regulation of vascular tone; TRPC4 in hypoxia-induced vascular remodeling; and TRPC3, TRPC4, and TRPM2 in oxidative stress-induced responses. However, in spite of the large body of data available, the functional role of many endothelial TRP channels is still poorly understood. Elucidating the mechanisms regulating the different endothelial TRP channels, and the associated development of drugs selectively to target the different isoforms, as a means to treat cardiovascular disease should, therefore, be a high priority.

Diacylglycerol-Containing Docosahexaenoic Acid in Acyl Chain Modulates Airway Smooth Muscle Tone

We synthesized and assessed the role of a diacylglycerol (DAG)-containing docosahexaenoic acid (DHA), that is, 1-stearoyl-2-docosahexaenoyl-sn-glycerol (SDHG), in the contraction of guinea pig airway smooth muscle (ASM). We compared its action with 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) and 1,2-dioctanoyl-sn-glycerol (1,2-DiC8), a stable DAG analog. The three DAGs (SAG, SDHG, and 1,2-DiC8) induced reversible concentration-dependent contraction of ASM. SDHG induced higher guinea pig ASM contraction than did SAG and 1,2-DiC8. The effects of SDHG were blocked, to different extents, by nifedipine (L-type Ca2+ channel blocker). By employing GF-109203X (protein kinase C [PKC] inhibitor) and lanthanum (La3+), a nonselective cation channel blocker, we observed that SDHG evoked ASM contractile response via PKC-dependent and PKC-independent (but Ca2+-dependent) pathways. Interestingly, SAG exerted its action only by increasing [Ca2+]i and did not require PKC activation. To probe the implication of calcium mobilization, we employed thapsigargin (TG), which also induced ASM contraction in a calcium-dependent manner. SDHG and 1,2-DiC8, in a PKC-dependent manner, induced the phosphorylation of CPI-17 (myosin light chain phosphatase inhibitor of 17 kD). Furthermore, SAG and TG failed to phosphorylate CPI-17 in ASM cells. Our results suggest that different DAG species, produced during a dietary supplementation with fatty acids, could modulate the reactivity of airway smooth muscles in a PKC-dependent and -independent manner, and hence, may play a critical role in health and disease.

Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes

Activation of Ca2+-permeable non-selective cation channels produces an increase in excitability of vascular smooth muscle cells which has an important role in vasoconstriction. These channels are activated by various physiological stimuli including vasoconstrictor agents such as noradrenaline, depletion of internal Ca2+ stores and cell stretching. In addition cation channels have been shown to be constitutively active and these channels are thought to contribute to resting membrane conductance and basal Ca2+ influx in vascular myocytes. Recent evidence has suggested that transient receptor potential (TRP) proteins represent strong candidates for these channels in the vasculature. This review discusses proposed signal transduction pathways and gating mechanisms which link physiological stimuli to opening of cation channels in vascular myocytes. It is apparent that G-protein-coupled pathways linked to stimulation of phospholipase activity have a profound effect on regulating channel activity and that generation of diacylglycerol (DAG) is a central event in these signalling cascades with this triglyceride having a pivotal role in gating cation channels via both PKC-independent and -dependent mechanisms. Moreover phosphorylation processes produced by stimulation of protein kinases have been proposed to have an important role in regulating cation channel activity.

Identification of a spontaneously active, Na+-permeable channel in guinea pig gallbladder smooth muscle

The action potential in gallbladder smooth muscle (GBSM) is caused by Ca2+ entry through voltage-dependent Ca2+ channels (VDCC), which contributes to the GBSM contractions. Action potential generation in GBSM is critically dependent on the resting membrane potential (about -50 mV), which is approximately 35 mV more positive of the K+ equilibrium potential. We hypothesized that a tonic, depolarizing conductance is present in GBSM and contributes to the regulation of the resting membrane potential and action potential frequency. GBSM cells were isolated from guinea pig gallbladders, and the whole cell patch-camp technique was used to record membrane currents. After eliminating the contribution of VDCC and K+ channels, we identified a novel spontaneously active cation conductance (I(cat)) in GBSM. This I(cat) was mediated predominantly by influx of Na+. Na+ substitution with N-methyl-D-glucamine (NMDG), a large relatively impermeant cation, caused a negative shift in the reversal potential of the ramp current and reduced the amplitude of the inward current at -50 mV by 65%. Membrane potential recordings with intracellular microelectrodes or in current-clamp mode of the patch-clamp technique indicated that the inhibition of I(cat) conductance by NMDG is associated with membrane hyperpolarization and inhibition of action potentials. Extracellular Ca2+, Mg2+, and Gd3+ attenuated the I(cat) in GBSM. Muscarinic stimulation did not activate the I(cat). Our results indicate that, in GBSM, an Na+-permeable channel contributes to the maintenance of the resting membrane potential and action potential generation and therefore plays a critical role in the regulation of GBSM excitability and contractility.

Critical role for transient receptor potential channel TRPM4 in myogenic constriction of cerebral arteries

Local control of cerebral blood flow is regulated in part through myogenic constriction of resistance arteries. Although this response requires Ca2+ influx via voltage-dependent Ca2+ channels secondary to smooth muscle cell depolarization, the mechanisms responsible for alteration of vascular smooth muscle (VSM) cell membrane potential are not fully understood. A previous study from our laboratory demonstrated a critical role for a member of the transient receptor potential (TRP) superfamily of ion channels, TRPC6, in this response. Several other of the approximately 22 identified TRP proteins are also present in cerebral arteries, but their functions have not been elucidated. Two of these channels, TRPM4 and TRPM5, exhibit biophysical properties that are consistent with a role for control of membrane potential of excitable cells. We hypothesized that TRPM4/TRPM5-dependent currents contribute to myogenic vasoconstriction of cerebral arteries. Cation channels with unitary conductance, ion selectivity and Ca2+-dependence similar to those of cloned TRPM4 and TRPM5 were present in freshly isolated VSM cells. We found that TRPM4 mRNA was detected in both whole cerebral arteries and in isolated VSM cells whereas TRPM5 message was absent from cerebral artery myocytes. We also found that pressure-induced smooth muscle cell depolarization was attenuated in isolated cerebral arteries treated with TRPM4 antisense oligodeoxynucleotides to downregulate channel subunit expression. In agreement with these data, myogenic vasoconstriction of intact cerebral arteries administered TRPM4 antisense was attenuated compared with controls, whereas KCl-induced constriction did not differ between groups. We concluded that activation of TRPM4-dependent currents contributed to myogenic vasoconstriction of cerebral arteries.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Properties of a tonically active, sodium-permeable current in mouse urinary bladder smooth muscle

Urinary bladder smooth muscle (UBSM) elicits depolarizing action potentials, which underlie contractile events of the urinary bladder. The resting membrane potential of UBSM is approximately −40 mV and is critical for action potential generation, with hyperpolarization reducing action potential frequency. We hypothesized that a tonic, depolarizing conductance was present in UBSM, functioning to maintain the membrane potential significantly positive to the equilibrium potential for K+ ( EK; −85 mV) and thereby facilitate action potentials. Under conditions eliminating the contribution of K+ and voltage-dependent Ca2+ channels, and with a clear separation of cation- and Cl−-selective conductances, we identified a novel background conductance ( Icat) in mouse UBSM cells. Icat was mediated predominantly by the influx of Na+, although a small inward Ca2+ current was detectable with Ca2+ as the sole cation in the bathing solution. Extracellular Ca2+, Mg2+, and Gd3+ blocked Icat in a voltage-dependent manner, with Ki values at −40 mV of 115, 133, and 1.3 μM, respectively. Although UBSM Icat is extensively blocked by physiological extracellular Ca2+ and Mg2+, a tonic, depolarizing Icat was detected at −40 mV. In addition, inhibition of Icat demonstrated a hyperpolarization of the UBSM membrane potential and decreased the amplitude of phasic contractions of isolated UBSM strips. We suggest that Icat contributes tonically to the depolarization of the UBSM resting membrane potential, facilitating action potential generation and thereby a maintenance of urinary bladder tone.