The existence of a highly tetrodotoxin sensitive Na channel in freshly dispersed smooth muscle cells of the rabbit main pulmonary artery

Modulation of fast sodium current in airway smooth muscle cells by exchange protein directly activated by cAMP

Airway smooth muscle (ASM) cells from mouse bronchus express a fast sodium current mediated by NaV1.7. We present evidence that this current is regulated by cAMP. ASM cells were isolated by enzymatic dispersal and studied using the whole cell patch clamp technique at room temperature. A fast sodium current, INa, was observed on holding cells under voltage clamp at -100 mV and stepping to -20 mV. This current was reduced in a concentration-dependent manner by denopamine (10 and 30 µM), a β-adrenergic agonist. Forskolin (1 µM), an activator of adenylate cyclase, reduced the current by 35%, but 6-MB-cAMP (300 µM), an activator of protein kinase A (PKA), had no effect. In contrast, 8-pCPT-2-O-Me-cAMP-AM (007-AM, 10 µM), an activator of exchange protein directly activated by cAMP (Epac), reduced the current by 48%. The inhibitory effect of 007-AM was still observed in the presence of dantrolene (10 µM), an inhibitor of ryanodine receptors, and when cytosolic [Ca2+] was buffered by inclusion of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, Sigma (BAPTA) (50 µM) in the pipette solution, suggesting that the inhibition of INa was not due to Ca2+-release from intracellular stores. When 007-AM was tested on the current-voltage relationship, it reduced the current at potentials from -30 to 0 mV, but had no effect on the steady-state activation curve. However, the steady-state inactivation V1/2, the voltage causing inactivation of 50% of the current, was shifted in the negative direction from -76.6 mV to -89.7 mV. These findings suggest that cAMP regulates INa in mouse ASM via Epac, but not PKA.NEW & NOTEWORTHY β-adrenergic agonists are commonly used in inhalers to treat asthma and chronic obstructive pulmonary disease. These work by causing bronchodilation and reducing inflammation. The present study provides evidence that these drugs have an additional action, namely, to reduce sodium influx into airway smooth muscle cells via fast voltage-dependent channels. This may have the dual effect of promoting bronchodilation and reducing remodeling of the airways, which has a detrimental effect in these diseases.

Tetrodotoxin Decreases the Contractility of Mesenteric Arteries, Revealing the Contribution of Voltage-Gated Na+ Channels in Vascular Tone Regulation

Tetrodotoxin (TTX) poisoning through the consumption of contaminated fish leads to lethal symptoms, including severe hypotension. This TTX-induced hypotension is likely due to the downfall of peripheral arterial resistance through direct or indirect effects on adrenergic signaling. TTX is a high-affinity blocker of voltage-gated Na⁺ (Na_V) channels. In arteries, Na_V channels are expressed in sympathetic nerve endings, both in the intima and media. In this present work, we aimed to decipher the role of Na_V channels in vascular tone using TTX. We first characterized the expression of Na_V channels in the aorta, a model of conduction arteries, and in mesenteric arteries (MA), a model of resistance arteries, in C57Bl/6J mice, by Western blot, immunochemistry, and absolute RT-qPCR. Our data showed that these channels are expressed in both endothelium and media of aorta and MA, in which scn2a and scn1b were the most abundant transcripts, suggesting that murine vascular Na_V channels consist of Na_V1.2 channel subtype with Na_Vβ1 auxiliary subunit. Using myography, we showed that TTX (1 µM) induced complete vasorelaxation in MA in the presence of veratridine and cocktails of antagonists (prazosin and atropine with or without suramin) that suppressed the effects of neurotransmitter release. In addition, TTX (1 µM) strongly potentiated the flow-mediated dilation response of isolated MA. Altogether, our data showed that TTX blocks Na_V channels in resistance arteries and consecutively decreases vascular tone. This could explain the drop in total peripheral resistance observed during mammal tetrodotoxications.

Ranolazine: An Old Drug with Emerging Potential; Lessons from Pre-Clinical and Clinical Investigations for Possible Repositioning

Ischemic heart disease is a significant public health problem with high mortality and morbidity. Extensive scientific investigations from basic sciences to clinics revealed multilevel alterations from metabolic imbalance, altered electrophysiology, and defective Ca²⁺/Na⁺ homeostasis leading to lethal arrhythmias. Despite the recent identification of numerous molecular targets with potential therapeutic interest, a pragmatic observation on the current pharmacological R&D output confirms the lack of new therapeutic offers to patients. By contrast, from recent trials, molecules initially developed for other fields of application have shown cardiovascular benefits, as illustrated with some anti-diabetic agents, regardless of the presence or absence of diabetes, emphasizing the clear advantage of “old” drug repositioning. Ranolazine is approved as an antianginal agent and has a favorable overall safety profile. This drug, developed initially as a metabolic modulator, was also identified as an inhibitor of the cardiac late Na⁺ current, although it also blocks other ionic currents, including the hERG/Ikr K⁺ current. The latter actions have been involved in this drug’s antiarrhythmic effects, both on supraventricular and ventricular arrhythmias (VA). However, despite initial enthusiasm and promising development in the cardiovascular field, ranolazine is only authorized as a second-line treatment in patients with chronic angina pectoris, notwithstanding its antiarrhythmic properties. A plausible reason for this is the apparent difficulty in linking the clinical benefits to the multiple molecular actions of this drug. Here, we review ranolazine’s experimental and clinical knowledge on cardiac metabolism and arrhythmias. We also highlight advances in understanding novel effects on neurons, the vascular system, skeletal muscles, blood sugar control, and cancer, which may open the way to reposition this “old” drug alone or in combination with other medications.

Fast voltage-dependent sodium (NaV ) currents are functionally expressed in mouse corpus cavernosum smooth muscle cells

BACKGROUND AND PURPOSE

Corpus cavernosum smooth muscle (CCSM) exhibits phasic contractions that are coordinated by ion channels. Mouse models are commonly used to study erectile dysfunction, but there are few published electrophysiological studies of mouse CCSM. We describe, for the first time, voltage-dependent sodium (Na_V ) currents in mouse CCSM and investigate their function.

EXPERIMENTAL APPROACH

Electrophysiological, pharmacological, and immunocytochemical studies on isolated CCSM cells. Tension measurements in whole tissue.

KEY RESULTS

A fast, voltage-dependent sodium current was induced by depolarising steps. Steady-state activation and inactivation curves revealed a window current between -60 and -30 mV. Two populations of Na_V currents, ('TTX-sensitive') and ('TTX-insensitive'), were distinguished. TTX-sensitive current showed 48% block with the Na_V -subtype-specific blockers ICA-121431 (Na_V 1.1-1.3), PF-05089771 (Na_V 1.7), and 4,9-anhydro-TTX (Na_V 1.6). TTX-insensitive current was insensitive to A803467, a Na_V 1.8 blocker. Immunocytochemistry confirmed the expression of Na_V 1.5 and Na_V 1.4 in freshly dispersed CCSM cells. Veratridine, a Na_V activator, reduced time-dependent inactivation of the current and increased the duration of evoked action potentials. Veratridine induced phasic contractions in CCSM strips. This effect was reversible with TTX and nifedipine but not by KB-R7943.

CONCLUSION AND IMPLICATIONS

We report, for the first time, a fast voltage-dependent sodium current in mouse CCSM. Stimulation of this current increases the contractility of corpus cavernosum in vitro, suggesting that it may contribute to the mechanisms of detumescence, and potentially serve as a clinically relevant target for pharmaceutical intervention in erectile dysfunction. Further work will be necessary to define its role.

Voltage-dependent inward currents in smooth muscle cells of skeletal muscle arterioles

Voltage-dependent inward currents responsible for the depolarizing phase of action potentials were characterized in smooth muscle cells of 4^th order arterioles in mouse skeletal muscle. Currents through L-type Ca²⁺ channels were expected to be dominant; however, action potentials were not eliminated in nominally Ca²⁺-free bathing solution or by addition of L-type Ca²⁺ channel blocker nifedipine (10 μM). Instead, Na⁺ channel blocker tetrodotoxin (TTX, 1 μM) reduced the maximal velocity of the upstroke at low, but not at normal (2 mM), Ca²⁺ in the bath. The magnitude of TTX-sensitive currents recorded with 140 mM Na⁺ was about 20 pA/pF. TTX-sensitive currents decreased five-fold when Ca²⁺ increased from 2 to 10 mM. The currents reduced three-fold in the presence of 10 mM caffeine, but remained unaltered by 1 mM of isobutylmethylxanthine (IBMX). In addition to L-type Ca²⁺ currents (15 pA/pF in 20 mM Ca²⁺), we also found Ca²⁺ currents that are resistant to 10 μM nifedipine (5 pA/pF in 20 mM Ca²⁺). Based on their biophysical properties, these Ca²⁺ currents are likely to be through voltage-gated T-type Ca²⁺ channels. Our results suggest that Na⁺ and at least two types (T- and L-) of Ca²⁺ voltage-gated channels contribute to depolarization of smooth muscle cells in skeletal muscle arterioles. Voltage-gated Na⁺ channels appear to be under a tight control by Ca²⁺ signaling.

Endothelial and smooth muscle cell ion channels in pulmonary vasoconstriction and vascular remodeling

The pulmonary circulation is a low resistance and low pressure system. Sustained pulmonary vasoconstriction and excessive vascular remodeling often occur under pathophysiological conditions such as in patients with pulmonary hypertension. Pulmonary vasoconstriction is a consequence of smooth muscle contraction. Many factors released from the endothelium contribute to regulating pulmonary vascular tone, while the extracellular matrix in the adventitia is the major determinant of vascular wall compliance. Pulmonary vascular remodeling is characterized by adventitial and medial hypertrophy due to fibroblast and smooth muscle cell proliferation, neointimal proliferation, intimal, and plexiform lesions that obliterate the lumen, muscularization of precapillary arterioles, and in situ thrombosis. A rise in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction, while increased release of mitogenic factors, upregulation (or downregulation) of ion channels and transporters, and abnormalities in intracellular signaling cascades are key to the remodeling of the pulmonary vasculature. Changes in the expression, function, and regulation of ion channels in PASMC and pulmonary arterial endothelial cells play an important role in the regulation of vascular tone and development of vascular remodeling. This article will focus on describing the ion channels and transporters that are involved in the regulation of pulmonary vascular function and structure and illustrating the potential pathogenic role of ion channels and transporters in the development of pulmonary vascular disease.

Peristalsis and propulsion of colonic content can occur after blockade of major neuroneuronal and neuromuscular transmitters in isolated guinea pig colon

We recently identified hexamethonium-resistant peristalsis in the guinea pig colon. We showed that, following acute blockade of nicotinic receptors, peristalsis recovers, leading to normal propagation velocities of fecal pellets along the colon. This raises the fundamental question: what mechanisms underlie hexamethonium-resistant peristalsis? We investigated whether blockade of the major receptors that underlie excitatory neuromuscular transmission is required for hexamethonium-resistant peristalsis. Video imaging of colonic wall movements was used to make spatiotemporal maps and determine the velocity of peristalsis. Propagation of artificial fecal pellets in the guinea pig distal colon was studied in hexamethonium, atropine, ω-conotoxin (GVIA), ibodutant (MEN-15596), and TTX. Hexamethonium and ibodutant alone did not retard peristalsis. In contrast, ω-conotoxin abolished peristalsis in some preparations and reduced the velocity of propagation in all remaining specimens. Peristalsis could still occur in some animals in the presence of hexamethonium + atropine + ibodutant + ω-conotoxin. Peristalsis never occurred in the presence of TTX. The major finding of the current study is the unexpected observation that peristalsis can occur after blockade of the major excitatory neuroneuronal and neuromuscular transmitters. Also, the colon retained an intrinsic polarity in the presence of these antagonists and was only able to expel pellets in an aboral direction. The nature of the mechanism(s)/neurotransmitter(s) that generate(s) peristalsis and facilitate(s) natural fecal pellet propulsion, after blockade of major excitatory neurotransmitters, at the neuroneuronal and neuromuscular junction remains to be identified.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Functional ion channels in human pulmonary artery smooth muscle cells: Voltage-dependent cation channels

The activity of voltage-gated ion channels is critical for the maintenance of cellular membrane potential and generation of action potentials. In turn, membrane potential regulates cellular ion homeostasis, triggering the opening and closing of ion channels in the plasma membrane and, thus, enabling ion transport across the membrane. Such transmembrane ion fluxes are important for excitation–contraction coupling in pulmonary artery smooth muscle cells (PASMC). Families of voltage-dependent cation channels known to be present in PASMC include voltage-gated K⁺ (Kv) channels, voltage-dependent Ca²⁺-activated K⁺ (Kca) channels, L- and T- type voltage-dependent Ca²⁺ channels, voltage-gated Na⁺ channels and voltage-gated proton channels. When cells are dialyzed with Ca²⁺-free K⁺- solutions, depolarization elicits four components of 4-aminopyridine (4-AP)-sensitive Kvcurrents based on the kinetics of current activation and inactivation. In cell-attached membrane patches, depolarization elicits a wide range of single-channel K⁺ currents, with conductances ranging between 6 and 290 pS. Macroscopic 4-AP-sensitive Kv currents and iberiotoxin-sensitive Kca currents are also observed. Transcripts of (a) two Na⁺ channel α-subunit genes (SCN5A and SCN6A), (b) six Ca²⁺ channel α–subunit genes (α_1A, α_1B, α_1X, α_1D, α_1Eand α_1G) and many regulatory subunits (α₂δ₁, β_1-4, and γ₆), (c) 22 Kv channel α–subunit genes (Kv1.1 - Kv1.7, Kv1.10, Kv2.1, Kv3.1, Kv3.3, Kv3.4, Kv4.1, Kv4.2, Kv5.1, Kv 6.1-Kv6.3, Kv9.1, Kv9.3, Kv10.1 and Kv11.1) and three Kv channel β-subunit genes (Kvβ1-3) and (d) four Kca channel α–subunit genes (Sloα1 and SK2-SK4) and four Kca channel β-subunit genes (Kcaβ1-4) have been detected in PASMC. Tetrodotoxin-sensitive and rapidly inactivating Na⁺ currents have been recorded with properties similar to those in cardiac myocytes. In the presence of 20 mM external Ca²⁺, membrane depolarization from a holding potential of -100 mV elicits a rapidly inactivating T-type Ca²⁺ current, while depolarization from a holding potential of -70 mV elicits a slowly inactivating dihydropyridine-sensitive L-type Ca²⁺ current. This review will focus on describing the electrophysiological properties and molecular identities of these voltage-dependent cation channels in PASMC and their contribution to the regulation of pulmonary vascular function and its potential role in the pathogenesis of pulmonary vascular disease.

Function and role of voltage-gated sodium channel NaV1.7 expressed in aortic smooth muscle cells

Voltage-gated Na(+) channel currents (I(Na)) are expressed in several types of smooth muscle cells. The purpose of this study was to evaluate the expression of I(Na), its functional role, pathophysiology in cultured human (hASMCs) and rabbit aortic smooth muscle cells (rASMCs), and its association with vascular intimal hyperplasia. In whole cell voltage clamp, I(Na) was observed at potential positive to -40 mV, was blocked by tetrodotoxin (TTX), and replacing extracellular Na(+) with N-methyl-d-glucamine in cultured hASMCs. In contrast to native aorta, cultured hASMCs strongly expressed SCN9A encoding Na(V)1.7, as determined by quantitative RT-PCR. I(Na) was abolished by the treatment with SCN9A small-interfering (si)RNA (P < 0.01). TTX and SCN9A siRNA significantly inhibited cell migration (P < 0.01, respectively) and horseradish peroxidase uptake (P < 0.01, respectively). TTX also significantly reduced the secretion of matrix metalloproteinase-2 6 and 12 h after the treatment (P < 0.01 and P < 0.05, respectively). However, neither TTX nor siRNA had any effect on cell proliferation. L-type Ca(2+) channel current was recorded, and I(Na) was not observed in freshly isolated rASMCs, whereas TTX-sensitive I(Na) was recorded in cultured rASMCs. Quantitative RT-PCR and immunostaining for Na(V)1.7 revealed the prominent expression of SCN9A in cultured rASMCs and aorta 48 h after balloon injury but not in native aorta. In conclusion, these studies show that I(Na) is expressed in cultured and diseased conditions but not in normal aorta. The Na(V)1.7 plays an important role in cell migration, endocytosis, and secretion. Na(V)1.7 is also expressed in aorta after balloon injury, suggesting a potential role for Na(V)1.7 in the progression of intimal hyperplasia.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.

Electrophysiological and molecular identification of voltage-gated sodium channels in murine vascular myocytes

A voltage-gated Na+ current was characterised in freshly dissociated mouse portal vein (PV) smooth muscle myocytes. The current was found superimposed upon the relatively slow L-type Ca2+ current and was resistant to conventional Ca2+ channel blockers but was abolished by external Na+ replacement and tetrodotoxin (TTX, 1 microM). The molecular identity of the channel responsible for this conductance was determined by RT-PCR where only the transcripts for Na+ channel genes SCN7a, 8a and 9a were detected. The presence of the protein counterparts to the SCN8a and 9a genes (NaV1.6 and NaV1.7, respectively) on the individual smooth muscle myocytes were confirmed in immunocytochemistry, which showed diffuse staining around a predominantly plasmalemmal location. TTX inhibited the action potential in individual myocytes generated in the current clamp mode but isometric tissue tension experiments revealed that TTX (1 and 5 microM) had no effect on the inherent mouse PV rhythmicity. However, the Na+ channel opener veratridine (10 and 50 microM) significantly increased the length of contraction and the interval between contractions. This effect was not influenced by pre-incubation with atropine, prazosin and propranolol, but was reversed by TTX (1 microM) and completely abolished by nicardipine (1 microM). Furthermore, preincubation with the reverse-mode Na+-Ca2+ exchange blocker KB-R7943 (10 microM) also inhibited the veratridine response. We have established for the first time the molecular identity of the voltage-gated Na+ channel in freshly dispersed smooth muscle cells and have shown that these channels can modulate contractility through a novel mechanism of action possibly involving reverse mode Na+-Ca2+ exchange.

TTX-sensitive voltage-gated Na+ channels are expressed in mesenteric artery smooth muscle cells

The presence and properties of voltage-gated Na+ channels in mesenteric artery smooth muscle cells (SMCs) were studied using whole cell patch-clamp recording. SMCs from mouse and rat mesenteric arteries were enzymatically dissociated using two dissociation protocols with different enzyme combinations. Na+ and Ca2+ channel currents were present in myocytes isolated with collagenase and elastase. In contrast, Na+ currents were not detected, but Ca2+ currents were present in cells isolated with papain and collagenase. Ca2+ currents were blocked by nifedipine. The Na+ current was insensitive to nifedipine, sensitive to changes in the extracellular Na+ concentration, and blocked by tetrodotoxin with an IC50 at 4.3 nM. The Na+ conductance was half maximally activated at -16 mV, and steady-state inactivation was half-maximal at -53 mV. These values are similar to those reported in various SMC types. In the presence of 1 microM batrachotoxin, the Na+ conductance-voltage relationship was shifted by 27 mV in the hyperpolarizing direction, inactivation was almost completely eliminated, and the deactivation rate was decreased. The present study indicates that TTX-sensitive, voltage-gated Na+ channels are present in SMCs from the rat and mouse mesenteric artery. The presence of these channels in freshly isolated SMC depends critically on the enzymatic dissociation conditions. This could resolve controversy about the presence of Na+ channels in arterial smooth muscle.

Identification of functional voltage-gated Na(+) channels in cultured human pulmonary artery smooth muscle cells

Electrical excitability, which plays an important role in excitation-contraction coupling in the pulmonary vasculature, is regulated by transmembrane ion flux in pulmonary artery smooth muscle cells (PASMC). This study aimed to characterize the electrophysiological properties and molecular identities of voltage-gated Na(+) channels in cultured human PASMC. We recorded tetrodotoxin (TTX) sensitive and rapidly inactivating Na(+) currents with properties similar to those described in cardiac myocytes. Using RT-PCR, we detected transcripts of seven Na(+) channel alpha genes (SCN2A, 3A, 4A, 7A, 8A, 9A, and 11A), and two beta subunit genes (SCN1B and 2B). Our results demonstrate that human PASMC express TTX-sensitive voltage-gated Na(+) channels. Their physiological functions remain unresolved, although our data suggest that Na(+) channel activity does not directly influence membrane potential, intracellular Ca(2+) release, or proliferation in normal human PASMC. Whether their expression and/or activity are heightened in the pathological state is discussed.

Vascular ENaC proteins are required for renal myogenic constriction

The myogenic response is an essential component of renal blood flow autoregulation and is the inherent ability of vascular smooth muscle cells (VSMCs) to contract in response to increases in intraluminal pressure. Although mechanosensitive ion channels are thought to initiate VSMC stretch-induced contraction, their molecular identity is unknown. Recent reports suggest degenerin/epithelial Na(+) channels (DEG/ENaC) may form mechanotransducers in sensory neurons and VSMCs; however, the role of DEG/ENaC proteins in myogenic constriction of mouse renal arteries has not been established. To test the hypothesis that DEG/ENaC proteins are required for myogenic constriction in renal vessels, we first determined expression of ENaC transcripts and proteins in mouse renal VSMCs. Then, we determined pressure- and agonist-induced constriction and changes in vascular smooth muscle cytosolic Ca(2+) and Na(+) in isolated mouse renal interlobar arteries following DEG/ENaC inhibition with amiloride and benzamil. We detect alpha-, beta-, and gammaENaC transcript and protein expression in cultured mouse renal VSMC. In contrast, we detect only beta- and gamma- but not alphaENaC protein in freshly dispersed mrVMSC. Selective DEG/ENaC inhibition, with low doses of amiloride and benzamil, abolishes pressure-induced constriction and increases in cytosolic Ca(2+) and Na(+) without diminishing agonist-induced responses in isolated mouse interlobar arteries. Our findings indicate that DEG/ENaC proteins are required for myogenic constriction in mouse interlobar arteries and are consistent with our hypothesis that DEG/ENaC proteins may be components of mechanosensitive ion channel complexes required for myogenic vasoconstriction.

Voltage-gated sodium channel expressed in cultured human smooth muscle cells: involvement of SCN9A

Voltage-gated Na(+) channel (I(Na)) is expressed under culture conditions in human smooth muscle cells (hSMCs) such as coronary myocytes. The aim of this study is to clarify the physiological, pharmacological and molecular characteristics of I(Na) expressed in cultured hSMCs obtained from bronchus, main pulmonary and coronary artery. I(Na), was recorded in these hSMCs and inhibited by tetrodotoxin (TTX) with an IC(50) value of approximately 10 nM. Reverse transcriptase/polymerase chain reaction (RT-PCR) analysis of mRNA showed the prominent expression of transcripts for SCN9A, which was consistent with the results of real-time quantitative RT-PCR. These results provide novel evidence that TTX-sensitive Na(+) channel expressed in cultured hSMCs is mainly composed of Na(v)1.7.

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.

Regulation of Ca2+ homeostasis by atypical Na+ currents in cultured human coronary myocytes

Primary cultured human coronary myocytes (HCMs) derived from ischemic human hearts express an atypical voltage-gated tetrodotoxin (TTX)-sensitive sodium current (I(Na)). The whole-cell patch-clamp technique was used to study the properties of I(Na) in HCMs. The variations of intracellular calcium ([Ca2+]i) and sodium ([Na+]i) were monitored in non-voltage-clamped cells loaded with Fura-2 or benzofuran isophthalate, respectively, using microspectrofluorimetry. The activation and steady-state inactivation properties of I(Na) determined a "window" current between -50 and -10 mV suggestive of a steady-state Na+ influx at the cell resting membrane potential. Consistent with this hypothesis, the resting [Na+]i was decreased by TTX (1 micromol/L). In contrast, it was increased by Na+ channel agonists that also promoted a large rise in [Ca2+]i. Veratridine (10 micromol/L), toxin V from Anemonia sulcata (0.1 micromol/L), and N-bromoacetamide (300 micromol/L) increased [Ca2+]i by 7- to 15-fold. This increase was prevented by prior application of TTX or lidocaine (10 micromol/L) and by the use of Na(+)-free or Ca(2+)-free external solutions. The Ca(2+)-channel antagonist nicardipine (5 micromol/L) blocked the effect of veratridine on [Ca2+]i only partially. The residual component disappeared when external Na+ was replaced by Li+ known to block the Na+/Ca2+ exchanger. The resting [Ca2+]i was decreased by TTX in some cells. In conclusion, I(Na) regulates [Ca2+]i in primary cultured HCMs. This regulation, effective at baseline, involves a tonic control of Ca2+ influx via depolarization-gated Ca2+ channels and, to a lesser extent, via a Na+/Ca2+ exchanger working in the reverse mode.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Tetrodotoxin-sensitive sodium current in sheep lymphatic smooth muscle

1. Fast inward currents were elicited in freshly isolated sheep lymphatic smooth muscle cells by depolarization from a holding potential of -80 mV using the whole-cell patch-clamp technique. The currents activated at voltages positive to -40 mV and peaked at 0 mV. 2. When sodium chloride in the bathing solution was replaced isosmotically with choline chloride inward currents were abolished at all potentials. 3. These currents were very sensitive to tetrodotoxin (TTX). Peak current was almost abolished at 1 microM with half-maximal inhibition at 17 nM. 4. Examination of the voltage dependence of steady state inactivation showed that more than 90% of the current was available at the normal resting potential of these cells (-60 mV). 5. The time course of recovery from inactivation was studied using a double-pulse protocol and showed that recovery was complete within 100 ms with a time constant of recovery of 20 ms. 6. Under current clamp, action potentials were elicited by depolarizing current pulses. These had a rapid upstroke and a short duration and could be blocked with 1 microM TTX. 7. Spontaneous contractions of isolated rings of sheep mesenteric lymphatic vessels were abolished or significantly depressed by 1 microM TTX.

Voltage-gated ionic channels in cultured rabbit articular chondrocytes

The membrane properties of cultured cells of rabbit articular chondrocytes were studied using the whole-cell patch clamp technique. The average cell capacitance was 37.9 +/- 9.0 pF (n = 13), and the cell resting potential was -41.0 +/- 7.0 mV (n = 11). We were unable to induce an action potential by applying a depolarizing current. Upon step depolarization, under voltage clamp conditions, one kind of inward and two kinds of outward currents were elicited. The inward current was initially observed at around -30 mV, peaked at 0 mV, and reversed at around +90 mV. Tetrodotoxin (TTX; 1 microM) was shown to completely block this inward current. At steady state, the inward current was half-inactivated at -51 mV, with a slope factor of 6.3 mV. Two outward currents were determined from measurements of activation threshold, reversal potential, and pharmacological responses. One was observed at around -30 mV, and its amplitude increased with membrane depolarization. Extracellularly applied 4-aminopyridine (4 AP) (1 mM) and tetraethyl ammonium chloride (TEA) (5 mM) blocked this current. The other outward current was observed at around +10 mV, and its direction reversed at a potential close to that predicted by the Nernst equation for a Cl- selective channel. This current fluctuated markedly, and the fluctuation did not decline throughout the 100 ms of the step pulse. Extracellularly applied 4-acetamido-4'-isothiocyanostilbenezene-2,2-disulfonic acid (SITS) (0.25 mM) blocked this current, but the same dose of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) had little effect. These results suggest the presence of TTX-sensitive Na+, 4-AP- and TEA-sensitive K+, and SITS-sensitive Cl- channels in rabbit articular chondrocyte membrane. The functional significance of these channels is discussed.

Electrical activity underlying rhythmic contraction in human pial arteries

Human pial arteries obtained during surgery frequently exhibit spontaneous periodic contractions. Simultaneous measurements of membrane potential and vessel wall force were used to examine whether these contractions are associated with electrical activity of smooth muscle cells (SMCs). A total of 53 segments from 38 patients were studied, and of these, 26 showed spontaneous contractions related to periodic depolarization and generation of action potentials (APs). The resting membrane potential during the silent periods was -44.0 +/- 0.5 mV. APs without "overshoot'' were observed when spontaneous depolarization reached levels of -40 to -35 mV. Just over half of the arterial segments failed to exhibit spontaneous activity; however, APs could be generated during K+-induced depolarization. The mean SMC resting membrane potential of these vessels was -53.5 +/- 1.5 mV, and this value differed significantly from that of SMCs in spontaneously active arteries. Application of tetrodotoxin did not change the amplitude and duration of APs. Removal of Ca2+ from the bathing solution and addition of nifedipine completely inhibited the spontaneous APs and associated contractions. K+ depolarization failed to induce APs and contraction in the presence of nifedipine. We conclude that periodic spontaneous depolarization and AP generation underlie the periodic spontaneous contractions of human pial arteries. Both the APs and associated contractions are related to the activation of dihydropyridine-sensitive voltage-dependent Ca2+ channels. It is suggested that AP generation can be responsible for vasomotion of human pial arteries in vivo.

Biophysical properties of Ca(2+)- and Mg-ATP-activated K+ channels in pulmonary arterial smooth muscle cells isolated from the rat

A novel class of Ca(2+)-activated K+ channel, also activated by Mg-ATP, exists in the main pulmonary artery of the rat. In view of the sensitivity of these "KCa,ATP" channels to such charged intermediates it is possible that they may be involved in regulating cellular responses to hypoxia. However, their electrophysiological profile is at present unknown. We have therefore characterised the sensitivity of KCa,ATP channels to voltage, intracellular Ca2+ ([Ca2+]i) and Mg-ATP. They have a conductance of 245 pS in symmetrical K+ and are approximately 20 times more selective for K+ ions than Na+ ions, with a K+ permeability (PK) of 4.6 x 10(-13) cm s-1.Ca2+ ions applied to the intracellular membrane surface of KCa,ATP channels causes a marked enhancement of their activity. This activation is probably the result of simultaneous binding of at least two Ca2+ ions, determined using Hill analysis, to the channel or some closely associated protein. This results in a shift of the voltage activation threshold to more hyperpolarized membrane potentials. The activation of KCa,ATP channels by Mg-ATP has an EC50 of approximately 50 microM. Although the EC50 is unaffected by [Ca2+]i, channel activation by Mg-ATP is enhanced by increasing [Ca2+]i. One possible interpretation of these data is that Mg-ATP increases the sensitivity of KCa,ATP channels to Ca2+. It is therefore possible that under hypoxic conditions, where lower levels of Mg-ATP may be encountered, the sensitivity of KCa,ATP channels to Ca2+ and therefore voltage is reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Fast Na+ current in circular smooth muscle cells of the large intestine

Whole-cell voltage clamp was carried out on freshly dispersed single smooth muscle cells from adult rat and human colons to investigate the regulation of the Ca2+ channels. In this study, we unexpectedly discovered the existence of a fast Na+ channel current. With normal physiological salt solution (PSS) plus 4-amino-pyridine (3 mM) in the bath and high-Cs+ solution in the pipette to inhibit outward K+ currents, an inward current possessing fast and slow components was observed when the cell membrane was depolarized to a value more positive than -20 mV from a holding potential of -100 mV. When Ca2+ ions were removed from the PSS, or when nifedipine (10 microM) and Ni2+ (30 microM) were simultaneously applied, the slow component disappeared and the fast component remained. The fast current component became almost completely inactivated within 10 ms. This fast component was dependent on extracellular Na+ concentration and was inhibited by tetrodotoxin (TTX) dose dependently (IC50 of 130 nM in rat and 14 nM in human). These results suggest that the slow component of inward current was a Ca2+ channel current, whereas the fast component was a TTX-sensitive fast Na+ channel current. The threshold voltage, the voltage for peak current, and the reversal potential for the fast Na+ current were, respectively, about -50, -20, and +50 mV in rats, and -40, 0, and +60 mV in humans. The incidence of cells possessing fast Na+ currents depended on the region of the colon.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ channel antagonists and inhibition of protein kinase C each block contraction but not depolarization to 5-hydroxytryptamine in the rabbit basilar artery

The Ca2+ channel antagonists nifedipine and verapamil each significantly inhibited (50-100%) the smooth muscle contraction induced in response to either 5-hydroxytryptamine (1 microM, 5-HT) or 20 mM K+ (K(+)-physiological salt solution) in the basilar artery. Simultaneous measurements of smooth muscle membrane potential showed that changes in potential were not modified at this time. A similar inhibitory action against the smooth muscle contraction but not the depolarization to 5-HT was obtained with the putative protein kinase C and phospholipase C inhibitors, 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (10 microM, H7) and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (70 microM, NCDC). These data indicate that 5-HT-induced Ca2+ influx through voltage sensitive channels is important for smooth muscle contraction but not depolarization in the rabbit basilar artery.

Membrane conductances involved in amplification of small signals by sodium channels in photoreceptors of drone honey bee

1. Voltage signals of about 1 mV evoked in photoreceptors of the drone honey bee by shallow modulation of a background illumination of an intensity useful for behaviour are thought to be amplified by voltage-dependent Na+ channels. To elucidate the roles of the various membrane conductances in this amplification we have studied the effects of the Na+ channel blocker tetrodotoxin (TTX) and various putative K+ channel blockers on the membrane potential, Vm. 2. Superfusion of a slice of retina with 0.5-10 mM-4-aminopyridine (4-AP) depolarized the membrane and, in fifty of sixty-three cells induced repetitive action potentials. Ionophoretic injection of tetraethylammonium produced similar effects. 3. In order to measure the depolarization caused by 4-AP, action potentials were prevented by application of TTX: 4-AP was applied when the membrane was depolarized to different levels by light. 4-AP induced an additional depolarization at all membrane potentials tested (-64 to -27 mV). We conclude that there are 4-AP-sensitive K+ channels that are open at constant voltage over this range. 4. 4-AP slowed down the recovery phase of the action potential induced by a light flash by a factor that ranged from 0.51 to 0.16. This reduction could be accounted for by the reduction in a voltage-independent K+ conductance estimated from the steady-state depolarization. 5. After the voltage-gated Na+ channels had been blocked by TTX, exposure to 4-AP further changed the amplitude of the response to a small (approximately 10%) decremental light stimulus. The change was an increase when the background illumination brought Vm to potentials more negative than about -40 mV; it was a decrease when Vm > -40 mV. The data could be fitted by a circuit representation of the membrane with a light-activated conductance and a K+ conductance (EK = -66 mV) that was partly blocked by 4-AP. The voltage range studied was from -52 to -27 mV; neither conductance in the model was voltage dependent. 6. The responses to small changes in light intensity in the absence of TTX were mimicked by a model. We conclude that a voltage-dependent Na+ conductance described by the Hodgkin-Huxley equations can amplify small voltage changes in a cell membrane that is also capable of generating action potentials; the magnitude of the K+ conductance is critical for optimization of signals while avoiding membrane instability.

Veratridine activates a silent sodium channel in rat isolated aorta

To investigate the existence of silent Na+ channels, isolated rat aorta was treated with veratridine (0.1 mM) and the resulting Ca2+ uptake was determined. After 30-min incubation the total tissue uptake of Ca2+ and Ca2+ uptake increased from 2.325 +/- 0.017 to 2.614 +/- 0.080 nmol/mg wet weight (ww) and from 162.6 +/- 9.7 to 218.1 +/- 13.0 pmol/mg ww, respectively. The veratridine-induced Ca2+ uptake was blocked by tetrodotoxin (1 microM; to 17 +/- 5%) but not altered by amiloride (10 microM-1 mM). Activation of Na+/Ca2+ exchange by Na+ removal increased Ca2+ uptake from 74.2 +/- 4.5 to 97.3 +/- 5.3 pmol/mg ww, which was suppressed by amiloride (10 microM-1 mM). Nifedipine (10 nM) and verapamil (0.1 microM) at concentrations at which depolarization-induced Ca2+ uptake was diminished did not attenuate veratridine-induced Ca2+ uptake. Phenytoin at 0.1 mM reduced the Ca2+ uptake induced by veratridine or by depolarization. R 56865 (0.1 microM) and R 59494 (1 microM), novel anti-ischemic compounds inhibiting slowly inactivating Na+ channels, suppressed the veratridine-induced but not the depolarization-induced Ca2+ uptake. Guanidinium uptake was increased by veratridine (0.1 mM) from 371.2 +/- 7.2 to 574.8 +/- 45.9 pmol/mg ww. These results suggest that the rat aorta possesses a Na+ channel which is electrically silent under normal conditions but could be activated by veratridine.

Potential-dependent inward currents in single isolated smooth muscle cells of the rat ileum

1. Calcium (ICa) and sodium (INa) currents were studied in single smooth muscle cells freshly isolated from both the newborn (1-3 days old) and adult rat ileum, using the patch-clamp technique (whole-cell configuration). 2. Under conditions when INa was blocked, two components of ICa, low-voltage activated or ICa,low and high-voltage activated or ICa,high, were observed in the newborn rat ileal cells. ICa,high and ICa,low have differing voltage ranges of activation and steady-state inactivation and time courses of recovery from inactivation. Potential dependence of ICa,low was much steeper and shifted toward negative membrane potential than that for ICa,high (slope factors and the potential of half-maximal inactivation were 13.6 and -60.6 and 8.8 and -49 mV for ICa,low and ICa,high, correspondingly). 3. Nifedipine at the high concentration of 30 microM exerted no effect on ICa,low and only slightly suppressed ICa,high, decreasing its peak to 0.81 +/- 0.04 (n = 7) at the holding potential of -80 mV and to 0.66 +/- 0.05 (n = 3) at -50 mV. ICa,high was suppressed significantly by Cd2+ ions, while ICa,low was more sensitive to Ni2+ ions. 4. Results presented here suggest that the properties of high-voltage-activated (HVA) Ca2+ channels in the rat small intestine are quite different to those described for L-type Ca2+ channels found in other smooth muscles. It is proposed that HVA Ca2+ channels are similar to N-type Ca2+ channels. 5. Comparison of Ca2+ currents in newborn and adult rat ileal cells showed that the contribution of ICa,low to the net Ca2+ current was negligible in adults, whereas the properties of HVA Ca2+ channels were similar in the neonatal and adult animals. 6. INa, studied in nominally Ca(2+)-free physiological salt solution, activated in the voltage range between -50 and -40 mV and reached its peak at -10 mV. INa was blocked in a dose-dependent manner by TTX with an apparent dissociation constant of 4.5 nM. 7. INa decay was monoexponential in the voltage range studied and its time constant decreased monotonically with membrane depolarization from 4.7 +/- 0.2 ms (n = 6) at -30 mV to 0.51 +/- 0.03 ms (n = 7) at 20 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of dihydropyridines on calcium channels in isolated smooth muscle cells from rat vena cava

1. Whole-cell patch-clamp method was applied to single smooth muscle cells freshly isolated from the rat inferior vena cava. 2. Depolarizing pulses, applied from a holding potential of -90 mV, activated both Na+ and Ca2+ channels. The fast Na+ current was inhibited by nanomolar concentrations of tetrodotoxin (TTX). The slow Ba2+ current (measured in 5 mM Ba2+ solution) was inhibited by Cd2+ and modulated by dihydropyridine derivatives. When the cells were held at a holding potential of -80 mV, racemic Bay K 8644 increased the Ba2+ current (ED50 = 10 nM) while racemic isradipine inhibited the current (IC50 = 21 nM). 3. The voltage-dependency of isradipine blockade was assessed by determining the steady-state availability of the Ca2+ channels. From the shift of the inactivation curve in the presence of isradipine, we calculated a dissociation constant of 1.11 nM for inactivated Ca2+ channels. Scatchard plots of the specific binding of (+)-[3H]-isradipine obtained in intact strips incubated in 5.6 mM or 135 mM K+ solutions confirmed the voltage-dependency of isradipine binding. 4. Specific binding of (+)-[3H]-isradipine was completely displaced by unlabelled (+/-)-isradipine, with an IC50 of 15.1 nM. This value is similar to the IC50 for inhibition of the Ba2+ current (21 nM) in cells maintained at a holding potential of -80 mV. 5. Bay K 8644 had no effects on the Ba2+ current kinetics during a depolarizing test pulse. The steady-state inactivation-activation curves of Ba2+ current were not significantly shifted along the voltage axis.6. The present data suggest the existence of two distinct dihydropyridine binding sites which can be bound preferentially by agonist or antagonist derivatives.

Direct role for potassium channel inhibition in hypoxic pulmonary vasoconstriction

Cellular mechanisms responsible for hypoxic pulmonary vasoconstriction were investigated in pulmonary arterial cells, isolated perfused lung, and pulmonary artery rings. Three K+ channel antagonists, Leiurus quinquestriatus venom, tetraethylammonium, and 4-aminopyridine, mimicked the effects of hypoxia in isolated lung and arterial rings by increasing pulmonary artery pressure and tension and also inhibited whole cell K+ currents in isolated pulmonary arterial cells. Reduction of oxygen tension from normoxic to hypoxic levels directly inhibited K+ currents and caused membrane depolarization in isolated canine pulmonary arterial smooth muscle cells but not in canine renal arterial smooth muscle cells. Nisoldipine or high buffering of intracellular Ca2+ concentration with [1,2-bis(2)aminophenoxy] ethane-N,N,N',N'-tetraacetic acid prevented hypoxic inhibition of K+ current, suggesting that a Ca(2+)-sensitive K+ channel may be responsible for the hypoxic response. These results indicate that K+ channel inhibition may be a key event that links hypoxia to pulmonary vasoconstriction by causing membrane depolarization and subsequent Ca2+ entry.

Vascular smooth muscle contraction induced by Na+ channel activators, veratridine and batrachotoxin

The effects of the sodium channel activators veratridine and batrachotoxin on isolated rat aorta were investigated. Veratridine caused gradual contraction, independent of the presence of endothelium, with an EC50 of 35 microM. Batrachotoxin (1 microM) also induced contraction. Both effects were completely inhibited by the sodium channel blocker tetrodotoxin (1 microM). The veratridine (60 microM)-induced contraction was inhibited by nifedipine (0.1 microM). In the absence of extracellular Ca2+, veratridine (60 microM) did not cause contraction. Sodium nitroprusside (80 nM), acetylcholine (10 microM) and isoproterenol (1 microM) caused relaxation of rings precontracted with veratridine (60 microM). An inhibitor of endothelium-derived relaxing factor (EDRF) synthase, N omega-nitro-L-arginine methyl ester (L-NAME) (0.65 mM), enhanced the veratridine-induced contraction in rings with an intact endothelium, which suggests that EDRF was being released during the veratridine-induced contraction. These results show that the activation of sodium channels on smooth muscle cells induces a contraction that is probably mediated by Ca2+ influx through voltage-dependent Ca2+ channels.

A new non-voltage-dependent, epithelial-like Na+ channel in vascular smooth muscle cells

A new type of Na+ channel was identified in smooth muscle cells of the rat aortic cell line A7r5, and in smooth muscle cells cultured from rat aorta and rat portal vein. The channel is highly selective for Na+ (PNa/PK greater than 11). It is active in cell-attached patches, and independent of the trans-patch membrane potential. The single channel conductance is low (10.7 pS). Two substates were identified. This channel is insensitive to effectors of other types of Na+ channels, such as amiloride (100 microM) or tetrodotoxin (100 microM). It is inhibited by phenamil at high concentrations (greater than 10 microM). The mean open state probability P(O) varied from patch to patch (0.05-0.88). Kinetics analysis reveals a complex behaviour: open times separate in short (tau 1 = 84 ms) and long (tau 2 = 845 ms) openings and closed times separate into short (tau 1 = 60 ms) and long closures (tau 2 = 272-3130 ms). Short openings and long closures are preponderant at a low P(O). Long openings are absent in the presence of phenamil (50 microM) and are unaffected by amiloride (100 microM). Fluctuations of the channel activity in cell-attached patches and the fast disappearance after excision suggest that this channel is under metabolic control. This vascular smooth muscle channel appears to be a potentially important Na+ entry pathway for vascular cells and an amiloride-resistant homologue of the epithelial Na+ channel.

Sodium currents in smooth muscle cells freshly isolated from stomach fundus of the rat and ureter of the guinea-pig

1. Inward currents elicited by depolarization from holding potentials of -80 to -10 mV in single smooth muscle cells isolated from stomach fundus of the rat and ureter of the guinea-pig had two components. The initial fast component (Ifi) was activated and mostly inactivated within 1-2 and 10 ms, respectively, at 21 degrees C. The following sustained component (Isi) lasted over 50 and 500 ms in fundus and ureter cells, respectively. Ifi was blocked by tetrodotoxin but not affected by 0.5 microM-mu-conotoxin in both types of cells. Isi was abolished by the substitution of extracellular Ca2+ with Mn2+. 2. The sensitivity of Ifis to TTX was markedly different in fundus and ureter cells. The half-inhibition was obtained at 870 and 11 nM, respectively. The amplitude of Ifi was highly dependent on extracellular Na+ concentration in a solution containing 2.2 mM-Mn2+ and 0 mM-Ca2+ in both cells. It is concluded that Ifis in these cells are TTX-sensitive and mu-conotoxin-insensitive Na+ currents. 3. Some of the kinetics of INa measured at 10 degrees C were markedly different in fundus and ureter cells. The current-voltage relationships for Ifi in fundus and ureter cells had peaks at about -10 and 0 mV, respectively. The voltage dependence of the steady-state inactivation of Ifi was also significantly different in these cell types. The half-inactivation voltages were about -74 and -45 mV, respectively. The recovery time course from inactivation in fundus cells was about 10 times slower than that in ureter at -80 mV, where it was 25 ms. 4. The contribution of Ifi to the rising phase of an action potential was examined using TTX under current clamp mode at 21 degrees C. A fast notch-like potential elicited by a subthreshold stimulus for action potential generation was blocked by TTX in both types of cells. Action potentials elicited by a stimulus around threshold were occasionally suppressed by TTX, whereas an action potential was never observed when extracellular Ca2+ was replaced with Mn2+. 5. In conclusion, the existence of at least two types of Na+ channel currents, which were distinguished by their TTX sensitivity and kinetics, was strongly suggested in smooth muscle cells from the rat fundus and the guinea-pig ureter. INa in these cells may have a physiological role to accelerate the generation of an action potential by triggering a rapid activation of ICa, while not being essential for activation of action potentials.

Modulation of calcium movements by nitroprusside in isolated vascular smooth muscle cells

Using the patch-clamp technique, we have characterised the inward current from enzymatically dispersed rabbit pulmonary arterial cells, and investigated the effects of the vasodilator, nitroprusside (NP), on these and other membrane currents. With Cs(+)-filled pipettes, inward currents were recorded during brief depolarizing voltage steps in both physiological Ca2+ and 10 mM Ba2+. The threshold for current activation was positive to -40 mV and the current peaked at 0 mV for Ca2+ and +10 mV for Ba2+. During the first few minutes of recording, inward currents increased or "ran-up". This could not be attributed to blockade of outward current or the inclusion of adenosine triphosphate (ATP) in the patch pipette. Experiments revealed that all the inward current was carried through a single type of voltage-activated Ca2+ channel, namely the high-threshold, dihydropyridine-sensitive channel. It was unaffected by tetrodotoxin but was abolished at all potentials by low concentrations of Cd2+ (100 microM) or nifedipine (1-2 microM). NP (1 microM) suppressed peak inward Ba2+ current at +10 mV by approximately 45%. Higher concentrations (50 microM) did not produce further blockade of the current. This decrease was associated with increased inactivation of the current, and both effects required the presence of ATP in the patch pipette. In physiological Ca2+, using K(+)-filled pipettes, NP was found to induce spontaneous bursts of outward currents, which are probably activated by the release of Ca2+ from Ca(2+)-overloaded stores. These results are consistent with NP lowering cytosolic Ca2+, and hence causing vasodilation, by inhibiting Ca2+ influx through voltage-gated Ca2+ channels and by promoting Ca2+ uptake into the sarcoplasmic reticulum.

Voltage-dependent sodium and potassium, but no calcium conductances in DDT1 MF-2 smooth muscle cells

Voltage-dependent inward and outward membrane currents were investigated in the DDT1 MF-2 smooth muscle cell line using the whole-cell patch-clamp technique. Application of a pulse protocol with subsequent depolarizing voltage steps elicited an inactivating inward current and a non-inactivating outward current. The outward current was activated at membrane potentials more positive than -35 mV, with tau act = 30 -40 ms. The outward current was blocked by tetraethylammonium (NEt4Cl) and 3,4-aminopyridine in a dose-dependent manner (EC50 of 5 mM and 0.5 mM, respectively). The amplitude of the outward current was linked to the potassium equilibrium potential (Vek), and tail currents reversed near Vek. The outward current was completely abolished when intracellular potassium was substituted by 106 mM caesium and 20 mM NEt4Cl. The inward current was activated at potentials more positive than -30 mV with tau act of 1.6-2.5 ms, and with tau inact of 1.7-3.0 ms. Steady-state inactivation was 50% at a holding potential of -40 mV. The inward current was blocked by tetrodotoxin (EC50 of 0.15 microM) and dependent on the reversal potential for sodium. Voltage-dependent calcium currents could not be detected. Further, the cytoplasmic free calcium concentration, as measured using Indo-1 fluorescence, was not changed during high-potassium (40 mM)-induced depolarization. In contrast, contraction of freshly obtained hamster vas deferens tissue elicited by high-potassium(40 mM)-induced depolarization was largely inhibited by diltiazem (20 microM). These findings showed that voltage-dependent calcium channels are not functional in DDT1 MF-2 smooth muscle cells in contrast to freshly obtained Syrian hamster vas deferens smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of sympathetic neuromuscular transmission and smooth muscle cell membranes in vascular beds

In vascular smooth muscle tissues, the cycle of contraction-relaxation is mainly regulated by the cytosolic Ca, and many other factors, such as substances released from endothelial cells and perivascular nerve terminals (mainly sympathetic nerves). In this article, we introduce regional differences in specific features of ionic channels in vascular smooth muscle membranes (mainly on features of Ca, Na and K channels) in relation to mobilization of the cytosolic Ca. In many vascular tissues, neurotransmitters released from sympathetic nerve terminals activate post-junctional receptors, and subsequently modify ion channels (receptor-activated cation channel and voltage-dependent Ca channel), whereas in some tissues, ionic channels are not modified by receptor activations (pharmaco-mechanical coupling). However, activation of receptors, with or without modulation of ionic channels, regulates the cytosolic Ca through synthesis of second messengers. In addition, receptors distributed on prejunctional nerve terminals positively or negatively regulate the release of transmitters. Roles of neurotransmitters (mainly ATP and noradrenaline) are also discussed in relation to the generation of excitatory junction potentials.

High-affinity binding sites for [3H]saxitoxin are associated with voltage-dependent sodium channels in portal vein smooth muscle

Saturable, high-affinity binding sites for [3H]saxitoxin were identified in equine portal vein smooth muscle membranes. These sites had a dissociation constant of 0.29 nM and a maximal binding capacity of 115 fmol.mg-1 of protein. A similar dissociation constant was obtained with cells prepared from rat portal vein. Specific binding of [3H]saxitoxin was completely displaced by unlabelled saxitoxin and tetrodotoxin, with inhibition constants of 0.42 and 2.10 nM, respectively. Tetrodotoxin blocked the fast Na+ current in single cells of rat portal vein in a concentration-dependent manner, with an IC50 of 3.15 nM. These results suggest that the high-affinity binding sites for tetrodotoxin and saxitoxin may be associated with voltage-dependent Na+ channels in vascular myocytes.

The whole-cell Ca2+ channel current in single smooth muscle cells of the guinea-pig ureter

1. Calcium channel currents were recorded in single, enzymatically isolated smooth muscle cells of the guinea-pig ureter using a single-electrode whole-cell voltage clamp technique. Calcium and barium currents through voltage-activated Ca2+ channels were recorded in cells dialysed with Cs(+)- or Na(+)-containing saline which suppressed K+ currents. 2. Inward currents in Ca2+ (1.5-7.5 mM) or Ba2+ (1.5-7.5 mM) were recorded at potentials positive to -50 to -30 mV. Inward currents were maximal at 0 mV in 1.5 mM-Ca2+ and at +10 mV in 7.5 mM-Ba2+. Current flow through Ca2+ channels in Cs(+)-filled cells (in 1.5 mM-Ca2+ or 7.5 mM-Ba2+) changed from inward to outward at potentials positive to +70 mV. In Na(+)-filled cells this reversal potential was between +50 and +60 mV. 3. Replacing Ca2+ or Ba2+ with Co2+ (1.5 mM) blocked all inward current flow through these Ca2+ channels; outward currents at potentials positive to +40 mV, however, were increased. Cadmium (100 microM) and nifedipine (0.1-10 microM) reduced both inward and outward current flow. 4. Calcium channel activation showed a sigmoidal relationship with membrane potential; the potential of half-maximal activation was -8.4 mV in 1.5 mM-Ca2+ and -10.8 mV in 7.5 mM-Ba2+. The maximum membrane conductance to Ca2+ (in 1.5 mM-Ca2+) was 2.57 nS/cell or approximately 0.05 mS/cm2. 5. Evidence for a voltage-dependent inactivation mechanism included (a) the time-dependent relaxation of the outward currents at potentials positive to the reversal potential and (b) a steady-state inactivation (f infinity (V] vs. membrane potential relationship (in 7.5 mM-Ba2+) which ranged between -80 and 0 mV, with a half-maximal availability at -40.5 mV. 6. The voltage dependencies of the inward current elicited from -80 and -30 mV were similar, suggesting that depolarization activated only L-type Ca2+ channels. 7. It was concluded that the processes controlling the time course of the Ca2+ current in single ureteral cells bathed in physiological concentrations of Ca2+ were mostly voltage-dependent.

Fast Na+ and slow Ca2+ channels in single uterine muscle cells from pregnant rats

Whole cell voltage-clamp method was applied to single smooth muscle cells freshly isolated from the longitudinal layer of 18-day pregnant rat uterus. Inward currents were isolated after outward currents were minimized by use of high Cs+ in the pipette solution and 4-aminopyridine (3 mM) in the bath solution. Depolarizing pulses, applied from a holding potential of -90 mV, evoked two types of inward current, fast and slow. The fast inward current decayed and disappeared within 30 ms and depended on extracellular Na+ concentration. This fast current was inhibited by tetrodotoxin (TTX) dose dependently (KD = 27 nM). These results suggest that the fast inward current was a TTX-sensitive Na+ channel current. In contrast, the slow inward current decayed slowly, dependent on extracellular Ca2+ (or Ba2+) concentration, and was inhibited by the Ca2+ channel blocker, nifedipine, dose dependently (10 nM-10 microM). These results suggest that the slow inward current was a Ca2+ channel current. A fast-inactivating Ca2+ channel current was not evident when Ba2+ was the charge carrier. We conclude that the major ion channels in the cell membrane of pregnant rat uterus, which generate inward currents, are TTX-sensitive fast Na+ channels and dihydropyridine-sensitive slow Ca2+ channels (L-type, high-threshold type).

Ionic currents in single smooth muscle cells from the ureter of the guinea-pig

1. Ionic currents underlying the action potential were recorded from enzymatically isolated smooth muscle cells of guinea-pig ureter. 2. The action potential recorded from a single cell was similar to that from a multicellular preparation. It showed repetitive spikes on a plateau potential which followed the first spike. Treatment with 10 mM-tetraethylammonium (TEA) increased the amplitude and duration of the plateau phase and abolished the repetitive spikes. 3. Under voltage clamp mode, at least two (maybe three) kinds of outward currents were activated during depolarizing pulses. The main outward current was Ca2+-dependent K+ current (IK(Ca], which was mostly blocked in Ca2+-free solution, or by application of 1 mM-cadmium (Cd2+) or 2 mM-tetraethylammonium (TEA). IK(Ca) was greatly decreased by treatment with 5 mM-caffeine or an addition of 10 mM-EGTA in a pipette solution. 4. In the presence of 1 mM-Cd2+ and 2 mM-TEA, a small transient outward current remained. 4-Aminopyridine (1 mM) suppressed the transient outward current by about 40%. Time- and voltage-dependent delayed rectifier outward currents were small in ureter cells. An inwardly rectifying K+ current was not detected. 5. An application of 1 mM-Cd2+, 5 mM-cobalt (Co2+), 1 mM-lanthanum (La3+) or 0.1 microM-nifedipine completely blocked the action potential. Replacement of 80-90% of extracellular Na+ with Li+ or Tris almost abolished the plateau potential and repetitive spikes but did not change significantly the first spike. 6. In the presence of 30 mM-TEA, the inward current elicited by depolarization was monophasic and lasted for more than 1 s. Application of 1 mM-Cd2+, 1 mM-La3+, 0.1 microM-nifedipine, or 5 mM-Co2+ completely blocked inward current. The replacement (87%) of extracellular Na+ ions with Li+, Tris, sucrose or TEA speeded up the decay of inward current; the inward current decreased by 10-60% at the end of a 500 ms pulse. 7. Even in low-Na+ solution (120 mM-TEA), the inactivation of ICa had a quite slow component (tau = 1 s), in addition to another faster component (tau = 100 ms) at 0 mV. When short depolarizing clamp pulses (50 ms) were repetitively applied at short intervals (50 ms) and with interpulse voltage of -10 or -20 mV to mimic the repetitive spikes on the plateau of the action potential, the decline of peak Ca2+ current during the train of pulses was smaller than the decay of Ca2+ current during a long pulse.(ABSTRACT TRUNCATED AT 400 WORDS)