Ca2+ current and Ca(2+)-activated chloride current in isolated smooth muscle cells of the sheep urethra

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.

Lymphatic Collecting Vessel: New Perspectives on Mechanisms of Contractile Regulation and Potential Lymphatic Contractile Pathways to Target in Obesity and Metabolic Diseases

Obesity and metabolic syndrome pose a significant risk for developing cardiovascular disease and remain a critical healthcare challenge. Given the lymphatic system's role as a nexus for lipid absorption, immune cell trafficking, interstitial fluid and macromolecule homeostasis maintenance, the impact of obesity and metabolic disease on lymphatic function is a burgeoning field in lymphatic research. Work over the past decade has progressed from the association of an obese phenotype with Prox1 haploinsufficiency and the identification of obesity as a risk factor for lymphedema to consistent findings of lymphatic collecting vessel dysfunction across multiple metabolic disease models and organisms and characterization of obesity-induced lymphedema in the morbidly obese. Critically, recent findings have suggested that restoration of lymphatic function can also ameliorate obesity and insulin resistance, positing lymphatic targeted therapies as relevant pharmacological interventions. There remain, however, significant gaps in our understanding of lymphatic collecting vessel function, particularly the mechanisms that regulate the spontaneous contractile activity required for active lymph propulsion and lymph return in humans. In this article, we will review the current findings on lymphatic architecture and collecting vessel function, including recent advances in the ionic basis of lymphatic muscle contractile activity. We will then discuss lymphatic dysfunction observed with metabolic disruption and potential pathways to target with pharmacological approaches to improve lymphatic collecting vessel function.

Anoctamin 1 antagonism potentiates conventional tocolytic-mediated relaxation of pregnant human uterine smooth muscle

BACKGROUND

Currently available tocolytic agents are not effective treatment for preterm labor beyond 48 h. A major reason is the development of maternal side effects which preclude the maintenance of an effective steady-state drug concentration. One strategy that can mitigate these side effects is utilizing synergistic drug combinations to reduce the drug concentrations necessary to elicit a clinical effect. We have previously shown that three anoctamin 1 (ANO1) antagonists mediate potent relaxation of precontracted human uterine smooth muscle (USM). In this study, we aimed to determine whether a combination of sub-relaxatory doses of tocolytic drugs in current clinical use [the L-type voltage-gated calcium channel (VGCC) blocker, nifedipine (NIF); and the β2-adrenergic (β2AR) agonist, terbutaline (TRB)] will potentiate USM relaxation with two ANO1 antagonists [benzbromarone (BB) and MONNA (MN)].

OBJECTIVE

This study sought to examine the synergistic potency and mechanistic basis of two ANO1 antagonists with currently available tocolytic drugs. Functional endpoints assessed included relaxation of pre-contracting pregnant human USM tissue, inhibition of intracellular calcium release, and reduction of spontaneous transient inward current (STIC) recordings in human uterine smooth muscle cells.

METHODS

Human myometrial strips and primary human USM cells were used in organ bath and calcium flux experiments with different combinations of sub-threshold doses of ANO1 antagonists and terbutaline or nifedipine to determine if ANO1 antagonists potentiate tocolytic drugs.

RESULTS

The combination of sub-threshold doses of two ANO1 antagonists and current tocolytic drugs demonstrate a significant degree of synergy to relax human pregnant USM compared to the effects achieved when these drugs are administered individually.

CONCLUSION

A combination of sub-threshold doses of VGCC blocker and β2AR agonist with ANO1 antagonists potentiates relaxation of oxytocin-induced contractility and calcium flux in human USM ex vivo. Our findings may serve as a foundation for novel tocolytic drug combinations.

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.

ANO1 in urethral SMCs contributes to sex differences in urethral spontaneous tone

Stress urinary incontinence (SUI) is more common in women than in men, and sex differences in anatomic structure and physiology have been suggested as causes; however, the underlying cellular and molecular mechanisms remain unclear. The spontaneous tone (STT) of the urethra has been shown to have a fundamental effect on preventing the occurrence of SUI. Here, we investigated whether the urethral STT exhibited sex differences. First, we isolated urethral smooth muscle (USM) and detected STT in female mice and women. No STT was found in male mice or men. Furthermore, caffeine induced increased contractility and intracellular Ca²⁺ concentration in urethrae from female mice compared with male mice. EACT [an N-aroylaminothiazole, anoctamin-1 (ANO1) activator] elicited increased intracellular Ca²⁺ concentration and stronger currents in female mice than in male mice. Moreover, ANO1 expression in single USM cells from women and female mice was almost twofold higher than that found in cells from men and male mice. In summary, ANO1 in USM contributes to sex differences in urethral spontaneous tone. This finding may provide new guidance for the treatment of SUI in women and men.

Functional comparison of anoctamin 1 antagonists on human uterine smooth muscle contractility and excitability

Background: Pre-term birth is a major health care challenge throughout the world, and preterm labor represents a potentially reversible component of this problem. Current tocolytics do not improve preterm labor beyond 48 h. We have previously shown that anoctamin 1 (ANO1) channel blockade results in relaxation of pre-contracted human uterine smooth muscle (USM). Three drug classes with reported medicinal effects in humans also have members with ANO1 antagonism. In this study, we compared the ability of representatives from these 3 classes to reduce human USM contractility and excitability. Objective: This study sought to examine the comparative potency of 3 ANO1 antagonists on pregnant human USM relaxation, contraction frequency reduction, inhibition of intracellular calcium release and membrane hyperpolarization. Methods: Experiments were performed using: 1) Ex vivo organ bath (human pregnant tissue), 2) Oxytocin-induced calcium flux (in vitro human USM cells) and 3) Membrane potential assay (in vitro human USM cells). Results: Benzbromarone (BB) demonstrated the greatest potency among the compounds tested with respect to force, frequency inhibition, reducing calcium elevation and depolarizing membrane potential. Conclusion: While all 3 ANO1 antagonists attenuate pregnant human uterine tissue contractility and excitability, BB is the most potent tocolytic drug. Our findings may serve as a foundation for future structure-function analyses for novel tocolytic drug development.

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.

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.

Ion channels of the mammalian urethra

The mammalian urethra is a muscular tube responsible for ensuring that urine remains in the urinary bladder until urination. In order to prevent involuntary urine leakage, the urethral musculature must be capable of constricting the urethral lumen to an extent that exceeds bladder intravesicular pressure during the urine-filling phase. The main challenge in anti-incontinence treatments involves selectively-controlling the excitability of the smooth muscles in the lower urinary tract. Almost all strategies to battle urinary incontinence involve targeting the bladder and as a result, this tissue has been the focus for the majority of research and development efforts. There is now increasing recognition of the value of targeting the urethral musculature in the treatment and management of urinary incontinence. Newly-identified and characterized ion channels and pathways in the smooth muscle of the urethra provides a range of potential therapeutic targets for the treatment of urinary incontinence. This review provides a summary of the current state of knowledge of the ion channels discovered in urethral smooth muscle cells that regulate their excitability.

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.

Multiple effects of anthracene-9-carboxylic acid on the TMEM16B/anoctamin2 calcium-activated chloride channel

Ca(2+)-activated Cl(-) currents (CaCCs) play important roles in many physiological processes. Recent studies have shown that TMEM16A/anoctamin1 and TMEM16B/anoctamin2 constitute CaCCs in several cell types. Here we have investigated for the first time the extracellular effects of the Cl(-) channel blocker anthracene-9-carboxylic acid (A9C) and of its non-charged analogue anthracene-9-methanol (A9M) on TMEM16B expressed in HEK 293T cells, using the whole-cell patch-clamp technique. A9C caused a voltage-dependent block of outward currents and inhibited a larger fraction of the current as depolarization increased, whereas the non-charged A9M produced a small, not voltage dependent block of outward currents. A similar voltage-dependent block by A9C was measured both when TMEM16B was activated by 1.5 and 13μM Ca(2+). However, in the presence of 1.5μM Ca(2+) (but not in 13μM Ca(2+)), A9C also induced a strong potentiation of tail currents measured at -100mV after depolarizing voltages, as well as a prolongation of the deactivation kinetics. On the contrary, A9M did not produce potentiation of tail currents, showing that the negative charge is required for potentiation. Our results provide the first evidence that A9C has multiple effects on TMEM16B and that the negative charge of A9C is necessary both for voltage-dependent block and for potentiation. Future studies are required to identify the molecular mechanisms underlying these complex effects of A9C on TMEM16B. Understanding these mechanisms will contribute to the elucidation of the structure and functional properties of TMEM16B channels.

9-Anthracene carboxylic acid is more suitable than DIDS for characterization of calcium-activated chloride current during canine ventricular action potential

Understanding the role of ionic currents in shaping the cardiac action potential (AP) has great importance as channel malfunctions can lead to sudden cardiac death by inducing arrhythmias. Therefore, researchers frequently use inhibitors to selectively block a certain ion channel like 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and 9-anthracene carboxylic acid (9-AC) for calcium-activated chloride current (ICl(Ca)). This study aims to explore which blocker is preferable to study ICl(Ca). Whole-cell voltage-clamp technique was used to record ICa,L, IKs, IKr and IK1, while action potentials were measured using sharp microelectrodes. DIDS- (0.2 mM) and 9-AC-sensitive (0.5 mM) currents were identical in voltage-clamp conditions, regardless of intracellular Ca(2+) buffering. DIDS-sensitive current amplitude was larger with the increase of stimulation rate and correlated well with the rate-induced increase of calcium transients. Both drugs increased action potential duration (APD) to the same extent, but the elevation of the plateau potential was more pronounced with 9-AC at fast stimulation rates. On the contrary, 9-AC did not influence either the AP amplitude or the maximal rate of depolarization (V max), but DIDS caused marked reduction of V max. Both inhibitors reduced the magnitude of phase-1, but, at slow stimulation rates, this effect of DIDS was larger. All of these actions on APs were reversible upon washout of the drugs. Increasing concentrations of 9-AC between 0.1 and 0.5 mM in a cumulative manner gradually reduced phase-1 and increased APD. 9-AC at 1 mM had no additional actions upon perfusion after 0.5 mM. The half-effective concentration of 9-AC was approximately 160 μM with a Hill coefficient of 2. The amplitudes of ICa,L, IKs, IKr and IK1 were not changed by 0.5 mM 9-AC. These results suggest that DIDS is equally useful to study ICl(Ca) during voltage-clamp but 9-AC is superior in AP measurements for studying the physiological role of ICl(Ca) due to the lack of sodium channel inhibition. 9-AC has also no action on other ion currents (ICa,L, IKr, IKs, IK1); however, ICa,L tracings can be contaminated with ICl(Ca) when measured in voltage-clamp condition.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

Possibility of inhibition of calcium-activated chloride channel rescuing erectile failures in diabetes

Although calcium-activated chloride channel (CaCC) blockers, niflumic acid (NFA) and anthracene-9-carboxylic acid (A9C), have been shown as potential erectogenic agents in healthy corpus cavernosum (CC) tissues, the pharmacological characteristics of CaCC blockers in diabetic state are relatively unknown. This study compares the direct muscle relaxant property of NFA and A9C with their influence on contraction and nitrergic relaxation as elicited by electrical field stimulation in normal and 16-week-old diabetic rabbit CC (n=8). Mean blood glucose level in alloxan-treated rabbits was elevated threefold (21.9±0.5 mmol  l(-1) vs 7.1±0.2 mmol l(-1) in untreated rabbits; P<0.05). There was no significant alteration in the efficacies of NFA and A9C in eliciting a concentration-dependent relaxation of noradrenaline-induced cavernosum tone and in inhibiting neurogenic contraction of CC from diabetic rabbits. The capability of NFA (100 μM) and A9C (1 mM) in augmenting nitrergic transmission was also not adversely affected by diabetes. However, in CC from diabetic rabbits, A9C markedly increased nitrergic relaxation response to 1-10 Hz by 10.6-36.6% (vs -5.1-0.8% in nondiabetic control). CaCC sensitivity to A9C appears to be enhanced in diabetic CC tissue. Inhibiting the CaCC activity in diabetes-related ED may tip the balance between proerectile/relaxant and antierectile/contractile mechanisms in favor of cavernosum relaxation.

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.

Interstitial cells of Cajal in the urinary tract

The study of novel interstitial cells in the tissues of the urinary tract has defined advances in the field in the last decade. These intriguing cells belong to the same family as the better known interstitial cells of Cajal (ICC) of the gastrointestinal tract, and their discovery has been interpreted to suggest that pacemaker cells may be present in the urinary tract, driving the spontaneous or myogenic activity of the neighboring smooth muscle. This scenario may be true for the urethra where ICC have been described as "loose pacemakers" providing multiple, random inputs to modulate urethral smooth muscle activity. However, there is a paucity of direct evidence available to support this hypothesis in the bladder (where the smooth muscle cells are spontaneously active) or the renal pelvis (where atypical smooth muscle cells are the pacemakers), and it now seems more likely that urinary tract ICC act as modulators of smooth muscle activity.Interestingly, the literature suggests that the role of urinary tract ICC may be more apparent in pathophysiological conditions such as the overactive bladder. Several reports have indicated that the numbers of ICC present in overactive bladder tissues are greater than those from normal tissues; moreover, the contractility of tissues from overactive bladders in vitro appears to be more sensitive to the Kit antagonist, glivec, than those from normal bladder. Future research on urinary tract ICC in the short to medium term is likely to be dynamic and exciting and will lead to increasing our understanding of the roles of these cells in both normal and dysfunctional bladder.

Heterogeneous CPA sensitivity of spontaneous excitation in smooth muscle of the rabbit urethra

1. To investigate the role of intracellular Ca stores in generating spontaneous excitation of the urethra, the effects of cyclopiazonic acid (CPA) on spontaneous contractions, transient increases in intracellular calcium concentration ([Ca2+]i; Ca transients) and depolarizations were examined in smooth muscles of the rabbit urethra. 2. In about 90% of circular smooth muscle (CSM) preparations, CPA (10 microM) increased the amplitude of spontaneous contractions by about 180% and reduced their frequency to some 25% of control values (CPA-resistant), while it readily abolished the contractions in the remaining preparations. 3. In about 70% of CSM preparations, CPA prevented the generation of spontaneous depolarizations termed slow waves, but increased their amplitude and duration in the remainder. CPA also prevented the generation of spontaneous Ca transients in about 40% of CSM preparations, while increasing their amplitude and duration in the remaining preparations. In CPA-resistant preparations that had been exposed to nicardipine (1 microM), subsequent CPA invariably abolished residual spontaneous depolarizations or Ca transients. CPA abolished caffeine-induced Ca transients in Ca-free solutions, suggesting that it effectively depleted intracellular Ca stores. 4. Longitudinal smooth muscles generated spontaneous action potentials, which had a shape distinct from that of slow waves in CSM. Spontaneous action potentials were abolished by nicardipine but not CPA. 5. Transmural nerve stimulation increased the frequency of Ca transients to give a sustained rise in [Ca2+]i, but inhibited their generation after blocking alpha-adrenoceptors with phentolamine (1 microM). These nerve-evoked responses were preserved in preparations that had been exposed to CPA. Similarly, both in control and CPA-treated CSM preparations, spontaneous Ca transients were accelerated by noradrenaline (NAd, 1 microM) and were suppressed by 3-morpholino-sydnonimine (SIN-1, 10 microM), a nitric oxide (NO) donor. 6. In conclusion, CSM of the urethra generates spontaneous activity, which depends on Ca release from intracellular Ca stores. However, after blocking this primary pacemaking mechanism, L-type Ca channel-dependent action potentials may drive CSM. Irrespective of the origin of pacemaking, neurally-released NAd and NO are capable of modulating spontaneous excitation.

Calcium currents in interstitial cells from the guinea-pig bladder

OBJECTIVE

To determine whether there are inward currents in interstitial cells (IC) isolated from the guinea-pig detrusor and if so, to characterise them using the patch-clamp technique and pharmacological agents.

MATERIALS AND METHODS

Using the whole-cell patch-clamp technique, inward currents were studied in IC enzymatically isolated from the detrusor of the guinea-pig bladder. Currents were evoked by stepping positively from a holding potential of - 80 mV.

RESULTS

Outward K+ currents were blocked by Cs+ internal solution to reveal inward currents, which activated at voltages more positive than - 50 mV, peaked at 0 mV, reversed near + 50 mV and were half-maximally activated at - 27 mV. The inward currents showed voltage-dependent inactivation and were half-maximally inactivated at - 36 mV. Fitting the activation and inactivation data with a Boltzmann function revealed a window current between - 40 mV and + 20 mV. The decay of the current evoked at 0 mV could be fitted with a single exponential with a mean time-constant of 88 ms. Replacing external Ca2+ with Ba2+ significantly increased this to 344 ms. The current amplitude was augmented by Ba2+, and by Bay K 8644. Inward currents were significantly reduced by 1 microm nifedipine, across the voltage range, but the blockade was more effective on the current evoked at 0 mV than that evoked by a step to - 20 mV, perhaps indicating voltage-dependence of the action of nifedipine or another component of inward current. Increasing the concentration of the drug to 10 microm caused no further significant reduction either at 0 mV or at -20 mV. However, in the presence of 1 microm nifedipine the latter current was significantly reduced by 100 microm Ni2+. Both currents were significantly reduced in Ca2+-free solution.

CONCLUSIONS

IC from the guinea-pig detrusor possess inward currents with typical characteristics of L-type Ca2+ current. They also have a component of inward Ca2+ current, which was resistant to nifedipine, but sensitive to Ni2+. Further work is needed to characterise the latter conductance.

Mechanisms of Disease: specialized interstitial cells of the urinary tract--an assessment of current knowledge

Scientists interested in the smooth muscles of the urinary tract, and their control, have recently been studying cells in the interstitium of tissues that express the c-kit antigen (Kit(+) cells). These cells have morphologic features that are reminiscent of the well-described pacemaker cells in the gut, the interstitial cells of Cajal (ICC). The spontaneous contractile behavior of muscles in the urinary tract varies widely, and it is clear that urinary tract Kit(+) interstitial cells cannot be playing an identical role to that played by the ICC in the gut. Nevertheless, there is increasing evidence that they do play a role in modulating the contractile behavior of adjacent smooth muscle, and might also be involved in mediating neural control. This review outlines the properties of ICC in the gut, and gives an account of the discovery of cells in the interstitium of the main components of the urinary tract. The physiologic properties of such cells and the functional implications of their presence are discussed, with particular reference to the bladder. In this organ, Kit(+) cells are found under the lamina propria, where they might interact with the urothelium and with sensory nerves, and also between and within the smooth-muscle bundles. Confocal microscopy and calcium imaging are being used to assess the physiology of ICC and their interactions with smooth muscles. Differences in the numbers of ICC are seen in smooth muscle specimens obtained from patients with various pathologies; in particular, bladder overactivity is associated with increased numbers of these cells.

Mefenamic acid as a novel activator of L-type voltage-dependent Ca2+ channels in smooth muscle cells from pig proximal urethra

The effects of mefenamic acid and Bay K 8644 on voltage-dependent nifedipine-sensitive inward Ba2+ currents in pig urethral myocytes were investigated by use of conventional whole-cell configuration patch clamp. Mefenamic acid increased the peak amplitude of voltage-dependent nifedipine-sensitive inward Ba2+ current without shifting the position of the current-voltage relationship. Mefenamic acid (300 microM) caused little shift in the activation curve although the voltage dependence of the steady-state inactivation was shifted to more positive potentials by 11 mV in the presence of mefenamic acid. Bay K 8644 (> or = 100 nM) enhanced voltage-dependent nifedipine-sensitive inward Ba2+ currents in a concentration- and voltage-dependent manner, shifting the maximum of the current-voltage relationship by 10 mV in the hyperpolarizing direction. Bay K 8644 (1 microM) significantly shifted the voltage dependence of the activation curve to more negative potentials by approximately 9 mV although Bay K 8644 caused little shift in the steady-state inactivation curve. These results indicate that mefenamic acid increased voltage-dependent nifedipine-sensitive inward Ba2+ currents through the activation of L-type Ca2+ channels with different kinetics from those of Bay K 8644 in pig urethral myocytes.

Activation of chloride currents in murine portal vein smooth muscle cells by membrane depolarization involves intracellular calcium release

The present study describes the first characterization of Ca2+-activated Cl− currents ( IClCa) in single smooth muscle cells from a murine vascular preparation (portal veins). IClCa was recorded using the perforated patch version of the whole cell voltage-clamp technique and was evoked using membrane depolarization. Generation of IClCa relied on Ca2+ entry through dihydropyridine-sensitive Ca2+ channels because IClCa was abolished by 1 μM nicardipine and enhanced by raising external Ca2+ concentration or by application of BAY K 8644. IClCa was characterized by the sensitivity to Cl− channel blockers and the effect of altering the external anion on reversal potential. Activation of IClCa after membrane depolarization was dependent on Ca2+ release from intracellular stores. Thus the amplitude of IClCa was diminished by the SR-ATPase inhibitor cyclopiazonic acid, the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate (2-APB), and the ryanodine receptor blocker tetracaine. The degree of inhibition produced by the application of 2-APB and tetracaine together was significantly greater than the effect of each agent applied alone. In current-clamp mode, injection of depolarizing current elicited a biphasic action potential, with the later depolarization being sensitive to niflumic acid (NFA; 10 μM). In isometric tension recordings, NFA inhibited spontaneous contractions. These data support a role for this conductance in portal vein excitability.

Female pig urethral tone is dependent on Rho guanosine triphosphatases and Rho-associated kinase

PURPOSE

Circular smooth muscle of the urethra generates spontaneous myogenic tone of relevance for the maintenance of continence. We tested if Rho guanosine triphosphatases (GTPases) and Rho-associated kinase (ROK) are involved in the generation of urethral tone.

MATERIALS AND METHODS

Small circular strips of female pig urethra were dissected out and mounted for recording isometric force. The effect of pharmacological agents known to modulate the activity of Rho GTPases or ROK was examined. The intracellular calcium concentration was measured using fura-2.

RESULTS

Urethral tone was abolished by removing extracellular calcium or by adding the calcium antagonist felodipine. The decrease in force was closely related to a decrease in intracellular calcium concentration, indicating that tone depends on membrane associated mechanisms. Toxin B, which inactivates Rho GTPases, and Y 27632, which inhibits ROK, completely abolished tone in the female pig urethra. The latter effect occurred without any change in the intracellular calcium concentration.

CONCLUSIONS

The results suggests that urethral tone depends on activity in G-protein coupled pathways and inhibition of this activity is sufficient for urethral tone relaxation. Thus, to our knowledge a new pathway in the generation of urethral tone, which might be acted on by autonomic nerves during micturition, has been identified.

Modulation of spontaneous Ca2+-activated Cl- currents in the rabbit corpus cavernosum by the nitric oxide-cGMP pathway

The whole-cell perforated patch clamp technique was used to study membrane currents in isolated rabbit corpus cavernosum smooth muscle cells. Depolarization from -80 mV to the range -40 to -10 mV evoked a nifedipine-sensitive Ca(2+) current that was followed by a slower inward current that activated over several hundred milliseconds. The slow current reversed near the Cl(-) equilibrium potential (E(Cl)) and was reduced by anthracene-9-carboxylic acid (A9C; 1 mm) and niflumic acid (100 microm), suggesting that it was a Ca(2+)-activated Cl(-) current. When held constantly at -60 mV, over 70% of cells fired spontaneous transient inward currents (STICs), the amplitudes of which were reduced by A9C and niflumic acid. STICs reversed near E(Cl) in a symmetrical Cl(-) gradient and when [Cl(-)](o) was substituted with glutamate or I(-), the reversal potential shifted to more positive or more negative values, respectively, confirming that STICs were mediated by Cl(-) channels. STICS were also blocked by cyclopiazonic acid, 2-aminoethoxydiphenyl borate (2-APB) and 2-nitro-4-carboxyl-N,N-diphenylcarbamate (NCDC), suggesting that they depended on IP(3)-mediated Ca(2+)-release from the sarcoplasmic reticulum. Modulation by the NO-cGMP pathway was investigated by applying nitrosocysteine, 3-(5-hydroxymethyl-2-furyl)-1-benzyl indazole (YC-1), and 8-bromo cGMP, all three of which abolished STIC activity. YC-1 also reduced noradrenaline-evoked inward currents, but had no effect on similar currents evoked by caffeine, suggesting that cGMP selectively inhibited IP(3)-mediated Ca(2+) release. We propose that Ca(2+)-activated Cl(-) currents underlie detumescent tone in the corpus cavernosum, and that modulation of this mechanism by the NO-cGMP pathway is important during penile erection.

Electrophysiological characterization and functional importance of calcium-activated chloride channel in rat uterine myocytes

In order to better understand the mechanisms underlying excitation of the uterus, we have elucidated the characteristics and functional importance of Ca(2+)-activated Cl(-) currents ( I(Cl-Ca)) in pregnant rat myometrium. In 101/320 freshly isolated myocytes, there was a slowly inactivating tail current (162+/-48 pA) upon repolarization following depolarising steps. This current has a reversal potential close to that for chloride, and was shifted when [Cl(-)] was altered. It was activated by Ca(2+) (but not Ba(2+)) entry through L-type Ca(2+) channels, enhanced by the Ca(2+) channel agonist Bay K8644 (2 microM), and inhibited by the Cl(-) channel blockers, niflumic acid (10 microM) and anthracene-9-carboxylic acid (9-AC, 100 microM). We therefore conclude that the pregnant rat myometrium contains Ca(2+)-activated Cl(-) channels producing inward current in ~30% of its cells. When these channels were inhibited by niflumic acid or 9-AC in intact tissues, the frequency of spontaneous contractions, was significantly reduced. Niflumic acid was also shown to inhibit oxytocin-induced contractions and Ca(2+) transients. Neither 9-AC nor niflumic acid had any effect on high-K-invoked contractions. Taken together these data suggest that Ca(2+)-activated Cl(-) channels are activated by Ca(2+) entry and play a functionally important role in myometrium, probably by contributing to membrane potential and firing frequency (pacemakers) in these cells.

The Cl(-) channel blocker niflumic acid releases Ca(2+) from an intracellular store in rat pulmonary artery smooth muscle cells

The effect of the Cl- channel blockers niflumic acid (NFA), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and anthracene-9-carboxylic acid (A-9-C), on Ca2+ signalling in rat pulmonary artery smooth muscle cells was examined. Intracellular Ca2+ concentration ([Ca2+]i) was monitored with either fura-2 or fluo-4, and caffeine was used to activate the ryanodine receptor, thereby releasing Ca2+ from the sarcoplasmic reticulum (SR). NFA and NPPB significantly increased basal [Ca2+]i and attenuated the caffeine-induced increase in [Ca2+]i. These Cl- channel blockers also increased the half-time (t1/2) to peak for the caffeine-induced [Ca2+]i transient, and slowed the removal of Ca2+ from the cytosol following application of caffeine. Since DIDS and A-9-C were found to adversely affect fura-2 fluorescence, fluo-4 was used to monitor intracellular Ca2+ in studies involving these Cl- channel blockers. Both DIDS and A-9-C increased basal fluo-4 fluorescence, indicating an increase in intracellular Ca2+, and while DIDS had no significant effect on the t1/2 to peak for the caffeine-induced Ca2+ transient, it was significantly increased by A-9-C. In the absence of extracellular Ca2+, NFA significantly increased basal [Ca2+]i, suggesting that the release of Ca2+ from an intracellular store was responsible for the observed effect. Depleting the SR with the combination of caffeine and cyclopiazonic acid prevented the increase in basal [Ca2+]i induced by NFA. Additionally, incubating the cells with ryanodine also prevented the increase in basal [Ca2+]i induced by NFA. These data show that Cl- channel blockers have marked effects on Ca2+ signalling in pulmonary artery smooth muscle cells. Furthermore, examination of the NFA-induced increase in [Ca2+]i indicates that it is likely due to Ca2+ release from an intracellular store, most probably the SR.

Role of sarcoplasmic reticulum in control of membrane potential and nitrergic response in opossum lower esophageal sphincter

1. We previously demonstrated that a balance of Ca2+-activated Cl- current (ICl(Ca)) and K+ current activity sets the resting membrane potential of opossum lower esophageal sphincter (LES) circular smooth muscle at approximately -41 mV, which leads to continuous spike-like action potentials and the generation of basal tone. Ionic mechanisms underlying this basal ICl(Ca) activity and its nitrergic regulation remain unclear. Recent studies suggest that spontaneous Ca2+ release from sarcoplasmic reticulum (SR) and myosin light chain kinase (MLCK) play important roles. The current study investigated this possibility. Conventional intracellular recordings were performed on circular smooth muscle of opossum LES. Nerve responses were evoked by electrical square wave pulses of 0.5 ms duration at 20 Hz. 2. In the presence of nifedipine (1 microm), substance P (1 microm), atropine (3 microm) and guanethidine (3 microm), intracellular recordings demonstrated a resting membrane potential (MP) of -38.1+/-0.7 mV (n=25) with spontaneous membrane potential fluctuations (MPfs) of 1-3 mV. Four pulses of nerve stimulation induced slow inhibitory junction potentials (sIJPs) with an amplitude of 6.1+/-0.3 mV and a half-amplitude duration of 1926+/-147 ms (n=25). 3. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific guanylyl cyclase inhibitor, abolished sIJPs, but had no effects on MPfs. Caffeine, a ryanodine receptor agonist, hyperpolarized MP and abolished sIJPs and MPfs. Ryanodine (20 microm) inhibited the sIJP and induced biphasic effects on MP, an initial small hyperpolarization followed by a large depolarization. sIJPs and MPfs were also inhibited by cyclopiazonic acid, an SR Ca2+ ATPase inhibitor. Specific ICl(Ca) and MLCK inhibitors hyperpolarized the MP and inhibited MPfs and sIJPs. 4. These data suggest that (1). spontaneous release of Ca2+ from the SR activates ICl(Ca), which in turn contributes to resting membrane potential; (2). MLCK is involved in activation of ICl(Ca); (3). inhibition of ICl(Ca) is likely to underlie sIJPs induced by nitrergic innervation.

Activation of Ca2+-activated Cl- current by depolarizing steps in rabbit urethral interstitial cells

Interstitial cells were isolated from strips of rabbit urethra for study using the amphotericin B perforated-patch technique. Depolarizing steps to -30 mV or greater activated a Ca2+ current (ICa), followed by a Ca2+-activated Cl- current, and, on stepping back to -80 mV, large Cl- tail currents were observed. Both currents were abolished when the cells were superfused with Ca2+-free bath solution, suggesting that Ca2+ influx was necessary for activation of the Cl- current. The Cl- current was also abolished when Ba2+ was substituted for Ca2+ in the bath or the cell was dialyzed with EGTA (2 mM). The Cl- current was also reduced by cyclopiazonic acid, ryanodine, 2-aminoethoxydiphenyl borate (2-APB), and xestospongin C, suggesting that Ca2+-induced Ca2+ release (CICR) involving both ryanodine and inositol 1,4,5-trisphosphate receptors contributes to its activation.

T- and L-type Ca2+ currents in freshly dispersed smooth muscle cells from the human proximal urethra

The purpose of the present study was to characterise Ca2+ currents in smooth muscle cells isolated from biopsy samples taken from the proximal urethra of patients undergoing surgery for bladder or prostate cancer. Cells were studied at 37 degrees C using the amphotericin B perforated-patch configuration of the patch-clamp technique. Currents were recorded using Cs+-rich pipette solutions to block K+ currents. Two components of current, with electrophysiological and pharmacological properties typical of T- and L-type Ca2+ currents, were present in these cells. When steady-state inactivation curves for the L current were fitted with a Boltzmann equation, this yielded a V1/2 of -45+/-5 mV. In contrast, the T current inactivated with a V1/2 of -80+/-3 mV. The L currents were reduced in a concentration-dependent manner by nifedipine (ED50=159+/-54 nM) and Ni2+ (ED50=65+/-16 microM) but were enhanced when external Ca2+ was substituted with Ba2+. The T current was little affected by TTX, reduction in external Na+, application of nifedipine at concentrations below 300 nM or substitution of external Ca2+ with Ba2+, but was reduced by Ni2+ with an ED50 of 6+/-1 microM. When cells were stepped from -100 to -30 mV in Ca2+-free conditions, small inward currents could be detected. These were enhanced 40-fold in divalent-cation-free solution and blocked in a concentration-dependent manner by Mg2+ with an ED50 of 32+/-16 microM. These data support the idea that human urethral myocytes possess currents with electrophysiological and pharmacological properties typical of T- and L-type Ca2+ currents.

Anomalous effect of anthracene-9-carboxylic acid on calcium-activated chloride currents in rabbit pulmonary artery smooth muscle cells

1 Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) evoked by K(+)-free pipette solutions containing 500 nM Ca(2+) were recorded in rabbit pulmonary artery smooth muscle cells. A voltage step protocol in which the cells were stepped to +70 mV and then to -80 mV produced outward and inward Cl(-) currents respectively that exhibited distinctive voltage- and time-dependent kinetics that remained consistent for the recording period. 2 Application of the Cl(-) channel inhibitor anthracene-9-carboxylic acid (A-9-C, 500 micro M), produced a small inhibition of the maximum outward Cl(-) current at +70 mV (21+/-10%) but augmented the amplitude of the instantaneous inward relaxation at -80 mV by 321+/-34% (n=12). 3 The current recorded in the absence and presence of A-9-C reversed at the theoretical Cl(-) equilibrium potential and the reversal potential was shifted by about -40 mV upon replacement of external chloride ion by the more permeant anion thiocyanate. Currents in the absence and presence of A-9-C were similarly affected by 100 micro M niflumic acid. 4 Augmentation of the inward current at -80 mV by A-9-C required prior depolarization, i.e. A-9-C did not simply activate a Cl(-) current at negative membrane potentials. Moreover the degree of augmentation was independent of the internal Ca(2+) for concentrations between 100 nM and 1 micro M Ca(2+). 5 The data from the present study confirm previous observations that the inhibitory effect of Cl(-) channel blockers is modified when [Ca(2+)](i) is maintained at higher than normal resting concentrations.

Characterization of norepinephrine-evoked inward currents in interstitial cells isolated from the rabbit urethra

Freshly dispersed interstitial cells from the rabbit urethra were studied by using the perforated-patch technique. When cells were voltage clamped at -60 mV and exposed to 10 microM norepinephrine (NE) at 80-s intervals, either large single inward currents or a series of oscillatory inward currents of diminishing amplitude were evoked. These currents were blocked by either phentolamine (1 microM) or prazosin (1 microM), suggesting that the effects of NE were mediated via alpha(1)-adrenoceptors. NE-evoked currents were depressed by the blockers of Ca(2+)-activated Cl(-) currents, niflumic acid (10 microM), and 9-anthracenecarboxylic acid (9-AC, 1 mM). The reversal potential of the above currents changed in a predictable manner when the Cl(-) equilibrium potential was altered, again suggesting that they were due to activation of a Cl(-) conductance. NE-evoked currents were decreased by 10 microM cyclopiazonic acid, suggesting that they were dependent on store-released Ca(2+). Inhibition of NE-evoked currents by the phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (100 microM) suggested that NE releases Ca(2+) via an inositol 1,4,5-trisphosphate (IP(3))-dependent mechanism. These results support the idea that stimulation of alpha(1)-adrenoceptors releases Ca(2+) from an IP(3)-sensitive store, which in turn activates Ca(2+)-activated Cl(-) current in freshly dispersed interstitial cells of the rabbit urethra. This elevates slow wave frequency in these cells and may underlie the mechanism responsible for increased urethral tone during nerve stimulation.

The involvement of L-type Ca(2+) channels in the relaxant effects of the ATP-sensitive K(+) channel opener ZD6169 on pig urethral smooth muscle

1. The effects of ZD6169, a novel ATP-sensitive K(+) channel (K(ATP) channel) opener, were investigated on membrane currents in isolated myocytes using patch-clamp techniques. Tension measurement was also performed to study the effects of ZD6169 on the resting tone of pig urethral smooth muscle. 2. Levcromakalim was more potent than ZD6169 in lowering the resting urethral tone. Relaxation induced by low concentrations of ZD6169 (< or =3 microM) was completely suppressed by additional application of glibenclamide (1 microM). In contrast, glibenclamide (1-10 microM) only partially inhibited the relaxation induced by higher concentrations of ZD6169 (> or = microM). 3. Bay K8644 (1 microM) reduced the maximum relaxation produced by ZD6169 (> or =10 microM). 4. In whole-cell configuration, ZD6169 suppressed the peak amplitude of voltage-dependent Ba(2+) currents in a concentration- and voltage-dependent manner, and at 100 microM, shifted the steady-state inactivation curve of the voltage-dependent Ba(2+) currents to the left at a holding potential of -90 mV. 5. In cell-attached configuration, open probability of unitary voltage-dependent Ba(2+) channels (27 pS, 90 mM Ba(2+)) was inhibited by 100 microM ZD6169 and by 10 microM nifedipine. 6. Reverse transcriptase-polymerase chain reaction (RT - PCR) analysis revealed the presence of the transcript of the alpha(1C) subunit of L-type Ca(2+) channels in pig urethra. 7. These results demonstrate that ZD6169 causes urethral relaxation through two distinct mechanisms, activation of K(ATP) channels at lower concentrations and inhibition of voltage-dependent Ca(2+) channels at higher concentrations (about 10 microM).

Chloride channel antagonists perturb growth and morphology of Neurospora crassa

The chloride channel antagonists anthracene-9-carboxylic acid, ethacrynic acid and niflumic acid were found to be fungistatic and morphogenic when tested against the ascomycete Neurospora crassa. Potency increased with decreasing pH, suggesting that the protonated forms of the compounds were active. Niflumic acid produced the most pronounced growth aberrations which may reflect an ability to acidify the cytoplasm and block the plasma membrane anion channel of N. crassa.

Ca(2+)-activated Cl(-) current in sheep lymphatic smooth muscle

Freshly dispersed sheep mesenteric lymphatic smooth muscle cells were studied at 37 degrees C using the perforated patch-clamp technique with Cs(+)- and K(+)-filled pipettes. Depolarizing steps evoked currents that consisted of L-type Ca(2+) [I(Ca(L))] current and a slowly developing current. The slow current reversed at 1 +/- 1.5 mV with symmetrical Cl(-) concentrations compared with 23.2 +/- 1.2 mV (n = 5) and -34.3 +/- 3.5 mV (n = 4) when external Cl(-) was substituted with either glutamate (86 mM) or I(-) (125 mM). Nifedipine (1 microM) blocked and BAY K 8644 enhanced I(Ca(L)), the slow-developing sustained current, and the tail current. The Cl(-) channel blocker anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing inward and tail currents. Application of caffeine (10 mM) to voltage-clamped cells evoked currents that reversed close to the Cl(-) equilibrium potential and were sensitive to 9-AC. Small spontaneous transient depolarizations and larger action potentials were observed in current clamp, and these were blocked by 9-AC. Evoked action potentials were triphasic and had a prominent plateau phase that was selectively blocked by 9-AC. Similarly, fluid output was reduced by 9-AC in doubly cannulated segments of spontaneously pumping sheep lymphatics, suggesting that the Ca(2+)-activated Cl(-) current plays an important role in the electrical activity underlying spontaneous activity in this tissue.

The effects of caffeine on ATP-sensitive K(+) channels in smooth muscle cells from pig urethra

The effects of caffeine on both levcromakalim-induced macroscopic and unitary currents in pig proximal urethra were investigated by the use of patch-clamp techniques (conventional whole-cell configuration and cell-attached configuration). The effects of caffeine were also examined on currents in inside-out patches of COS7 cells expressing carboxy terminus truncated inwardly rectifying K(+) channel (Kir6.2) subunits (i.e. Kir6.2DeltaC36) which form ATP-sensitive K(+) channels (K(ATP) channels). In conventional whole-cell configuration, the levcromakalim (100 microM)-induced inward current (symmetrical 140 mM K(+) conditions) was inhibited by caffeine (> or =1 mM) at a holding potential of -50 mV. In contrast, ryanodine (10 microM) caused no significant inhibitory effect on the gradual decay of the levcromakalim-induced current at -50 mV. The amplitude of the 30 microM levcromakalim-induced current was enhanced by 3-isobutyl-1-methylxanthine (IBMX, 100 microM). In cell-attached configuration, the levcromakalim-induced K(+) channel openings were inhibited by subsequent application of 10 mM caffeine, decreasing the channel open probability at -50 mV. Reverse transcriptase-polymerase chain reaction (RT - PCR) analysis revealed the presence of Kir6.2 transcript in pig urethra. Caffeine (> or =3 mM) inhibited the channel activity of Kir6.2DeltaC36 expressed in COS7 cells (3 mM caffeine, 65+/-6%, n=4; 10 mM caffeine, 29+/-2%, n=4). These results suggest that caffeine can inhibit the activity of K(ATP) channels through a direct blocking effect on the pore-forming Kir subunit.

Characterization of outward K(+) currents in isolated smooth muscle cells from sheep urethra

The perforated-patch technique was used to measure membrane currents in smooth muscle cells from sheep urethra. Depolarizing pulses evoked large transient outward currents and several components of sustained current. The transient current and a component of sustained current were blocked by iberiotoxin, penitrem A, and nifedipine but were unaffected by apamin or 4-aminopyridine, suggesting that they were mediated by large-conductance Ca(2+)-activated K(+) (BK) channels. When the BK current was blocked by exposure to penitrem A (100 nM) and Ca(2+)-free bath solution, there remained a voltage-sensitive K(+) current that was moderately sensitive to blockade with tetraethylammonium (TEA; half-maximal effective dose = 3.0 +/- 0.8 mM) but not 4-aminopyridine. Penitrem A (100 nM) increased the spike amplitude and plateau potential in slow waves evoked in single cells, whereas addition of TEA (10 mM) further increased the plateau potential and duration. In conclusion, both Ca(2+)-activated and voltage-dependent K(+) currents were found in urethral myocytes. Both of these currents are capable of contributing to the slow wave in these cells, suggesting that they are likely to influence urethral tone under certain conditions.

Specialised pacemaking cells in the rabbit urethra

1. Collagenase dispersal of strips of rabbit urethra yielded, in addition to normal spindle-shaped smooth muscle cells, a small proportion of branched cells which resembled the interstitial cells of Cajal dispersed from canine colon. These were clearly distinguishable from smooth muscle in their appearance under the phase-contrast microscope, their immunohistochemistry and their ultrastructure. They had abundant vimentin filaments but no myosin, a discontinuous basal lamina, sparse rough endoplasmic reticulum, many mitochondria and a well-developed smooth endoplasmic reticulum. 2. Interstitial cells were non-contractile but exhibited regular spontaneous depolarisations in current clamp. These could be increased in frequency by noradrenaline and blocked by perfusion with calcium-free solution. In voltage clamp they showed abundant calcium-activated chloride current and spontaneous transient inward currents which could be blocked by chloride channel blockers. 3. The majority of smooth muscle cells were vigorously contractile when stimulated but did not show spontaneous electrical activity in current clamp. In voltage clamp, smooth muscle cells showed very little calcium-activated chloride current. 4. We conclude that there are specialised pacemaking cells in the rabbit urethra that may be responsible for initiating the slow waves recorded from smooth muscle cells in the intact syncitium.

Cellular mechanisms of myogenic activity in gastric smooth muscle

In many regions of the intestine, a thin layer of interstitial cells of Cajal (ICC) lie in the myenteric region, between the circular and longitudinal muscle layers. ICC are connected by gap junctions to surrounding ICC and also with circular and longitudinal smooth muscle cells, forming a large electrical syncytium. Damage of the ICC causes a disorder in the patterns of rhythmic activity. Isolated ICC produce a rhythmic oscillation of the membrane potential. All these observations have led to the suggestion that ICC may be the pacemaker cell responsible for intestinal activity. Gastric smooth muscles generate slow oscillatory membrane potential changes (slow waves) and spike potentials. The activity is considered to be linked to the metabolism in the cell. Three types of cells located in the gastric wall (circular and longitudinal smooth muscle cells and ICC) produce synchronized electrical responses with different shapes. The electrical responses appear to originate in ICC and then spread to the smooth muscle layers, indicating that ICC may also be the pacemaker cells responsible for gastric activity. However, isolated circular smooth muscle tissues spontaneously generate regenerative potentials, suggesting that there are at least two sites for the initiation of spontaneous activity in the stomach. Regenerative potentials persist in the presence of Ca-antagonists and are inhibited by agents which disrupt intracellular Ca(2+) homeostasis. Depolarization of the membrane elicits regenerative potentials after a long delay and the potentials have long refractory periods. This suggests that an unidentified 2nd messenger may be formed during the delay between membrane depolarization and the initiation of a regenerative potential. In gastric muscles of mutant mice which do not express inositol trisphosphate (InsP(3)) receptors, spike potentials but not slow waves are generated, suggesting the possible involvement of InsP(3) in the initiation of spontaneous activity.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Anion channel blockers differentially affect T-type Ca(2+) currents of mouse spermatogenic cells, alpha1E currents expressed in Xenopus oocytes and the sperm acrosome reaction

The direct electrophysiological characterization of sperm Ca(2+) channels has been precluded by their small size and flat shape. An alternative to study these channels is to use spermatogenic cells, the progenitors of sperm, which are larger and easier to patch-clamp. In mouse and rat, the only voltage-dependent Ca(2+) currents displayed by these cells are of the T type. Because compounds that block these currents inhibit the zona pellucida-induced Ca(2+) uptake and the sperm acrosome reaction (AR) at similar concentrations, it is likely that they are fundamental for this process. Recent single channel recordings in mouse sperm demonstrated the presence of a Cl(-) channel. This channel and the zona pellucida (ZP)-induced AR were inhibited by niflumic acid (NA), an anion channel blocker [Espinosa et al. (1998): FEBS Lett 426:47-51]. Because NA and other anion channel blockers modulate cationic channels as well, it became important to determine whether they affect the T-type Ca(2+) currents of spermatogenic cells. These currents were blocked in a voltage-dependent manner by NA, 1, 9-dideoxyforskolin (DDF), and 5-nitro-2-(3-phenylpropylamine)benzoic acid (NPPB). The IC(50) values at -20 mV were 43 microM for NA, 28 microM for DDF, and 15 microM for NPPB. Moreover, DDF partially inhibited the ZP-induced AR (40% at 1 microM) and NPPB displayed an IC(50) value of 6 microM for this reaction. These results suggest that NA and DDF do not inhibit the ZP-induced AR by blocking T-type Ca(2+) currents, while NPPB may do so. Interestingly 200 microM NA was basically unable to inhibit alpha1E Ca(2+) channels expressed in Xenopus oocytes, questioning that this alpha subunit codes for the T-type Ca(2+) channels present in spermatogenic cells. Evidence for the presence of alpha1C, alpha1G, and alpha1H in mouse pachytene spematocytes and in round and condensing spermatids is presented.

Tonic and phasic activity in smooth muscle

We have studied the electrical and mechanical behaviour of two very different smooth muscle preparations, mesenteric lymphatic ducts and proximal urethra. These tissues generate different patterns of spontaneous contraction adapted to fulfil their contrasting functions. While lymphatics undergo regular phasic contractions and relaxations, suited to their role in propelling lymph, the urethra remains in a state of contracture to maintain urinary continence. The challenge is to understand how both of these achieve their respective roles. Interestingly, electrical activity of lymphatics resembles that in the heart in having a one to one relationship between the action potential and phasic contraction. Patch clamp studies have shown that lymphatic cells express 3 ionic currents that are not present in urethral cells, but are shared with cardiac muscle. These are, i) fast Na+ current, ii) T-type Ca2+ current and iii) a hyperpolarization-activated cation current, Ir. The fast Na+ current is ideally suited to the propagation of the action potential over large distances, as required by a vessel capable of generating a rapid well co-ordinated contraction along its length. The T-current and Ir, on the other hand, appear to be involved in electrical pacemaking as they are in the heart. The urethra does not usually undergo regular phasic contractions and it lacks these currents. Instead, urethral tone may depend on an interaction between L-type Ca2+ current and a large Ca(2+)-activated Cl- current. Activation of Cl- channels (perhaps by spontaneous release of Ca2+ from intracellular stores) would depolarize the membrane potential to within the 'window current' range for L-type Ca2+ channels and result in Ca2+ influx and contraction. This process may be maintained for a time by positive feedback whereby the influx of Ca2+ continues to activate the Cl- channels.

Spontaneous and neurally activated depolarizations in smooth muscle cells of the guinea-pig urethra

1. Membrane potential recordings were made from longitudinal smooth muscle cells of the guinea-pig urethra using conventional microelectrode techniques. 2. Smooth muscle cells of the urethra developed spontaneous transient depolarizations (STDs) and slow waves. Single unit STDs had amplitudes of approximately 5 mV and slow waves seemed to occur as amplitude multiples of single unit STDs. 3. STDs and slow waves were abolished by niflumic acid or low chloride solution and also by cyclopiazonic acid (CPA), BAPTA or high concentrations of caffeine. Lower concentrations of caffeine abolished slow waves but not STDs. Nifedipine inhibited slow waves but not STDs. 4. When stochastic properties of STDs were examined, it was found that the intervals between occurrences were not well modelled by Poisson statistics, instead the STDs appeared to be clustered. 5. Transmural stimulation evoked excitatory junctional potentials (EJPs) and triggered slow waves which were abolished by either alpha,beta-methylene-ATP or tetrodotoxin. Evoked slow waves were also abolished by caffeine, co-application of caffeine and ryanodine or by CPA which left EJPs unaffected. 6. In conclusion, smooth muscle cells of urethra exhibit STDs which are clustered rather than random events, and are the result of spontaneous Ca2+ release from intracellular stores and subsequent activation of Ca2+-activated chloride channels. STDs sum to activate L-type Ca2+ channels which contribute to the sustained phase of slow waves. Stimulation of purinoceptors by neurally released ATP initiates EJPs and also causes the release of Ca2+ from intracellular stores to evoke slow waves.