Upregulation of Na+/Ca2+ exchanger and TRPC6 contributes to abnormal Ca2+ homeostasis in arterial smooth muscle cells from Milan hypertensive rats

STIM Proteins: The Gas and Brake of Calcium Entry in Neurons

Stromal interaction molecules (STIM)s are Ca2+ sensors in internal Ca2+ stores of the endoplasmic reticulum. They activate the store-operated Ca2+ channels, which are the main source of Ca2+ entry in non-excitable cells. Moreover, STIM proteins interact with other Ca2+ channel subunits and active transporters, making STIMs an important intermediate molecule in orchestrating a wide variety of Ca2+ influxes into excitable cells. Nevertheless, little is known about the role of STIM proteins in brain functioning. Being involved in many signaling pathways, STIMs replenish internal Ca2+ stores in neurons and mediate synaptic transmission and neuronal excitability. Ca2+ dyshomeostasis is a signature of many pathological conditions of the brain, including neurodegenerative diseases, injuries, stroke, and epilepsy. STIMs play a role in these disturbances not only by supporting abnormal store-operated Ca2+ entry but also by regulating Ca2+ influx through other channels. Here, we review the present knowledge of STIMs in neurons and their involvement in brain pathology.

Role of transient receptor potential channels in the regulation of vascular tone

Vascular tone is a major element in the control of hemodynamics. Transient receptor potential (TRP) channels conducting monovalent and/or divalent cations (e.g. Na+ and Ca2+) are expressed in the vasculature. Accumulating evidence suggests that TRP channels participate in regulating vascular tone by regulating intracellular Ca2+ signaling in both vascular smooth muscle cells (VSMCs) and endothelial cells (ECs). Aberrant expression/function of TRP channels in the vasculature is associated with vascular dysfunction in systemic/pulmonary hypertension and metabolic syndromes. This review intends to summarize our current knowledge of TRP-mediated regulation of vascular tone in both physiological and pathophysiological conditions and to discuss potential therapeutic approaches to tackle abnormal vascular tone due to TRP dysfunction.

Inhibition and potentiation of the exercise pressor reflex by pharmacological modulation of TRPC6 in male rats

We determined the role played by the transient receptor potential canonical 6 (TRPC6) channel in evoking the mechanical component of the exercise pressor reflex in male decerebrated Sprague-Dawley rats. TRPC6 channels were identified by quadruple-labelled (DiI, TRPC6, neurofilament-200 and peripherin) immunohistochemistry in dorsal root ganglion (DRG) cells innervating the triceps surae muscles (n = 12). The exercise pressor reflex was evoked by statically contracting the triceps surae muscles before and after injection of the TRPC6 antagonist BI-749327 (n = 11; 12 μg kg-1 ) or SAR7334 (n = 11; 7 μg kg-1 ) or the TRPC6 positive modulator C20 (n = 11; 18 μg kg-1 ). Similar experiments were conducted while the muscles were passively stretched (n = 8-12), a manoeuvre that isolated the mechanical component of the reflex. Blood pressure, tension, renal sympathetic nerve activity (RSNA) and blood flow were recorded. Of the DRG cells innervating the triceps surae muscles, 85% stained positive for the TRPC6 antigen, and 45% of those cells co-expressed neurofilament-200. Both TRPC6 antagonists decreased the reflex pressor responses to static contraction (-32 to -42%; P < 0.05) and to passive stretch (-35 to -52%; P < 0.05), whereas C20 increased these responses (55-65%; P < 0.05). In addition, BI-749327 decreased the peak and integrated RSNA responses to both static contraction (-39 to -43%; P < 0.05) and passive stretch (-56 to -62%; P < 0.05), whereas C20 increased the RSNA to passive stretch only. The onset latency of the decrease or increase in RSNA occurred within 2 s of the onset of the manoeuvres (P < 0.05). Collectively, our results show that TRPC6 plays a key role in evoking the mechanical component of the exercise pressor reflex. KEY POINTS: The exercise pressor reflex plays a key role in the sympathetic and haemodynamic responses to exercise. This reflex is composed of two components, namely the mechanoreflex and the metaboreflex. The receptors responsible for evoking the mechanoreflex are poorly documented. A good candidate for this function is the transient receptor potential canonical 6 (TRPC6) channel, which is activated by mechanical stimuli and expressed in dorsal root ganglia of rats. Using two TRPC6 antagonists and one positive modulator, we investigated the role played by TRPC6 in evoking the mechanoreflex in decerebrated rats. Blocking TRPC6 decreased the renal sympathetic and the pressor responses to both contraction and stretch, the latter being a manoeuvre that isolates the mechanoreflex. In contrast, the positive modulator increased the pressor reflex to contraction and stretch, in addition to the sympathetic response to stretch. Our results provide strong support for a role played by the TRPC6 channel in evoking the mechanoreflex.

Vascular smooth muscle ion channels in essential hypertension

Hypertension is a highly prevalent chronic disease and the major risk factor for cardiovascular diseases, the leading cause of death worldwide. Hypertension is characterized by an increased vascular tone determined by the contractile state of vascular smooth muscle cells that depends on intracellular calcium levels. The interplay of ion channels determine VSMCs membrane potential and thus intracellular calcium that controls the degree of contraction, vascular tone and blood pressure. Changes in ion channels expression and function have been linked to hypertension, but the mechanisms and molecular entities involved are not completely clear. Furthermore, the literature shows discrepancies regarding the contribution of different ion channels to hypertension probably due to differences both in the vascular preparation and in the model of hypertension employed. Animal models are essential to study this multifactorial disease but it is also critical to know their characteristics to interpret properly the results obtained. In this review we summarize previous studies, using the hypertensive mouse (BPH) and its normotensive control (BPN), focused on the identified changes in the expression and function of different families of ion channels. We will focus on L-type voltage-dependent Ca2+ channels (Cav1.2), canonical transient receptor potential channels and four different classes of K+ channels: voltage-activated (Kv), large conductance Ca2+-activated (BK), inward rectifiers (Kir) and ATP-sensitive (KATP) K+ channels. We will describe the role of these channels in hypertension and we will discuss the importance of integrating individual changes in a global context to understand the complex interplay of ion channels in hypertension.

The adducin saga: pleiotropic genomic targets for precision medicine in human hypertension-vascular, renal, and cognitive diseases

Hypertension is a leading risk factor for stroke, heart disease, chronic kidney disease, vascular cognitive impairment, and Alzheimer's disease. Previous genetic studies have nominated hundreds of genes linked to hypertension, and renal and cognitive diseases. Some have been advanced as candidate genes by showing that they can alter blood pressure or renal and cerebral vascular function in knockout animals; however, final validation of the causal variants and underlying mechanisms has remained elusive. This review chronicles 40 years of work, from the initial identification of adducin (ADD) as an ACTIN-binding protein suggested to increase blood pressure in Milan hypertensive rats, to the discovery of a mutation in ADD1 as a candidate gene for hypertension in rats that were subsequently linked to hypertension in man. More recently, a recessive K572Q mutation in ADD3 was identified in Fawn-Hooded Hypertensive (FHH) and Milan Normotensive (MNS) rats that develop renal disease, which is absent in resistant strains. ADD3 dimerizes with ADD1 to form functional ADD protein. The mutation in ADD3 disrupts a critical ACTIN-binding site necessary for its interactions with actin and spectrin to regulate the cytoskeleton. Studies using Add3 KO and transgenic strains, as well as a genetic complementation study in FHH and MNS rats, confirmed that the K572Q mutation in ADD3 plays a causal role in altering the myogenic response and autoregulation of renal and cerebral blood flow, resulting in increased susceptibility to hypertension-induced renal disease and cerebral vascular and cognitive dysfunction.

The effect of chloroquine on the TRPC1, TRPC6, and CaSR in the pulmonary artery smooth muscle cells in hypoxia-induced experimental pulmonary artery hypertension

Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by a constant high pulmonary artery pressure and the remodeling of the vessel. Chloroquine (CLQ) has been observed to inhibit calcium influx. The aim of this study is to investigate the effect of CLQ on transient receptor cationic proteins (TRPC1 and TRPC6) and extracellular calcium-sensitive receptor (CaSR) in a hypoxic PAH model. In this study, 8- to 12-week-old 32 male Wistar albino rats, weighing 200 to 300 g, were used. The rats were studied in four groups, including normoxy control, n = 8; normoxy CLQ (50 mg/kg/28 d), n = 8; hypoxia (HX; 10% oxygen/28 d) control, n = 8; and HX (10% oxygen/28 d) + CLQ (50 mg/kg), N = 8. Pulmonary arterial medial wall thickness, pulmonary arteriole wall, TRPC1, TRPC6, and CaSR expressions were evaluated by immunohistochemistry, polymerase chain reaction, and enzyme-linked immunosorbent assay methods. At the end of the experiment, a statistically significant increase in the medial wall thickness was observed in the hypoxic group as compared with the control group. However, in the HX + CLQ group, there was a statistically significant decrease in the vessel medial wall as compared with the HX group. In the TRPC1-, TRPC6-, and CaSR-immunopositive cell numbers, messenger RNA expressions and biochemical results showed an increase in the HX group, whereas they were decreased in the HX + CLQ group. The inhibitory effect of CLQ on calcium receptors in arterioles was observed in PAH.

Transient Receptor Potential Canonical (TRPC) Channels: Then and Now

Twenty-five years ago, the first mammalian Transient Receptor Potential Canonical (TRPC) channel was cloned, opening the vast horizon of the TRPC field. Today, we know that there are seven TRPC channels (TRPC1-7). TRPCs exhibit the highest protein sequence similarity to the Drosophila melanogaster TRP channels. Similar to Drosophila TRPs, TRPCs are localized to the plasma membrane and are activated in a G-protein-coupled receptor-phospholipase C-dependent manner. TRPCs may also be stimulated in a store-operated manner, via receptor tyrosine kinases, or by lysophospholipids, hypoosmotic solutions, and mechanical stimuli. Activated TRPCs allow the influx of Ca2+ and monovalent alkali cations into the cytosol of cells, leading to cell depolarization and rising intracellular Ca2+ concentration. TRPCs are involved in the continually growing number of cell functions. Furthermore, mutations in the TRPC6 gene are associated with hereditary diseases, such as focal segmental glomerulosclerosis. The most important recent breakthrough in TRPC research was the solving of cryo-EM structures of TRPC3, TRPC4, TRPC5, and TRPC6. These structural data shed light on the molecular mechanisms underlying TRPCs' functional properties and propelled the development of new modulators of the channels. This review provides a historical overview of the major advances in the TRPC field focusing on the role of gene knockouts and pharmacological tools.

TRPC and TRPV Channels' Role in Vascular Remodeling and Disease

Transient receptor potentials (TRPs) are non-selective cation channels that are widely expressed in vascular beds. They contribute to the Ca²⁺ influx evoked by a wide spectrum of chemical and physical stimuli, both in endothelial and vascular smooth muscle cells. Within the superfamily of TRP channels, different isoforms of TRPC (canonical) and TRPV (vanilloid) have emerged as important regulators of vascular tone and blood flow pressure. Additionally, several lines of evidence derived from animal models, and even from human subjects, highlighted the role of TRPC and TRPV in vascular remodeling and disease. Dysregulation in the function and/or expression of TRPC and TRPV isoforms likely regulates vascular smooth muscle cells switching from a contractile to a synthetic phenotype. This process contributes to the development and progression of vascular disorders, such as systemic and pulmonary arterial hypertension, atherosclerosis and restenosis. In this review, we provide an overview of the current knowledge on the implication of TRPC and TRPV in the physiological and pathological processes of some frequent vascular diseases.

Review of Transient Receptor Potential Canonical (TRPC5) Channel Modulators and Diseases

Transient receptor potential canonical (TRPC) channels are highly homologous, nonselective cation channels that form many homo- and heterotetrameric channels. These channels are highly abundant in the brain and kidney and have been implicated in numerous diseases, such as depression, addiction, and chronic kidney disease, among others. Historically, there have been very few selective modulators of the TRPC family in order to fully understand their role in disease despite their physiological significance. However, that has changed recently and there has been a significant increase in interest in this family of channels which has led to the emergence of selective tool compounds, and even preclinical drug candidates, over the past few years. This review will cover these new advancements in the discovery of TRPC modulators and the emergence of newly reported structural information which will undoubtedly lead to even greater advancements.

Na+/Ca2+ exchanger overexpression in smooth muscle augments cytosolic Ca2+ in femoral arteries of living mice

Plasma membrane Na+/Ca2+ exchanger-1 (NCX1) helps regulate the cytosolic Ca2+ concentration ([Ca2+]CYT) in arterial myocytes. NCX1 mediates both Ca2+ entry and exit and tends to promote net Ca2+ entry in partially constricted arteries. Mean blood pressure (telemetry) is elevated by ≈10 mmHg in transgenic (TG) mice that overexpress NCX1 specifically in smooth muscle. We tested the hypothesis that NCX1 overexpression mediates Ca2+ gain and elevated [Ca2+]CYT in exposed femoral arteries that also express the Ca2+ biosensor exogenous myosin light chain kinase. [Ca2+]CYT and the NCX1-dependent (SEA0400-sensitive) component, ≈15% of total basal constriction in controls, were increased in TG arteries, but constrictions to phenylephrine and ANG II were comparable in TG and control arteries. Normalized phenylephrine dose-response curves and constriction to 30 and 300 ng/kg iv ANG II were virtually identical in control and TG arteries. ANG II-evoked constrictions, superimposed on elevated basal tone, accounted for the larger blood pressure responses to ANG II in TG arteries. TG and control mouse arteries fit the same pCa-constriction relationship over a wide range of pCa (≈125-500 nM). Vasodilation to acetylcholine, normalized to passive diameter, was also comparable in TG and control arteries, implying normal endothelial function. TG artery Na+ nitroprusside (nitric oxide donor)-induced dilations were, however, shifted to lower Na+ nitroprusside concentrations, indicating that TG myocyte vasodilator mechanisms were augmented. Maximum arterial dilation was comparable in TG and control mice, although passive diameter was ≈6-7% smaller in TG mice. The changes in TG arteries were apparently largely functional rather than structural, despite the congenital hypertension. NEW & NOTEWORTHY Smooth muscle Na+/Ca2+ exchanger-1 transgene overexpression (TG mice) increases femoral artery basal cytosolic Ca2+ concentration ([Ca2+]CYT) and tone in vivo and raises blood pressure. Arterial constriction to phenylephrine and angiotensin II are normal but superimposed on the augmented basal [Ca2+]CYT and tone (constriction) in TG mouse arteries. Similar effects in resistance arteries would explain the elevated blood pressure. Acetylcholine-induced vasodilation is unimpaired, implying a normal endothelium, but TG arteries are hypersensitive to sodium nitroprusside.

The NCX1/TRPC6 Complex Mediates TGFβ-Driven Migration and Invasion of Human Hepatocellular Carcinoma Cells

TGFβ plays an important role in the progression and metastasis of hepatocellular carcinoma (HCC), yet the cellular and molecular mechanisms underlying this role are not completely understood. In this study, we investigated the roles of Na⁺/Ca²⁺ exchanger 1 (NCX1) and canonical transient receptor potential channel 6 (TRPC6) in regulating TGFβ in human HCC. In HepG2 and Huh7 cells, TGFβ-stimulated intracellular Ca²⁺ increases through NCX1 and TRPC6 and induced the formation of a TRPC6/NCX1 molecular complex. This complex-mediated Ca²⁺ signaling regulated the effect of TGFβ on the migration, invasion, and intrahepatic metastasis of human HCC cells in nude mice. TGFβ upregulated TRPC6 and NCX1 expression, and there was a positive feedback between TRPC6/NCX1 signaling and Smad signaling. Expression of both TRPC6 and NCX1 were markedly increased in native human HCC tissues, and their expression levels positively correlated with advancement of HCC in patients. These data reveal the role of the TRPC6/NCX1 molecular complex in HCC and in regulating TGFβ signaling, and they implicate TRPC6 and NCX1 as potential targets for therapy in HCC.Significance: TGFβ induces the formation and activation of a TRPC6/NCX1 molecular complex, which mediates the effects of TGFβ on the migration, invasion, and intrahepatic metastasis of HCC. Cancer Res; 78(10); 2564-76. ©2018 AACR.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

Update on angiotensin II: new endocrine connections between the brain, adrenal glands and the cardiovascular system

In the brain, angiotensinergic pathways play a major role in chronic regulation of cardiovascular and electrolyte homeostasis. Increases in plasma angiotensin II (Ang II), aldosterone, [Na⁺] and cytokines can directly activate these pathways. Chronically, these stimuli also activate a slow neuromodulatory pathway involving local aldosterone, mineralocorticoid receptors (MRs), epithelial sodium channels and endogenous ouabain (EO). This pathway increases AT₁R and NADPH oxidase subunits and maintains/further increases the activity of angiotensinergic pathways. These brain pathways not only increase the setpoint of sympathetic activity per se, but also enhance its effectiveness by increasing plasma EO and EO-dependent reprogramming of arterial and cardiac function. Blockade of any step in this slow pathway or of AT₁R prevents Ang II-, aldosterone- or salt and renal injury-induced forms of hypertension. MR/AT₁R activation in the CNS also contributes to the activation of sympathetic activity, the circulatory and cardiac RAAS and increase in circulating cytokines in HF post MI. Chronic central infusion of an aldosterone synthase inhibitor, MR blocker or AT₁R blocker prevents a major part of the structural remodeling of the heart and the decrease in LV function post MI, indicating that MR activation in the CNS post MI depends on aldosterone, locally produced in the CNS. Thus, Ang II, aldosterone and EO are not simply circulating hormones that act on the CNS but rather they are also paracrine neurohormones, locally produced in the CNS, that exert powerful effects in key CNS pathways involved in the long-term control of sympathetic and neuro-endocrine function and cardiovascular homeostasis.

Physiological and pathophysiological role of transient receptor potential canonical channels in cardiac myocytes

Transient receptor potential canonical (TRPC) channels constitute a family of seven Ca²⁺ permeable ion channels, named TRPC1 to 7. These channels are abundantly expressed in the mammalian heart, yet mechanisms underlying activation of TRPC channels and their precise role in cardiac physiology remain poorly understood. In this review, we perused original literature regarding TRPC channels in cardiomyocytes. We first reviewed studies on TRPC channel assembly and sub-cellular localization across multiple species and cell types. Our review indicates that TRPC localization in cardiac cells is still a topic of controversy. We then examined common molecular biology tools used to infer on location and physiological roles of TRPC channels in the heart. We subsequently reviewed pharmacological tools used to modulate TRPC activity in both cardiac and non-cardiac cells. Suggested physiological roles in the heart include modulation of heart rate and sensing of mechanical strain. We examined studies on the contribution of TRPC to cardiac pathophysiology, mainly hypertrophic signaling. Several TRPC channels, particularly TRPC1, 3 and 6 were proposed to play a crucial role in hypertrophic signaling. Finally, we discussed gaps in our understanding of the location and physiological role of TRPC channels in cardiomyocytes. Closing these gaps will be crucial to gain a full understanding of the role of TRPC channels in cardiac pathophysiology and to further explore these channels as targets for treatments for cardiac diseases, in particular, hypertrophy.

Muscling in on TRP channels in vascular smooth muscle cells and cardiomyocytes

The human TRP protein family comprises a family of 27 cation channels with diverse permeation and gating properties. The common theme is that they are very important regulators of intracellular Ca²⁺ signaling in diverse cell types, either by providing a Ca²⁺ influx pathway, or by depolarising the membrane potential, which on one hand triggers the activation of voltage-gated Ca²⁺ channels, and on the other limits the driving force for Ca²⁺ entry. Here we focus on the role of these TRP channels in vascular smooth muscle and cardiac striated muscle. We give an overview of highlights from the recent literature, and highlight the important and diverse roles of TRP channels in the pathophysiology of the cardiovascular system. The discovery of the superfamily of Transient Receptor Potential (TRP) channels has significantly enhanced our knowledge of multiple signal transduction mechanisms in cardiac muscle and vascular smooth muscle cells (VSMC). In recent years, multiple studies have provided evidence for the involvement of these channels, not only in the regulation of contraction, but also in cell proliferation and remodeling in pathological conditions. The mammalian family of TRP cation channels is composed by 28 genes which can be divided into 6 subfamilies groups based on sequence similarity: TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipins), TRPV (Vanilloid), TRPP (Policystin) and TRPA (Ankyrin-rich protein). Functional TRP channels are believed to form four-unit complexes in the plasma, each of them expressed with six transmembrane domain and intracellular N and C termini. Here we review the current knowledge on the expression of TRP channels in both muscle types, and discuss their functional properties and role in physiological and pathophysiological processes.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Local coupling of TRPC6 to ANO1/TMEM16A channels in smooth muscle cells amplifies vasoconstriction in cerebral arteries

Anoctamin-1 [ANO1, also known as transmembrane protein 16A (TMEM16A)] is a Ca(2+)-activated Cl(-) channel expressed in arterial myocytes that regulates membrane potential and contractility. Signaling mechanisms that control ANO1 activity in arterial myocytes are poorly understood. In cerebral artery myocytes, ANO1 channels are activated by local Ca(2+) signals generated by plasma membrane nonselective cation channels, but the molecular identity of these proteins is unclear. Arterial myocytes express several different nonselective cation channels, including multiple members of the transient receptor potential receptor (TRP) family. The goal of this study was to identify localized ion channels that control ANO1 currents in cerebral artery myocytes. Coimmunoprecipitation and immunofluorescence resonance energy transfer microscopy experiments indicate that ANO1 and canonical TRP 6 (TRPC6) channels are present in the same macromolecular complex and localize in close spatial proximity in the myocyte plasma membrane. In contrast, ANO1 is not near TRPC3, TRP melastatin 4, or inositol trisphosphate receptor 1 channels. Hyp9, a selective TRPC6 channel activator, stimulated Cl(-) currents in myocytes that were blocked by T16Ainh-A01, an ANO1 inhibitor, ANO1 knockdown using siRNA, and equimolar replacement of intracellular EGTA with BAPTA, a fast Ca(2+) chelator that abolishes local Ca(2+) signaling. Hyp9 constricted pressurized cerebral arteries, and this response was attenuated by T16Ainh-A01. In contrast, T16Ainh-A01 did not alter depolarization-induced (60 mM K(+)) vasoconstriction. These data indicate that TRPC6 channels generate a local intracellular Ca(2+) signal that activates nearby ANO1 channels in myocytes to stimulate vasoconstriction.

Excitation-Contraction Coupling and Excitation-Transcription Coupling in Blood Vessels: Their Possible Interactions in Hypertensive Vascular Remodeling

Vascular smooth muscle cells (VSMC) display considerable phenotype plasticity which can be studied in vivo on vascular remodeling which occurs during acute or chronic vascular injury. In differentiated cells, which represent contractile phenotype, there are characteristic rapid transient changes of intracellular Ca2+ concentration ([Ca2+]i), while the resting cytosolic [Ca2+]i concentration is low. It is mainly caused by two components of the Ca2+ signaling pathways: Ca2+ entry via L-type voltage-dependent Ca2+ channels and dynamic involvement of intracellular stores. Proliferative VSMC phenotype is characterized by long-lasting [Ca2+]i oscillations accompanied by sustained elevation of basal [Ca2+]i. During the switch from contractile to proliferative phenotype there is a general transition from voltage-dependent Ca2+ entry to voltage-independent Ca2+ entry into the cell. These changes are due to the altered gene expression which is dependent on specific transcription factors activated by various stimuli. It is an open question whether abnormal VSMC phenotype reported in rats with genetic hypertension (such as spontaneously hypertensive rats) might be partially caused by a shift from contractile to proliferative VSMC phenotype.

TRP-Na(+)/Ca(2+) Exchanger Coupling

Na(+)/Ca(2+) exchangers (NCXs) have traditionally been viewed principally as a means of Ca(2+) removal from non-excitable cells. However there has recently been increasing interest in the operation of NCXs in reverse mode acting as a means of eliciting Ca(2+) entry into these cells. Reverse mode exchange requires a significant change in the normal resting transmembrane ion gradients and membrane potential, which has been suggested to occur principally via the coupling of NCXs to localised Na(+) entry through non-selective cation channels such as canonical transient receptor potential (TRPC) channels. Here we review evidence for functional or physical coupling of NCXs to non-selective cation channels, and how this affects NCX activity in non-excitable cells. In particular we focus on the potential role of nanojunctions, where the close apposition of plasma and intracellular membranes may help create the conditions needed for the generation of localised rises in Na(+) concentration that would be required to trigger reverse mode exchange.

Arterial α2-Na+ pump expression influences blood pressure: lessons from novel, genetically engineered smooth muscle-specific α2 mice

Arterial myocytes express α1-catalytic subunit isoform Na(+) pumps (75-80% of total), which are ouabain resistant in rodents, and high ouabain affinity α2-Na(+) pumps. Mice with globally reduced α2-pumps (but not α1-pumps), mice with mutant ouabain-resistant α2-pumps, and mice with a smooth muscle (SM)-specific α2-transgene (α2 (SM-Tg)) that induces overexpression all have altered blood pressure (BP) phenotypes. We generated α2 (SM-DN) mice with SM-specific α2 (not α1) reduction (>50%) using nonfunctional dominant negative (DN) α2. We compared α2 (SM-DN) and α2 (SM-Tg) mice to controls to determine how arterial SM α2-pumps affect vasoconstriction and BP. α2 (SM-DN) mice had elevated basal mean BP (mean BP by telemetry: 117 ± 4 vs. 106 ± 1 mmHg, n = 7/7, P < 0.01) and enhanced BP responses to chronic ANG II infusion (240 ng·kg(-1)·min(-1)) and high (6%) NaCl. Several arterial Ca(2+) transporters, including Na(+)/Ca(2+) exchanger 1 (NCX1) and sarcoplasmic reticulum and plasma membrane Ca(2+) pumps [sarco(endo)plasmic reticulum Ca(2+)-ATPase 2 (SERCA2) and plasma membrane Ca(2+)-ATPase 1 (PMCA1)], were also reduced (>50%). α2 (SM-DN) mouse isolated small arteries had reduced myogenic reactivity, perhaps because of reduced Ca(2+) transporter expression. In contrast, α2 (SM-Tg) mouse aortas overexpressed α2 (>2-fold), NCX1, SERCA2, and PMCA1 (43). α2 (SM-Tg) mice had reduced basal mean BP (104 ± 1 vs. 109 ± 2 mmHg, n = 15/9, P < 0.02) and attenuated BP responses to chronic ANG II (300-400 ng·kg(-1)·min(-1)) with or without 2% NaCl but normal myogenic reactivity. NCX1 expression was inversely related to basal BP in SM-α2 engineered mice but was directly related in SM-NCX1 engineered mice. NCX1, which usually mediates arterial Ca(2+) entry, and α2-Na(+) pumps colocalize at plasma membrane-sarcoplasmic reticulum junctions and functionally couple via the local Na(+) gradient to help regulate cell Ca(2+). Altered Ca(2+) transporter expression in SM-α2 engineered mice apparently compensates to minimize Ca(2+) overload (α2 (SM-DN)) or depletion (α2 (SM-Tg)) and attenuate BP changes. In contrast, Ca(2+) transporter upregulation, observed in many rodent hypertension models, should enhance Ca(2+) entry and signaling and contribute significantly to BP elevation.

Transient receptor potential channels in the vasculature

The mammalian genome encodes 28 distinct members of the transient receptor potential (TRP) superfamily of cation channels, which exhibit varying degrees of selectivity for different ionic species. Multiple TRP channels are present in all cells and are involved in diverse aspects of cellular function, including sensory perception and signal transduction. Notably, TRP channels are involved in regulating vascular function and pathophysiology, the focus of this review. TRP channels in vascular smooth muscle cells participate in regulating contractility and proliferation, whereas endothelial TRP channel activity is an important contributor to endothelium-dependent vasodilation, vascular wall permeability, and angiogenesis. TRP channels are also present in perivascular sensory neurons and astrocytic endfeet proximal to cerebral arterioles, where they participate in the regulation of vascular tone. Almost all of these functions are mediated by changes in global intracellular Ca(2+) levels or subcellular Ca(2+) signaling events. In addition to directly mediating Ca(2+) entry, TRP channels influence intracellular Ca(2+) dynamics through membrane depolarization associated with the influx of cations or through receptor- or store-operated mechanisms. Dysregulation of TRP channels is associated with vascular-related pathologies, including hypertension, neointimal injury, ischemia-reperfusion injury, pulmonary edema, and neurogenic inflammation. In this review, we briefly consider general aspects of TRP channel biology and provide an in-depth discussion of the functions of TRP channels in vascular smooth muscle cells, endothelial cells, and perivascular cells under normal and pathophysiological conditions.

Discovery and pharmacological characterization of a novel potent inhibitor of diacylglycerol-sensitive TRPC cation channels

BACKGROUND AND PURPOSE

The cation channel transient receptor potential canonical (TRPC) 6 has been associated with several pathologies including focal segmental glomerulosclerosis, pulmonary hypertension and ischaemia reperfusion-induced lung oedema. We set out to discover novel inhibitors of TRPC6 channels and investigate the therapeutic potential of these agents.

EXPERIMENTAL APPROACH

A library of potential TRPC channel inhibitors was designed and synthesized. Activity of the compounds was assessed by measuring intracellular Ca(2+) levels. The lead compound SAR7334 was further characterized by whole-cell patch-clamp techniques. The effects of SAR7334 on acute hypoxic pulmonary vasoconstriction (HPV) and systemic BP were investigated.

KEY RESULTS

SAR7334 inhibited TRPC6, TRPC3 and TRPC7-mediated Ca(2+) influx into cells with IC50 s of 9.5, 282 and 226 nM, whereas TRPC4 and TRPC5-mediated Ca(2+) entry was not affected. Patch-clamp experiments confirmed that the compound blocked TRPC6 currents with an IC50 of 7.9 nM. Furthermore, SAR7334 suppressed TRPC6-dependent acute HPV in isolated perfused lungs from mice. Pharmacokinetic studies of SAR7334 demonstrated that the compound was suitable for chronic oral administration. In an initial short-term study, SAR7334 did not change mean arterial pressure in spontaneously hypertensive rats (SHR).

CONCLUSIONS AND IMPLICATIONS

Our results confirm the role of TRPC6 channels in hypoxic pulmonary vasoregulation and indicate that these channels are unlikely to play a major role in BP regulation in SHR. SAR7334 is a novel, highly potent and bioavailable inhibitor of TRPC6 channels that opens new opportunities for the investigation of TRPC channel function in vivo.

Neuroendocrine humoral and vascular components in the pressor pathway for brain angiotensin II: a new axis in long term blood pressure control

Central nervous system (CNS) administration of angiotensin II (Ang II) raises blood pressure (BP). The rise in BP reflects increased sympathetic outflow and a slower neuromodulatory pressor mechanism mediated by CNS mineralocorticoid receptors (MR). We investigated the hypothesis that the sustained phase of hypertension is associated also with elevated circulating levels of endogenous ouabain (EO), and chronic stimulation of arterial calcium transport proteins including the sodium-calcium exchanger (NCX1), the type 6 canonical transient receptor potential protein (TRPC6), and the sarcoplasmic reticulum calcium ATPase (SERCA2). Wistar rats received a chronic intra-cerebroventricular infusion of vehicle (C) or Ang II (A, 2.5 ng/min, for 14 days) alone or combined with the MR blocker, eplerenone (A+E, 5 µg/day), or the aldosterone synthase inhibitor, FAD286 (A+F, 25 µg/day). Conscious mean BP increased (P<0.05) in A (123±4 mm Hg) vs all other groups. Blood, pituitary and adrenal samples were taken for EO radioimmunoassay (RIA), and aortas for NCX1, TRPC6 and SERCA2 immunoblotting. Central infusion of Ang II raised plasma EO (0.58±0.08 vs C 0.34±0.07 nM (P<0.05), but not in A + E and A + F groups as confirmed by off-line liquid chromatography (LC)-RIA and LC-multistage mass spectrometry. Two novel isomers of EO were elevated by Ang II; the second less polar isomer increased >50-fold in the A+F group. Central Ang II increased arterial expression of NCX1, TRPC6 and SERCA2 (2.6, 1.75 and 3.7-fold, respectively; P<0.01)) but not when co-infused with E or F. Adrenal and pituitary EO were unchanged. We conclude that brain Ang II activates a CNS-humoral axis involving plasma EO. The elevated EO reprograms peripheral ion transport pathways known to control arterial Na⁺ and Ca²⁺ homeostasis; this increases contractility and augments sympathetic effects. The new axis likely contributes to the chronic pressor effect of brain Ang II.

Association of calcium concentration with pulse pressure in older women: data from a large population-based multicentric study

OBJECTIVE

High arterial pulse pressure is a predictor of cardiovascular morbimortality. Mineral metabolism has been associated with blood pressure regulation. Our objective was to determine which variable among serum calcium, parathyroid hormone and 25-hydroxyvitamin D concentrations, was associated with pulse pressure among older adults.

DESIGN

Cross-sectional study corresponding to the baseline assessment of the EPIDOS study.

SETTING

Five French cities including Amiens, Lyon, Montpellier, Paris and Toulouse.

PARTICIPANTS

Randomized sample of 610 community-dwelling older women (mean age 80.2±3.5years) using no antihypertensive drugs.

MEASUREMENTS

Serum calcium, parathyroid hormone and 25-hydroxyvitamin D concentrations; supine pulse pressure after 15 minutes of rest (hypertension defined as pulse pressure >50mmHg). Age, body mass index, the number of morbidities and of drugs daily taken, diabetes mellitus, dysthyroidy, the use of estrogenic drugs, smoking, alcohol consumption, practice of a regular physical activity, creatinine clearance, and the effects of season and study centers were used as potential confounders.

RESULTS

Hypertensive participants (n=539) had higher calcium concentrations than normotensive ones (94.33±4.12mg/L versus 93.28±3.36mg/L respectively, P=0.040). There were no between-group differences for serum parathyroid hormone and 25-hydroxyvitamin D concentrations. The multiple logistic regressions examining the serum calcium, parathyroid hormone and 25-hydroxyvitamin D concentrations as predictors of hypertension found an association only with calcium (adjusted odds ratio=1.19, P=0.015), but not with parathyroid hormone (adjusted OR=1.01, P=0.349) or 25-hydroxyvitamin D concentration (adjusted OR=0.99, P=0.971).

CONCLUSION

Increased serum calcium concentration was independently and positively associated with high pulse pressure in our study, possibly due to increased arterial stiffness. Interventions aimed at normalizing calcaemia may be attractive to prevent hypertension and cardiovascular risk in older adults.

Ouabain-digoxin antagonism in rat arteries and neurones

'Classic' cardiotonic steroids (CTSs) such as digoxin and ouabain selectively inhibit Na+, K+ -ATPase (the Na+ pump) and, via Na+ / Ca2+ exchange (NCX), exert cardiotonic and vasotonic effects. CTS action is more complex than previously thought: prolonged subcutaneous administration of ouabain, but not digoxin, induces hypertension, and digoxin antagonizes ouabain's hypertensinogenic effect. We studied the acute interactions between CTSs in two indirect assays of Na+ pump function: myogenic tone (MT) in isolated, pressurized rat mesenteric small arteries, and Ca2+ signalling in primary cultured rat hippocampal neurones. The 'classic' CTSs (0.3-10 nm) behaved as 'agonists': all increased MT70 (MT at 70 mmHg) and augmented glutamate-evoked Ca2+ (Fura-2) signals. We then tested one CTS in the presence of another. Most CTSs could be divided into ouabain-like (ouabagenin, dihydroouabain (DHO), strophanthidin) or digoxin-like CTS (digoxigenin, digitoxin, bufalin). Within each group, the CTSs were synergistic, but ouabain-like and digoxin-like CTSs antagonized one another in both assays: For example, the ouabain-evoked (3 nm) increases in MT70 and neuronal Ca2+ signals were both greatly attenuated by the addition of 10 nm digoxin or 10 nm bufalin, and vice versa. Rostafuroxin (PST2238), a digoxigenin derivative that displaces 3H-ouabain from Na+, K+ -ATPase, and attenuates some forms of hypertension, antagonized the effects of ouabain, but not digoxin. SEA0400, a Na+ / Ca2+ exchanger (NCX) blocker, antagonized the effects of both ouabain and digoxin. CTSs bind to the α subunit of pump αβ protomers. Analysis of potential models suggests that, in vivo, Na+ pumps function as tetraprotomers ((αβ)4) in which the binding of a single CTS to one protomer blocks all pumping activity. The paradoxical ability of digoxin-like CTSs to reactivate the ouabain-inhibited complex can be explained by de-oligomerization of the tetrameric state. The interactions between these common CTSs may be of considerable therapeutic relevance.

Serotonin contracts the rat mesenteric artery by inhibiting 4-aminopyridine-sensitive Kv channels via the 5-HT2A receptor and Src tyrosine kinase

Serotonin (5-hydroxytryptamine (5-HT)) is a neurotransmitter that regulates a variety of functions in the nervous, gastrointestinal and cardiovascular systems. Despite such importance, 5-HT signaling pathways are not entirely clear. We demonstrated previously that 4-aminopyridine (4-AP)-sensitive voltage-gated K⁺ (Kv) channels determine the resting membrane potential of arterial smooth muscle cells and that the Kv channels are inhibited by 5-HT, which depolarizes the membranes. Therefore, we hypothesized that 5-HT contracts arteries by inhibiting Kv channels. Here we studied 5-HT signaling and the detailed role of Kv currents in rat mesenteric arteries using patch-clamp and isometric tension measurements. Our data showed that inhibiting 4-AP-sensitive Kv channels contracted arterial rings, whereas inhibiting Ca²⁺-activated K⁺, inward rectifier K⁺ and ATP-sensitive K⁺ channels had little effect on arterial contraction, indicating a central role of Kv channels in the regulation of resting arterial tone. 5-HT-induced arterial contraction decreased significantly in the presence of high KCl or the voltage-gated Ca²⁺ channel (VGCC) inhibitor nifedipine, indicating that membrane depolarization and the consequent activation of VGCCs mediate the 5-HT-induced vasoconstriction. The effects of 5-HT on Kv currents and arterial contraction were markedly prevented by the 5-HT_2A receptor antagonists ketanserin and spiperone. Consistently, α-methyl 5-HT, a 5-HT₂ receptor agonist, mimicked the 5-HT action on Kv channels. Pretreatment with a Src tyrosine kinase inhibitor, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, prevented both the 5-HT-mediated vasoconstriction and Kv current inhibition. Our data suggest that 4-AP-sensitive Kv channels are the primary regulator of the resting tone in rat mesenteric arteries. 5-HT constricts the arteries by inhibiting Kv channels via the 5-HT_2A receptor and Src tyrosine kinase pathway.

Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions

NCX proteins explore the electrochemical gradient of Na(+) to mediate Ca(2+)-fluxes in exchange with Na(+) either in the Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, whereas the directionality depends on ionic concentrations and membrane potential. Mammalian NCX variants (NCX1-3) and their splice variants are expressed in a tissue-specific manner to modulate the heartbeat rate and contractile force, the brain's long-term potentiation and learning, blood pressure, renal Ca(2+) reabsorption, the immune response, neurotransmitter and insulin secretion, apoptosis and proliferation, mitochondrial bioenergetics, etc. Although the forward mode of NCX represents a major physiological module, a transient reversal of NCX may contribute to EC-coupling, vascular constriction, and synaptic transmission. Notably, the reverse mode of NCX becomes predominant in pathological settings. Since the expression levels of NCX variants are disease-related, the selective pharmacological targeting of tissue-specific NCX variants could be beneficial, thereby representing a challenge. Recent structural and biophysical studies revealed a common module for decoding the Ca(2+)-induced allosteric signal in eukaryotic NCX variants, although the phenotype variances in response to regulatory Ca(2+) remain unclear. The breakthrough discovery of the archaebacterial NCX structure may serve as a template for eukaryotic NCX, although the turnover rates of the transport cycle may differ ~10(3)-fold among NCX variants to fulfill the physiological demands for the Ca(2+) flux rates. Further elucidation of ion-transport and regulatory mechanisms may lead to selective pharmacological targeting of NCX variants under disease conditions.

Age-related autoregulatory dysfunction and cerebromicrovascular injury in mice with angiotensin II-induced hypertension

Hypertension in the elderly substantially contributes to cerebromicrovascular damage and promotes the development of vascular cognitive impairment. Despite the importance of the myogenic mechanism in cerebromicrovascular protection, it is not well understood how aging affects the functional adaptation of cerebral arteries to high blood pressure. Hypertension was induced in young (3 months) and aged (24 months) C57/BL6 mice by chronic infusion of angiotensin II (AngII). In young hypertensive mice, the range of cerebral blood flow autoregulation was extended to higher pressure values, and the pressure-induced tone of middle cerebral artery (MCA) was increased. In aged hypertensive mice, autoregulation was markedly disrupted, and MCAs did not show adaptive increases in myogenic tone. In young mice, the mechanism of adaptation to hypertension involved upregulation of the 20-HETE (20-hydroxy-5,8,11,14-eicosatetraenoic acid)/transient receptor potential cation channel, subfamily C (TRPC6) pathway and this mechanism was impaired in aged hypertensive mice. Downstream consequences of cerebrovascular autoregulatory dysfunction in aged AngII-induced hypertensive mice included exacerbated disruption of the blood-brain barrier and neuroinflammation (microglia activation and upregulation of proinflammatory cytokines and chemokines), which were associated with impaired hippocampal dependent cognitive function. Collectively, aging impairs autoregulatory protection in the brain of mice with AngII-induced hypertension, potentially exacerbating cerebromicrovascular injury and neuroinflammation.

The SLC8 gene family of sodium-calcium exchangers (NCX) - structure, function, and regulation in health and disease

The SLC8 gene family encoding Na(+)/Ca(2+) exchangers (NCX) belongs to the CaCA (Ca(2+)/Cation Antiporter) superfamily. Three mammalian genes (SLC8A1, SLC8A2, and SLC8A3) and their splice variants are expressed in a tissue-specific manner to mediate Ca(2+)-fluxes across the cell-membrane and thus, significantly contribute to regulation of Ca(2+)-dependent events in many cell types. A long-wanted mitochondrial Na(+)/Ca(2+) exchanger has been recently identified as NCLX protein, representing a gene product of SLC8B1. Distinct NCX isoform/splice variants contribute to excitation-contraction coupling, long-term potentiation of the brain and learning, blood pressure regulation, immune response, neurotransmitter and insulin secretion, mitochondrial bioenergetics, etc. Altered expression and regulation of NCX proteins contribute to distorted Ca(2+)-homeostasis in heart failure, arrhythmia, cerebral ischemia, hypertension, diabetes, renal Ca(2+) reabsorption, muscle dystrophy, etc. Recently, high-resolution X-ray structures of Ca(2+)-binding regulatory domains of eukaryotic NCX and of full-size prokaryotic NCX have become available and the dynamic properties have been analyzed by advanced biophysical approaches. Molecular silencing/overexpression of NCX in cellular systems and organ-specific KO mouse models provided useful information on the contribution of distinct NCX variants to cellular and systemic functions under various pathophysiological conditions. Selective inhibition or activation of predefined NCX variants in specific diseases might have clinical relevance, although this breakthrough has not yet been realized. A better understanding of the underlying molecular mechanisms as well as the development of in vitro procedures for high-throughput screening of "drug-like" compounds may lead to selective pharmacological targeting of NCX variants.

Plasma lipidomic profile signature of hypertension in Mexican American families: specific role of diacylglycerols

Both as a component of metabolic syndrome and as an independent entity, hypertension poses a continued challenge with regard to its diagnosis, pathogenesis, and treatment. Previous studies have documented connections between hypertension and indicators of lipid metabolism. Novel technologies, such as plasma lipidomic profiling, promise a better understanding of disorders in which there is a derangement of the lipid metabolism. However, association of plasma lipidomic profiles with hypertension in a high-risk population, such as Mexican Americans, has not been evaluated before. Using the rich data and sample resource from the ongoing San Antonio Family Heart Study, we conducted plasma lipidomic profiling by combining high-performance liquid chromatography with tandem mass spectroscopy to characterize 319 lipid species in 1192 individuals from 42 large and extended Mexican American families. Robust statistical analyses using polygenic regression models, liability threshold models, and bivariate trait analyses implemented in the SOLAR software were conducted after accounting for obesity, insulin resistance, and relative abundance of various lipoprotein fractions. Diacylglycerols, in general, and the DG 16:0/22:5 and DG 16:0/22:6 lipid species, in particular, were significantly associated with systolic blood pressure (SBP), diastolic blood pressure (DBP), and mean arterial pressure (MAP), as well as liability of incident hypertension measured during 7140.17 person-years of follow-up. Four lipid species, including the DG 16:0/22:5 and DG 16:0/22:6 species, showed significant genetic correlations with the liability of hypertension in bivariate trait analyses. Our results demonstrate the value of plasma lipidomic profiling in the context of hypertension and identify disturbance of diacylglycerol metabolism as an independent biomarker of hypertension.

Salt sensitivity, endogenous ouabain and hypertension

PURPOSE OF REVIEW

Endogenous cardiotonic steroids (CTS) exert long-term effects on salt and blood pressure homeostasis. Here we discuss recent observations on mechanisms of salt sensitivity that involve endogenous ouabain and novel pathways in the brain and discuss their possible relationship to arterial and renal function in hypertension.

RECENT FINDINGS

Chronic elevation of brain sodium promotes sustained hypertension mediated by central endogenous ouabain and the Na(+) pump α-2 catalytic subunit. The intermediary pressor mechanism in the brain involves aldosterone biosynthesis, activation of mineralocorticoid receptors and increased epithelial sodium channel activity. In the periphery, elevated plasma CTS raise contractility and blood pressure by augmentation of sympathetic nerve responses, increasing arterial Ca(2+) signaling and blunting nitric oxide production in the renal medulla and collecting ducts.

SUMMARY

Endogenous ouabain in the brain appears to play a critical role in salt sensitivity and hypertension. In the periphery, the J-shaped relationship of plasma endogenous ouabain in response to short-term changes in salt balance in humans raises the possibility that endogenous ouabain contributes to the increased risk of adverse cardiovascular events associated with both low and high salt intakes.

Livin' with NCX and lovin' it: a 45 year romance

This conference commemorates, almost to the day, the 45th anniversary of the discovery of the Na(+)/Ca(2+) exchanger (NCX). The discovery was serendipitous, as is so often the case with scientific breakthroughs. Indeed, that is what is so fascinating and romantic about scientific research. I will describe the discovery of NCX, but will begin by explaining how I got there, and will then discuss how the discovery influenced my career path.

Cross talk between plasma membrane Na(+)/Ca (2+) exchanger-1 and TRPC/Orai-containing channels: key players in arterial hypertension

Arterial smooth muscle (ASM) Na(+)/Ca(2+) exchanger type 1 (NCX1) and TRPC/Orai-containing receptor/store-operated cation channels (ROC/SOC) are clustered with α2 Na(+) pumps in plasma membrane microdomains adjacent to the underlying junctional sarcoplasmic reticulum. This arrangement enables these transport proteins to function as integrated units to help regulate local Na(+) metabolism, Ca(2+) signaling, and arterial tone. They thus influence vascular resistance and blood pressure (BP). For instance, upregulation of NCX1 and TRPC6 has been implicated in the pathogenesis of high BP in several models of essential hypertension. The models include ouabain-induced hypertensive rats, Milan hypertensive rats, and Dahl salt-sensitive hypertensive rats, all of which exhibit elevated plasma ouabain levels. We suggest that these molecular mechanisms are key contributors to the increased vascular resistance ("whole body autoregulation") that elevates BP in essential hypertension. Enhanced expression and function of ASM NCX1 and TRPC/Orai1-containing channels in hypertension implies that these proteins are potential targets for pharmacological intervention.

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

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

Activation of c-SRC underlies the differential effects of ouabain and digoxin on Ca(2+) signaling in arterial smooth muscle cells

Cardiotonic steroids (CTS) of the strophanthus and digitalis families have opposing effects on long-term blood pressure (BP). This implies hitherto unrecognized divergent signaling pathways for these CTS. Prolonged ouabain treatment upregulates Ca(2+) entry via Na(+)/Ca(2+) exchanger-1 (NCX1) and TRPC6 gene-encoded receptor-operated channels in mesenteric artery smooth muscle cells (ASMCs) in vivo and in vitro. Here, we test the effects of digoxin on Ca(2+) entry and signaling in ASMC. In contrast to ouabain treatment, the in vivo administration of digoxin (30 μg·kg(-1)·day(-1) for 3 wk) did not raise BP and had no effect on resting cytolic free Ca(2+) concentration ([Ca(2+)](cyt)) or phenylephrine-induced Ca(2+) signals in isolated ASMCs. Expression of transporters in the α2 Na(+) pump-NCX1-TRPC6 Ca(2+) signaling pathway was not altered in arteries from digoxin-treated rats. Upregulated α2 Na(+) pumps and a phosphorylated form of the c-SRC protein kinase (pY419-Src, ~4.5-fold) were observed in ASMCs from rats treated with ouabain but not digoxin. Moreover, in primary cultured ASMCs from normal rats, treatment with digoxin (100 nM, 72 h) did not upregulate NCX1 and TRPC6 but blocked the ouabain-induced upregulation of these transporters. Pretreatment of ASMCs with the c-Src inhibitor PP2 (1 μM; 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) but not its inactive analog eliminated the effect of ouabain on NCX1 and TRPC6 expression and ATP-induced Ca(2+) entry. Thus, in contrast to ouabain, the interaction of digoxin with α2 Na(+) pumps is unable to activate c-Src phosphorylation and upregulate the downstream NCX1-TRPC6 Ca(2+) signaling pathway in ASMCs. The inability of digoxin to upregulate c-Src may underlie its inability to raise long-term BP.

An elevation in physical coupling of type 1 inositol 1,4,5-trisphosphate (IP3) receptors to transient receptor potential 3 (TRPC3) channels constricts mesenteric arteries in genetic hypertension

Hypertension is associated with an elevation in agonist-induced vasoconstriction, but mechanisms involved require further investigation. Many vasoconstrictors bind to phospholipase C-coupled receptors, leading to an elevation in inositol 1,4,5-trisphosphate (IP(3)) that activates sarcoplasmic reticulum IP(3) receptors. In cerebral artery myocytes, IP(3) receptors release sarcoplasmic reticulum Ca(2+) and can physically couple to canonical transient receptor potential 3 (TRPC3) channels in a caveolin-1-containing macromolecular complex, leading to cation current activation that stimulates vasoconstriction. Here, we investigated mechanisms by which IP(3) receptors control vascular contractility in systemic arteries and IP(3)R involvement in elevated agonist-induced vasoconstriction during hypertension. Total and plasma membrane-localized TRPC3 protein was ≈2.7- and 2-fold higher in mesenteric arteries of spontaneously hypertensive rats (SHRs) than in Wistar-Kyoto (WKY) rat controls, respectively. In contrast, IP(3)R1, TRPC1, TRPC6, and caveolin-1 expression was similar. TRPC3 expression was also similar in arteries of pre-SHRs and WKY rats. Control, IP(3)-induced and endothelin-1 (ET-1)-induced fluorescence resonance energy transfer between IP3R1 and TRPC3 was higher in SHR than WKY myocytes. IP3-induced cation current was ≈3-fold larger in SHR myocytes. Pyr3, a selective TRPC3 channel blocker, and calmodulin and IP(3) receptor binding domain peptide, an IP(3)R-TRP physical coupling inhibitor, reduced IP(3)-induced cation current and ET-1-induced vasoconstriction more in SHR than WKY myocytes and arteries. Thapsigargin, a sarcoplasmic reticulum Ca(2+)-ATPase blocker, did not alter ET-1-stimulated vasoconstriction in SHR or WKY arteries. These data indicate that ET-1 stimulates physical coupling of IP(3)R1 to TRPC3 channels in mesenteric artery myocytes, leading to vasoconstriction. Furthermore, an elevation in IP(3)R1 to TRPC3 channel molecular coupling augments ET-1-induced vasoconstriction during hypertension.

Nanomolar ouabain increases NCX1 expression and enhances Ca2+ signaling in human arterial myocytes: a mechanism that links salt to increased vascular resistance?

The mechanisms by which NaCl raises blood pressure (BP) in hypertension are unresolved, but much evidence indicates that endogenous ouabain is involved. In rodents, arterial smooth muscle cell (ASMC) Na(+) pumps with an α(2)-catalytic subunit (ouabain EC(50) ≤1.0 nM) are crucial for some hypertension models, even though ≈80% of ASMC Na(+) pumps have an α(1)-subunit (ouabain EC(50) ≈ 5 μM). Human α(1)-Na(+) pumps, however, have high ouabain affinity (EC(50) ≈ 10-20 nM). We used immunoblotting, immunocytochemistry, and Ca(2+) imaging (fura-2) to examine the expression, distribution, and function of Na(+) pump α-subunit isoforms in human arteries and primary cultured human ASMCs (hASMCs). hASMCs express α(1)- and α(2)-Na(+) pumps. Further, α(2)-, but not α(1)-, pumps are confined to plasma membrane microdomains adjacent to sarcoplasmic reticulum (SR), where they colocalize with Na/Ca exchanger-1 (NCX1) and C-type transient receptor potential-6 (receptor-operated channels, ROCs). Prolonged inhibition (72 h) with 100 nM ouabain (blocks nearly all α(1)- and α(2)-pumps) was toxic to most cultured hASMCs. Treatment with 10 nM ouabain (72 h), however, increased NCX1 and sarco(endo)plasmic reticulum Ca(2+)-ATPase expression and augmented ATP (10 μM)-induced SR Ca(2+) release in 0 Ca(2+), ouabain-free media, and Ca(2+) influx after external Ca(2+) restoration. The latter was likely mediated primarily by ROCs and store-operated Ca(2+) channels. These hASMC protein expression and Ca(2+) signaling changes are comparable with previous observations on myocytes isolated from arteries of many rat hypertension models. We conclude that the same structurally and functionally coupled mechanisms (α(2)-Na(+) pumps, NCX1, ROCs, and the SR) regulate Ca(2+) homeostasis and signaling in hASMCs and rodent ASMCs. These ouabain/endogenous ouabain-modulated mechanisms underlie the whole body autoregulation associated with increased vascular resistance and elevation of BP in human, salt-sensitive hypertension.

Increased arterial smooth muscle Ca2+ signaling, vasoconstriction, and myogenic reactivity in Milan hypertensive rats

The Milan hypertensive strain (MHS) rats are a genetic model of hypertension with adducin gene polymorphisms linked to enhanced renal tubular Na(+) reabsorption. Recently we demonstrated that Ca(2+) signaling is augmented in freshly isolated mesenteric artery myocytes from MHS rats. This is associated with greatly enhanced expression of Na(+)/Ca(2+) exchanger-1 (NCX1), C-type transient receptor potential (TRPC6) protein, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) compared with arteries from Milan normotensive strain (MNS) rats. Here, we test the hypothesis that the enhanced Ca(2+) signaling in MHS arterial smooth muscle is directly reflected in augmented vasoconstriction [myogenic and phenylephrine (PE)-evoked responses] in isolated mesenteric small arteries. Systolic blood pressure was higher in MHS (145 ± 1 mmHg) than in MNS (112 ± 1 mmHg; P < 0.001; n = 16 each) rats. Pressurized mesenteric resistance arteries from MHS rats had significantly augmented myogenic tone and reactivity and enhanced constriction to low-dose (1-100 nM) PE. Isolated MHS arterial myocytes exhibited approximately twofold increased peak Ca(2+) signals in response to 5 μM PE or ATP in the absence and presence of extracellular Ca(2+). These augmented responses are consistent with increased vasoconstrictor-evoked sarcoplasmic reticulum (SR) Ca(2+) release and increased Ca(2+) entry, respectively. The increased SR Ca(2+) release correlates with a doubling of inositol 1,4,5-trisphosphate receptor type 1 and tripling of SERCA2 expression. Pressurized MHS arteries also exhibited a ∼70% increase in 100 nM ouabain-induced vasoconstriction compared with MNS arteries. These functional alterations reveal that, in a genetic model of hypertension linked to renal dysfunction, multiple mechanisms within the arterial myocytes contribute to enhanced Ca(2+) signaling and myogenic and vasoconstrictor-induced arterial constriction. MHS rats have elevated plasma levels of endogenous ouabain, which may initiate the protein upregulation and enhanced Ca(2+) signaling. These molecular and functional changes provide a mechanism for the increased peripheral vascular resistance (whole body autoregulation) that underlies the sustained hypertension.

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

Hypertension from chronic central sodium chloride in mice is mediated by the ouabain-binding site on the Na,K-ATPase α₂-isoform

A chronic increase in the concentration of sodium chloride in the cerebrospinal fluid (CSF) (↑CSF [NaCl]) appears to be critically important for the development of salt-dependent hypertension. In agreement with this concept, increasing CSF [NaCl] chronically by intracerebroventricular (icv) infusion of NaCl-rich artificial CSF (aCSF-HiNaCl) in rats produces hypertension by the same mechanisms (i.e., aldosterone-ouabain pathway in the brain) as that produced by dietary sodium in salt-sensitive strains. We first demonstrate here that icv aCSF-HiNaCl for 10 days also causes hypertension in wild-type (WT) mice. We then used both WT and gene-targeted mice to explore the mechanisms. In WT mice with a ouabain-sensitive Na,K-ATPase α(2)-isoform (α2(S/S)), mean arterial pressure rose by ~25 mmHg within 2 days of starting aCSF-HiNaCl (0.6 nmol Na/min) and remained elevated throughout the study. Ouabain (171 pmol/day icv) increased blood pressure to a similar extent. aCSF-HiNaCl or ouabain given at the same rates subcutaneously instead of intracerebroventricularly had no effect on blood pressure. The pressor response to icv aCSF-HiNaCl was abolished by an anti-ouabain antibody given intracerebroventricularly but not subcutaneously, indicating that it is mediated by an endogenous ouabain-like substance in the brain. We compared the effects of icv aCSF-HiNaCl or icv ouabain on blood pressure in α2(S/S) versus knockout/knockin mice with a ouabain-resistant endogenous α(2)-subunit (α2(R/R)). In α2(R/R), there was no pressor response to icv aCSF-HiNaCl in contrast to WT mice. The α2(R/R) genotype also lacked a pressor response to icv ouabain. These data demonstrate that chronic ↑CSF [NaCl] causes hypertension in mice and that the blood pressure response is mediated by the ouabain-like substance in the brain, specifically by its binding to the α(2)-isoform of the Na,K-ATPase.

Attenuated renal vascular responses to acute angiotensin II infusion in smooth muscle-specific Na+/Ca2+ exchanger knockout mice

Recent studies in smooth muscle-specific Na(+)/Ca(2+) exchanger-1 knockout (NCX1(sm-/-)) mice reveal reduced arterial pressure and impaired myogenic responses compared with heterozygous littermates. In this study, we determined renal function in male anesthetized NCX1(sm-/-) mice and NCX1 heterozygous (NCX1(+/-)) littermates before and during acute ANG II infusions. Systolic blood pressure in awake mice was lower in NCX1(sm-/-) mice compared with NCX1(+/-) mice (119 ± 4 vs. 131 ± 3 mmHg, P < 0.05). Acute ANG II infusions (5 ng·min(-1)·g(-1) body wt) increased mean arterial pressure in anesthetized NCX1(+/-) (109 ± 2 to 134 ± 3 mmHg, P < 0.001, n = 8) and NCX1(sm-/-) (101 ± 8 to 129 ± 8 mmHg, P < 0.01, n = 6) mice to a similar extent (Δ25 ± 1 vs. Δ28 ± 4 mmHg, P > 0.05). In response to ANG II infusions, PAH clearance (C(PAH)) decreased from 1.39 ± 0.27 to 0.98 ± 0.22 ml·min(-1)·g(-1) (P < 0.05) and glomerular filtration rate (GFR) was reduced from 0.50 ± 0.09 to 0.32 ± 0.06 ml·min(-1)·g(-1) (P < 0.05) in NCX1(+/-) mice. In contrast, the NCX1(sm-/-) did not exhibit significant reductions in either C(PAH) (1.16 ± 0.30 to 1.22 ± 0.34 ml·min(-1)·g(-1), P > 0.05) or GFR (0.48 ± 0.08 to 0.41 ± 0.05 ml·min(-1)·g(-1), P > 0.05) during acute ANG II infusions. Using flometry to measure renal blood flow continuously, NCX1(sm-/-) mice had significantly attenuated responses to ANG II infusions (-34.2 ± 3.9%, P < 0.05) compared with those in NCX1(+/-) mice (-48 ± 2%) or in wild-type mice (-69 ± 7%). These data indicate that renal vascular responses to ANG II are attenuated in NCX1(sm-/-) mice compared with NCX1(+/-) mice and that NCX1 contributes to the renal vasoconstriction response to acute ANG II infusions.

Stable EET urea agonist and soluble epoxide hydrolase inhibitor regulate rat pulmonary arteries through TRPCs

Epoxyeicosatrienoic acids (EETs), cytochrome P450-derived metabolites of arachidonic acid, have been reported to increase intracellular calcium concentration in aortic vascular smooth muscle cells (SMCs). As EETs are labile, we synthesized a new stable urea EET analog with agonist and soluble epoxide hydrolase (sEH) inhibitor properties. We refer to this analog, 12-(3-hexylureido)dodec-8-enoic acid, as 8-HUDE. Measuring tension of vascular rings, intracellular calcium signaling by confocal laser scanning microscopy and gene expression by reverse-transcription-PCR and western blots, we examined the effects of 8-HUDE on pulmonary vascular tone and calcium signaling in rat pulmonary artery (PA) SMCs (PASMCs). 8-HUDE increased the tension of rat PAs to 145% baseline, whereas it had no effect on the tension of mesenteric arteries (MAs). The 8-HUDE-induced increase in vascular tone was abolished by removal of extracellular Ca(2+) or by pretreatment with either La(3+) or SKF96365, which are inhibitors of canonical transient receptor potential channels (TRPCs). Furthermore, 8-HUDE-evoked increases in [Ca(2+)](i) in PASMCs could be blunted by inhibition of TRPC with SKF96365, removal of extracellular calcium or depletion of intracellular calcium stores with caffeine, cyclopiazonic acid or 2-aminoethoxydiphenyl borate, but not by the voltage-activated calcium channel blocker nifedipine. In addition to immediate effects on calcium signaling, 8-HUDE upregulated the expression of TRPC1 and TRPC6 at both mRNA and protein levels in rat PASMCs, whereas it suppressed the expression of sEH. Our observations suggest that 8-HUDE increases PA vascular tone through increased release of calcium from intracellular stores, enhanced [Ca(2+)](i) influx in PASMCs through store-operated Ca(2+) channels and modulated the expression of TRPC and sEH proteins in a proconstrictive manner.

The Na+/Ca2+ exchanger-1 mediates left ventricular dysfunction in mice with chronic intermittent hypoxia

Chronic intermittent hypoxia (CIH) and cardiovascular dysfunction occur in patients with obstructive sleep apnea. We hypothesized that the Na(+)/Ca(2+) exchanger-1 (NCX1) mediates, at least partially, left ventricular (LV) dysfunction in CIH. Four groups of mice (N = 15-17 per group), either cardiac-specific NCX1 knockouts (KO) or wild types (WT), were exposed to either CIH or normoxia [i.e., handled controls (HC)] 10 h/day for 8 wk. As expected, myocardial expression of NCX1 was greater in WT than in KO animals, both in HC and CIH-exposed groups. In both CIH groups (WT or KO), but not the HC groups, blood pressure increased by 10% at week 1 over their baseline and remained elevated for all 8 wk, with no differences between WT and KO. LV dilation (increased diastolic and systolic dimension) and hypertrophy (increased left heart weight), along with LV dysfunction (greater end-diastolic pressure and lower ejection fraction), were observed in the WT animals compared with the KO following CIH exposure. Compared with HC, CIH exposure was associated with apoptosis (terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling and caspase-3) in WT, but not KO, mice. We conclude that myocardial NCX1 does not mediate changes in blood pressure, but is one of the mediators for LV global dysfunction and cardiomyocyte injury in CIH.