Voltage-dependent calcium currents and cytosolic calcium in equine airway myocytes

Physiology of nitric oxide in the respiratory system

Nitric oxide (NO) is an important endogenous neurotransmitter and mediator. It participates in regulation of physiological processes in different organ systems including airways. Therefore, it is important to clarify its role in the regulation of both airway and vascular smooth muscle, neurotransmission and neurotoxicity, mucus transport, lung development and in the. surfactant production. The bioactivity of NO is highly variable and depends on many factors: the presence and activity of NO-producing enzymes, activity of competitive enzymes (e.g. arginase), the amount of substrate for the NO production, the presence of reactive oxygen species and others. All of these can change NO primary physiological role into potentially harmful. The borderline between them is very fragile and in many cases not entirely clear. For this reason, the research focuses on a comprehensive understanding of NO synthesis and its metabolic pathways, genetic polymorphisms of NO synthesizing enzymes and related effects. Research is also motivated by frequent use of exhaled NO monitoring in the clinical manifestations of respiratory diseases. The review focuses on the latest knowledge about the production and function of this mediator and understanding the basic physiological processes in the airways.

Molecular expression and functional role of canonical transient receptor potential channels in airway smooth muscle cells

Multiple canonical or classic transient receptor potential (TRPC) molecules are expressed in animal and human airway smooth muscle cells (SMCs). TRPC3, but not TRPC1, is a major molecular component of native non-selective cation channels (NSCCs) to contribute to the resting [Ca(2+)](i) and muscarinic increase in [Ca(2+)](i) in freshly isolated airway SMCs. TRPC3-encoded NSCCs are significantly increased in expression and activity in airway SMCs from ovalbumin-sensitized/challenged "asthmatic" mice, whereas TRPC1-encoded channel activity, but not its expression, is largely augmented. The upregulated TRPC3- and TRPC1-encoded NSCC activity both mediate "asthmatic" membrane depolarization in airway SMCs. Supportively, tumor necrosis factor-α (TNFα), an important asthma mediator, increases TRPC3 expression, and TRPC3 gene silencing inhibits TNFα-mediated augmentation of acetylcholine-evoked increase in [Ca(2+)](i) in passaged airway SMCs. In contrast, TRPC6 gene silencing has no effect on 1-oleoyl-2-acetyl-sn-glycerol (OAG)-evoked increase in [Ca(2+)](i) in primary isolated cells. These findings provide compelling information indicating that TRPC3-encoded NSCCs are important for physiological and pathological cellular responses in airway SMCs. However, continual studies are necessary to further determine whether, which, and how TRPC-encoded channels are involved in cellular responses in normal and diseased (e.g., asthmatic) airway SMCs.

Ca2+ entry following P2X receptor activation induces IP3 receptor-mediated Ca2+ release in myocytes from small renal arteries

BACKGROUND AND PURPOSE

P2X receptors mediate sympathetic control and autoregulation of the renal circulation triggering contraction of renal vascular smooth muscle cells (RVSMCs) via an elevation of intracellular Ca(2+) concentration ([Ca(2+) ](i) ). Although it is well-appreciated that the myocyte Ca(2+) signalling system is composed of microdomains, little is known about the structure of the [Ca(2+) ](i) responses induced by P2X receptor stimulation in vascular myocytes.

EXPERIMENTAL APPROACHES

Using confocal microscopy, perforated-patch electrical recordings, immuno-/organelle-specific staining, flash photolysis and RT-PCR analysis we explored, at the subcellular level, the Ca(2+) signalling system engaged in RVSMCs on stimulation of P2X receptors with the selective agonist αβ-methylene ATP (αβ-meATP).

KEY RESULTS

RT-PCR analysis of single RVSMCs showed the presence of genes encoding inositol 1,4,5-trisphosphate receptor type 1(IP(3) R1) and ryanodine receptor type 2 (RyR2). The amplitude of the [Ca(2+) ](i) transients depended on αβ-meATP concentration. Depolarization induced by 10 µmol·L(-1) αβ-meATP triggered an abrupt Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum enriched with IP(3) Rs but poor in RyRs. Depletion of calcium stores, block of voltage-gated Ca(2+) channels (VGCCs) or IP(3) Rs suppressed the sub-plasmalemmal [Ca(2+) ](i) upstroke significantly more than block of RyRs. The effect of calcium store depletion or IP(3) R inhibition on the sub-plasmalemmal [Ca(2+) ](i) upstroke was attenuated following block of VGCCs.

CONCLUSIONS AND IMPLICATIONS

Depolarization of RVSMCs following P2X receptor activation induces IP(3) R-mediated Ca(2+) release from sub-plasmalemmal ('junctional') sarcoplasmic reticulum, which is activated mainly by Ca(2+) influx through VGCCs. This mechanism provides convergence of signalling pathways engaged in electromechanical and pharmacomechanical coupling in renal vascular myocytes.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Membrane depolarization causes a direct activation of G protein-coupled receptors leading to local Ca2+ release in smooth muscle

Membrane depolarization activates voltage-dependent Ca(2+) channels (VDCCs) inducing Ca(2+) release via ryanodine receptors (RyRs), which is obligatory for skeletal and cardiac muscle contraction and other physiological responses. However, depolarization-induced Ca(2+) release and its functional importance as well as underlying signaling mechanisms in smooth muscle cells (SMCs) are largely unknown. Here we report that membrane depolarization can induce RyR-mediated local Ca(2+) release, leading to a significant increase in the activity of Ca(2+) sparks and contraction in airway SMCs. The increased Ca(2+) sparks are independent of VDCCs and the associated extracellular Ca(2+) influx. This format of local Ca(2+) release results from a direct activation of G protein-coupled, M(3) muscarinic receptors in the absence of exogenous agonists, which causes activation of Gq proteins and phospholipase C, and generation of inositol 1,4,5-triphosphate (IP(3)), inducing initial Ca(2+) release through IP(3) receptors and then further Ca(2+) release via RyR2 due to a local Ca(2+)-induced Ca(2+) release process. These findings demonstrate an important mechanism for Ca(2+) signaling and attendant physiological function in SMCs.

Acetylcholine-Induced Asynchronous Calcium Waves in Intact Human Bronchial Muscle Bundle

Calcium (Ca2+) is an important activator of the contractile machinery in airway smooth muscle (ASM). While agonist-induced Ca2+ signals are well characterized in animal ASM, little is known about what occurs in adult human ASM. In this study, we examined the Ca2+ signal elicited by acetylcholine (ACh) in smooth muscle cells of the intact human bronchial muscle strips obtained from fresh surgical specimens in relation to muscle contraction. We found that ACh induces repetitive Ca2+ waves that spread along the longitudinal axis of individual cells in the intact human bronchial smooth muscle strips. These Ca2+ waves display no apparent synchronization between neighboring cells, and their generation precedes force development. Comparison of the ACh concentration dependence of tissue contraction and selected parameters of the asynchronous Ca2+ waves (ACW) reveals that the graded force generation by ACh-stimulated human bronchial muscle strips is achieved by differential recruitment of cells to initiate Ca2+ waves and by enhancement of the frequency of ACW once the cells are recruited. Furthermore, pharmacologic characterization shows that the ACW are produced by repetitive cycles of SR Ca2+ release via ryanodine-sensitive channels followed by SR Ca2+ reuptake by sarco(endo)plasmic reticulum Ca2+ ATPase. Extracellular Ca2+ entry involving receptor-operated channels/store-operated channels, reverse-mode Na+/Ca2+ exchange, and to a lesser extent L-type voltage-gated Ca2+ channels is required to maintain the ACW. These findings for the first time demonstrate the occurrence and the role of ACW in excitation-contraction coupling in adult human ASM.

Two distinct signaling pathways for regulation of spontaneous local Ca2+ release by phospholipase C in airway smooth muscle cells

Spontaneous local Ca(2+) release events have been observed in airway smooth muscle cells (SMCs), but the underlying mechanisms are largely unknown. Considering that each type of SMCs may use its own mechanisms to regulate local Ca(2+) release events, we sought to investigate the signaling pathway for spontaneous local Ca(2+) release events in freshly isolated mouse airway SMCs using a laser scanning confocal microscope. Application of ryanodine to block ryanodine receptors (RyRs) abolished spontaneous local Ca(2+) release events, indicating that these events are RyR-mediated Ca(2+) sparks. Inhibition of inositol 1,4,5-triphosphate receptors (IP(3)Rs) by 2-aminoethoxydiphenyl-borate (2-APB) or xestospongin-C significantly blocked the activity of Ca(2+) sparks. Under patch clamp conditions, dialysis of IP(3) to activate IP(3)Rs increased the activity of local Ca(2+) events in control cells but had no effect in ryanodine-pretreated cells. The RyR agonist caffeine augmented the frequency of Ca(2+) sparks in cells pretreated with and without 2-APB or xestospongin-C. The specific phospholipase C (PLC) blocker U73122 decreased the activity of Ca(2+) sparks and prevented xestospongin-C from producing the inhibitory effect. The protein kinase C (PKC) activator 1-oleoyl-2-acetyl-glycerol or phorbol-12-myristate-13-acetate inhibited Ca(2+) sparks, whereas the PKC inhibitor chelerythrine, PKCvarepsilon inhibitory peptide, or PKCvarepsilon gene knockout produced an opposite effect. Collectively, our data suggest that the basal activation of PLC regulates the activity of RyR-mediated, spontaneous Ca(2+) sparks in airway SMCs through two distinct signaling pathways: a positive IP(3)-IP(3)R pathway and a negative diacylglycerol-PKCvarepsilon pathway.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Mechanism of ACh-induced asynchronous calcium waves and tonic contraction in porcine tracheal muscle bundle

Stimulation of the tracheal muscle bundle by acetylcholine (ACh) results in the generation of asynchronous repetitive Ca2+ waves (ACW) in intact tracheal smooth muscle (TSM) cells. We showed previously that ACW underlie cholinergic excitation-contraction coupling in porcine TSM and that Ca2+ entry through the L-type voltage-gated Ca2+ channel (VGCC) contributes partially to maintenance of the ACW. However, the mechanism of the ACW remains undefined. In this study, we pharmacologically characterized the mechanism of ACh-induced ACW in the intact porcine tracheal muscle bundle. We found that inhibition of receptor-operated channels/store-operated channels (ROC/SOC) by SKF-96365 completely abolished the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Blockade of Na+/Ca2+ exchange with KB-R7943 or 2′,4′-dichlorobenzamil or removal of extracellular Na+ resulted in nearly complete inhibition of the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Inhibition of the sarco(endo)plasmic reticulum Ca2+-ATPase by cyclopiazonic acid abolished the ongoing ACW. Application of 2-aminoethoxydiphenyl borate (2-APB) or xestospongin C to inhibit the inositol 1,4,5-trisphosphate-sensitive sarcoplasmic reticulum (SR) Ca2+ release channels produced no effect on ACh-mediated ACW and tonic contraction. However, pretreatment with caffeine or ryanodine inhibited ACh-induced ACW. Furthermore, application of procaine or tetracaine prevented the generation and abolished the ongoing ACh-mediated ACW and tonic contraction. Collectively, these results indicate that the ACh-stimulated ACW in porcine TSM are produced by repetitive cycles of Ca2+ release from SR through 2-APB- and xestospongin C-insensitive Ca2+ release channels, and plasmalemmal Ca2+ entry involving reverse-mode Na+/Ca2+ exchange, ROC/SOC, and L-type VGCC is required to refill the SR via SERCA to support the ongoing ACW.

Two-step Ca2+ intracellular release underlies excitation-contraction coupling in mouse urinary bladder myocytes

The relative contributions of Ca(2+)-induced Ca(2+) release (CICR) versus Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) to excitation-contraction coupling has not been defined in most smooth muscle cells (SMCs). The present study was undertaken to address this issue in mouse urinary bladder (UB) smooth muscle cells (UBSMCs). Confocal Ca(2+) images were obtained under voltage- or current-clamp conditions. When UBSMCs were activated by a 30-ms depolarization to 0 mV, intracellular Ca(2+) concentration ([Ca(2+)](i)) increased in several small, discrete areas just beneath the cell membrane. These Ca(2+) "hot spots" then spread slowly through the myoplasm as Ca(2+) waves, which continued even after repolarization. Shorter depolarizations (5 ms) elicited only a few Ca(2+) sparks, which declined quickly. The number of Ca(2+) sparks, or hot spots, was closely related to the depolarization duration in the range of approximately 5-20 ms. There was an apparent threshold depolarization duration of approximately 10 ms within which to induce enough Ca(2+) transients to spread globally and then induce a contraction. Application of 100 microM ryanodine to the pipette solution did not change the resting [Ca(2+)](i) or the VDCC current, but it did abolish Ca(2+) hot spots elicited by depolarization. Application of 3 microM xestospongin C reduced ACh-induced Ca(2+) release but did not affect depolarization-induced Ca(2+) events. The addition of 100 microM ryanodine to tissue segments markedly reduced the amplitude of contractions triggered by direct electrical stimulation. In conclusion, global [Ca(2+)](i) rise triggered by a single action potential is not due mainly to Ca(2+) influx through VDCCs but is attributable to the subsequent two-step CICR.

Regulation of Rho/ROCK signaling in airway smooth muscle by membrane potential and [Ca2+]i

Recently, we have shown that Rho and Rho-activated kinase (ROCK) may become activated by high-millimolar KCl, which had previously been widely assumed to act solely through opening of voltage-dependent Ca(2+) channels. In this study, we explored in more detail the relationship between membrane depolarization, Ca(2+) currents, and activation of Rho/ROCK in bovine tracheal smooth muscle. Ca(2+) currents began to activate at membrane voltages more positive than -40 mV and were maximally activated above 0 mV; at the same time, these underwent time- and voltage-dependent inactivation. Depolarizing intact tissues by KCl challenge evoked contractions that were blocked equally, and in a nonadditive fashion, by nifedipine or by the ROCK inhibitor Y-27632. Other agents that elevate intracellular calcium concentration ([Ca(2+)](i)) by pathways independent of G protein-coupled receptors, namely the SERCA-pump inhibitor cyclopiazonic acid and the Ca(2+) ionophore A-23187, evoked contractions that were also largely reduced by Y-27632. KCl directly increased Rho and ROCK activities in a concentration-dependent fashion that paralleled closely the effect of KCl on tone and [Ca(2+)](i), as well as the voltage-dependent Ca(2+) currents that were measured over the voltage ranges that are evoked by 0-120 mM KCl. Through the use of various pharmacological inhibitors, we ruled out roles for Ca(2+)/calmodulin-dependent CaM kinase II, protein kinase C, and protein kinase A in mediating the KCl-stimulated changes in tone and Rho/ROCK activities. In conclusion, Rho is activated by elevation of [Ca(2+)](i) (although the signal transduction pathway underlying this Ca(2+) dependence is still unclear) and possibly also by membrane depolarization per se.

Expression of transient receptor potential C6 and related transient receptor potential family members in human airway smooth muscle and lung tissue

Elevation of the intracellular free Ca(2+) concentration regulates many functional responses in airway smooth muscle, including contraction, proliferation, adhesion, and cell survival. This increase in calcium can be achieved by a release from internal stores (sarcoplasmic reticulum) and/or entry across the cell membrane from the extracellular environment. The molecular identity of this calcium influx pathway in human airway smooth muscle (HASM) remains unclear. Functional studies using Fluo 4-loaded HASM suggest the presence of a histamine H(1) receptor-activated Ca(2+) entry pathway with characteristics similar to those seen with transient receptor potential (TRP) family homologs. Using a range of molecular and cell biological approaches we defined the expression pattern of transient receptor potential classics (TRPC) homologs in airway cells and tissue. Here we show that HASM and human bronchial epithelial cells both express TRPC1, -4, and -6, with HASM also expressing TRPC3 at the mRNA level. Identification of TRPC6 protein by western blot and confocal microscopy indicated that the protein is localized in specific cell types, suggesting that it plays an important role in regulating key functions in airway cells. These data demonstrate the expression of a range of TRPC homologs in the airway and the presence of a functional Ca(2+) entry pathway with characteristics typical of TRPC family members. TRPC homologs may provide an important novel target for the treatment of airway disease.

Calcium-induced calcium release in smooth muscle: the case for loose coupling

This article reviews the key experiments demonstrating calcium-induced calcium release (CICR) in smooth muscle and contrasts the biophysical and molecular features of coupling between the sarcolemmal (L-type Ca(2+) channel) and sarcoplasmic reticulum (ryanodine receptor) Ca(2+) channels in smooth and cardiac muscle. Loose coupling refers to the coupling process in smooth muscle in which gating of ryanodine receptors is non-obligate and may occur with a variable delay following opening of the sarcolemmal Ca(2+) channels. These features have been observed in the earliest studies of CICR in smooth muscle and are in marked contrast to cardiac CICR, where a close coupling between T-tubular and SR membranes results in tight coupling between the gating events. The relationship between this "loose coupling" and distinct subcellular release sites within smooth muscle cells, termed frequent discharge sites, is discussed.

FKBP12.6 and cADPR regulation of Ca2+ release in smooth muscle cells

Intracellular Ca2+ release through ryanodine receptors (RyRs) plays important roles in smooth muscle excitation-contraction coupling, but the underlying regulatory mechanisms are poorly understood. Here we show that FK506 binding protein of 12.6 kDa (FKBP12.6) associates with and regulates type 2 RyRs (RyR2) in tracheal smooth muscle. FKBP12.6 binds to RyR2 but not other RyR or inositol 1,4,5-trisphosphate receptors, and FKBP12, known to bind to and modulate skeletal RyRs, does not associate with RyR2. When dialyzed into tracheal myocytes, cyclic ADP-ribose (cADPR) alters spontaneous Ca2+ release at lower concentrations and produces macroscopic Ca2+ release at higher concentrations; neurotransmitter-evoked Ca2+ release is also augmented by cADPR. These actions are mediated through FKBP12.6 because they are inhibited by molar excess of recombinant FKBP12.6 and are not observed in myocytes from FKBP12.6-knockout mice. We also report that force development in FKBP12.6-null mice, observed as a decrease in the concentration/tension relationship of isolated trachealis segments, is impaired. Taken together, these findings point to an important role of the FKBP12.6/RyR2 complex in stochastic (spontaneous) and receptor-mediated Ca2+ release in smooth muscle.

Ca2+-dependent and Ca2+-independent regulation of smooth muscle contraction

An increase in the cytosolic Ca2+ concentration is a prerequisite in activation of contractile activity of smooth muscle. The shape of the Ca2+-signal is determined by spatial distribution and kinetics of Ca2+-binding sites in the cell. The increase in cytosolic Ca2+ activates myosin light chain kinase (MLCK) which in turn phosphorylates the regulatory light chains of myosin II. This Ca2+-dependent MLC20 phosphorylation is modulated in a Ca2+-independent manner by inhibiting the constitutive active myosin light chain phosphatase mediated by the monomeric GTPase Rho and the Rho-associated kinase as well as protein kinase C or by increasing its activity through cGMP. Furthermore, the activity of MLCK may be decreased due to phosphorylation by CaM kinase II and perhaps p21 activated protein kinase. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades which not only show a temporal but also a spatial activation pattern.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Ca(2+) regulation in guinea-pig colonic smooth muscle: the role of the Na(+)-Ca(2+) exchanger and the sarcoplasmic reticulum

To study the contribution of the Na(+)-Ca(2+) exchanger to Ca(2+) regulation and its interaction with the sarcoplasmic reticulum (SR), changes in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) were measured in single, voltage clamped, smooth muscle cells. Increases in [Ca(2+)](c) were evoked by either depolarisation (-70 mV to 0 mV) or by release from the SR by caffeine (10 mM) or flash photolysis of caged InsP(3) (InsP(3)). Depletion of the SR of Ca(2+) (verified by the absence of a response to caffeine and InsP(3)) by either ryanodine (50 microM), to open the ryanodine receptors (RyRs), or thapsigargin (500 nM) or cyclopiazonic acid (CPA, 10 microM), to inhibit the SR Ca(2+) pumps, reduced neither the magnitude of the Ca(2+) transient nor the relationship between the influx of and the rise in [Ca(2+)](c) evoked by depolarisation. This suggested that Ca(2+)-induced Ca(2+) release (CICR) from the SR did not contribute significantly to the depolarisation-evoked rise in [Ca(2+)](c). However, although Ca(2+) was not released from it, the SR accumulated the ion following depolarisation since ryanodine and thapsigargin each slowed the rate of decline of the depolarisation-evoked Ca(2+) transient. Indeed, the SR Ca(2+) content increased following depolarisation as assessed by the increased magnitude of the [Ca(2+)](c) levels evoked each by InsP(3) and caffeine, relative to controls. The increased SR Ca(2+) content following depolarisation returned to control values in approximately 12 min via Na(+)-Ca(2+) exchanger activity. Thus inhibition of the Na(+)-Ca(2+) exchanger by removal of external Na(+) (by either lithium or choline substitution) prevented the increased SR Ca(2+) content from returning to control levels. On the other hand, the Na(+)-Ca(2+) exchanger did not appear to regulate bulk average Ca(2+) directly since the rates of decline in [Ca(2+)](c), following either depolarisation or the release of Ca(2+) from the SR (by either InsP(3) or caffeine), were neither voltage nor Na(+) dependent. Thus, no evidence for short term (seconds) control of [Ca(2+)](c) by the Na(+)-Ca(2+) exchanger was found. Together, the results suggest that despite the lack of CICR, the SR removes Ca(2+) from the cytosol after its elevation by depolarisation. This Ca(2+) is then removed from the SR to outside the cell by the Na(+)-Ca(2+) exchanger. However, the exchanger does not contribute significantly to the decline in bulk average [Ca(2+)](c) following transient elevations in the ion produced either by depolarisation or by release from the store.

Muscarinic excitation-contraction coupling mechanisms in tracheal and bronchial smooth muscles

We investigated the mechanisms underlying muscarinic excitation-contraction coupling in canine airway smooth muscle using organ bath, fura 2 fluorimetric, and patch-clamp techniques. Cyclopiazonic acid (CPA) augmented the responses to submaximal muscarinic stimulation in both tracheal (TSM) and bronchial smooth muscles (BSM), consistent with disruption of the barrier function of the sarcoplasmic reticulum. During maximal stimulation, however, CPA evoked substantial relaxation in TSM but not BSM. CPA reversal of carbachol tone persisted in the presence of tetraethylammoium or high KCl, suggesting that hyperpolarization is not involved; CPA relaxations were absent in tissues preconstricted with KCl alone or by permeabilization with beta-escin, ruling out a nonspecific effect on the contractile apparatus. Peak contractions were sensitive to inhibitors of tyrosine kinase (genistein) or Rho kinase (Y-27632). Sustained responses were dependent on Ca(2+) influx in TSM but not BSM; this influx was sensitive to Ni(2+) but not La(3+). In conclusion, there are several mechanisms underlying excitation-contraction coupling in airway smooth muscle, the relative importance of which varies depending on tissue and degree of stimulation.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Sarcoplasmic reticulum Ca2+ depletion by caffeine and changes of [Ca2+](i) during refilling in bovine airway smooth muscle cells

BACKGROUND

In airway smooth muscle (ASM), Ca2+ influx in response to the Ca2+ depletion of the sarcoplasmic reticulum (SR) seems to play a role in the regulation of intracellular free Ca2+ concentrations ([Ca2+](i)). This study evaluates some possible Ca2+ entry pathways activated during SR-Ca2+ depletion induced by 10 mM caffeine.

METHODS

Enzymatically dispersed bovine ASM cells were loaded with Fura-2/AM to permit measurement of [Ca2+](i) changes in single cells.

RESULTS

Caffeine (10 mM) induced a transient increase in [[Ca2+](i) that depleted SR-Ca(2)+ content. After caffeine washout, a decrease in basal [Ca2+](i) (undershoot) was invariably observed, followed by a slow recovery. This phenomenon was inhibited by cyclopiazonic acid (5 microM). External Ca(2)+ removal in depolarized and nondepolarized cells induced a decrease in basal [Ca2+](i) that continued until depletion of the SR-Ca2+ content. The decrease in [Ca2+](i) induced by Ca2+-free physiological saline solution (PSS) was accelerated in caffeine-stimulated cells. Recovery from undershoot was not observed in Ca2+-free PSS. Depolarization with KCl and addition of D600 (30 microM) did not modify recovery. Similar results were obtained when the Na(+)/Ca2+ exchanger was blocked by substituting NaCl with KCl in normal PSS (Na(+)-free PSS) or by adding benzamil amiloride (25 microM).

CONCLUSIONS

SR-Ca2+ content plays an important role in the Ca2+ leak induced by Ca2+-free medium, and does not depend on membrane potential. Additionally, recovery from undershoot after caffeine depends on extracellular Ca2+, and neither voltage-dependent Ca2+ channels nor the Na(+)/Ca2+ exchanger are involved.

Calcium-induced calcium release in smooth muscle: loose coupling between the action potential and calcium release

Calcium-induced calcium release (CICR) has been observed in cardiac myocytes as elementary calcium release events (calcium sparks) associated with the opening of L-type Ca(2+) channels. In heart cells, a tight coupling between the gating of single L-type Ca(2+) channels and ryanodine receptors (RYRs) underlies calcium release. Here we demonstrate that L-type Ca(2+) channels activate RYRs to produce CICR in smooth muscle cells in the form of Ca(2+) sparks and propagated Ca(2+) waves. However, unlike CICR in cardiac muscle, RYR channel opening is not tightly linked to the gating of L-type Ca(2+) channels. L-type Ca(2+) channels can open without triggering Ca(2+) sparks and triggered Ca(2+) sparks are often observed after channel closure. CICR is a function of the net flux of Ca(2+) ions into the cytosol, rather than the single channel amplitude of L-type Ca(2+) channels. Moreover, unlike CICR in striated muscle, calcium release is completely eliminated by cytosolic calcium buffering. Thus, L-type Ca(2+) channels are loosely coupled to RYR through an increase in global [Ca(2+)] due to an increase in the effective distance between L-type Ca(2+) channels and RYR, resulting in an uncoupling of the obligate relationship that exists in striated muscle between the action potential and calcium release.

Multiple pathways responsible for the stretch-induced increase in Ca2+ concentration in toad stomach smooth muscle cells

1. A digital imaging microscope with fura-2 as the Ca2+ indicator was used to determine the sources for the rise in intracellular calcium concentration ([Ca2+]i) that occurs when the membrane in a cell-attached patch is stretched. Unitary ionic currents from stretch-activated channels and [Ca2+]i images were recorded simultaneously. 2. When suction was applied to the patch pipette to stretch a patch of membrane, Ca2+-permeable cation channels (stretch-activated channels) opened and a global increase in [Ca2+]i occurred, as well as a greater focal increase in the vicinity of the patch pipette. The global changes in [Ca2+]i occurred only when stretch-activated currents were sufficient to cause membrane depolarization, as indicated by the reduction in amplitude of the unitary currents. 3. When Ca2+ was present only in the pipette solution, just the focal change in [Ca2+]i was obtained. This focal change was not seen when the contribution from Ca2+ stores was eliminated using caffeine and ryanodine. 4. These results suggest that the opening of stretch-activated channels allows ions, including Ca2+, to enter the cell. The entry of positive charge triggers the influx of Ca2+ into the cell by causing membrane depolarization, which presumably activates voltage-gated Ca2+ channels. The entry of Ca2+ through stretch-activated channels is also amplified by Ca2+ release from internal stores. This amplification appears to be greater than that obtained by activation of whole-cell Ca2+ currents. These multiple pathways whereby membrane stretch causes a rise in [Ca2+]i may play a role in stretch-induced contraction, which is a characteristic of many smooth muscle tissues.

Signalling pathway for histamine activation of non-selective cation channels in equine tracheal myocytes

1. The signalling pathway underlying histamine activation of non-selective cation channels was investigated in single equine tracheal myocytes. Application of histamine (100 microM) activated the transient calcium-activated chloride current (ICl(Ca)) and sustained, low amplitude non-selective cation current (ICat). The H1 receptor antagonist pyrilamine (10 microM) blocked activation of ICl(Ca) and ICat. Simultaneous application of histamine (100 microM) and caffeine (8 mM) during H1 receptor blockade activated ICl(Ca), but not ICat. Neither the H2 receptor antagonist cimetidine (20 microM) nor the H3 receptor antagonist thioperamide (20 microM) prevented activation of ICl(Ca) and ICat. 2. Intracellular dialysis of anti-Galphai/Galphao antibodies completely blocked activation of ICat by histamine, whereas ICl(Ca) was not affected. By contrast, anti-Galphaq/Galpha11 antibodies greatly inhibited ICl(Ca), but did not alter activation of ICat. 3. 1-Oleoyl-2-acetyl-sn-glycerol (OAG, 20-100 microM) did not induce any current or affect currents activated by histamine or methacholine (mACH). Simultaneous application of OAG and caffeine activated ICl(Ca), but not ICat, indicating that a rise in [Ca2+]i and stimulation of diacylglycerol-sensitive protein kinase C (PKC) is not sufficient to activate ICat. The phospholipase C inhibitor U73122 (2 microM) blocked histamine activation of ICl(Ca) and ICat, but simultaneous exposure of myocytes to histamine and caffeine restored both ICl(Ca) and ICat in the presence of U73122. 4. Histamine and mACH activated currents with equivalent I-V relationships. The currents activated by these agonists were not additive; following activation of ICat by mACH, histamine failed to induce an additional membrane current. Similarly, mACH did not induce an additional current after full activation of ICat by histamine. 5. We conclude that H1 histamine receptors activate ICat through coupling to Gi/Go proteins. Activation of ICat also requires intracellular calcium release, mediated by H1 receptors coupling to Gq/G11 proteins. This coupling is analogous to the activation of ICat by co-stimulation of M2 and M3 receptors.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Mechanisms that regulate [Ca2+]i following depolarization in rat systemic arterial smooth muscle cells

1. We have used the patch-clamp technique in combination with fluorimetric recording to study the mechanisms that regulate intracellular Ca2+, [Ca2+]i, following depolarization in cells isolated from the rat femoral artery. 2. Depolarization to 0 mV from a holding potential of -70 mV increased [Ca2+]i. Little Ca2+ release from sarcoplasmic reticulum, SR, was detected during depolarization since application of 30 microM ryanodine, a Ca2+-release inhibitor, had no significant effect on total Ca2+ buffering power. 3. Upon repolarization to -70 mV, 7 out of 13 cells showed three phases of Ca2+ removal; an initial rapid first phase, a slow second phase, and a faster third phase. Six cells, in which Ca2+ recovered quickly, lacked the third phase. The third phase was also absent in cells treated with a SR Ca2+-pump inhibitor, cyclopiazonic acid. 4. The peak first-phase Ca2+ removal rate observed upon repolarization to -70 mV was significantly reduced in cells treated with a mitochondrial Ca2+ uptake inhibitor, carbonyl cyanide m-chlorophenylhydrazone. However, an ATP-synthase inhibitor, oligomycin B, had no significant effect. 5. The Ca2+ removal rate was little affected by clamping the cell at +120 mV rather than -70 mV, suggesting that Ca2+ removal processes are largely voltage independent. Also, little inward current was associated with Ca2+ clearance, indicating that Ca2+ removal does not involve an electrogenic process. 6. Our results suggest that Ca2+-induced Ca2+ release contributes little to the elevation of Ca2+ in these cells. The SR Ca2+ pump may contribute to Ca2+ removal over a low [Ca2+]i range in cells where [Ca2+]i remains high for long enough, while mitochondrial Ca2+ uptake may be important when [Ca2+]i is high.

An estimate of rapid cytoplasmic calcium buffering in a single smooth muscle cell

Cytoplasmic calcium increments in the absence of sarco (endo) plasmic reticulum function were measured with a low-affinity fluorophore Indo-1FF in single isolated smooth muscle cells from guinea-pig urinary bladder. To evaluate the Ca(2+)-buffering properties of the myoplasm, Ca2+ influx, measured as time integral of the Ica (integral of Ica), was compared with corresponding free Ca2+ increments (delta [Ca2+]i) in the cytoplasm. The ratio between integral of ICa and delta [Ca2+]i (integral Ica/delta [Ca2+]i), reflecting the Ca2+ buffering properties of the cytosol, was in the range of 4.9-9.3 pC/microM (mean 6.2 +/- 1.2, n = 12). It remained approximately constant (6.4 +/- 1.4 pC/microM, n = 8) during recordings lasting up to 25 min, suggesting that cytoplasmic Ca2+ binding does not change markedly during cell dialysis and that the endogenous Ca2+ buffer is not significantly washed out of the cell through the patch pipette. Wash-in or wash-out of BAPTA, a mobile high-affinity Ca2+ buffer, into or from the cell markedly changed the relationship between Ca2+ influx through Ca2+ channels and delta [Ca2+]i within minutes. Changes in integral of ICa/delta [Ca2+]i during the sequence of depolarizing steps, which increased free [Ca2+]i up to 5 microM, suggested lower limits for the apparent affinity of a rapid Ca2+ buffer (16 microM) and for the total buffer concentration (530 microM). Introduction of 4 mM DPTA (Kd for Ca2+ = 81 microM) into the cell more than doubled the total cytoplasmic Ca2+ buffer capacity. These results suggest that cytoplasmic Ca2+ buffer in smooth muscle cells has a low affinity for free Ca2+. The Ca(2+)-binding ratio of the cytoplasm in most cells was estimated to be between 30 and 40. The Ca(2+)-binding ratio did not differ markedly between cells isolated from neonatal (< or = 5 days) and adult animals.

Ca2+ removal mechanisms in rat cerebral resistance size arteries

Tissue blood flow and blood pressure are each regulated by the contractile behavior of resistance artery smooth muscle. Vascular diseases such as hypertension have also been attributed to changes in vascular smooth muscle function as a consequence of altered Ca2+ removal. In the present study of Ca2+ removal mechanisms, in dissociated single cells from resistance arteries using fura-2 microfluorimetry and voltage clamp, Ca2+ uptake by the sarcoplasmic reticulum and extrusion by the Ca2+ pump in the cell membrane were demonstrably important in regulating Ca2+. In contrast, the Na+-Ca2+ exchanger played no detectable role in clearing Ca2+. Thus a voltage pulse to 0 mV, from a holding potential of -70 mV, triggered a Ca2+ influx and increased intracellular Ca2+ concentration ([Ca2+]i). On repolarization, [Ca2+]i returned to the resting level. The decline in [Ca2+]i consisted of three phases. Ca2+ removal was fast immediately after repolarization (first phase), then plateaued (second phase), and finally accelerated just before [Ca2+]i returned to resting levels (third phase). Thapsigargin or ryanodine, which each inhibit Ca2+ uptake into stores, did not affect the first but significantly inhibited the third phase. On the other hand, Na+ replacement with choline+ did not affect either the phasic features of Ca2+ removal or the absolute rate of its decline. Ca2+ removal was voltage-independent; holding the membrane potential at 120 mV, rather than at -70 mV, after the voltage pulse to 0 mV, did not attenuate Ca2+ removal rate. These results suggest that Ca2+ pumps in the sarcoplasmic reticulum and the plasma membrane, but not the Na+-Ca2+ exchanger, are important in Ca2+ removal in cerebral resistance artery cells.

Carbachol-induced oscillations in membrane potential and [Ca2+]i in guinea-pig ileal smooth muscle cells

1. Cytosolic free Ca2+ concentration ([Ca2+]i) and membrane potential were simultaneously recorded from single smooth muscle cells of guinea-pig ileum, using a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques. 2. Carbachol (CCh, 2 microM) produced oscillatory changes in [Ca2+]i and membrane potential which coincided well in time with each other, and peaks of membrane potential oscillations reached a saturated level of around -7 mV. Thapsigargin (1 microM) abolished these effects of 2 microM CCh. La3+ (3 microM) immediately prevented the discharge of spike potentials, but allowed both on-going oscillatory responses to persist for a while. 3. CCh (0.25-0.75 microM) caused membrane potential and [Ca2+]i to oscillate in some 20 % of cells studied. Every membrane potential oscillation was preceded by the discharge of single or multiple spike potentials. The effects of CCh were readily abolished by La3+ (3 microM). 4. In cells exhibiting no oscillatory response to 0.25-0.75 microM CCh, an electrically evoked action potential usually generated changes in [Ca2+]i and membrane potential similar to those following spontaneously evoked action potentials, and sometimes it did so only after [Ca2+]i or InsP3 had been slightly elevated by repeatedly evoking action potentials or by increasing CCh concentration in the bath medium. 5. The results suggest that in ileal smooth muscle cells, the oscillations of [Ca2+]i and membrane potential arising from muscarinic stimulation result from release of Ca2+ from internal stores and that there is a Ca2+-induced potentiation of coincidently elicited cation channel openings. Under weak muscarinic stimulation, Ca2+ entry upon action potential discharge can trigger such a release of stored Ca2+, resulting in synchronous generation of a large rise in [Ca2+]i and a slow, large membrane depolarization.

Delayed rectifier K+ current of dog bronchial myocytes: effect of pollen sensitization and PKC activation

The properties of delayed rectifier K+ current [IK(dr)] of canine airway smooth muscle cells isolated from small bronchi and its modulation by protein kinase C (PKC) were studied by whole cell patch clamp. IK(dr) activated positive to -40 mV, with half-maximal activation at -16 +/- 1.2 mV (n = 15) and average current density of 31 +/- 2.6 pA/pF (n = 15) at +30 mV. The capacitive surface area, current density, and voltage dependence of activation of IK(dr) of myocytes of ragweed pollen-sensitized dogs were not different from age-matched control dogs. However, the sensitization reduced the availability of IK(dr) between -40 and -20 mV due to a hyperpolarizing shift in the voltage dependence of steady-state inactivation (-29.9 +/- 1.2 in sensitized versus -26.0 +/- 0.7 mV in control dogs, n = 9 and 11, respectively; P < 0.05). PKC activation with diacylglycerol analog or phorbol ester depressed IK(dr) amplitude, whereas an inactive diacylglycerol analog had no effect. The hyperpolarizing shift in voltage dependence of inactivation and/or modulation of IK(dr) by PKC may be two mechanisms that contribute to the enhanced reactivity of bronchial tissues from ragweed pollen-sensitized dogs.

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

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

Regulation of arterial diameter and wall [Ca2+] in cerebral arteries of rat by membrane potential and intravascular pressure

1. The regulation of intracellular [Ca2+] in the smooth muscle cells in the wall of small pressurized cerebral arteries (100-200 micron) of rat was studied using simultaneous digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Elevation of intravascular pressure (from 10 to 100 mmHg) caused a membrane depolarization from -63 +/- 1 to -36 +/- 2 mV, increased arterial wall [Ca2+] from 119 +/- 10 to 245 +/- 9 nM, and constricted the arteries from 208 +/- 10 micron (fully dilated, Ca2+ free) to 116 +/- 7 micron or by 45 % ('myogenic tone'). 3. Pressure-induced increases in arterial wall [Ca2+] and vasoconstriction were blocked by inhibitors of voltage-dependent Ca2+ channels (diltiazem and nisoldipine) or to the same extent by removal of external Ca2+. 4. At a steady pressure (i.e. under isobaric conditions at 60 mmHg), the membrane potential was stable at -45 +/- 1 mV, intracellular [Ca2+] was 190 +/- 10 nM, and arteries were constricted by 41 % (to 115 +/- 7 micron from 196 +/- 8 micron fully dilated). Under this condition of -45 +/- 5 mV at 60 mmHg, the voltage sensitivity of wall [Ca2+] and diameter were 7.5 nM mV-1 and 7.5 micron mV-1, respectively, resulting in a Ca2+ sensitivity of diameter of 1 mum nM-1. 5. Membrane potential depolarization from -58 to -23 mV caused pressurized arteries (to 60 mmHg) to constrict over their entire working range, i.e. from maximally dilated to constricted. This depolarization was associated with an elevation of arterial wall [Ca2+] from 124 +/- 7 to 347 +/- 12 nM. These increases in arterial wall [Ca2+] and vasoconstriction were blocked by L-type voltage-dependent Ca2+ channel inhibitors. 6. The relationship between arterial wall [Ca2+] and membrane potential was not significantly different under isobaric (60 mmHg) and non-isobaric conditions (10-100 mmHg), suggesting that intravascular pressure regulates arterial wall [Ca2+] through changes in membrane potential. 7. The results are consistent with the idea that intravascular pressure causes membrane potential depolarization, which opens voltage-dependent Ca2+ channels, acting as 'voltage sensors', thus increasing Ca2+ entry and arterial wall [Ca2+], which leads to vasoconstriction.

Inactivation of calcium-activated chloride channels in smooth muscle by calcium/calmodulin-dependent protein kinase

To determine the mechanisms responsible for the termination of Ca2+-activated Cl- currents (ICl(Ca)), simultaneous measurements of whole cell currents and intracellular Ca2+ concentration ([Ca2+]i) were made in equine tracheal myocytes. In nondialyzed cells, or cells dialyzed with 1 mM ATP, ICl(Ca) decayed before the [Ca2+]i decline, whereas the calcium-activated potassium current decayed at the same rate as [Ca2+]i. Substitution of AMP-PNP or ADP for ATP markedly prolonged the decay of ICl(Ca), resulting in a rate of current decay similar to that of the fall in [Ca2+]i. In the presence of ATP, dialysis of the calmodulin antagonist W7, the Ca2+/calmodulin-dependent kinase II (CaMKII) inhibitor KN93, or a CaMKII-specific peptide inhibitor the rate of ICl(Ca) decay was slowed and matched the [Ca2+]i decline, whereas H7, a nonspecific kinase inhibitor with low affinity for CaMKII, was without effect. When a sustained increase in [Ca2+]i was produced in ATP dialyzed cells, the current decayed completely, whereas in cells loaded with 5'-adenylylimidodiphosphate (AMP-PNP), KN93, or the CaMKII inhibitory peptide, ICl(Ca) did not decay. Slowly decaying currents were repeatedly evoked in ADP- or AMP-PNP-loaded cells, but dialysis of adenosine 5'-O-(3-thiotriphosphate) or okadaic acid resulted in a smaller initial ICl(Ca), and little or no current (despite a normal [Ca2+]i transient) with a second stimulation. These data indicate that CaMKII phosphorylation results in the inactivation of calcium-activated chloride channels, and that transition from the inactivated state to the closed state requires protein dephosphorylation.

Changes in intracellular Ca2+ concentration induced by L-type Ca2+ channel current in guinea pig gastric myocytes

We investigated the relationship between voltage-operated Ca2+ channel current and the corresponding intracellular Ca2+ concentration ([Ca2+]i) change (Ca2+ transient) in guinea pig gastric myocytes. Fluorescence microspectroscopy was combined with conventional whole cell patch-clamp technique, and fura 2 (80 microM) was added to CsCl-rich pipette solution. Step depolarization to 0 mV induced inward Ca2+ current (ICa) and concomitantly raised [Ca2+]i. Both responses were suppressed by nicardipine, an L-type Ca2+ channel blocker, and the voltage dependence of Ca2+ transient was similar to the current-voltage relation of ICa. When pulse duration was increased by up to 900 ms, peak Ca2+ transient increased and reached a steady state when stimulation was for longer. The calculated fast Ca2+ buffering capacity (B value), determined as the ratio of the time integral of ICa divided by the amplitude of Ca2+ transient, was not significantly increased after depletion of Ca2+ stores by the cyclic application of caffeine (10 mM) in the presence of ryanodine (4 microM). The addition of cyclopiazonic acid (CPA, 10 microM), a sarco(endo)plasmic reticulum Ca(2+)-ATPase inhibitor, decreased B value by approximately 20% in a reversible manner. When KCl pipette solution was used, Ca(2+)-activated K+ current [IK(Ca)] was also recorded during step depolarization. CPA sensitively suppressed the initial peak and oscillations of IK(Ca) with irregular effects on Ca2+ transients. The above results suggest that, in guinea pig gastric myocyte, Ca2+ transient is tightly coupled to ICa during depolarization, and global [Ca2+]i is not significantly affected by Ca(2+)-induced Ca2+ release from sarcoplasmic reticulum during depolarization.

Characterization of action potential-triggered [Ca2+]i transients in single smooth muscle cells of guinea-pig ileum

1. To characterize increases in cytosolic free Ca2+ concentration ([Ca2+]i) associated with discharge of action potentials, membrane potential and [Ca2+]i were simultaneously recorded from single smooth muscle cells of guinea-pig ileum by use of a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques. 2. A single action potential in response to a depolarizing current pulse elicited a transient rise in [Ca2+]i. When the duration of the current pulse was prolonged, action potentials were repeatedly discharged during the early period of the pulse duration with a progressive decrease in overshoot potential, upstroke rate and repolarization rate. However, such action potentials could each trigger [Ca2+]i transients with an almost constant amplitude. 3. Nicardipine (1 microM) and La3+ (10 microM), blockers of voltage-dependent Ca2+ channels (VDCCs), abolished both the action potential discharge and the [Ca2+]i transient. 4. Charybdotoxin (ChTX, 300 nM) and tetraethylammonium (TEA, 2 mM), blockers of large conductance Ca2+-activated K+ channels, decreased the rate of repolarization of action potentials but increased the amplitude of [Ca2+]i transients. 5. Thapsigargin (1 microM), an inhibitor of SR Ca2+-ATPase, slowed the falling phase and somewhat increased the amplitude, of action potential-triggered [Ca2+]i transients without affecting action potentials. In addition. in voltage-clamped cells, the drug had little effect on the voltage step-evoked Ca2+ current but exerted a similar effect on its concomitant rise in [Ca2+]i to that on the action potential-triggered [Ca2+]i transient. 6. Similar action potential-triggered [Ca2+]i transients were induced by brief exposures to high-K+ solution. They were not decreased, but rather increased, after depletion of intracellular Ca2+ stores by a combination of ryanodine (30 microM) and caffeine (10 mM) through an open-lock of Ca2+-induced Ca2+ release (CICR)-related channels. 7. The results show that action potentials, discharged repeatedly during the early period of a long membrane depolarization, undergo a progressive change in configuration but can each trigger a constant rise in [Ca2+]i. Intracellular Ca2+ stores have a role, especially in accelerating the falling phase of the action potential-triggered [Ca2+]i transients by replenishing cytosolic Ca2+. No evidence was provided for the involvement of CICR in the action potential-triggered [Ca2+]i transient.

Muscarinic signaling pathway for calcium release and calcium-activated chloride current in smooth muscle

We investigated the muscarinic activation of Ca(2+)-activated Cl- currents [ICl(Ca)] in voltage-clamped equine tracheal myocytes. The threshold of cytosolic free Ca2+ concentration ([Ca2+]i) required for activation of ICl(Ca) was 202 +/- 22 nM, and full activation of the current occurred at 771 +/- 31 nM. Hexahydro-sila-difenidol (M3 antagonist) inhibited the methacholine-induced phasic [Ca2+]i increase and ICl(Ca) in a concentration-dependent manner, whereas methoctramine (M2 antagonist) only slightly attenuated the [Ca2+]i increase and ICl(Ca) (14.8 and 21.4%, respectively), consistent with incomplete selectivity. Dialysis of heparin (10 mg/ml) blocked methacholine-induced [Ca2+]i and ICl(Ca) but had no effect on the caffeine-induced Ca2+ release or ICl(Ca); inositol 1,4,5-trisphosphate (100 microM) induced ICl(Ca) and blocked the methacholine current. Conversely, ruthenium red (50 microM) prevented the caffeine-induced [Ca2+]i release and ICl(Ca) but had no effect on methacholine-induced [Ca2+]i or current. Intracellular dialysis of the calmodulin antagonist N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7, 500 microM) or the Ca2+/calmodulin-dependent protein kinase inhibitor KN93 (5 microM) had no effect on the [Ca2+]i increase or ICl(Ca). Pertussis toxin (0.5 mg/ml) did not affect the increase in [Ca2+]i or ICl(Ca). Dialysis with antibodies directed against the alpha-subunit of Gq/G11 (Gq alpha/ G alpha 11) blocked the methacholine-induced ICl(Ca) in a concentration-dependent manner, whereas anti-G alpha i-1/G alpha 1-2 antibodies (1:35) and anti-G alpha i-3/G(o) alpha antibodies (1:35) were without effect. The results indicate that stimulation of phospholipase C via M3/Gq proteins is the predominant signaling pathway for the activation of ICl(Ca); at high agonist concentrations, Ca(2+)-induced Ca2+ release does not appear to play a prominent role in muscarinic signaling.

M2 receptor activation of nonselective cation channels in smooth muscle cells: calcium and Gi/G(o) requirements

Muscarinic stimulation of fura 2-loaded smooth muscle cells evoked a rapidly inactivating Ca(2+)-activated Cl- current [ICl(Ca)] and a sustained nonselective cation current (Icat) as well as a transient (delta Ca(tran)) and a sustained (delta Ca(sus)) elevation of cytosolic Ca2+ concentration ([Ca2+]i). Caffeine and inositol 1,4,5-trisphosphate induced delta Ca(tran) and ICl(Ca) but not Icat or delta Ca(sus). M2 receptor antagonism blocked muscarinic activation of Icat and delta Ca(sus) but not ICl(Ca) and delta Ca(tran). M3 antagonism blocked activation of ICl(Ca) and Icat and a rise in [Ca2+]i, but application of caffeine with methacholine restored Icat and delta Ca(sus). After depletion of intracellular Ca2+ stores, methacholine failed to induce Icat or a [Ca2+]i increase and, in pertussis toxin-treated cells, ICl(Ca) and delta Ca(tran) but not Icat or delta Ca(sus) were evoked. Anti-G alpha i-1/G alpha i-2 antibodies and anti-G alpha i-3/ G(o) alpha antibodies blocked Icat but did not affect ICl(Ca). Anti-Gq alpha/ G alpha 11 antibodies greatly inhibited ICl(Ca) but did not affect Icat. Activation of M2 receptors leads to the opening of nonselective cation channels through Gi/G(o) proteins in smooth muscle cells, resulting in a sustained rise in [Ca2+]i. Arise in [Ca2+]i is necessary but not sufficient for activation of nonselective cation channels.

Ca(2+)-activated Cl- currents are activated by metabolic inhibition in rat pulmonary artery smooth muscle cells

We report the electrophysiological and functional properties of Ca(2+)-activated Cl- currents [ICl(Ca)] in rat pulmonary artery smooth muscle and the activation of these currents by the metabolic inhibitor cyanide. Caffeine and norepinephrine (NE) evoked both Ca(2+)-activated K+ currents [IK(Ca)] and ICl(Ca) currents in voltage-clamped myocytes (-50 mV). Niflumic acid (10 microM) reduced the caffeine-induced ICl(Ca) by approximately 64% and reversibly reduced NE-induced tension. Exposure of myocytes to cyanide (2-10 mM) induced a slowly developing inward current (-50 mV) in physiological and K(+)-free solutions, which was identified as ICl(Ca) on the basis of ion selectivity and Ca2+ dependence. Cyanide elevated cytosolic Ca2+ concentration, and this elevation was markedly inhibited by preexposure to caffeine and slightly inhibited by nisoldipine. During exposure to caffeine, the Ca(2+)-activated K+ current was also augmented. Cyanide markedly prolonged ICl(Ca) activated by caffeine, increasing the half-decay time from 3.5 (control) to 29 s (cyanide); the half-decay time of the caffeine-induced IK(Ca) was not significantly affected by cyanide. The results indicate that metabolic inhibition increases [Ca2+]i and activates a prolonged, depolarizing Cl- current in pulmonary artery myocytes.

Regulation of the cytosolic Ca2+ concentration by Ca2+ stores in single smooth muscle cells from rat cerebral arteries

1. There is no general agreement on the presence or role of Ca(2+)-induced Ca2+ release in smooth muscle. In this paper, Ca(2+)-induced Ca2+ release has been investigated in rat resistance-sized superior cerebral arteries to determine its role in regulating the cytosolic Ca2+ concentration ([Ca2+]i). 2. Pressurized superior cerebral arteries developed spontaneous oscillations in diameter. These oscillations were abolished by ryanodine (an inhibitor of Ca(2+)-induced Ca2+ release) and removal of extracellular Ca2+. This suggests, indirectly, that Ca(2+)-induced Ca2+ release may regulate [Ca2+]i in the resistance arteries. 3. To determine if Ca(2+)-induced Ca2+ release could regulate [Ca2+]i, single smooth muscle cells were isolated from the superior cerebral artery, voltage clamped in the whole cell configuration and high temporal resolution [Ca2+]i measurements made. The relationship between the Ca2+ current (ICa) and rise in [Ca2+]i was examined. 4. Depolarization triggered ICa and increased [Ca2+]i. The time course of the measured increase in [Ca2+]i closely followed the increase in [Ca2+]i expected from the time-integrated ICa, although about 140-fold more Ca2+ entered the cytosol than appeared as free Ca2+. When the cells were dialysed with ryanodine (30 microM), the Ca2+ transient evoked by the ICa was substantially reduced indicating that Ca2+ influx triggered Ca2+ release from an internal store. 5. Voltage pulses to negative membrane potentials were more effective in triggering Ca2+ release than pulses to positive potentials suggesting that the Ca(2+)-induced Ca2+ release was voltage dependent. However, the release of Ca2+ from the internal store triggered by caffeine was voltage independent. These results suggest that the voltage dependence of Ca2+ release is indirect and possibly related to the plasmalemma unitary Ca2+ current magnitude. 6. The results establish that Ca(2+)-induced Ca2+ release contributes to depolarization-evoked increases in [Ca2+]i in rat resistance-sized superior cerebral arteries over the physiological [Ca2+]i range (100-200 nM). Compared with more positive membrane potentials the efficacy of Ca2+ in triggering release is high at physiological membrane potentials.

Oscillatory Cl- current induced by angiotensin II in rat pulmonary arterial myocytes: Ca2+ dependence and physiological implication

We have previously reported that angiotensin II (ANG II) induces oscillations in the cytoplasmic calcium concentration ([Ca2+]i) of pulmonary vascular myocytes. The present work was undertaken to investigate the effect of ANG II in comparison with ATP and caffeine on membrane currents and to explore the relation between these membrane currents and [Ca2+]i. In cells clamped at -60 mV, ANG II (10 microM) or ATP (100 microM) induced an oscillatory inward current. Caffeine (5 mM) induced only one transient inward current. In control conditions, the reversal potential (Erev) of these currents was close to the equilibrium potential for Cl- ions (Ecl = -2.1 mV) and was shifted towards more positive values in low-Cl- solutions. Niflumic acid (10-50 microM) and DIDS (0.25-1 mM) inhibited this inward current. Combined recordings of membrane current and [Ca2+]i by indo-1 microspectrofluorimetry revealed that ANG II- and ATP-induced currents occurred simultaneously with oscillations in [Ca2+]i whereas the caffeine-induced current was accompanied by only one transient increase in [Ca2+]i. Niflumic acid (25 microM) had no effect on agonist-induced [Ca2+]i responses, whereas thapsigargin (1 microM) abolished both membrane current and the [Ca2+]i response. Heparin (5 mg/ml in the pipette solution) inhibited both [Ca2+]i responses and membrane currents induced by ANG II and ATP, but not by caffeine. In pulmonary arterial strips, ANG II-induced contraction was inhibited by niflumic acid (25 microM) or nifedipine (1 microM) to the same extent and the two substances did not have an additive effect. This study demonstrates that, in pulmonary vascular smooth muscle, ANG II, as well as ATP, activate an oscillatory calcium dependent chloride current which is triggered by cyclic increases in [Ca2+]i and that both oscillatory phenomena are primarily IP3-mediated. It is suggested that ANG II-induced oscillatory chloride current could depolarise the cell membrane leading to activation of voltage-operated Ca2+ channels. The resulting Ca2+ influx contributes to the component of ANG II-induced contraction that is equally sensitive to chloride or calcium channel blockade.

T-type and L-type Ca2+ currents in canine bronchial smooth muscle: characterization and physiological roles

We examined the voltage-dependent Ca2+ currents in freshly dissociated smooth muscle cells obtained from canine bronchi (3rd to 5th order). When cells were depolarized from -40 mV, we observed an inward current that 1) exhibited threshold and peak activation at approximately -35 mV and +10 mV, respectively; 2) inactivated slowly with half-inactivation at -20 mV; 3) deactivated rapidly (time constant < 1 ms) upon repolarization; and 4) was abolished by nifedipine and suppressed by cholinergic agonist. These characteristics are typical of L-type Ca2+ current. During depolarization from -70 or -80 mV, however, many cells exhibited a second inward current superimposed on the L-type Ca2+ current. Activation of this other current was first noted at -60 mV, was maximal at approximately -20 mV, and was very rapid (reaching a peak within 10 ms). Inactivation of the other current was also rapid (time constant approximately 3 ms) and half-maximal at approximately -70 mV. There was a persistent "window" current over the physiologically relevant range of potentials (i.e., -60 to -30 mV). This current was also sensitive to nifedipine (although less so than the L-type current) and to Ni2+, but not to cholinergic agonist. Finally, the tail currents evoked upon repolarization to the holding potential decayed approximately 10 times more slowly than did L-type tail currents. These characteristics are all typical of T-type Ca2+ current. We conclude that there is a prominent T-type Ca2+ current in canine bronchial smooth muscle; this current may play a central role in excitation-contraction coupling, in refilling of the internal Ca2+ pool, and in electrical slow waves. Because airflow resistance is determined primarily by the smaller airways and not the trachea, these findings may have important implications with respect to airway physiology and the mechanisms underlying airway hyperreactivity and asthma.

Modulation of maxi-K+ channels by voltage-dependent Ca2+ channels and methacholine in single airway myocytes

The role of Ca2+ influx through voltage-dependent Ca2+ channels and the inhibitory effects of methacholine on large-conductance Ca2+-activated K+ (K(Ca)) channels (maxi-K+ channels) were studied in voltage-clamped (nystatin), fura 2-loaded airway smooth muscle cells. Spontaneous transient outward currents (STOCs) were strongly coupled to voltage-dependent Ca2+ channel activity; activity was suppressed by nisoldipine and Cd2+ and increased by BAY K 8644 within seconds. Moreover, release of intracellular Ca2+ by caffeine or cyclopiazonic acid only partially suppressed STOCs, and the remainder were almost completely blocked by nisoldipine. Methacholine suppressed STOCs but also significantly decreased the mean outward current. Whole cell current inhibition was observed in the presence of 4-aminopyridine but not in the presence of charybdotoxin. Caffeine inhibited STOCs but macroscopic outward currents were not altered. In the continued presence of caffeine, methacholine abolished the remaining STOCs and decreased the mean K+ current. We conclude that STOCs are activated by influx of Ca2+ through plasmalemmal voltage-dependent Ca2+ channels, as well as by release of Ca2+ from intracellular stores, and muscarinic stimulation depresses the mean K(Ca) current via a pathway independent of the depletion of intracellular Ca2+ stores.

Muscarinic activation and calcium permeation of nonselective cation currents in airway myocytes

We examined the activation and Ca2+ permeation of nonselective cation channels in voltage-clamped (nystatin), fura 2-loaded equine tracheal myocytes at 35 degrees C. Methacholine (50 microM) induced a biphasic increase in intracellular Ca2+ concentration ([Ca2+]i) and a biphasic inward current consisting of a large, rapidly inactivating Ca(2+)-activated Cl current [ICl(Ca)] and a smaller, sustained nonselective cation current (Icat) ICl(Ca) but not Icat was activated by caffeine. Neither Icat nor the sustained rise in [Ca2+]i was blocked by nisoldipine, whereas both were rapidly blocked by Ni2+; Icat was determined to be Ca2+ permeant, since 1) a sustained elevation of [Ca2+]i occurred when Icat was activated, and blockade of Icat produced a rapid decline in [Ca2+]i; 2) increasing extracellular Ca2+ during Icat increased [Ca2+]i; 3) 110 mM extracellular Ca2+ shifted the reversal potential of Icat to 12 mV (Ca(2+)-to-Cs+ permeability ratio = 3.6); and 4) instantaneous voltage-clamp steps to negative potentials during Icat increased the current and [Ca2+]i, whereas depolarizing steps decreased the current and [Ca2+]i. The fraction of Icat carried by Ca2+ under physiological conditions was estimated to be 14% at -60 mV.

The amount of acetylcholine mobilisable Ca2+ in single smooth muscle cells measured with the exogenous cytoplasmic Ca2+ buffer, Indo-1

Single smooth muscle cells from guinea pig urinary bladder were voltage clamped with patch electrodes containing 1 mM Indo-1. As Indo-1 entered the cell, delta[Ca2+]i in response to Ca2+ influx with ICa (1 s steps to -10 mV) was progressively decreased. delta F410 was used as a measure of the Ca2+ amount bound to Indo-1. Within less than 2 min after establishment of the whole-cell configuration, the fraction of Ca2+ entering the cell with ICa which binds to Indo-1 became constant, suggesting that Indo-1 completely overrides the endogenous Ca2+ buffers. Under these conditions, delta F410 was satisfactorily fitted with the time integral of ICa during 1 s long steps. Acetylcholine (ACh, 50 microM) was rapidly applied to Indo-1 loaded cells to induce IP3-induced Ca2+ release (IICR), which peaked within about 1 s. From delta F410 in response to ICa and ACh and from the time integral of ICa the amount of Ca2+ released during IICR was estimated to be 680 attomole (680 x 10(-18) mole), corresponding to 230 microM for 3 pl of accessible cytoplasmic volume.