Ca-dependent K channels in smooth muscle cells permeabilized by beta-escin recorded using the cell-attached patch-clamp technique

Uncovering the arrhythmogenic potential of TRPM4 activation in atrial-derived HL-1 cells using novel recording and numerical approaches

Aims

Transient receptor potential cation channel subfamily melastatin member 4 (TRPM4), a Ca2+-activated nonselective cation channel abundantly expressed in the heart, has been implicated in conduction block and other arrhythmic propensities associated with cardiac remodelling and injury. The present study aimed to quantitatively evaluate the arrhythmogenic potential of TRPM4.

Methods and results

Patch clamp and biochemical analyses were performed using expression system and an immortalized atrial cardiomyocyte cell line (HL-1), and numerical model simulation was employed. After rapid desensitization, robust reactivation of TRPM4 channels required high micromolar concentrations of Ca2+. However, upon evaluation with a newly devised, ionomycin-permeabilized cell-attached (Iono-C/A) recording technique, submicromolar concentrations of Ca2+ (apparent Kd = ∼500 nM) were enough to activate this channel. Similar submicromolar Ca2+ dependency was also observed with sharp electrode whole-cell recording and in experiments coexpressing TRPM4 and L-type voltage-dependent Ca2+ channels. Numerical simulations using a number of action potential (AP) models (HL-1, Nygren, Luo-Rudy) incorporating the Ca2+- and voltage-dependent gating parameters of TRPM4, as assessed by Iono-C/A recording, indicated that a few-fold increase in TRPM4 activity is sufficient to delay late AP repolarization and further increases (≥ six-fold) evoke early afterdepolarization. These model predictions are consistent with electrophysiological data from angiotensin II-treated HL-1 cells in which TRPM4 expression and activity were enhanced.

Conclusions

These results collectively indicate that the TRPM4 channel is activated by a physiological range of Ca2+ concentrations and its excessive activity can cause arrhythmic changes. Moreover, these results demonstrate potential utility of the first AP models incorporating TRPM4 gating for in silico assessment of arrhythmogenicity in remodelling cardiac tissue.

Beta-escin inhibits colonic aberrant crypt foci formation in rats and regulates the cell cycle growth by inducing p21(waf1/cip1) in colon cancer cells

Extracts of Aesculus hippocastanum (horse chestnut) seed have been used in the treatment of chronic venous insufficiency, edema, and hemorrhoids. Most of the beneficial effects of horse chestnut are attributed to its principal component beta-escin or aescin. Recent studies suggest that beta-escin may possess anti-inflammatory, anti-hyaluronidase, and anti-histamine properties. We have evaluated the chemopreventive efficacy of dietary beta-escin on azoxymethane-induced colonic aberrant crypt foci (ACF). In addition, we analyzed the cell growth inhibitory effects and the induction of apoptosis in HT-29 human colon cancer cell line. To evaluate the inhibitory properties of beta-escin on colonic ACF, 7-week-old male F344 rats were fed experimental diets containing 0%, 0.025%, or 0.05% beta-escin. After 1 week, the rats received s.c. injections of azoxymethane (15 mg/kg body weight, once weekly for 2 weeks) or an equal volume of normal saline (vehicle). Rats were continued on respective experimental diets and sacrificed 8 weeks after the azoxymethane treatment. Colons were evaluated histopathologically for ACF. Administration of dietary 0.025% and 0.05% beta-escin significantly suppressed total colonic ACF formation up to approximately 40% (P < 0.001) and approximately 50% (P < 0.0001), respectively, when compared with control diet group. Importantly, rats fed beta-escin showed dose-dependent inhibition (approximately 49% to 65%, P < 0.0001) of foci containing four or more aberrant crypts. To understand the growth inhibitory effects, HT-29 human colon carcinoma cell lines were treated with various concentrations of beta-escin and analyzed by flow cytometry for apoptosis and cell cycle progression. Beta-escin treatment in HT-29 cells induced growth arrest at the G1-S phase, which was associated with the induction of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1), and this correlated with reduced phosphorylation of retinoblastoma protein. Results also indicate that beta-escin inhibited growth of colon cancer cells with either wild-type or mutant p53. This novel feature of beta-escin, a triterpene saponin, may be a useful candidate agent for colon cancer chemoprevention and treatment.

Local Ca(2+) transients and distribution of BK channels and ryanodine receptors in smooth muscle cells of guinea-pig vas deferens and urinary bladder

1. The relationship between Ca(2+) sparks spontaneously occurring at rest and local Ca(2+) transients elicited by depolarization was analysed using two-dimensional confocal Ca(2+) images of single smooth muscle cells isolated from guinea-pig vas deferens and urinary bladder. The current activation by these Ca(2+) events was also recorded simultaneously under whole-cell voltage clamp. 2. Spontaneous transient outward currents (STOCs) and Ca(2+) sparks were simultaneously detected at -40 mV in approximately 50 % of myocytes of either type. Ca(2+) sparks and corresponding STOCs occurred repetitively in several discrete sites in the subplasmalemmal area. Large conductance Ca(2+)-dependent K(+) (BK) channel density in the plasmalemma near the Ca(2+) spark sites generating STOCs was calculated to be 21 channels microm(-2). 3. When myocytes were depolarized from -60 to 0 mV, several local Ca(2+) transients were elicited within 20 ms in exactly the same peripheral sites where sparks occurred at rest. The local Ca(2+) transients often lasted over 300 ms and spread into other areas. The appearance of local Ca(2+) transients occurred synchronously with the activation of Ca(2+)-dependent K(+) current (I(K,Ca)). 4. Immunofluorescence staining of the BK channel alpha-subunit (BKalpha) revealed a spot-like pattern on the plasmalemma, in contrast to the uniform staining of voltage-dependent Ca(2+) channel alpha1C subunits along the plasmalemma. Ryanodine receptor (RyR) immunostaining also suggested punctate localization predominantly in the periphery. Double staining of BKalpha and RyRs revealed spot-like co-localization on/beneath the plasmalemma. 5. Using pipettes of relatively low resistance, inside-out patches that included both clustered BK channels at a density of over 20 channels microm(-2) and functional Ca(2+) storage sites were obtained at a low probability of approximately 5%. The averaged BK channel density was 3-4 channels microm(-2) in both types of myocyte. 6. These results support the idea that a limited number of discrete sarcoplasmic reticulum (SR) fragments in the subplasmalemmal area play key roles in the control of BK channel activity in two ways: (i) by generating Ca(2+) sparks at rest to activate STOCs and (ii) by generating Ca(2+) transients presumably triggered by sparks during an action potential to activate a large I(K,Ca) and also induce a contraction. BK channels and RyRs may co-localize densely at the junctional areas of plasmalemma and SR fragments, where Ca(2+) sparks occur to elicit STOCs.

Comparative study of effects of isoproterenol and vasoactive intestinal polypeptide on voltage-dependent Ca2+ and Ca(2+)-activated K+ currents in porcine tracheal smooth muscle cells

Effects of isoproterenol (Iso) and vasoactive intestinal polypeptide (VIP) on voltage-dependent Ca2+ channel (ICa) and Ca(2+)-activated K+ channel current (IK-Ca) in porcine tracheal smooth muscle cells were examined. When K+ currents were inhibited using a Cs-rich pipette solution, application of 0.1-1 microM Iso or 1-10 nM VIP increased ICa by 20-30%. On the other hand, IK-Ca elicited upon depolarization and spontaneous transient outward K+ currents (STOCs) recorded at a holding potential of -50 mV were enhanced by 80-100% in the presence of 0.1 microM Iso or 1 nM VIP.

Effects of Elevated Cytosolic Calcium on ACh-Induced Swine Tracheal Smooth Muscle Contraction

Increased intracellular calcium concentration ([Ca(2+)](i)) is required for smooth muscle contraction. In tracheal and other tonic smooth muscles, contraction and elevated [Ca(2+)](i) are maintained as long as an agonist is present. To evaluate the physiological role of steady-state increases in Ca(2+) on tension maintenance, [Ca(2+)](i) was elevated using ionomycin, a Ca(2+) ionophore or charybdotoxin, a large-conductance calcium-activated potassium channel (K(Ca)) blocker prior to or during exposure of tracheal smooth muscle strips to ACh (10(-9) to 10(-4) M). Ionomycin (5 &mgr;M) in resting muscles induced increases in [Ca(2+)](i) to 500 +/- 230 nM and small increases in force of 2.6 +/- 2.3 N/cm(2). This tension is only 10% of the maximal tension induced by ACh. Charybdotoxin had no effect on [Ca(2+)](i) or tension in resting muscle. After pretreatment of muscle with ionomycin, the concentration-response relationship for ACh-induced changes in tension shifted to the left (EC(50) = 0.07 +/- 0.05 &mgr;M ionomycin; 0.17 +/- 0.07 &mgr;M, control, p < 0.05). When applied to the muscles during steady-state responses to submaximal concentrations of ACh, both ionomycin and charybdotoxin induced further increases in tension. The same magnitude increase in tension occurs after ionomycin and charybdotoxin treatment, even though the increase in [Ca(2+)](i) induced by charybdotoxin is much smaller than that induced by ionomycin. We conclude that the resting muscle is much less sensitive to elevation of [Ca(2+)](i) when compared to muscles stimulated with ACh. Steady-state [Ca(2+)](i) limits tension development induced by submaximal concentrations of ACh. The activity of K(Ca) moderates the response of the muscle to ACh at concentrations less than 1 &mgr;M. Copyright 1996 S. Karger AG, Basel

Time course of Ca(2+)-dependent K+ and Cl- currents in single smooth muscle cells of guinea-pig trachea

The time course of two types of Ca(2+)-dependent currents were compared in single smooth muscle cells freshly isolated from guinea-pig trachea. When the pipette solution contained mainly 140 mM KCl, depolarization from -60 mV to 0 mV evoked an initial inward current followed by an outward current which consisted of transient (I(to)) and sustained components. In addition, a long-lasting inward tail current (Itail) was occasionally observed after the repolarization to -60 mV. Although I(to) often occurred repetitively during depolarization, the first I(to) reached the peak of approximately 50 ms after the start of depolarization and had the largest amplitude in most cells examined. The amplitude of Itail increased with the increase in depolarization period up to about 500 ms. Pharmacological analyses indicate that I(to) and Itail are Ca(2+)-dependent K+ and Cl- currents (IK-Ca and ICl-Ca), respectively, and suggest that not only Ca(2+)-influx through Ca2+ channels but also subsequent Ca2+ release from stores contributes to activate these currents. Spontaneous transient outward and inward currents, IK-Ca and ICl-Ca, respectively, were simultaneously recorded at -40 mV. In over 80% of the spontaneous current events, outward and inward currents coupled one to one and always occurred in this order. Puff-application of 10 mM caffeine also induced IK-Ca and ICl-Ca in this order at -40 mV. When caffeine was applied twice with various intervals, the current amplitude in the second application depended upon the period of the interval. The recovery of ICl-Ca during the interval was faster than that of IK-Ca. The results indicate that the activation and decay time courses of ICl-Ca are slower but its recovery is faster than those of IK-Ca.

Relaxant actions of isoprenaline on guinea-pig isolated tracheal smooth muscle

1. The effects of isoprenaline on membrane potential and intracellular Ca2+ concentration ([Ca2+]i) in guinea-pig isolated tracheal muscle were studied by use of intracellular micro-electrodes and fura-2 signals respectively. Measurements of membrane potential were carried out in the presence of spontaneously-generated muscle tone, whereas fura-2 signals were measured during contraction produced by exogenous prostaglandin E2 (100 nM). The potency of isoprenaline in causing relaxation was the same in these two different situations. 2. Isoprenaline (0.01 microM) produced relaxation accompanied by 5 mV hyperpolarization. A combination of tetraethylammonium (TEA, 10 mM) and verapamil (3 microM) did not alter the effects of isoprenaline. Removal of external K+ did not increase the degree of hyperpolarization produced by isoprenaline. 3. In the presence of TEA (10 mM) and verapamil (3 microM), isoprenaline (0.03-1 microM) reduced [Ca2+]i concentration-dependently. A similar degree of inhibition was observed when isoprenaline was applied during the maintained contraction induced by prostaglandin E2 and against the contraction evoked by the addition of Ca2+ to tissues bathed in a Ca(2+)-free medium and pretreated with both isoprenaline and prostaglandin E2. 4. It is concluded that activation of TEA-sensitive Ca(2+)-dependent K+ channels does not play a significant role in isoprenaline-induced relaxation. We propose that, in the guinea-pig tracheal muscle, isoprenaline may produce relaxation mainly by inhibiting a receptor-operated pathway for Ca2+ influx across the plasma membrane which is normally activated by prostaglandins.

The patch clamp technique

The introduction of the patch clamp technique less than two decades ago revolutionized the study of cellular physiology by providing a high-resolution method of observing the function of individual ionic channels in a variety of normal and pathological cell types. By the use of variations of the basic recording methodology, cellular function and regulation can be studied at a molecular level by observing currents through individual ionic channels. At a cellular level, processes such as signaling, secretion, and synaptic transmission can be examined. In addition, by combining the information from high-resolution electrophysiological recordings obtained by the patch clamp method with modern molecular biological techniques, further insight can be gained into the gene expression and protein structure of ionic channels. Given the ubiquity and importance of ionic channels, it is not surprising that their study has led to a new understanding of the mechanisms of certain disease processes and has given insight into treatments for these diseases. This review gives an historical perspective of the development of the patch clamp technique and an overview of the methodologies currently in use. Examples are shown to illustrate typical uses of the patch clamp technique with emphasis on the variety of recording configurations available and the advantages and drawbacks of each method.

Muscarinic stimulation of tracheal smooth muscle cells activates large-conductance Ca(2+)-dependent K+ channel

We investigated the regulation of the large-conductance Ca(2+)-dependent K+ (KCa) channel by acetylcholine (ACh) in freshly dissociated tracheal smooth muscle cells. Channels were recorded in the cell-attached patch configuration, and cells were stimulated with ACh, muscarine, or caffeine. We identified KCa channel activity based on 1) the voltage dependence of channel opening; 2) the large unitary conductance (242 +/- 5 pS with symmetrical 135 mM K+); 3) dependence of the reversal potential on the [K+] gradient, shifting 56 +/- 3 mV/10-fold change in extracellular [K+]; and 4) opening of channels after elevation of cytosolic free Ca2+ concentration ([Ca2+]i) using the Ca2+ ionophore A23187. When cells were bathed either in a physiological saline solution or a solution containing 135 mM K+ (to clamp cell membrane potential near 0 mV), ACh caused contraction of cells and activation of voltage-dependent channels. With 135 mM extracellular K+, the channels activated by ACh had a unitary conductance of 247 +/- 10 pS, and currents reversed near the K+ equilibrium potential (-1 +/- 1 mV). The effects of ACh were reversible, blocked by atropine, and mimicked by muscarine. From these characteristics we conclude that muscarinic stimulation of canine tracheal smooth muscle cells leads to activation of the large-conductance KCa channel. Because the KCa channels were isolated from ACh by the patch pipette, the increased channel activity was probably mediated by a cytosolic second messenger. ACh shifted the threshold for KCa channel opening to less positive membrane potentials, similar to that seen with elevation of [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.