TRPV2 modulates mechanically Induced ATP Release from Human bronchial epithelial cells

Repetitive bouts of coughing expose the large airways to significant cycles of shear stress. This leads to the release of alarmins and the tussive agent adenosine triphosphate (ATP) which may be modulated by the activity of ion channels present in the human airway. This study aimed to investigate the role of the transient receptor potential subfamily vanilloid member 2 (TRPV2) channel in mechanically induced ATP release from primary bronchial epithelial cells (PBECs).PBECs were obtained from individuals undergoing bronchoscopy. They were cultured in vitro and exposed to mechanical stress in the form of compressive and fluid shear stress (CFSS) or fluid shear stress (FSS) alone at various intensities. ATP release was measured using a luciferin-luciferase assay. Functional TRPV2 protein expression in human PBECs was investigated by confocal calcium imaging. The role of TRPV2 inhibition on FSS-induced ATP release was investigated using the TRPV2 inhibitor tranilast or siRNA knockdown of TRPV2. TRPV2 protein expression in human lung tissue was also determined by immunohistochemistry.ATP release was significantly increased in PBECs subjected to CFSS compared with control (unstimulated) PBECs (N = 3, ***P < 0.001). PBECs expressed functional TRPV2 channels. TRPV2 protein was also detected in fixed human lung tissue. ATP release from FFS stimulated PBECs was decreased by the TRPV2 inhibitor tranilast (N = 3, **P < 0.01) (vehicle: 159 ± 17.49 nM, tranilast: 25.08 ± 5.1 nM) or by TRPV2 siRNA knockdown (N = 3, *P < 0.05) (vehicle: 197 ± 24.52 nM, siRNA: 119 ± 26.85 nM).In conclusion, TRPV2 is expressed in the human airway and modulates ATP release from mechanically stimulated PBECs.

House dust mite allergens induce Ca2+ signalling and alarmin responses in asthma airway epithelial cells

Type 2 inflammation in asthma develops with exposure to stimuli to include inhaled allergens from house dust mites (HDM). Features include mucus hypersecretion and the formation of pro-secretory ion transport characterised by elevated basal Cl- current. Studies using human sinonasal epithelial cells treated with HDM extract report a higher protease activated receptor-2 (PAR-2) agonist-induced calcium mobilisation that may be related to airway sensitisation by allergen-associated proteases. Herein, this study aimed to investigate the effect of HDM on Ca2+ signalling and inflammatory responses in asthmatic airway epithelial cells. Primary bronchial epithelial cells (hPBECs) from asthma donors cultured at air-liquid interface were used to assess electrophysiological, Ca2+ signalling and inflammatory responses. Differences were observed regarding Ca2+ signalling in response to PAR-2 agonist 2-Furoyl-LIGRLO-amide (2-FLI), and equivalent short-circuit current (Ieq) in response to trypsin and 2-FLI, in ALI-asthma and healthy hPBECs. HDM treatment led to increased levels of intracellular cations (Ca2+, Na+) and significantly reduced the 2-FLI-induced change of Ieq in asthma cells. Apical HDM-induced Ca2+ mobilisation was found to mainly involve the activation of PAR-2 and PAR-4-associated store-operated Ca2+ influx and TRPV1. In contrast, PAR-2, PAR-4 antagonists and TRPV1 antagonist only showed slight impact on basolateral HDM-induced Ca2+ mobilisation. HDM trypsin-like serine proteases were the main components leading to non-amiloride sensitive Ieq and also increased interleukin-33 (IL-33) and thymic stromal lymphopoietin (TSLP) from asthma hPBECs. These studies add further insight into the complex mechanisms associated with HDM-induced alterations in cell signalling and their relevance to pathological changes within asthma.

Regulation of nerve-evoked contractions of the murine vas deferens

Stimulation of sympathetic nerves in the vas deferens yields biphasic contractions consisting of a rapid transient component resulting from activation of P2X1 receptors by ATP and a secondary sustained component mediated by activation of α1-adrenoceptors by noradrenaline. Noradrenaline can also potentiate the ATP-dependent contractions of the vas deferens, but the mechanisms underlying this effect are unclear. The purpose of the present study was to investigate the mechanisms underlying potentiation of transient contractions of the vas deferens induced by activation of α1-adrenoceptors. Contractions of the mouse vas deferens were induced by electric field stimulation (EFS). Delivery of brief (1s duration) pulses (4 Hz) yielded transient contractions that were inhibited tetrodotoxin (100 nM) and guanethidine (10 µM). α,β-meATP (10 µM), a P2X1R desensitising agent, reduced the amplitude of these responses by 65% and prazosin (100 nM), an α1-adrenoceptor antagonist, decreased mean contraction amplitude by 69%. Stimulation of α1-adrenoceptors with phenylephrine (3 µM) enhanced EFS and ATP-induced contractions and these effects were mimicked by the phorbol ester PDBu (1 µM), which activates PKC. The PKC inhibitor GF109203X (1 µM) prevented the stimulatory effects of PDBu on ATP-induced contractions of the vas deferens but only reduced the stimulatory effects of phenylephrine by 40%. PDBu increased the amplitude of ATP-induced currents recorded from freshly isolated vas deferens myocytes and HEK-293 cells expressing human P2X1Rs by 93%. This study indicates that: (1) potentiation of ATP-evoked contractions of the mouse vas deferens by α1-adrenoceptor activation were not fully blocked by the PKC inhibitor GF109203X and (2) that the stimulatory effect of PKC on ATP-induced contractions of the vas deferens is associated with enhanced P2X1R currents in vas deferens myocytes.

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.

Blockade of Kv7 channels reverses the inhibitory effects of EPAC activation on purinergic contractions of the murine detrusor

Purinergic contractions of the detrusor are reduced by cAMP, but the underlying mechanisms are unclear. We examined the effects of BK and Kv7 channel modulators on purinergic contractions of the detrusor and tested if the inhibitory effects of activators of the cAMP effectors, PKA and EPAC, were reduced by blockade of BK or Kv7 channels. Purinergic contractions of the murine detrusor were induced by electric field stimulation (EFS) or application of the P2X receptor agonist α,β-MeATP. EFS-responses were inhibited by the L-type Ca²⁺ channel blocker nifedipine, but not by the SERCA inhibitor CPA or the SOCE blocker GSK7975A. The Kv7 channel opener retigabine and BK channel activator compound X inhibited purinergic responses, while blockade of Kv7 or BK channels with XE991 or iberiotoxin, respectively, augmented these responses. Application of the EPAC activator 007-AM or PKA activator 6-MB-cAMP inhibited EFS responses. These effects were unaffected by iberiotoxin, however XE991 reduced the effects of 007-AM, but not 6-MB-cAMP. Kv7.5 was the only Kv7 transcript detected in isolated detrusor myocytes. These data suggest that purinergic contractions of the detrusor are regulated by BK and Kv7 channels and the latter may also play a role in EPAC-dependent inhibition of this activity.

Excitatory cholinergic responses in mouse primary bronchial smooth muscle require both Ca2+ entry via l-type Ca2+ channels and store operated Ca2+ entry via Orai channels

Malfunctions in airway smooth muscle Ca²⁺-signalling leads to airway hyperresponsiveness in asthma and chronic obstructive pulmonary disease. Ca²⁺-release from intracellular stores is important in mediating agonist-induced contractions, but the role of influx via l-type Ca²⁺ channels is controversial. We re-examined roles of the sarcoplasmic reticulum Ca²⁺ store, refilling of this store via store-operated Ca²⁺ entry (SOCE) and l-type Ca²⁺ channel pathways on carbachol (CCh, 0.1-10 µM)-induced contractions of mouse bronchial rings and intracellular Ca²⁺ signals of mouse bronchial myocytes. In tension experiments, the ryanodine receptor (RyR) blocker dantrolene (100 µM) reduced CCh-responses at all concentrations, with greater effects on sustained rather than initial components of contraction. 2-Aminoethoxydiphenyl borate (2-APB, 100 μM), in the presence of dantrolene, abolished CCh-responses, suggesting the sarcoplasmic reticulum Ca²⁺ store is essential for contraction. The SOCE blocker GSK-7975A (10 µM) reduced CCh-contractions, with greater effects at higher (e.g. 3 and 10 µM) CCh concentrations. Nifedipine (1 µM), abolished remaining contractions in GSK-7975A (10 µM). A similar pattern was observed on intracellular Ca²⁺-responses to 0.3 µM CCh, where GSK-7975A (10 µM) substantially reduced Ca²⁺ transients induced by CCh, and nifedipine (1 µM) abolished remaining responses. When nifedipine (1 µM) was applied alone it had less effect, reducing tension responses at all CCh concentrations by 25% - 50%, with greater effects at lower (e.g. 0.1 and 0.3 µM) CCh concentrations. When nifedipine (1 µM) was examined on the intracellular Ca²⁺-response to 0.3 µM CCh, it only modestly reduced Ca²⁺ signals, while GSK-7975A (10 µM) abolished remaining responses. In conclusion, Ca²⁺-influx from both SOCE and l-type Ca²⁺ channels contribute to excitatory cholinergic responses in mouse bronchi. The contribution of l-type Ca²⁺ channels was especially pronounced at lower doses of CCh, or when SOCE was blocked. This suggests l-type Ca²⁺ channels might be a potential target for bronchoconstriction under certain circumstances.

The TMEM16A blockers benzbromarone and MONNA cause intracellular Ca2+-release in mouse bronchial smooth muscle cells

We investigated effects of TMEM16A blockers benzbromarone, MONNA, CaCC_inhA01 and Ani9 on isometric contractions in mouse bronchial rings and on intracellular calcium in isolated bronchial myocytes. Separate concentrations of carbachol (0.1-10 μM) were applied for 10 min periods to bronchial rings, producing concentration-dependent contractions that were well maintained throughout each application period. Benzbromarone (1 μM) markedly reduced the contractions with a more pronounced effect on their sustained component (at 10 min) compared to their initial component (at 2 min). Iberiotoxin (0.3 μM) enhanced the contractions, but they were still blocked by benzbromarone. MONNA (3 μM) and CaCC_inhA01 (10 μM) had similar effects to benzbromarone, but were less potent. In contrast, Ani9 (10 μM) had no effect on carbachol-induced contractions. Confocal imaging revealed that benzbromarone (0.3 μM), MONNA (1 μM) and CaCC_inhA01 (10 μM) increased intracellular calcium in isolated myocytes loaded with Fluo-4AM. In contrast, Ani9 (10 μM) had no effect on intracellular calcium. Benzbromarone and MONNA also increased calcium in calcium-free extracellular solution, but failed to do so when intracellular stores were discharged with caffeine (10 mM). Caffeine was unable to cause further discharge of the store when applied in the presence of benzbromarone. Ryanodine (100 μM) blocked the ability of benzbromarone (0.3 μM) to increase calcium, while tetracaine (100 μM) reversibly reduced the rise in calcium induced by benzbromarone. We conclude that benzbromarone and MONNA caused intracellular calcium release, probably by opening ryanodine receptors. Their ability to block carbachol contractions was likely due to this off-target effect.

Oxidation modulates LINGO2-induced inactivation of large conductance, Ca2+-activated potassium channels

Ca²⁺ and voltage-activated K⁺ (BK) channels are ubiquitous ion channels that can be modulated by accessory proteins, including β, γ, and LINGO1 BK subunits. In this study, we utilized a combination of site-directed mutagenesis, patch clamp electrophysiology, and molecular modeling to investigate if the biophysical properties of BK currents were affected by coexpression of LINGO2 and to examine how they are regulated by oxidation. We demonstrate that LINGO2 is a regulator of BK channels, since its coexpression with BK channels yields rapid inactivating currents, the activation of which is shifted ∼-30 mV compared to that of BKα currents. Furthermore, we show the oxidation of BK:LINGO2 currents (by exposure to epifluorescence illumination or chloramine-T) abolished inactivation. The effect of illumination depended on the presence of GFP, suggesting that it released free radicals which oxidized cysteine or methionine residues. In addition, the oxidation effects were resistant to treatment with the cysteine-specific reducing agent DTT, suggesting that methionine rather than cysteine residues may be involved. Our data with synthetic LINGO2 tail peptides further demonstrate that the rate of inactivation was slowed when residues M603 or M605 were oxidized, and practically abolished when both were oxidized. Taken together, these data demonstrate that both methionine residues in the LINGO2 tail mediate the effect of oxidation on BK:LINGO2 channels. Our molecular modeling suggests that methionine oxidation reduces the lipophilicity of the tail, thus preventing it from occluding the pore of the BK channel.

Ca2+ -activated Cl- channels (TMEM16A) underlie spontaneous electrical activity in isolated mouse corpus cavernosum smooth muscle cells

Penile detumescence is maintained by tonic contraction of corpus cavernosum smooth muscle cells (CCSMC), but the underlying mechanisms have not been fully elucidated. The purpose of this study was to characterize the mechanisms underlying activation of TMEM16A Ca2+ -activated Cl- channels in freshly isolated murine CCSMC. Male C57BL/6 mice aged 10-18 weeks were euthanized via intraperitoneal injection of sodium pentobarbital (100 mg.kg-1 ). Whole-cell patch clamp, pharmacological, and immunocytochemical experiments were performed on isolated CCSM. Tension measurements were performed in whole tissue. TMEM16A expression in murine corpus cavernosum was confirmed using immunocytochemistry. Isolated CCSMC developed spontaneous transient inward currents (STICs) under voltage clamp and spontaneous transient depolarizations (STDs) in current clamp mode of the whole cell, perforated patch clamp technique. STICs reversed close to the predicted Cl- equilibrium potential and both STICs and STDs were blocked by the TMEM16A channel blockers, Ani9 and CaCC(inh)-A01. These events were also blocked by GSK7975A (ORAI inhibitor), cyclopiazonic acid (CPA, sarcoplasmic reticulum [SR] Ca2+- ATPase blocker), tetracaine (RyR blocker), and 2APB (IP3 R blocker), suggesting that they were dependent on Ca2+ release from intracellular Ca2+ stores. Nifedipine (L-type Ca2+ channel blocker) did not affect STICs, but reduced the duration of STDs. Phenylephrine induced transient depolarizations and transient inward currents which were blocked by Ani9. Similarly, phenylephrine induced phasic contractions of intact corpus cavernosum muscle strips and these events were also inhibited by Ani9. This study suggests that contraction of CCSM is regulated by activation of TMEM16A channels and therefore inhibition of these channels could lead to penile erection.

M2 Muscarinic Receptor-Dependent Contractions of Airway Smooth Muscle are Inhibited by Activation of β-Adrenoceptors

Beta-adrenoceptor (β-AR) agonists inhibit cholinergic contractions of airway smooth muscle (ASM), but the underlying mechanisms are unclear. ASM cells express M3 and M2 muscarinic receptors, but the bronchoconstrictor effects of acetylcholine are believed to result from activation of M3Rs, while the role of the M2Rs is confined to offsetting β-AR-dependent relaxations. However, a profound M2R-mediated hypersensitization of M3R-dependent contractions of ASM was recently reported, indicating an important role for M2Rs in cholinergic contractions of ASM. Here, we investigated if M2R-dependent contractions of murine bronchial rings were inhibited by activation of β-ARs. M2R-dependent contractions were apparent at low frequency (2Hz) electric field stimulation (EFS) and short (10s) stimulus  intervals. The β1-AR agonist, denopamine inhibited EFS-evoked contractions of ASM induced by reduction in stimulus interval from 100 to 10 s and was more effective at inhibiting contractions evoked by EFS at 2 than 20 Hz. Denopamine also abolished carbachol-evoked contractions that were resistant to the M3R antagonist 4-DAMP, similar to the effects of the M2R antagonists, methoctramine and AFDX-116. The inhibitory effects of denopamine on EFS-evoked contractions of ASM were smaller in preparations taken from M2R ^-/- mice, compared to wild-type (WT) controls. In contrast, inhibitory effects of the β3-AR agonist, BRL37344, on EFS-evoked contractions of detrusor strips taken from M2R ^-/- mice were greater than WT controls. These data suggest that M2R-dependent contractions of ASM were inhibited by activation of β1-ARs and that genetic ablation of M2Rs decreased the efficacy of β-AR agonists on cholinergic contractions.

Functional expression of NaV1.7 channels in freshly dispersed mouse bronchial smooth muscle cells

Isolated smooth muscle cells (SMC) from mouse bronchus were studied using the whole-cell patch clamp technique at ~21oC. Stepping from -100 mV to -20 mV evoked inward currents of mean amplitude -275 pA. These inactivated (tau=1.1 ms) and were abolished when external Na+ was substituted with N-Methyl-D-glucamine. In current-voltage protocols, current peaked at -10 mV and reversed between +20 and +30 mV. The V1/2s of activation and inactivation were -25 & -86 mV, respectively. The current was highly sensitive to tetrodotoxin (IC50=1.5 nM) and the NaV1.7 subtype selective blocker, PF-05089771 (IC50=8.6 nM), consistent with NaV1.7 as the underlying pore-forming a subunit. Two NaV1.7-selective antibodies caused membrane-delineated staining of isolated SMC, as did a non-selective pan-NaVantibody. RT-PCR, performed on groups of ~15 isolated SMC, revealed transcripts for NaV1.7 in 7/8 samples. Veratridine (30 mM), a non-selective NaV channel activator, reduced peak current evoked by depolarization but induced a sustained current of 40 pA. Both effects were reversed by tetrodotoxin (100 nM). In tension experiments veratridine (10 mM) induced contractions that were entirely blocked by atropine (1 mM). However, in the presence of atropine, veratridine was able to modulate the pattern of activity induced by a combination of U-46619 (a thromboxane A2 mimetic) & PGE2(prostaglandin E2), by eliminating bursts in favour of sustained phasic contractions. These effects were readily reversed to control-like activity by tetrodotoxin (100 nM). In conclusion, mouse bronchial SMC functionally express NaV1.7 channels that are capable of modulating contractile activity, at least under experimental conditions.

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.

Contribution of Postjunctional M2 Muscarinic Receptors to Cholinergic Nerve-Mediated Contractions of Murine Airway Smooth Muscle

Postjunctional M2Rs on airway smooth muscle (ASM) outnumber M3Rs by a ratio of 4:1 in most species, however, it is the M3Rs that are thought to mediate the bronchoconstrictor effects of acetylcholine. In this study, we describe a novel and profound M2R-mediated hypersensitization of M3R-dependent contractions of ASM at low stimulus frequencies.. Contractions induced by 2Hz EFS were augmented by > 2.5-fold when the stimulus interval was reduced from 100 to 10 s. This effect was reversed by the M2R antagonists, methoctramine, and AFDX116, and was absent in M2R null mice. The M3R antagonist 4-DAMP abolished the entire response in both WT and M2R KO mice. The M2R-mediated potentiation of EFS-induced contractions was not observed when the stimulus frequency was increased to 20 Hz. A subthreshold concentration of carbachol enhanced the amplitude of EFS-evoked contractions in WT, but not M2R null mice. These data highlight a significant M2R-mediated potentiation of M3R-dependent contractions of ASM at low frequency stimulation that could be relevant in diseases such as asthma and COPD.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

Perspectives on neuroinflammation contributing to chronic cough

Chronic cough can be a troublesome clinical problem. Current thinking is that increased activity and/or enhanced sensitivity of the peripheral and central neural pathways mediates chronic cough via processes similar to those associated with the development of chronic pain. While inflammation is widely thought to be involved in the development of chronic cough, the true mechanisms causing altered neural activity and sensitisation remain largely unknown. In this back-to-basics perspective article we explore evidence that inflammation in chronic cough may, at least in part, involve neuroinflammation orchestrated by glial cells of the nervous system. We summarise the extensive evidence for the role of both peripheral and central glial cells in chronic pain, and hypothesise that the commonalities between pain and cough pathogenesis and clinical presentation warrant investigations into the neuroinflammatory mechanisms that contribute to chronic cough. We open the debate that glial cells may represent an underappreciated therapeutic target for controlling troublesome cough in disease.

β3-Adrenoceptor agonists inhibit purinergic receptor-mediated contractions of the murine detrusor

β₃-Adrenoceptor (β₃-AR) agonists are used to treat overactive bladder syndrome; however, their mechanism of action has not been determined. The aims of this study were to compare the effects of β₃-AR agonists on cholinergic versus purinergic receptor-mediated contractions of the detrusor and to examine the mechanisms underlying inhibition of the purinergic responses by β₃-AR agonists. Isometric tension recordings were made from strips of murine detrusor and whole cell current recordings were made from freshly isolated detrusor myocytes using the patch-clamp technique. Transcriptional expression of exchange protein directly activated by cAMP (EPAC) subtypes in detrusor strips was assessed using RT-PCR and real-time quantitative PCR. The β₃-AR agonists BRL37344 and CL316243 (100 nM) inhibited cholinergic nerve-mediated contractions of the detrusor by 19 and 23%, respectively, but did not reduce contractions induced by the cholinergic agonist carbachol (300 nM). In contrast, BRL37344 and CL316243 inhibited purinergic nerve-mediated responses by 55 and 56%, respectively, and decreased the amplitude of contractions induced by the P2X receptor agonist α,β-methylene ATP by 40 and 45%, respectively. The adenylate cyclase activator forskolin inhibited purinergic responses, and these effects were mimicked by a combination of the PKA activator N⁶-monobutyryl-cAMP and the EPAC activator 8-pCPT-2'-O-methyl-cAMP-AM (007-AM). Application of ATP (1 μM) evoked reproducible P2X currents in isolated detrusor myocytes voltage-clamped at -60 mV. These responses were reduced in amplitude in the presence of BRL37344 and also by 007-AM. This study demonstrates that β₃-AR agonists reduce postjunctional purinergic responses in the detrusor via a pathway involving activation of the cAMP effector EPAC.

Ion Channels and Intracellular Calcium Signalling in Corpus Cavernosum

The corpus cavernosum smooth muscle is important for both erection of the penis and for maintaining penile flaccidity. Most of the time, the smooth muscle cells are in a contracted state, which limits filling of the corpus sinuses with blood. Occasionally, however, they relax in a co-ordinated manner, allowing filling to occur. This results in an erection. When contractions of the corpus cavernosum are measured, it can be deduced that the muscle cells work together in a syncytium, for not only do they spontaneously contract in a co-ordinated manner, but they also synchronously relax. It is challenging to understand how they achieve this.In this review we will attempt to explain the activity of the corpus cavernosum, firstly by summarising current knowledge regarding the role of ion channels and how they influence tone, and secondly by presenting data on the intracellular Ca²⁺ signals that interact with the ion channels. We propose that spontaneous Ca²⁺ waves act as a primary event, driving transient depolarisation by activating Ca²⁺-activated Cl^- channels. Depolarisation then facilitates Ca²⁺ influx via L-type voltage-dependent Ca²⁺ channels. We propose that the spontaneous Ca²⁺ oscillations depend on Ca²⁺ release from both ryanodine- and inositol trisphosphate (IP₃)-sensitive stores and that modulation by signalling molecules is achieved mainly by interactions with the IP₃-sensitive mechanism. This pacemaker mechanism is inhibited by nitric oxide (acting through cyclic GMP) and enhanced by noradrenaline. By understanding these mechanisms better, it might be possible to design new treatments for erectile dysfunction.

Inhibitory effects of openers of large-conductance Ca2+-activated K+ channels on agonist-induced phasic contractions in rabbit and mouse bronchial smooth muscle

Airway smooth muscle expresses abundant BK_Ca channels, but their role in regulating contractions remains controversial. This study examines the effects of two potent BK_Ca channel openers on agonist-induced phasic contractions in rabbit and mouse bronchi. First, we demonstrated the ability of 10 μM GoSlo-SR5-130 to activate BK_Ca channels in inside-out patches from rabbit bronchial myocytes, where it shifted the activation V_1/2 by -88 ± 11 mV (100 nM Ca²⁺, n = 7). In mouse airway smooth muscle cells, GoSlo-SR5-130 dose dependently shifted V_1/2 by 12-83 mV over a concentration range of 1-30 μM. Compound X, a racemic mixture of two enantiomers, reported to be potent BK_Ca channel openers, shifted V_1/2 by 20-79 mV over a concentration range of 0.3-3 μM. In rabbit bronchial rings, exposure to histamine (1 μM) induced phasic contractions after a delay of ~35 min. These were abolished by GoSlo-SR5-130 (30 μM). Nifedipine (100 nM) and CaCC_inhA01 (10 μM), a TMEM16A blocker, also abolished histamine-induced phasic contractions. In mouse bronchi, similar phasic contractions were evoked by exposure to U46619 (100 nM) and carbachol (100 nM). In each case, these were inhibited by concentrations of GoSlo-SR5-130 and compound X that shifted the activation V_1/2 of BK_Ca channels in the order of -80 mV. In conclusion, membrane potential-dependent regulation of L-type Ca²⁺ channels appears to be important for histamine-, U46619-, and carbachol-induced phasic contractions in airway smooth muscle. Contractions can be abolished by BK_Ca channel openers, suggesting that these channels are potential targets for treating some causes of airway obstruction.

Ca2+ signalling in mouse urethral smooth muscle in situ: role of Ca2+ stores and Ca2+ influx mechanisms

KEY POINTS

Contraction of urethral smooth muscle cells (USMCs) contributes to urinary continence. Ca2+ signalling in USMCs was investigated in intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs were spontaneously active in situ, firing intracellular Ca2+ waves that were asynchronous at different sites within cells and between adjacent cells. Spontaneous Ca2+ waves in USMCs were myogenic but enhanced by adrenergic or purinergic agonists and decreased by nitric oxide. Ca2+ waves arose from inositol trisphosphate type 1 receptors and ryanodine receptors, and Ca2+ influx by store-operated calcium entry was required to maintain Ca2+ release events. Ca2+ release and development of Ca2+ waves appear to be the primary source of Ca2+ for excitation-contraction coupling in the mouse urethra, and no evidence was found that voltage-dependent Ca2+ entry via L-type or T-type channels was required for responses to α adrenergic responses.

ABSTRACT

Urethral smooth muscle cells (USMCs) generate myogenic tone and contribute to urinary continence. Currently, little is known about Ca2+ signalling in USMCs in situ, and therefore little is known about the source(s) of Ca2+ required for excitation-contraction coupling. We characterized Ca2+ signalling in USMCs within intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs fired spontaneous intracellular Ca2+ waves that did not propagate cell-to-cell across muscle bundles. Ca2+ waves increased dramatically in response to the α1 adrenoceptor agonist phenylephrine (10 μm) and to ATP (10 μm). Ca2+ waves were inhibited by the nitric oxide donor DEA NONOate (10 μm). Ca2+ influx and release from sarcoplasmic reticulum stores contributed to Ca2+ waves, as Ca2+ free bathing solution and blocking the sarcoplasmic Ca2+ -ATPase abolished activity. Intracellular Ca2+ release involved cooperation between ryanadine receptors and inositol trisphosphate receptors, as tetracaine and ryanodine (100 μm) and xestospongin C (1 μm) reduced Ca2+ waves. Ca2+ waves were insensitive to L-type Ca2+ channel modulators nifedipine (1 μm), nicardipine (1 μm), isradipine (1 μm) and FPL 64176 (1 μm), and were unaffected by the T-type Ca2+ channel antagonists NNC-550396 (1 μm) and TTA-A2 (1 μm). Ca2+ waves were reduced by the store operated Ca2+ entry blocker SKF 96365 (10 μm) and by an Orai antagonist, GSK-7975A (1 μm). The latter also reduced urethral contractions induced by phenylephrine, suggesting that Orai can function effectively as a receptor-operated channel. In conclusion, Ca2+ waves in mouse USMCs are a source of Ca2+ for excitation-contraction coupling in urethral muscles.

β3-adrenoceptor agonists inhibit carbachol-evoked Ca2+ oscillations in murine detrusor myocytes

OBJECTIVE

To test if carbachol (CCh)-evoked Ca²⁺ oscillations in freshly isolated murine detrusor myocytes are affected by β3-adrenoceptor (β-AR) modulators.

MATERIALS AND METHODS

Isometric tension recordings were made from strips of murine detrusor, and intracellular Ca²⁺ measurements were made from isolated detrusor myocytes using confocal microscopy. Transcriptional expression of β-AR sub-types in detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time quantitative PCR (qPCR). Immunocytochemistry experiments, using a β3-AR selective antibody, were performed to confirm that β3-ARs were present on detrusor myocytes.

RESULTS

The RT-PCR and qPCR experiments showed that β1-, β2- and β3-AR were expressed in murine detrusor, but that β3-ARs were the most abundant sub-type. The selective β3-AR agonist BRL37344 reduced the amplitude of CCh-induced contractions of detrusor smooth muscle. These responses were unaffected by addition of the BK channel blocker iberiotoxin. BRL37344 also reduced the amplitude of CCh-induced Ca²⁺ oscillations in freshly isolated murine detrusor myocytes. This effect was mimicked by CL316,243, another β3-AR agonist, and inhibited by the β3-AR antagonist L748,337, but not by propranolol, an antagonist of β1- and β2-ARs. BRL37344 did not affect caffeine-evoked Ca²⁺ transients or L-type Ca²⁺ current in isolated detrusor myocytes.

CONCLUSION

Inhibition of cholinergic-mediated contractions of the detrusor by β3-AR agonists was associated with a reduction in Ca²⁺ oscillations in detrusor myocytes.

TRPA1 activation in a human sensory neuronal model: relevance to cough hypersensitivity?

The cough reflex becomes hyperresponsive in acute and chronic respiratory diseases, but understanding the underlying mechanism is hampered by difficulty accessing human tissue containing both nerve endings and neuronal cell bodies. We refined an adult stem cell sensory neuronal model to overcome the limited availability of human neurones and applied the model to study transient receptor potential ankyrin 1 (TRPA1) channel expression and activation.Human dental pulp stem cells (hDPSCs) were differentiated towards a neuronal phenotype, termed peripheral neuronal equivalents (PNEs). Using molecular and immunohistochemical techniques, together with Ca²⁺ microfluorimetry and whole cell patch clamping, we investigated roles for nerve growth factor (NGF) and the viral mimic poly I:C in TRPA1 activation.PNEs exhibited morphological, molecular and functional characteristics of sensory neurons and expressed functional TRPA1 channels. PNE treatment with NGF for 20 min generated significantly larger inward and outward currents compared to untreated PNEs in response to the TRPA1 agonist cinnamaldehyde (p<0.05). PNE treatment with poly I:C caused similar transient heightened responses to TRPA1 activation compared to untreated cells.Using the PNE neuronal model we observed both NGF and poly I:C mediated sensory neuronal hyperresponsiveness, representing potential neuro-inflammatory mechanisms associated with heightened nociceptive responses recognised in cough hypersensitivity syndrome.

The role of Ca2+-activated Cl- current in tone generation in the rabbit corpus cavernosum

Rabbit corpus cavernosum smooth muscle (RCCSM) cells express ion channels that produce Ca²⁺-activated Cl^- (I_ClCa) current, but low sensitivity to conventional antagonists has made its role in tone generation difficult to evaluate. We have reexamined this question using two new generation I_ClCa blockers, T16A_inh-A01 and CaCC_inh-A01. Isolated RCCSM cells were studied using the perforated patch method. Current-voltage protocols revealed that both L-type Ca²⁺ current and I_ClCa T16A_inh-A01 and CaCC_inh-A01 (10 μM) reduced I_ClCa by ~85%, while 30 μM abolished it. L-type Ca²⁺ current was unaffected by 10 μM CaCC_inh-A01 but was reduced by 50% at 30 μM CaCC_inh-A01, 46% at 10 μM T16A_inh-A01, and 78% at 30 μM T16A_inh-A01. Both drugs reduced spontaneous isometric tension in RCCSM strips, by 60-70% at 10 μM and >90% at 30 μM. Phenylephrine (PE)-enhanced tension was also reduced (ED₅₀ = 3 μM, CaCC_inh-A01; 14 μM, T16A_inh-A01). CaCC_inh-A01 at 10 μM had little effect on 60 mM KCl contractures, though they were reduced by 30 μM CaCC_inh-A01 and T16A_inh-A01 (10 μM and 30 μM) consistent with their effects on L-type Ca²⁺ current. Both drugs also reversed the stimulatory effect of PE on intracellular Ca²⁺ waves, studied with laser scanning confocal microscopy in isolated RCCSM cells. In conclusion, although both drugs were effective blockers of I_ClCa, the effect of T16A_inh-A01 on L-type Ca²⁺ current precludes its use for evaluating the role of I_ClCa in tone generation. However, 10 μM CaCC_inh-A01 selectively blocked I_ClCa versus L-type Ca²⁺ current and reduced spontaneous and PE-induced tone, suggesting that I_ClCa is important in maintaining penile detumescence.

Effects of new-generation TMEM16A inhibitors on calcium-activated chloride currents in rabbit urethral interstitial cells of Cajal

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit Ca²⁺-activated Cl^- currents (I _ClCa) that are important for the development of urethral tone. Here, we examined if TMEM16A (ANO1) contributed to this activity by examining the effect of "new-generation" TMEM16A inhibitors, CACC_inh-A01 and T16A_inh-A01, on I _ClCa recorded from freshly isolated rabbit urethral ICC (RUICC) and on contractions of intact strips of rabbit urethra smooth muscle. Real-time quantitative PCR experiments demonstrated that TMEM16A was highly expressed in rabbit urethra smooth muscle, in comparison to TMEM16B and TMEM16F. Single-cell RT-PCR experiments revealed that only TMEM16A was expressed in freshly isolated RUICC. Depolarization-evoked I _ClCa in isolated RUICC, recorded using voltage clamp, were inhibited by CACC_inh-A01 and T16A_inh-A01 with IC₅₀ values of 1.2 and 3.4 μM, respectively. Similarly, spontaneous transient inward currents (STICs) recorded from RUICC voltage clamped at -60 mV and spontaneous transient depolarizations (STDs), recorded in current clamp, were also inhibited by CACC_inh-A01 and T16A_inh-A01. In contrast, spontaneous Ca²⁺ waves in isolated RUICC were only partially reduced by CACC_inh-A01 and T16A_inh-A01. Finally, neurogenic contractions of strips of rabbit urethra smooth muscle (RUSM), evoked by electric field stimulation (EFS), were also significantly reduced by CACC_inh-A01 and T16A_inh-A01. These data are consistent with the idea that TMEM16A is involved with CACCs in RUICC and in contraction of rabbit urethral smooth muscle.

Differential efficacy of GoSlo-SR compounds on BKα and BKαγ1-4 channels

Large conductance, voltage and Ca²⁺ activated K⁺ channels (BK channels) are abundantly expressed throughout the body and are important regulators of smooth muscle tone and neuronal excitability. Their dysfunction is implicated in various diseases including overactive bladder, hypertension and erectile dysfunction. Therefore, BK channel openers bear significant therapeutic potential to treat the above diseases. GoSlo-SR compounds were designed to be potent and efficacious BK channel openers. Although their structural activity relationships, activation in both BKα and BKαβ channels and the hypothetical mode of action of these compounds has been studied in detail in recent years, their effectiveness to open the BKαγ channels still remains unexplored. In this study, we have examined the efficacy of 3 closely related GoSlo-SR openers, GoSlo-SR-5-6 (SR-5-6), GoSlo-SR-5-44 (SR-5-44) and GoSlo-SR-5-130 (SR-5-130) using inside out patches on BKα channels coexpressed with 4 different LRRC (γ_1-4) subunits in HEK293 cells. Our data suggests that the activation effects due to SR-5-6 were not significantly affected in the presence of γ_1-4 subunits. Interestingly, the effects of more efficacious BK channel opener SR-5-44 were altered by different γ subunits. In cells expressing BKα channels, the shift in V_1/2 (ΔV_1/2) induced by SR-5-44 (3 μM) was -76 ± 3 mV, whereas it was significantly reduced by ∼70 % in BKαγ₁ channels (ΔV_1/2= -23 ± 3, p < 0.001, ANOVA). In BKαγ₂ channels the ΔV_1/2 was -36 ± 1 mV, which was less than that observed in BKαγ₃ and BKαγ₄ channels where the ΔV_1/2 was -47 ± 5 mV, and -82 ± 5 mV, respectively. Additionally, the excitatory effects of a 'β specific' BK channel opener, SR-5-130 were only partially restored in the patches containing BKαγ_1-4 channels. Together this data highlights that subtle modifications in GoSlo-SR structures alter their effectiveness on BK channels with accessory γ subunits and this study might provide a scaffold for the development of more tissue specific BK channel openers.

Muscarinic Receptor Induced Contractions of the Detrusor are Mediated by Activation of TRPC4 Channels

PURPOSE

Muscarinic receptor mediated contractions of the detrusor rely on Ca²⁺ influx through voltage-gated Ca²⁺ channels but to our knowledge the mechanism linking stimulation of M3Rs to the activation of voltage dependent Ca²⁺ channels has not been established. TRPC4 channels are receptor operated cation channels that couple muscarinic receptor activation to depolarization of intestinal smooth muscle cells, voltage-activated Ca²⁺ influx and contraction. We investigated whether TRPC4 channels are involved in cholinergic mediated contractions of the detrusor.

MATERIALS AND METHODS

Isometric tension recordings were made on strips of murine detrusor and intracellular Ca²⁺ measurements were made on isolated detrusor myocytes using confocal microscopy. Transcriptional expression of TRPC and IP₃R subtypes in intact detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction.

RESULTS

Cholinergic stimulation of the detrusor induced by electrical field stimulation or exogenous application of carbachol or neostigmine evoked contractions consisting of a transient plus a tonic response, which was blocked by ML204, an inhibitor of TRPC4 channels. A phasic oscillatory component was blocked by the IP₃R inhibitor 2-APB. Carbachol evoked reproducible Ca²⁺ responses in isolated detrusor myocytes, consisting of an initial Ca²⁺ transient followed by Ca²⁺ oscillations. ML204 inhibited the initial Ca²⁺ transient whereas 2-APB inhibited the Ca²⁺ oscillations. Reverse transcriptase-polymerase chain reaction experiments showed that TRPC4β, TRPC6 and IP₃R1 were selectively expressed in isolated detrusor myocytes. Control experiments demonstrated that ML204 did not affect L-type Ca²⁺ or BK current amplitude, caffeine induced Ca²⁺ transients or KCl induced contractions of the detrusor.

CONCLUSIONS

Muscarinic receptor mediated contractions of the detrusor involve the activation of TRPC4β channels.

Regulation of nerve-evoked contractions of rabbit vas deferens by acetylcholine

Stimulation of intramural nerves in the vas deferens of many species yields a classical biphasic contraction comprised of an initial fast component, mediated by P2X receptors and a second slower component, mediated by α1-adrenoceptors. It is also recognized that sympathetic nerve-mediated contractions of the vas deferens can be modulated by acetylcholine (Ach), however there is considerable disagreement in the literature regarding the precise contribution of cholinergic nerves to contraction of the vas deferens. In this study we examined the effect of cholinergic modulators on electric field stimulation (EFS)-evoked contractions of rabbit vas deferens and on cytosolic Ca(2+) levels in isolated vas deferens smooth muscle cells (VDSMC). The sustained component of EFS-evoked contractions was inhibited by atropine and by the selective M3R antagonist, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP). EFS-evoked contractions were potentiated by Ach, carbachol (Cch), and neostigmine. The sustained phase of the EFS-evoked contraction was inhibited by prazosin, an α1-adrenoceptor antagonist and guanethidine, an inhibitor of noradrenaline release, even in the continued presence of Ach, Cch or neostigmine. The soluble guanylate cyclase (sGC) inhibitor, 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one enhanced the amplitude of EFS-evoked contractions and reduced the inhibitory effects of 4-DAMP. Isolated VDSMC displayed spontaneous Ca(2+) oscillations, but did not respond to Cch. However, the α1-adrenoceptor agonist, phenylephrine, evoked a Ca(2+) transient and contracted the cells. These data suggest that EFS-evoked contractions of the rabbit vas deferens are potentiated by activation of M3 receptors and reduced by activation of a sGC-dependent inhibitory pathway.

The role of Ca(2+) influx in spontaneous Ca(2+) wave propagation in interstitial cells of Cajal from the rabbit urethra

KEY POINTS

Tonic contractions of rabbit urethra are associated with spontaneous electrical slow waves that are thought to originate in pacemaker cells termed interstitial cells of Cajal (ICC). ICC pacemaker activity results from their ability to generate propagating Ca(2+) waves, although the exact mechanisms of propagation are not understood. In this study, we have identified spontaneous localised Ca(2+) events for the first time in urethral ICC; these were due to Ca(2+) release from the endoplasmic reticulum (ER) via ryanodine receptors (RyRs) and, while they often remained localised, they sometimes initiated propagating Ca(2+) waves. We show that propagation of Ca(2+) waves in urethral ICC is critically dependent upon Ca(2+) influx via reverse mode NCX. Our data provide a clearer understanding of the intracellular mechanisms involved in the generation of ICC pacemaker activity. Interstitial cells of Cajal (ICC) are putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous propagating Ca(2+) waves that are modulated by [Ca(2+)]o and whose propagation is inhibited by inositol tri-phosphate receptor (IP3 R) blockers. The purpose of this study was to further examine the role of Ca(2+) influx and Ca(2+) release in the propagation of Ca(2+) waves. Intracellular Ca(2+) was measured in Fluo-4-loaded ICC using a Nipkow spinning disc confocal microscope at fast acquisition rates (50 fps). We identified previously undetected localised Ca(2+) events originating from ryanodine receptors (RyRs). Inhibiting Ca(2+) influx by removing [Ca(2+)]o or blocking reverse mode sodium-calcium exchange (NCX) with KB-R 7943 or SEA-0400 abolished Ca(2+) waves, while localised Ca(2+) events persisted. Stimulating RyRs with 1 mm caffeine restored propagation. Propagation was also inhibited when Ca(2+) release sites were uncoupled by buffering intracellular Ca(2+) with EGTA-AM. This was reversed when Ca(2+) influx via NCX was increased by reducing [Na(+)]o to 13 mm. Low [Na(+)]o also increased the frequency of Ca(2+) waves and this effect was blocked by tetracaine and ryanodine but not 2-aminoethoxydiphenyl borate (2-APB). RT-PCR revealed that isolated ICC expressed both RyR2 and RyR3 subtypes. We conclude: (i) RyRs are required for the initiation of Ca(2+) waves, but wave propagation normally depends on activation of IP3 Rs; (ii) under resting conditions, propagation by IP3 Rs requires sensitisation by influx of Ca(2+) via reverse mode NCX; (iii) propagation can be maintained by RyRs if they have been sensitised to Ca(2+).

Pharmacological characterization of TMEM16A currents

Recent studies have shown that transmembrane protein 16 A (TMEM16A) is a subunit of calcium-activated chloride channels (CACCs). Pharmacological agents have been used to probe the functional role of CACCs, however their effect on TMEM16A currents has not been systematically investigated. In the present study, we characterized the voltage and concentration-dependent effects of 2 traditional CACC inhibitors (niflumic acid and anthracene-9-carboxcylic acid) and 2 novel CACC / TMEM16A inhibitors (CACC(inh)A01 and T16A(inh)A01) on TMEM16A currents. The whole cell patch clamp technique was used to record TMEM16A currents from HE K 293 cells that stably expressed human TMEM16A. Niflumic acid, A-9-C, CACC(inh)A01 and T16A(inh)A01 inhibited TMEM16A currents with IC50 values of 12, 58, 1.7 and 1.5 μM, respectively, however, A-9-C and niflumic acid were less efficacious at negative membrane potentials. A-9-C and niflumic acid reduced the rate of TMEM16A tail current deactivation at negative membrane potentials and A-9-C (1 mM) enhanced peak TMEM16A tail current amplitude. In contrast, the inhibitory effects of CACC(inh)A01 and T16A(inh)A01 were independent of voltage and they did not prolong the rate of TMEM16A tail current deactivation. The effects of niflumic acid and A-9-C on TMEM16A currents were similar to previous observations on CACCs in vascular smooth muscle, strengthening the hypothesis that they are encoded by TMEM16A. However, CACC(inh)A01 and T16A(inh)A01 were more potent inhibitors of TMEM16A channels and their effects were not diminished at negative membrane potentials making them attractive candidates to interrogate the functional role of TMEM16A channels in future studies.

Effects of the novel BK (KCa 1.1) channel opener GoSlo-SR-5-130 are dependent on the presence of BKβ subunits

BACKGROUND AND PURPOSE

GoSlo-SR compounds are efficacious BK (KCa 1.1) channel openers, but little is known about their mechanism of action or effect on bladder contractility. We examined the effects of two closely related compounds on BK currents and bladder contractions.

EXPERIMENTAL APPROACH

A combination of electrophysiology, molecular biology and synthetic chemistry was used to examine the effects of two novel channel agonists on BK channels from bladder smooth muscle cells and in HEK cells expressing BKα alone or in combination with either β1 or β4 subunits.

KEY RESULTS

GoSlo-SR-5-6 shifted the voltage required for half maximal activation (V1/2 ) of BK channels approximately -100 mV, irrespective of the presence of regulatory β subunits. The deaminated derivative, GoSlo-SR-5-130, also shifted the activation V1/2 in smooth muscle cells by approximately -100 mV; however, this was reduced by ∼80% in HEK cells expressing only BKα subunits. When β1 or β4 subunits were co-expressed with BKα, efficacy was restored. GoSlo-SR-5-130 caused a concentration-dependent reduction in spontaneous bladder contraction amplitude and this was abolished by iberiotoxin, consistent with an effect on BK channels.

CONCLUSIONS AND IMPLICATIONS

GoSlo-SR-5-130 required β1 or β4 subunits to mediate its full effects, whereas GoSlo-SR-5-6 worked equally well in the absence or presence of β subunits. GoSlo-SR-5-130 inhibited spontaneous bladder contractions by activating BK channels. The novel BK channel opener, GoSlo-SR-5-130, is approximately fivefold more efficacious on BK channels with regulatory β subunits and may be a useful scaffold in the development of drugs to treat diseases such as overactive bladder.

The role of cAMP dependent protein kinase in modulating spontaneous intracellular Ca²⁺ waves in interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) serve as electrical pacemakers in the rabbit urethra. Pacemaking activity in ICC results from spontaneous intracellular Ca(2+) waves that rely on Ca(2+) release from endoplasmic reticulum (ER) stores. The purpose of this study was to investigate if the action of protein kinase A (PKA) affected the generation of Ca(2+) waves in ICC. Intracellular [Ca(2+)] was measured in fluo-4 loaded ICC, freshly isolated from the rabbit urethra using a Nipkow spinning disc confocal microscope. Application of the PKA inhibitor H-89 (10 μM) significantly inhibited the generation of spontaneous Ca(2+) waves in ICC and this was associated with a significant decrease in the ER Ca(2+) load, measured with 10mM caffeine responses. Ca(2+) waves could be rescued in the presence of H-89 by stimulating ryanodine receptors (RyRs) with 1mM caffeine but not by activation of inositol 1,4,5 tri-phosphate receptors (IP3Rs) with 10 μM phenylephrine. Increasing intracellular PKA with the cAMP agonists forskolin and 8-bromo-cAMP failed to yield an increase in Ca(2+) wave activity. We conclude that PKA may be maximally active under basal conditions in ICC and that inhibition of PKA with H-89 leads to a decreased ER Ca(2+) load sufficient to inactivate IP3Rs but not RyRs.

Development of GoSlo-SR-5-69, a potent activator of large conductance Ca2+-activated K+ (BK) channels

We have designed, synthesised and characterised the effects of a number of novel anthraquinone derivatives and assessed their effects on large conductance, Ca(2+) activated K(+) (BK) channels recorded from rabbit bladder smooth muscle cells using the excised, inside/out configuration of the patch clamp technique. These compounds are members of the GoSlo-SR family of compounds, which potently open BK channels and shift the voltage required for half maximal activation (V1/2) negatively. The efficacy of the anilinoanthraquinone derivatives was enhanced when the size of ring D was increased, since the cyclopentane and cyclohexane derivatives shifted the V1/2, by -24 ± 6 mV and -54 ± 8 mV, respectively, whereas the cycloheptane and cyclooctane derivatives shifted the V1/2 by -61 ± 6 mV and -106 ± 6 mV. To examine if a combination of hydrophobicity and steric bulking of this region further enhanced their ability to open BK channels, we synthesised a number of naphthalene and tetrahydro-naphthalene derivatives. The tetrahydro-2-naphthalene derivative GoSlo-SR-5-69 was the most potent and efficacious of the series since it was able to shift the activation V1/2 by greater than -100 mV when applied at a concentration of 1 μM and had an EC50 of 251 nM, making it one of the most potent and efficacious BK channel openers synthesised to date.

The effect of high [K(+)]o on spontaneous Ca(2+) waves in freshly isolated interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The purpose of this study was to elucidate the mechanism of this response. Intracellular [Ca²⁺] was measured in fluo‐4‐loaded smooth muscle cells (SMCs) and ICC using a Nipkow spinning disk confocal microscope. Increasing [K⁺]_o to 60 mmol/L caused an increase in [Ca²⁺]_i accompanied by contraction in SMCs. Raising [K⁺]_o did not cause contraction in ICC, but the frequency of firing of spontaneous calcium waves increased. Reducing [Ca²⁺]_o to 0 mmol/L abolished the response in both cell types. Nifedipine of 1 μmol/L blocked the response of SMC to high [K⁺]_o, but did not affect the increase in firing in ICC. This latter effect was blocked by 30 μmol/L NiCl₂ but not by the T‐type Ca²⁺ channel blocker mibefradil (300 nmol/L). However, inhibition of Ca²⁺ influx via reverse‐mode sodium/calcium exchange (NCX) using either 1 μmol/L SEA0400 or 5 μmol/L KB‐R7943 did block the effect of high [K⁺]_o on ICC. These data suggest that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The experiments described support the hypothesis that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

ATP evokes inward currents in corpus cavernosum myocytes

INTRODUCTION

Although adenosine triphosphate (ATP) has often been reported to relax the corpus cavernosum, this may be mediated by indirect effects, such as release of nitric oxide from the endothelium. Recent data suggest that P2X(1) receptors may be up-regulated in diabetes, and these exert an anti-erectile effect by causing the corpus cavernosum smooth muscle cells (CCSMCs) to contract. However, to date, there is no functional evidence that ATP can directly stimulate CCSMC.

AIMS

This study aims to (i) to directly examine the effect of ATP on membrane currents in freshly isolated CCSMC, where influences of endothelium and other cells are absent; and (ii) to determine the receptor subtypes, ionic currents, and Ca(2+) signals stimulated by ATP.

METHODS

CCSMCs were enzymatically dispersed from male New Zealand White rabbits for patch clamp recording and measurement of intracellular Ca(2+) in fluo-4-loaded cells using spinning disk confocal microscopy.

MAIN OUTCOME MEASURES

Patch clamp recordings were made of ATP-evoked membrane currents and spontaneous membrane currents. Spinning disk confocal imaging of intracellular Ca(2+) was performed, and the response to ATP was recorded.

RESULTS

ATP evoked repeatable inward currents in CCSMC (1st application: -675 ± 101 pA; 2nd application: -694 ± 120 pA, N = 9, P = 0.77). ATP-induced currents were reduced by suramin from -380 ± 121 to -124 ± 37 pA (N = 8, P < 0.05), by α,β-methylene ATP from -755 ± 235 to 139 ± 49 pA (N = 5, P < 0.05), and by NF449 from -419 ± to -51 ± 13 pA (N = 6, P < 0.05). In contrast, MRS2500, a P2Y1(1,12,13) antagonist, had no effect on ATP responses (control: -838 ± 139 pA; in MRS2500: -822 ± 184 pA, N = 13, P = 0.84) but blocked inward currents evoked by 2-MeSATP, a P2Y1,12,13 agonist (control: -623 ± 166 pA; in MRS2500: -56 ± 25 pA, N = 6, P < 0.05). The ATP-evoked inward current was unaffected by changing the transmembrane Cl(-) gradient but reversed in direction when extracellular Na(+) was reduced, indicating that it was a cation current.

CONCLUSIONS

ATP directly stimulates CCSMC by evoking a P2X-mediated cation current.

Mechanisms underlying activation of transient BK current in rabbit urethral smooth muscle cells and its modulation by IP3-generating agonists

We used the perforated patch-clamp technique at 37°C to investigate the mechanisms underlying the activation of a transient large-conductance K(+) (tBK) current in rabbit urethral smooth muscle cells. The tBK current required an elevation of intracellular Ca(2+), resulting from ryanodine receptor (RyR) activation via Ca(2+)-induced Ca(2+) release, triggered by Ca(2+) influx through L-type Ca(2+) (CaV) channels. Carbachol inhibited tBK current by reducing Ca(2+) influx and Ca(2+) release and altered the shape of spike complexes recorded under current-clamp conditions. The tBK currents were blocked by iberiotoxin and penitrem A (300 and 100 nM, respectively) and were also inhibited when external Ca(2+) was removed or the CaV channel inhibitors nifedipine (10 μM) and Cd(2+) (100 μM) were applied. The tBK current was inhibited by caffeine (10 mM), ryanodine (30 μM), and tetracaine (100 μM), suggesting that RyR-mediated Ca(2+) release contributed to the activation of the tBK current. When IP3 receptors (IP3Rs) were blocked with 2-aminoethoxydiphenyl borate (2-APB, 100 μM), the amplitude of the tBK current was not reduced. However, when Ca(2+) release via IP3Rs was evoked with phenylephrine (1 μM) or carbachol (1 μM), the tBK current was inhibited. The effect of carbachol was abolished when IP3Rs were blocked with 2-APB or by inhibition of muscarinic receptors with the M3 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (1 μM). Under current-clamp conditions, bursts of action potentials could be evoked with depolarizing current injection. Carbachol reduced the number and amplitude of spikes in each burst, and these effects were reduced in the presence of 2-APB. In the presence of ryanodine, the number and amplitude of spikes were also reduced, and carbachol was without further effect. These data suggest that IP3-generating agonists can modulate the electrical activity of rabbit urethral smooth muscle cells and may contribute to the effects of neurotransmitters on urethral tone.

The cardiac sodium current Na(v)1.5 is functionally expressed in rabbit bronchial smooth muscle cells

A collagenase-proteinase mixture was used to isolate airway smooth muscle cells (ASMC) from rabbit bronchi, and membrane currents were recorded using the whole cell patch-clamp technique. Stepping from -100 mV to a test potential of -40 mV evoked a fast voltage-dependent Na(+) current, sometimes with an amplitude of several nanoamperes. The current disappeared within 15 min of exposure to papain + DTT (n = 6). Comparison of the current in ASMC with current mediated by NaV1.5 α-subunits expressed in human embryonic kidney cells revealed similar voltage dependences of activation (V1/2 = -42 mV for NaV1.5) and sensitivities to TTX (IC50 = 1.1 and 1.2 μM for ASMC and NaV1.5, respectively). The current in ASMC was also blocked by lidocaine (IC50 = 160 μM). Although veratridine, an agonist of voltage-gated Na(+) channels, reduced the peak current by 33%, it slowed inactivation, resulting in a fourfold increase in sustained current (measured at 25 ms after onset). In current-clamp mode, veratridine prolonged evoked action potentials from 37 ± 9 to 1,053 ± 410 ms (n = 8). Primers for NaV1.2-1.9 were used to amplify mRNA from groups of ∼20 isolated ASMC and from whole bronchial tissue by RT-PCR. Transcripts for NaV1.2, NaV1.3, and NaV1.5-1.9 were detected in whole tissue, but only NaV1.2 and NaV1.5 were detected in single cells. We conclude that freshly dispersed rabbit ASMC express a fast voltage-gated Na(+) current that is mediated mainly by the NaV1.5 subtype.

Investigation of L-type Ca(2+) current in the aganglionic bowel segment in Hirschsprung's disease

BACKGROUND

Studies on animal models of Hirschsprung's disease (HD) suggest that L-type Ca(2+) channels are down-regulated in the aganglionic bowel segment, however, this has yet to be confirmed in HD patients. The objective of this study was to test the hypothesis that L-type Ca(2+) current density is decreased in smooth muscle cells (SMC) obtained from the aganglionic bowel segment of patients with HD in comparison with those from the ganglionic segment.

METHODS

Smooth muscle cells were freshly isolated from colon samples obtained from HD patients undergoing pull-through surgery. L-type Ca(2+) currents were recorded using the perforated patch configuration of the whole cell voltage clamp technique and the expression levels of CACNA1C transcripts (which encode L-type Ca(2+) channels) in the ganglionic and aganglionic bowel segments were compared using real-time quantitative PCR.

KEY RESULTS

All SMC displayed robust currents that had activation/inactivation kinetics typical of L-type Ca(2+) current, were inhibited by nifedipine and enhanced by the L-type Ca(2+) channel agonists FPL 64176 and Bay K 8644. Moreover, FPL 64176 activated currents were also inhibited by nifedipine. However, there was no significant difference in L-type Ca(2+) current density, CACNA1C subunit expression or sensitivity to the pharmacological agents noted above, between SMC isolated from the ganglionic and aganglionic regions of the HD colon.

CONCLUSIONS & INFERENCES

In contrast to studies on genetic animal models of HD, L-type Ca(2+) currents are not down-regulated in the aganglionic bowel segment of HD patients and are therefore unlikely to account for the impaired colonic peristalsis observed in these patients.

Platelet-derived growth factor receptor-α cells in mouse urinary bladder: a new class of interstitial cells

Specific classes of interstitial cells exist in visceral organs and have been implicated in several physiological functions including pacemaking and mediators in neurotransmission. In the bladder, Kit(+) interstitial cells have been reported to exist and have been suggested to be neuromodulators. More recently a second interstitial cell, which is identified using antibodies against platelet-derived growth factor receptor-α (PDGFR-α) has been described in the gastrointestinal (GI) tract and has been implicated in enteric motor neurotransmission. In this study, we examined the distribution of PDGFR-α(+) cells in the murine urinary bladder and the relation that these cells may have with nerve fibres and smooth muscle cells. Platelet-derived growth factor receptor-α(+) cells had a spindle shape or stellate morphology and often possessed multiple processes that contacted one another forming a loose network. These cells were distributed throughout the bladder wall, being present in the lamina propria as well as throughout the muscularis of the detrusor. These cells surrounded and were located between smooth muscle bundles and often came into close morphological association with intramural nerve fibres. These data describe a new class of interstitial cells that express a specific receptor within the bladder wall and provide morphological evidence for a possible neuromodulatory role in bladder function.

Structure-activity relationships of a novel group of large-conductance Ca(2+)-activated K(+) (BK) channel modulators: the GoSlo-SR family

Opening up ion channels: We synthesised a series of anthraquinone analogues, called the GoSlo-SR family. Their effects on bladder smooth muscle BK channels were examined and, as shown, shifted voltage dependent activation >-100 mV (at 10 μM). They were more efficacious than NS11021 and could provide a new scaffold for the design of efficacious BK openers.

Cajal beyond the gut: interstitial cells in the urinary system--towards general regulatory mechanisms of smooth muscle contractility?

Interstitial cells of Cajal (ICC), similar to GI pacemakers have been identified throughout the urinary system. Although each part of the system serves a different function, ranging from peristalsis of the ureters, storage of urine by the bladder, and a sphincteric action by the urethra, they share a common mechanism in being able to generate phasic myogenic contractions. Even the urethra, often considered to be a 'tonic' smooth muscle, achieves an apparently sustained contraction by averaging numerous small asynchronous 'phasic' contractions. This activity can occur in the absence of any neural input, implying the presence of an intrinsic pacemaker. Intracellular microelectrode recordings from urethral muscle strips reveal electrical slow waves similar to those of the GI tract. To study this further, we isolated single cells from rabbit urethra and found not only smooth muscle cells (SMC), but a second cell type comprising -10% of the total. The latter cells were branched and non-contractile and closely resembled intestinal ICC. Electrophyiological studies revealed that, while the isolated smooth muscle cells were electrically quiescent, the 'ICC' fired electrical slow waves similar to those observed in the whole tissue. The basis of this difference was the presence of a large pacemaker current involving the activation of calcium-activated Cl channels by spontaneous intracellular Ca2+ waves. These, in turn, have been shown to be modulated by neurotransmitters such as nitric oxide, noradrenaline and ATP, thus providing a possible mechanism whereby neural regulation of the urethra, as well as spontaneous tone, may be mediated via ICC.

Contribution of Kv2.1 channels to the delayed rectifier current in freshly dispersed smooth muscle cells from rabbit urethra

We have characterized the native voltage-dependent K(+) (K(v)) current in rabbit urethral smooth muscle cells (RUSMC) and compared its pharmacological and biophysical properties with K(v)2.1 and K(v)2.2 channels cloned from the rabbit urethra and stably expressed in human embryonic kidney (HEK)-293 cells (HEK(Kv2.1) and HEK(Kv2.2)). RUSMC were perfused with Hanks' solution at 37°C and studied using the patch-clamp technique with K(+)-rich pipette solutions. Cells were bathed in 100 nM Penitrem A (Pen A) to block large-conductance Ca(2+)-activated K(+) (BK) currents and depolarized to +40 mV for 500 ms to evoke K(v) currents. These were unaffected by margatoxin, κ-dendrotoxin, or α-dendrotoxin (100 nM, n = 3-5) but were blocked by stromatoxin-1 (ScTx, IC(50) ∼130 nM), consistent with the idea that the currents were carried through K(v)2 channels. RNA was detected for K(v)2.1, K(v)2.2, and the silent subunit K(v)9.3 in urethral smooth muscle. Immunocytochemistry showed membrane staining for both K(v)2 subtypes and K(v)9.3 in isolated RUSMC. HEK(Kv2.1) and HEK(Kv2.2) currents were blocked in a concentration-dependent manner by ScTx, with estimated IC(50) values of ∼150 nM (K(v)2.1, n = 5) and 70 nM (K(v)2.2, n = 6). The mean half-maximal voltage (V(1/2)) of inactivation of the USMC K(v) current was -56 ± 3 mV (n = 9). This was similar to the HEK(Kv2.1) current (-55 ± 3 mV, n = 13) but significantly different from the HEK(Kv2.2) currents (-30 ± 3 mV, n = 11). Action potentials (AP) evoked from RUSMC studied under current-clamp mode were unaffected by ScTx. However, when ScTx was applied in the presence of Pen A, the AP duration was significantly prolonged. Similarly, ScTx increased the amplitude of spontaneous contractions threefold, but only after Pen A application. These data suggest that K(v)2.1 channels contribute significantly to the K(v) current in RUSMC.

The PI-PLC inhibitor U-73122 is a potent inhibitor of the SERCA pump in smooth muscle

In this issue MacMillan and McCarron in 2010 demonstrated that the phospholipase C (PLC) inhibitor U-73122 can potently inhibit Ca(2+) release from isolated smooth muscle cells independent of its effect on PLC. Their data suggest that the PLC inhibitor can block the sarcoplasmic/endoplasmic reticulum calcium ATPase pump in smooth muscle and cast doubt on the reliability of U-73122 as the main pharmacological tool to assess the role of the phosphotidyl inositol-PLC pathway in cellular signalling.

Ionic currents in intimal cultured synoviocytes from the rabbit

Hyaluronan, a joint lubricant and regulator of synovial fluid content, is secreted by fibroblast-like synoviocytes lining the joint cavity, and secretion is greatly stimulated by Ca²⁺-dependent protein kinase C. This study aimed to define synoviocyte membrane currents and channels that may influence synoviocyte Ca²⁺ dynamics. Resting membrane potential ranged from −30 mV to −66 mV (mean −45 ± 8.60 mV, n = 40). Input resistance ranged from 0.54 GΩ to 2.6 GΩ (mean 1.28 ± 0.57 GΩ; ν = 33). Cell capacitance averaged 97.97 ± 5.93 pF. Voltage clamp using C^s+ pipette solution yielded a transient inward current that disappeared in Ca²⁺-free solutions and was blocked by 1 μM nifedipine, indicating an L-type calcium current. The current was increased fourfold by the calcium channel activator FPL 64176 (300 nM). Using K⁺ pipette solution, depolarizing steps positive to −40 mV evoked an outward current that showed kinetics and voltage dependence of activation and inactivation typical of the delayed rectifier potassium current. This was blocked by the nonspecific delayed rectifier blocker 4-aminopyridine. The synoviocytes expressed mRNA for four Kv1 subtypes (Kv1.1, Kv1.4, Kv1.5, and Kv1.6). Correolide (1 μM), margatoxin (100 nM), and α-dendrotoxin block these Kv1 subtypes, and all of these drugs significantly reduced synoviocyte outward current. The current was blocked most effectively by 50 nM κ-dendrotoxin, which is specific for channels containing a Kv1.1 subunit, indicating that Kv1.1 is critical, either as a homomultimeric channel or as a component of a heteromultimeric Kv1 channel. When 50 nM κ-dendrotoxin was added to current-clamped synoviocytes, the cells depolarized by >20 mV and this was accompanied by an increase in intracellular calcium concentration. Similarly, depolarization of the cells with high external potassium solution caused an increase in intracellular calcium, and this effect was greatly reduced by 1 μM nifedipine. In conclusion, fibroblast-like synoviocytes cultured from the inner synovium of the rabbit exhibit voltage-dependent inward and outward currents, including Ca²⁺ currents. They thus express ion channels regulating membrane Ca²⁺ permeability and electrochemical gradient. Since Ca²⁺-dependent kinases are major regulators of synovial hyaluronan secretion, the synoviocyte ion channels are likely to be important in the regulation of hyaluronan secretion.