Inositol 1,4,5-trisphosphate- and ryanodine-sensitive Ca2+ release channel-dependent Ca2+ signalling in rat portal vein myocytes

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

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

KEY POINTS

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

ABSTRACT

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

Presenilin 1 mutation decreases both calcium and contractile responses in cerebral arteries

Mutations or upregulation in presenilin 1 (PS1) gene are found in familial early-onset Alzheimer's disease or sporadic late-onset Alzheimer's disease, respectively. PS1 has been essentially studied in neurons and its mutation was shown to alter intracellular calcium (Ca²⁺) signals. Here, we showed that PS1 is expressed in smooth muscle cells (SMCs) of mouse cerebral arteries, and we assessed the effects of the deletion of exon 9 of PS1 (PS1dE9) on Ca²⁺ signals and contractile responses of vascular SMC. Agonist-induced contraction of cerebral vessels was significantly decreased in PS1dE9 both in vivo and ex vivo. Spontaneous activity of Ca²⁺ sparks through ryanodine-sensitive channels (RyR) was unchanged, whereas the RyR-mediated Ca²⁺-release activated by caffeine was shorter in PS1dE9 SMC when compared with control. Moreover, PS1dE9 mutation decreased the caffeine-activated capacitive Ca²⁺ entry, and inhibitors of SERCA pumps reversed the effects of PS1dE9 on Ca²⁺ signals. PS1dE9 mutation also leads to the increased expression of SERCA3, phospholamban, and RyR3. These results show that PS1 plays a crucial role in the cerebrovascular system and the vascular reactivity is decreased through altered Ca²⁺ signals in PS1dE9 mutant mice.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

Evoked centripetal Ca(2+) mobilization in cardiac Purkinje cells: insight from a model of three Ca(2+) release regions

Despite strong suspicion that abnormal Ca(2+) handling in Purkinje cells (P-cells) is implicated in life-threatening forms of ventricular tachycardias, the mechanism underlying the Ca(2+) cycling of these cells under normal conditions is still unclear. There is mounting evidence that P-cells have a unique Ca(2+) handling system. Notably complex spontaneous Ca(2+) activity was previously recorded in canine P-cells and was explained by a mechanistic hypothesis involving a triple layered system of Ca(2+) release channels. Here we examined the validity of this hypothesis for the electrically evoked Ca(2+) transient which was shown, in the dog and rabbit, to occur progressively from the periphery to the interior of the cell. To do so, the hypothesis was incorporated in a model of intracellular Ca(2+) dynamics which was then used to reproduce numerically the Ca(2+) activity of P-cells under stimulated conditions. The modelling was thus performed through a 2D computational array that encompassed three distinct Ca(2+) release nodes arranged, respectively, into three consecutive adjacent regions. A system of partial differential equations (PDEs) expressed numerically the principal cellular functions that modulate the local cytosolic Ca(2+) concentration (Cai). The apparent node-to-node progression of elevated Cai was obtained by combining Ca(2+) diffusion and 'Ca(2+)-induced Ca(2+) release'. To provide the modelling with a reliable experimental reference, we first re-examined the Ca(2+) mobilization in swine stimulated P-cells by 2D confocal microscopy. As reported earlier for the dog and rabbit, a centripetal Ca(2+) transient was readily visible in 22 stimulated P-cells from six adult Yucatan swine hearts (pacing rate: 0.1 Hz; pulse duration: 25 ms, pulse amplitude: 10% above threshold; 1 mm Ca(2+); 35°C; pH 7.3). An accurate replication of the observed centripetal Ca(2+) propagation was generated by the model for four representative cell examples and confirmed by statistical comparisons of simulations against cell data. Selective inactivation of Ca(2+) release regions of the computational array showed that an intermediate layer of Ca(2+) release nodes with an ~30-40% lower Ca(2+) activation threshold was required to reproduce the phenomenon. Our computational analysis was therefore fully consistent with the activation of a triple layered system of Ca(2+) release channels as a mechanism of centripetal Ca(2+) signalling in P-cells. Moreover, the model clearly indicated that the intermediate Ca(2+) release layer with increased sensitivity for Ca(2+) plays an important role in the specific intracellular Ca(2+) mobilization of Purkinje fibres and could therefore be a relevant determinant of cardiac conduction.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

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.

Spontaneous electrical and Ca2+ signals in the mouse renal pelvis that drive pyeloureteric peristalsis

1. Peristalsis in the smooth muscle cell (SMC) wall of the pyeloureteric system is unique in physiology in that the primary pacemaker resides in a population of atypical SMCs situated near the border of the renal papilla. 2. Atypical SMCs display high-frequency Ca(2+) transients upon the spontaneous release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-dependent stores that trigger cation-selective spontaneous transient depolarizations (STDs). In the presence of nifedipine, these Ca(2+) transients and STDs seldom propagate > 100 mum. Synchronization of STDs in neighbouring atypical SMCs into an electrical signal that can trigger action potential discharge and contraction in the typical SMC layer involves a coupled oscillator mechanism dependent on Ca(2+) entry through L-type voltage-operated Ca(2+) channels. 3. A population of spindle- or stellate-shaped cells, immunopositive for the tyrosine receptor kinase kit, is sparsely distributed throughout the pyeloureteric system. In addition, Ca(2+) transients and action potentials of long duration occurring at low frequencies have been recorded in a population of fusiform cells, which we have termed interstitial cells of Cajal (ICC)-like cells. 4. The electrical and Ca(2+) signals in ICC-like cells are abolished upon blockade of Ca(2+) release from either IP(3)- or ryanodine-dependent Ca(2+) stores. However, the spontaneous Ca(2+) signals in atypical SMCs or ICC-like cells are little affected in W/W(-v) transgenic mice, which have extensive lesions of their intestinal ICC networks. 5. In summary, we have developed a model of pyeloureteric pacemaking in which atypical SMCs are indeed the primary pacemakers, but the function of ICC-like cells has yet to be determined.

Protein kinase C-epsilon regulates local calcium signaling in airway smooth muscle cells

Protein kinase C (PKC) is known to regulate ryanodine receptor (RyR)-mediated local Ca(2+) signaling (Ca(2+) spark) in airway and vascular smooth muscle cells (SMCs), but its specific molecular mechanisms and functions still remain elusive. In this study, we reveal that, in airway SMCs, specific PKCepsilon peptide inhibitor and gene deletion significantly increased the frequency of Ca(2+) sparks, and decreased the amplitude of Ca(2+) sparks in the presence of xestospogin-C to eliminate functional inositol 1,4,5-triphosphate receptors. PKCepsilon activation with phorbol-12-myristate-13-acetate significantly decreased Ca(2+) spark frequency and increased Ca(2+) spark amplitude. The effect of PKCepsilon inhibition or activation on Ca(2+) sparks was completely lost in PKCepsilon(-/-) cells. PKCepsilon inhibition or PKCepsilon activation was unable to affect Ca(2+) sparks in RyR1(-/-) and RyR1(+/-) cells. Modification of RyR2 activity by FK506-binding protein 12.6 homozygous or RyR2 heterozygous gene deletion did not prevent the effect of PKCepsilon inhibition or activation. RyR3 homogenous gene deletion did not block the effect of PKCepsilon inhibition and activation, either. PKCepsilon inhibition promotes agonist-induced airway muscle contraction, whereas PKCepsilon activation produces an opposite effect. Taken together, these results indicate that PKCepsilon regulates Ca(2+) sparks by specifically interacting with RyR1, which plays an important role in the control of contractile responses in airway SMCs.

Role of Ca2+ entry and Ca2+ stores in atypical smooth muscle cell autorhythmicity in the mouse renal pelvis

BACKGROUND AND PURPOSE

Electrically active atypical smooth muscle cells (ASMCs) within the renal pelvis have long been considered to act as pacemaker cells driving pelviureteric peristalsis. We have investigated the role of Ca2+ entry and uptake into and release from internal stores in the generation of Ca2+ transients and spontaneous transient depolarizations (STDs) in ASMCs.

EXPERIMENTAL APPROACH

The electrical activity and separately visualized changes in intracellular Ca2+ concentration in typical smooth muscle cells (TSMCs), ASMCs and interstitial cells of Cajal-like cells (ICC-LCs) were recorded using intracellular microelectrodes and a fluorescent Ca2+ indicator, fluo-4.

RESULTS

In 1 microM nifedipine, high frequency (10-30 min(-1)) Ca2+ transients and STDs were recorded in ASMCs, while ICC-LCs displayed low frequency (1-3 min(-1)) Ca2+ transients. All spontaneous electrical activity and Ca2+ transients were blocked upon removal of Ca2+ from the bathing solution, blockade of Ca2+ store uptake with cyclopiazonic acid (CPA) and with 2-aminoethoxy-diphenylborate (2-APB). STD amplitudes were reduced upon removal of the extracellular Na+ or blockade of IP3 dependent Ca2+ store release with neomycin or U73122. Blockade of ryanodine-sensitive Ca2+ release blocked ICC-LC Ca2+ transients but only reduced Ca2+ transient discharge in ASMCs. STDs in ASMCS were also little affected by DIDS, La3+, Gd3+ or by the replacement of extracellular Cl(-) with isethionate.

CONCLUSIONS

ASMCs generated Ca2+ transients and cation-selective STDs via mechanisms involving Ca2+ release from IP3-dependent Ca2+ stores, STD stimulation of TSMCs was supported by Ca2+ entry through L type Ca2+ channels and Ca2+ release from ryanodine-sensitive stores.

Sub-plasmalemmal [Ca2+]i upstroke in myocytes of the guinea-pig small intestine evoked by muscarinic stimulation: IP3R-mediated Ca2+ release induced by voltage-gated Ca2+ entry

Membrane depolarization triggers Ca²⁺ release from the sarcoplasmic reticulum (SR) in skeletal muscles via direct interaction between the voltage-gated L-type Ca²⁺ channels (the dihydropyridine receptors; VGCCs) and ryanodine receptors (RyRs), while in cardiac muscles Ca²⁺ entry through VGCCs triggers RyR-mediated Ca²⁺ release via a Ca²⁺-induced Ca²⁺ release (CICR) mechanism. Here we demonstrate that in phasic smooth muscle of the guinea-pig small intestine, excitation evoked by muscarinic receptor activation triggers an abrupt Ca²⁺ release from sub-plasmalemmal (sub-PM) SR elements enriched with inositol 1,4,5-trisphosphate receptors (IP₃Rs) and poor in RyRs. This was followed by a lesser rise, or oscillations in [Ca²⁺]_i. The initial abrupt sub-PM [Ca²⁺]_i upstroke was all but abolished by block of VGCCs (by 5 μM nicardipine), depletion of intracellular Ca²⁺ stores (with 10 μM cyclopiazonic acid) or inhibition of IP₃Rs (by 2 μM xestospongin C or 30 μM 2-APB), but was not affected by block of RyRs (by 50–100 μM tetracaine or 100 μM ryanodine). Inhibition of either IP₃Rs or RyRs attenuated phasic muscarinic contraction by 73%. Thus, in contrast to cardiac muscles, excitation–contraction coupling in this phasic visceral smooth muscle occurs by Ca²⁺ entry through VGCCs which evokes an initial IP₃R-mediated Ca²⁺ release activated via a CICR mechanism.

Protein kinase C regulates vascular myogenic tone through activation of TRPM4

Myogenic vasoconstriction results from pressure-induced vascular smooth muscle cell depolarization and Ca(2+) influx via voltage-dependent Ca(2+) channels, a process that is significantly attenuated by inhibition of protein kinase C (PKC). It was recently reported that the melastatin transient receptor potential (TRP) channel TRPM4 is a critical mediator of pressure-induced smooth muscle depolarization and constriction in cerebral arteries. Interestingly, PKC activity enhances the activation of cloned TRPM4 channels expressed in cultured cells by increasing sensitivity of the channel to intracellular Ca(2+). Thus we postulated that PKC-dependent activation of TRPM4 might be a critical mediator of vascular myogenic tone. We report here that PKC inhibition attenuated pressure-induced constriction of cerebral vessels and that stimulation of PKC activity with phorbol 12-myristate 13-acetate (PMA) enhanced the development of myogenic tone. In freshly isolated cerebral artery myocytes, we identified a Ca(2+)-dependent, rapidly inactivating, outwardly rectifying, iberiotoxin-insensitive cation current with properties similar to those of expressed TRPM4 channels. Stimulation of PKC activity with PMA increased the intracellular Ca(2+) sensitivity of this current in vascular smooth muscle cells. To validate TRPM4 as a target of PKC regulation, antisense technology was used to suppress TRPM4 expression in isolated cerebral arteries. Under these conditions, the magnitude of TRPM4-like currents was diminished in cells from arteries treated with antisense oligonucleotides compared with controls, identifying TRPM4 as the molecular entity responsible for the PKC-activated current. Furthermore, the extent of PKC-induced smooth muscle cell depolarization and vasoconstriction was significantly decreased in arteries treated with TRPM4 antisense oligonucleotides compared with controls. We conclude that PKC-dependent regulation of TRPM4 activity contributes to the control of cerebral artery myogenic tone.

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.

Activation of the cGMP/PKG pathway inhibits electrical activity in rabbit urethral interstitial cells of Cajal by reducing the spatial spread of Ca2+ waves

In the present study we used a combination of patch clamping and fast confocal Ca2+ imaging to examine the effects of activators of the nitric oxide (NO)/cGMP pathway on pacemaker activity in freshly dispersed ICC from the rabbit urethra, using the amphotericin B perforated patch configuration of the patch-clamp technique. The nitric oxide donor, DEA-NO, the soluble guanylyl cyclase activator YC-1 and the membrane-permeant analogue of cGMP, 8-Br-cGMP inhibited spontaneous transient depolarizations (STDs) and spontaneous transient inward currents (STICs) recorded under current-clamp and voltage-clamp conditions, respectively. Caffeine-evoked Cl- currents were unaltered in the presence of SP-8-Br-PET-cGMPs, suggesting that activation of the cGMP/PKG pathway does not block Cl- channels directly or interfere with Ca2+ release via ryanodine receptors (RyR). However, noradrenaline-evoked Cl- currents were attenuated by SP-8-Br-PET-cGMPs, suggesting that activation of cGMP-dependent protein kinase (PKG) may modulate release of Ca2+ via IP3 receptors (IP3R). When urethral interstitial cells (ICC) were loaded with Fluo4-AM (2 microm), and viewed with a confocal microscope, they fired regular propagating Ca2+ waves, which originated in one or more regions of the cell. Application of DEA-NO or other activators of the cGMP/PKG pathway did not significantly affect the oscillation frequency of these cells, but did significantly reduce their spatial spread. These effects were mimicked by the IP3R blocker, 2-APB (100 microm). These data suggest that NO donors and activators of the cGMP pathway inhibit electrical activity of urethral ICC by reducing the spatial spread of Ca2+ waves, rather than decreasing wave frequency.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

Calcium events in smooth muscles and their interstitial cells; physiological roles of sparks

The observation of spontaneous sporadic releases of packets of stored calcium made 20 years ago has opened up a number of new concepts in smooth muscle physiology: (1) the calcium release sites are ryanodine and inositol 1,4,5-trisphosphate (IP3) receptor channels which contribute to cell-wide increases in [Ca2+]i in response to cell depolarization, activation of IP3-generating receptors, or other stimuli; (2) changes in [Ca2+]i act back on the cell membrane to activate or modulate K+, Cl- and cation channel activity so affecting contraction, in arterial smooth muscle for example affecting blood pressure; (3) IP3 production is voltage dependent and is believed to contribute to pacemaker potentials and to refractory periods which control the rhythmical motility of many hollow organs. Most smooth muscle tissues contain interstitial cells (ICs) in addition to contractile smooth muscle cells (SMCs). The interactions of these internal mechanisms, and in turn the interactions of SMCs and ICs in various smooth muscle tissues, are major factors in determining the unique physiological profiles of individual smooth muscles.

Mechanisms underlying the inhibitory effects induced by pituitary adenylate cyclase-activating peptide in mouse ileum

The aim of this study was to investigate the signal transduction mechanisms underlying the inhibitory effect induced by pituitary adenylate cyclase activating peptide (PACAP-27) on the spontaneous contractile activity of longitudinal muscle of mouse ileum. Mechanical activity of ileal segments was recorded isometrically in vitro. PACAP-27 produced apamin-sensitive reduction of the amplitude of the spontaneous contractions. 9-(Tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22,536), adenylate cyclase inhibitor, or genistein and tyrphostin 25, tyrosine kinase inhibitors, had negligible effects on PACAP-27-induced inhibition. PACAP-27 effects were significantly inhibited by U-73122, phopholipase C (PLC) inhibitor, by 2-aminoethoxy-diphenylborate (2-APB), permeable blocker of inositol 1,4,5-triphosphate (IP3) receptors and by depletion of Ca2+ stores with cyclopiazonic acid or thapsigargin. Ryanodine did not reduce PACAP-27-inhibitory responses. We suggest that, in mouse ileum, the inhibitory responses to PACAP-27 involve stimulation of PLC, increased production of IP3 and localised Ca2+ release from intracellular stores, which could provide the opening of apamin-sensitive Ca2+-dependent K+ channels.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Differential functional properties of Ca2+ stores in pulmonary arterial conduit and resistance myocytes

In smooth muscle cells, oscillations of intracellular Ca2+ concentration ([Ca2+]i) are controlled by inositol 1,4,5-trisphosphate (InsP3) and ryanodine (Ry) receptors on the sarcoplasmic reticulum (SR). Here we show that these Ca2+ oscillations are regulated differentially by InsP3 and Ry receptors in cells dispersed from the main trunk of the pulmonary artery (conduit myocytes) or from tertiary and quaternary arterial branches (resistance myocytes). Ry receptor antagonists inhibit either spontaneous or ATP-induced Ca2+ oscillations in resistance myocytes but they do not affect the oscillations in most conduit myocytes. In contrast, agents that inhibit InsP3 production or activation of InsP3 receptors do not alter the oscillations is resistance myocytes but block them in conduit myocytes. We have also examined the degree of overlap of Ry- and InsP3-sensitive stores in myocytes along the pulmonary arterial tree. In conduit myocytes, depletion of Ry-sensitive stores with repeated application of caffeine in the presence of Ry or in Ca2+ free solutions did not prevent the ATP-induced Ca2+ release from InsP3-dependent stores. However, responsiveness to ATP was completely abolished in resistance myocytes subjected to the same experimental protocol. Thus, InsP3- and Ry-dependent stores appear to be separated in conduit myocytes but joined in resistance myocytes. These data demonstrate for the first time differential properties of intracellular Ca2+ stores and receptors in myocytes distributed along the pulmonary arterial tree and help to explain the distinct functional responses of large and small pulmonary vessels to vasoactive agents.

Calcium oscillations in interstitial cells of the rabbit urethra

Measurements were made (using fast confocal microscopy) of intracellular Ca2+ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra. These cells exhibited regular Ca2+ oscillations which were associated with spontaneous transient inward currents recorded under voltage clamp. Interference with D-myo-inositol 1,4,5-trisphosphate (IP3) induced Ca2+ release using 100 microm 2-aminoethoxydiphenyl borate, and the phospholipase C (PLC) inhibitors 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate and U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when ryanodine receptors were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external Ca2+, and frequency was directly proportional to the external Ca2+ concentration. Frequency of Ca2+ oscillation was reduced by SKF-96365, but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. These results suggest that Ca2+ oscillations in isolated rabbit urethral interstitial cells are initiated by Ca2+ release from ryanodine-sensitive intracellular stores, that oscillation frequency is very sensitive to the external Ca2+ concentration and that conversion of the primary oscillation to a propagated Ca2+ wave depends upon IP3-induced Ca2+ release.

New aspects of nuclear calcium signalling

Nuclear calcium signalling has been a controversial battlefield for many years and the question of how permeable the nuclear pore complexes (NPCs) are to Ca2+ has been the subject of a particularly hot dispute. Recent data from isolated nuclei suggest that the NPCs are open even after depletion of the Ca2+ store in the nuclear envelope. Other research has suggested that a new Ca2+ -releasing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), can liberate Ca2+ only from acidic organelles, probably lysosomes, rather than from the traditional Ca2+ store in the endoplasmic reticulum (ER). Recent work indicates that NAADP can release Ca2+ from the nuclear envelope (NE), which has a thapsigargin-sensitive, ER-type Ca2+ store. NAADP acts in a manner similar to inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] or cyclic ADP-ribose (cADPR): all three messengers are equally able to reduce the Ca2+ concentration inside the NE and this is associated with a transient rise in the nucleoplasmic Ca2+ concentration. The NE contains ryanodine receptors (RyRs) and Ins(1,4,5)P3 receptors [Ins(1,4,5)P3Rs], and these can be activated separately and independently: the RyRs by either NAADP or cADPR, and the Ins(1,4,5)P3Rs by Ins(1,4,5)P3.

Interactions between inositol 1,4,5-trisphosphate receptors and ryanodine receptors in smooth muscle: one store or two?

This short review proposes a system of simplified functional models describing possible interactions between Ca(2+)-release channels associated with IP(3)Rs and RyRs in smooth muscle, and considers each of these models in the light of the available experimental evidence. Complete separation of IP(3)R- and RyR-gated stores seems to be unusual. Where both receptors release Ca(2+) from a common pool, simple interactions can occur since changes in the activation of one receptor type affects the availability of Ca(2+) for release through the other. Alterations in [Ca(2+)] within the sarcoplasmic reticulum can also affect the open probability of the release channels, and not just the Ca(2+)-flux through the channels when open, e.g., Ca(2+)-release through tonically active IP(3)Rs appears to limit SR Ca(2+)-content in some myocytes, and this modulates RyR activity, as indicated by changes in Ca(2+)-spark frequency. There is also evidence that intracellular release channels may co-operate, leading to positive feedback during activation. In particular, agonist-dependent activation of IP(3)Rs can promote activation of RyRs, amplifying and shaping the resulting Ca(2+)-signal. While there is little direct evidence as to the mechanism responsible for this interaction, some form of Ca(2+)-induced Ca(2+)-release in response to local increases in [Ca(2+)](c) seems likely.

Smooth muscle cells and interstitial cells of blood vessels

A rise in intracellular ionised calcium concentration ([Ca(2+)](i)) at sites adjacent to the contractile proteins is a primary signal for contraction in all types of muscles. Recent progress in the development of imaging techniques with special accent on the fluorescence confocal microscopy and new achievements in the synthesis of organelle- and ion-specific fluorochromes provide an experimental basis for study of the relationship between the structural organisation of the living smooth muscle myocyte and the features of calcium signalling at subcellular level. Applying fluorescent confocal microscopy and tight-seal recording of transmembrane ion currents to freshly isolated vascular myocytes we have demonstrated that: (1) Ca(2+) sparks originate from clustered opening of ryanodine receptors (RyRs) and build up a cell-wide increase in [Ca(2+)](i) upon myocyte excitation; (2) spontaneous Ca(2+) sparks occurred at the highest rate at certain preferred locations, frequent discharge sites (FDS), which are associated with a prominent portion of the sarcoplasmic reticulum (SR) located close to the cell membrane; (3) Ca(2+)-dependent K(+) and Cl(-) channels sense the local changes in [Ca(2+)](i) during a calcium spark and thereby couple changes in [Ca(2+)](i) within a microdomain to changes in the membrane potential, thus affecting excitability of the cell; (4) an intercommunication between RyRs and inositol trisphosphate receptors (IP(3)Rs) is one of the important determinants of intracellular calcium dynamics that, in turn, can modulate the cell membrane potential through differential targeting of calcium dependent membrane ion channels. Furthermore, using immunohystochemical approaches in combination with confocal imaging we identified non-contractile cells closely resembling interstitial cells (ICs) of Cajal (which are considered to be pacemaker cells in the gut) in the wall of portal vein and mesenteric artery. Using electron microscopy, tight-seal recording and fluorescence confocal imaging we obtained information on the morphology of ICs and their possible coupling to smooth muscle cells (SMCs), calcium signalling in ICs and their electrophysiological properties. The functions of these cells are not yet fully understood; in portal vein they may act as pacemakers driving the spontaneous activity of the muscle; in artery they may have other a yet unsuspected functions.

Agonist-induced mitochondrial Ca2+ transients in smooth muscle

We investigated the role of mitochondria (MT) in calcium signaling in a culture of rat aortic smooth muscle cells. We used targeted aequorin to selectively measure [Ca2+] in this organelle. Our results reveal that smooth muscle cell stimulation with agonists causes a large, transient increase in mitochondrial [Ca2+] ([Ca2+]m). This large transient can be blocked with inhibitors of the sarco-endoplasmic reticulum Ca2+-ATPase, suggesting a close relationship between the sarcoplasmic reticulum (SR) and the mitochondria. FCCP completely abolished the response to agonists, and targeted mitochondrial GFP revealed a vast tubular network of MT in these cells. When added before stimulation with ATP, IP3 inhibitors partially blocked the ATP-induced rise in mitochondrial Ca2+ release. The role of the Na+/Ca2+ exchanger (NCX) was examined by removing extracellular Na+. This procedure prevented the decrease in the [Ca2+]m transient normally seen on removal of extracellular Ca2+. We propose a functional linkage of MT and SR dependent on a narrow junctional space between the two organelles in which Ca2+ diffusion is restricted. Approximately half of the mitochondria appear to be associated with the superficial SR, which communicates with the extracellular space via NCX.

Nicotinic acid adenine dinucleotide phosphate mediates Ca2+ signals and contraction in arterial smooth muscle via a two-pool mechanism

Previous studies of arterial smooth muscle have shown that inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose mobilize Ca2+ from the sarcoplasmic reticulum. In contrast, little is known about Ca2+ mobilization by nicotinic acid adenine dinucleotide phosphate, a pyridine nucleotide derived from beta-NADP+. We show here that intracellular dialysis of nicotinic acid adenine dinucleotide phosphate (NAADP) induces spatially restricted "bursts" of Ca2+ release that initiate a global Ca2+ wave and contraction in pulmonary artery smooth muscle cells. Depletion of sarcoplasmic reticulum Ca2+ stores with thapsigargin and inhibition of ryanodine receptors with ryanodine, respectively, block the global Ca2+ waves by NAADP. Under these conditions, however, localized Ca2+ bursts are still observed. In contrast, xestospongin C, an IP3 receptor antagonist, had no effect on Ca2+ signals by NAADP. We propose that NAADP mobilizes Ca2+ via a 2-pool mechanism, and that initial Ca2+ bursts are amplified by subsequent sarcoplasmic reticulum Ca2+ release via ryanodine receptors but not via IP3 receptors.

Patterns of intracellular and intercellular Ca2+ waves in the longitudinal muscle layer of the murine large intestine in vitro

Ca2+ wave activity was monitored in the longitudinal (LM) layer of isolated murine caecum and proximal colon at 35 degrees C with fluo-4 AM and an iCCD camera. Both intracellular (within LM cells) and intercellular (also spreading from cell to cell) Ca2+ waves were observed. Intracellular Ca2+ waves were associated with a lack of muscle movement whereas intercellular Ca2+ waves, which were five times more intense than intracellular waves, were often associated with localized contractions. Several intracellular Ca2+ waves were present at the same time in individual LM cells. Waves in adjacent LM cells were not coordinated and were unaffected by TTX (1 microM) but were blocked by IP3 receptor antagonists xestospongin-C (Xe-C; 2 microM) or 2-aminoethyl diphenylborate (2-APB; 25 microM), and by ryanodine (10 microM). Caffeine (5 mM) restored wave activity following blockade with Xe-C. NiCl2 (1 mM) blocked intracellular Ca2+ waves, and nicardipine (2 microM) reduced their frequency and intensity, but did not affect their velocity, suggesting the sarcoplasmic reticulum may be fuelled by extracellular Ca2+ entry. Intercellular Ca2+ waves often occurred in bursts and propagated rapidly across sizeable regions of the LM layer and were blocked by heptanol (0.5 mM). Intercellular Ca2+ waves were dependent upon neural activity, external Ca2+ entry through L-type Ca2+ channels, and amplification via calcium-induced calcium release (CICR). In conclusion, intracellular Ca2+ waves, which may reduce muscle excitability, are confined to individual LM cells. They depend upon Ca2+ release from internal Ca2+ stores and are likely to be fuelled by extracellular Ca2+ entry. Intercellular Ca2+ waves, which are likely to underlie smooth muscle tone, mixing and propulsion, depend upon neural activity, muscle action potential propagation and amplification by CICR.

Crosstalk between ryanodine receptors and IP(3) receptors as a factor shaping spontaneous Ca(2+)-release events in rabbit portal vein myocytes

In smooth muscle cells freshly isolated from rabbit portal vein, there was only one site discharging the majority of spontaneous Ca(2+)-release events; the activity of this single site was studied using laser scanning confocal imaging after loading the cells with the fluorescent Ca(2+) indicator fluo-4 acetoxymethyl ester. Localised spontaneous Ca(2+)-release events visualised by line-scan imaging revealed two predominant spatiotemporal patterns: (i) small-amplitude, fast events similar to Ca(2+) sparks in cardiomyocytes and (ii) larger and slower events. The sum of two Gaussian profiles was well fitted to the amplitude histogram (peak frequencies at 1.8 and 3.2 F/F(0)) and spatial spread (full width at half-maximal amplitude) histogram (peak frequencies at 2 and 3.8 microm) for the 230 localised Ca(2+)-release events analysed. The existence of two populations of Ca(2+)-release events was also supported by the histograms of the rise times and half-decay times, which revealed modes at 38 and 65 ms, respectively. Shifting the scan line along the z-axis during imaging from a single discharge site suggested that the appearance of two populations of Ca(2+)-release events is not due to out-of-focus imaging. Both small and large events persisted upon 3-5 min exposure to 1-5 microM nicardipine, but were abolished after 10-15 min exposure to 50-100 microM ryanodine, 0.1 microM thapsigargin or 10 microM cyclopiazonic acid. Only small-amplitude, fast events persisted in the presence of inhibitors of inositol 1,4,5-trisphosphate (IP(3))-induced Ca(2+) release, 10 microM xestospongin C or 30 microM 2-aminoethoxy-diphenylborate (2-APB), or in the presence of 2.5 microM U-73122 (a phospholipase C (PLC) inhibitor). Coupling between neighbouring Ca(2+)-release domains giving rise to spontaneous [Ca(2+)](i) waves was abolished in the presence of 2-APB. Examination of the saltatory propagation of the waves suggested that the critical factor that determines propagation between domains is a time-dependent change in the sensitivity of ryanodine receptors and/or IP(3) receptors to Ca(2+), which can give rise to 'loose coupling' between release sites. These results suggest that activation of IP(3) receptors (due to the tonic activity of PLC and ongoing production of IP(3)) recruits neighbouring domains of ryanodine receptors, leading to larger Ca(2+) releases and saltatory propagation of [Ca(2+)](i) waves in portal vein myocytes.

Transformation of local Ca2+ spikes to global Ca2+ transients: the combinatorial roles of multiple Ca2+ releasing messengers

In pancreatic acinar cells, low, threshold concentrations of acetylcholine (ACh) or cholecystokinin (CCK) induce repetitive local cytosolic Ca2+ spikes in the apical pole, while higher concentrations elicit global signals. We have investigated the process that transforms local Ca2+ spikes to global Ca2+ transients, focusing on the interactions of multiple intracellular messengers. ACh-elicited local Ca2+ spikes were transformed into a global sustained Ca2+ response by cyclic ADP-ribose (cADPR) or nicotinic acid adenine dinucleotide phosphate (NAADP), whereas inositol 1,4,5-trisphosphate (IP3) had a much weaker effect. In contrast, the response elicited by a low CCK concentration was strongly potentiated by IP3, whereas cADPR and NAADP had little effect. Experiments with messenger mixtures revealed a local interaction between IP3 and NAADP and a stronger global potentiating interaction between cADPR and NAADP. NAADP strongly amplified the local Ca2+ release evoked by a cADPR/IP3 mixture eliciting a vigorous global Ca2+ response. Different combinations of Ca2+ releasing messengers can shape the spatio-temporal patterns of cytosolic Ca2+ signals. NAADP and cADPR are emerging as key messengers in the globalization of Ca2+ signals.

Identification and function of ryanodine receptor subtype 3 in non-pregnant mouse myometrial cells

Subtype 3 of the ryanodine receptor (RYR3) is a ubiquitous Ca2+ release channel which is predominantly expressed in smooth muscle tissues and certain regions of the brain. We show by reverse transcription-polymerase chain reaction (RT-PCR) that non-pregnant mouse myometrial cells expressed only RYR3 and therefore could be a good model for studying the role of endogenous RYR3. Expression of RYR3 was confirmed by Western blotting and immunostaining. Confocal Ca2+ measurements revealed that in 1.7 mM extracellular Ca2+, neither caffeine nor photolysis of caged Ca2+ were able to trigger any Ca2+ responses, whereas in the same cells oxytocin activated propagated Ca2+ waves. However, under conditions of increased sarcoplasmic reticulum (SR) Ca2+ loading, brought about by superfusing myometrial cells in 10 mM extracellular Ca2+, all the myometrial cells responded to caffeine and photolysis of caged Ca2+, indicating that it was possible to activate RYR3. The caffeine-induced Ca2+ responses were inhibited by intracellular application of an anti-RYR3-specific antibody. Immunodetection of RYR3 with the same antibody revealed a rather homogeneous distribution of fluorescence in confocal cell sections. In agreement with these observations, spontaneous or triggered Ca2+ sparks were not detected. In conclusion, our results suggest that under conditions of increased SR Ca2+ loading, endogenous RYR3 may contribute to the Ca2+ responses of myometrial cells.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

Bombesin-induced cytosolic Ca2+ spiking in pancreatic acinar cells depends on cyclic ADP-ribose and ryanodine receptors

Different hormones and neurotransmitters, using Ca2+ as their intracellular messenger, can generate specific cytosolic Ca2+ signals in different parts of a cell. In mouse pancreatic acinar cells, cytosolic Ca2+ oscillations are triggered by activation of acetylcholine (ACh), cholecystokinin (CCK) and bombesin receptors. Low concentrations of these three agonists all induce local Ca(2+)spikes, but in the case of bombesin and CCK these spikes can also trigger global Ca2+ signals. Here we monitor cytosolic Ca2+ oscillations induced by low (2-5 pM) concentrations of bombesin and show that, like ACh- and CCK-induced oscillations, the bombesin-elicited responses are inhibited by ryanodine(50 microM). We then demonstrate that, like CCK- but unlike ACh-induced oscillations, the responses to bombesin are abolished by intracellular infusion of the cyclic ADP ribose (cADPr) antagonist 8-NH2-cADPr (20 microM). We conclude that in mouse pancreatic acinar cells, bombesin, CCK and ACh all produce local Ca2+ spikes by recruiting common oscillator units composed of ryanodine and inositol trisphosphate receptors. However, bombesin and CCK also recruit cADPr receptors, which may account for the global Ca2+ signals that can be evoked by these two agonists. Our new results indicate that each Ca2+ -mobilizing agonist, acting on mouse pancreatic acinar cells, recruits a unique combination of intracellular Ca2+ channels.

Muscarinic stimulation increases basal Ca(2+) and inhibits spontaneous Ca(2+) transients in murine colonic myocytes

Localized Ca(2+) transients in isolated murine colonic myocytes depend on Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3)) receptors. Localized Ca(2+) transients couple to spontaneous transient outward currents (STOCs) and mediate hyperpolarization responses in these cells. We used confocal microscopy and whole cell patch-clamp recording to investigate how muscarinic stimulation, which causes formation of IP(3), can suppress Ca(2+) transients and STOCs that might override the excitatory nature of cholinergic responses. ACh (10 microM) reduced localized Ca(2+) transients and STOCs, and these effects were associated with a rise in basal cytosolic Ca(2+). These effects of ACh were mimicked by generalized rises in basal Ca(2+) caused by ionomycin (250-500 nM) or elevated external Ca(2+) (6 mM). Atropine (10 microM) abolished the effects of ACh. Pretreatment of cells with nicardipine (1 microM), or Cd(2+) (200 microM) had no effect on responses to ACh. An inhibitor of phospholipase C, U-73122, blocked Ca(2+) transients and STOCs but did not affect the increase in basal Ca(2+) after ACh stimulation. Xestospongin C (Xe-C; 5 microM), a membrane-permeable antagonist of IP(3) receptors, blocked spontaneous Ca(2+) transients but did not prevent the increase of basal Ca(2+) in response to ACh. Gd(3+) (10 microM), a nonselective cation channel inhibitor, prevented the increase in basal Ca(2+) after ACh and increased the frequency and amplitude of Ca(2+) transients and waves. Another inhibitor of receptor-mediated Ca(2+) influx channels, SKF-96365, also prevented the rise in basal Ca(2+) after ACh and increased Ca(2+) transients and development of Ca(2+) waves. FK-506, an inhibitor of FKBP12/IP(3) receptor interactions, had no effect on the rise in basal Ca(2+) but blocked the inhibitory effects of increased basal Ca(2+) and ACh on Ca(2+) transients. These results suggest that the rise in basal Ca(2+) that accompanies muscarinic stimulation of colonic muscles inhibits localized Ca(2+) transients that could couple to activation of Ca(2+)-activated K(+) channels and reduce the excitatory effects of ACh.

Heterogeneity of calcium stores and elementary release events in canine pulmonary arterial smooth muscle cells

To examine the nature of inositol 1,4,5-trisphosphate (IP(3))-sensitive and ryanodine (Ryn)-sensitive Ca(2+) stores in isolated canine pulmonary arterial smooth cells (PASMC), agonist-induced changes in global intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured using fura 2-AM fluorescence. Properties of elementary local Ca(2+) release events were characterized using fluo 3-AM or fluo 4-AM, in combination with confocal laser scanning microscopy. In PASMC, depletion of sarcoplasmic reticulum Ca(2+) stores with Ryn (300 microM) and caffeine (Caf; 10 mM) eliminated subsequent Caf-induced intracellular Ca(2+) transients but had little or no effect on the initial IP(3)-mediated intracellular Ca(2+) transient induced by ANG II (1 microM). Cyclopiazonic acid (CPA; 10 microM) abolished IP(3)-induced intracellular Ca(2+) transients but failed to attenuate the initial Caf-induced intracellular Ca(2+) transient. These results suggest that in canine PASMC, IP(3)-, and Ryn-sensitive Ca(2+) stores are organized into spatially distinct compartments while similar experiments in canine renal arterial smooth muscle cells (RASMC) reveal that these Ca(2+) stores are spatially conjoined. In PASMC, spontaneous local intracellular Ca(2+) transients sensitive to modulation by Caf and Ryn were detected, exhibiting spatial-temporal characteristics similar to those previously described for "Ca(2+) sparks" in cardiac and other types of smooth muscle cells. After depletion of Ryn-sensitive Ca(2+) stores, ANG II (8 nM) induced slow, sustained [Ca(2+)](i) increases originating at sites near the cell surface, which were abolished by depleting IP(3) stores. Discrete quantal-like events expected due to the coordinated opening of IP(3) receptor clusters ("Ca(2+) puffs") were not observed. These data provide new information regarding the functional properties and organization of intracellular Ca(2+) stores and elementary Ca(2+) release events in isolated PASMC.

Spatial control of Ca2+ signaling by nicotinic acid adenine dinucleotide phosphate diffusion and gradients

Intracellular Ca(2+) is able to control numerous cellular responses through complex spatiotemporal organization. Ca(2+) waves mediated by inositol trisphosphate or ryanodine receptors propagate by Ca(2+)-induced Ca(2+) release and therefore do not have an absolute requirement for a gradient in either inositol trisphosphate or cyclic ADP-ribose, respectively. In contrast, we report that although Ca(2+) increases induced by nicotinic acid adenine dinucleotide phosphate (NAADP) are amplified by Ca(2+)-induced Ca(2+) release locally, Ca(2+) waves mediated by NAADP have an absolute requirement for an NAADP gradient. If NAADP is increased such that its concentration is spatially uniform in one region of an egg, the Ca(2+) increase occurs simultaneously throughout this area, and only where there is diffusion out of this area to establish an NAADP gradient is there a Ca(2+) wave. A local increase in NAADP results in a Ca(2+) increase that spreads by NAADP diffusion. NAADP diffusion is restricted at low but not high concentrations of NAADP, indicating that NAADP diffusion is strongly influenced by binding to immobile and saturable sites, probably the NAADP receptor itself. Thus, the range of action of NAADP can be tuned by its concentration from that of a local messenger, like Ca(2+), to that of a global messenger, like IP(3) or cyclic ADP-ribose.

Intracellular calcium events activated by ATP in murine colonic myocytes

ATP is a candidate enteric inhibitory neurotransmitter in visceral smooth muscles. ATP hyperpolarizes visceral muscles via activation of small-conductance, Ca(2+)-activated K(+) (SK) channels. Coupling between ATP stimulation and SK channels may be mediated by localized Ca(2+) release. Isolated myocytes of the murine colon produced spontaneous, localized Ca(2+) release events. These events corresponded to spontaneous transient outward currents (STOCs) consisting of charybdotoxin (ChTX)-sensitive and -insensitive events. ChTX-insensitive STOCs were inhibited by apamin. Localized Ca(2+) transients were not blocked by ryanodine, but these events were reduced in magnitude and frequency by xestospongin C (Xe-C), a blocker of inositol 1,4,5-trisphosphate receptors. Thus we have termed the localized Ca(2+) events in colonic myocytes "Ca(2+) puffs. " The P(2Y) receptor agonist 2-methylthio-ATP (2-MeS-ATP) increased the intensity and frequency of Ca(2+) puffs. 2-MeS-ATP also increased STOCs in association with the increase in Ca(2+) puffs. Pyridoxal-phospate-6-azophenyl-2',4'-disculfonic acid tetrasodium, a P(2) receptor inhibitor, blocked responses to 2-MeS-ATP. Spontaneous Ca(2+) transients and the effects of 2-MeS-ATP on Ca(2+) puffs and STOCs were blocked by U-73122, an inhibitor of phospholipase C. Xe-C and ryanodine also blocked responses to 2-MeS-ATP, suggesting that, in addition to release from IP(3) receptor-operated stores, ryanodine receptors may be recruited during agonist stimulation to amplify release of Ca(2+). These data suggest that localized Ca(2+) release modulates Ca(2+)-dependent ionic conductances in the plasma membrane. Localized Ca(2+) release may contribute to the electrical responses resulting from purinergic stimulation.

Ca(2+) signals mediated by Ins(1,4,5)P(3)-gated channels in rat ureteric myocytes

Localized Ca(2+)-release signals (puffs) and propagated Ca(2+) waves were characterized in rat ureteric myocytes by confocal microscopy. Ca(2+) puffs were evoked by photorelease of low concentrations of Ins(1,4,5)P(3) from a caged precursor and by low concentrations of acetylcholine; they were also observed spontaneously in Ca(2+)-overloaded myocytes. Ca(2+) puffs showed some variability in amplitude, time course and spatial spread, suggesting that Ins(1,4,5)P(3)-gated channels exist in clusters containing variable numbers of channels and that within these clusters a variable number of channels can be recruited. Immunodetection of Ins(1,4,5)P(3) receptors revealed the existence of several spots of fluorescence in the confocal cell sections, supporting the existence of clusters of Ins(1,4,5)P(3) receptors. Strong Ins(1,4,5)P(3) photorelease and high concentrations of acetylcholine induced Ca(2+) waves that originated from an initiation site and propagated in the whole cell by spatial recruitment of neighbouring Ca(2+)-release sites. Both Ca(2+) puffs and Ca(2+) waves were blocked selectively by intracellular applications of heparin and an anti-Ins(1,4,5)P(3)-receptor antibody, but were unaffected by ryanodine and intracellular application of an anti-ryanodine receptor antibody. mRNAs encoding for the three subtypes of Ins(1,4,5)P(3) receptor and subtype 3 of ryanodine receptor were detected in these myocytes, and the maximal binding capacity of [(3)H]Ins(1,4,5)P(3) was 10- to 12-fold higher than that of [(3)H]ryanodine. These results suggest that Ins(1,4,5)P(3)-gated channels mediate a continuum of Ca(2+) signalling in smooth-muscle cells expressing a high level of Ins(1,4,5)P(3) receptors and no subtypes 1 and 2 of ryanodine receptors.

Two different but converging messenger pathways to intracellular Ca(2+) release: the roles of nicotinic acid adenine dinucleotide phosphate, cyclic ADP-ribose and inositol trisphosphate

Hormones and neurotransmitters mobilize Ca(2+) from the endoplasmic reticulum via inositol trisphosphate (IP(3)) receptors, but how a single target cell encodes different extracellular signals to generate specific cytosolic Ca(2+) responses is unknown. In pancreatic acinar cells, acetylcholine evokes local Ca(2+) spiking in the apical granular pole, whereas cholecystokinin elicits a mixture of local and global cytosolic Ca(2+) signals. We show that IP(3), cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP) evoke cytosolic Ca(2+) spiking by activating common oscillator units composed of IP(3) and ryanodine receptors. Acetylcholine activation of these common oscillator units is triggered via IP(3) receptors, whereas cholecystokinin responses are triggered via a different but converging pathway with NAADP and cyclic ADP-ribose receptors. Cholecystokinin potentiates the response to acetylcholine, making it global rather than local, an effect mediated specifically by cyclic ADP-ribose receptors. In the apical pole there is a common early activation site for Ca(2+) release, indicating that the three types of Ca(2+) release channels are clustered together and that the appropriate receptors are selected at the earliest step of signal generation.

Nitrergic relaxation of the mouse gastric fundus is mediated by cyclic GMP-dependent and ryanodine-sensitive mechanisms

1. Ryanodine-sensitive, Ca(2+) release ('Ca(2+) sparks') from the sarcoplasmic reticulum (SR) can activate plasmalemmal Ca(2+)-activated K(+) channels (K(Ca)) to cause membrane hyperpolarization and smooth muscle relaxation. Since cyclic guanosine monophosphate (cyclic GMP) can modulate Ca(2+) spark activity, the aim of the present study was to determine if Ca(2+) spark-like events are involved in NO-dependent, NANC relaxations to electrical field stimulation (EFS) of mouse, longitudinal smooth muscle of the gastric fundus in isolated strips contracted to approximately 40% of their maximum contraction. 2. NANC relaxations to EFS were almost abolished by both the NO synthase inhibitor, N(G)-nitro-L-arginine (L-NOARG; 100 microM) and the guanylate cyclase inhibitor, 1-H-oxodiazol-[1,2,4]-[4,3-alpha] quinoxaline-1-one (ODQ; 10 microM). Also, ODQ abolished relaxations to the NO donor, sodium nitroprusside (SNP; 1 nM - 30 microM). NANC relaxations and SNP-evoked relaxations were both partly ryanodine (10 microM)- and nifedipine (0.3 microM)-sensitive, but in each case, the inhibitory effects of ryanodine and nifedipine were additive. 3. Apamin (1 microM), charybdotoxin (0.1 microM), iberiotoxin (0.1 microM), tetraethylammonium (TEA; 1 mM), glibenclamide (10 microM) and 4-aminopyridine (1 mM) had no effect on either NANC- or SNP-evoked relaxations, the latter of which were also unaffected by high extracellular K(+) (68 mM). 4. Caffeine (0.1 - 1 mM) caused concentration-dependent relaxations of gastric fundus which were inhibited by ryanodine but unaffected by L-NOARG. 5. Relaxation to ATP (30 microM) was abolished by nifedipine, partly inhibited by apamin and ryanodine, but was unaffected by L-NOARG. 6. In conclusion, the results of the present study show that nitrergic relaxations in the mouse longitudinal gastric fundus occur via a cyclic GMP-activated ryanodine-sensitive mechanism, which does not appear to involve activation of K(+) channels.

Purinergic activation of spontaneous transient outward currents in guinea pig taenia colonic myocytes

Spontaneous transient outward currents (STOCs) were recorded from smooth muscle cells of the guinea pig taenia coli using the whole cell patch-clamp technique. STOCs were resolved at potentials positive to -50 mV. Treating cells with caffeine (1 mM) caused a burst of outward currents followed by inhibition of STOCs. Replacing extracellular Ca(2+) with equimolar Mn(2+) caused STOCs to "run down. " Iberiotoxin (200 nM) or charybdotoxin (ChTX; 200 nM) inhibited large-amplitude STOCs, but small-amplitude "mini-STOCs" remained in the presence of these drugs. Mini-STOCs were reduced by apamin (500 nM), an inhibitor of small-conductance Ca(2+)-activated K(+) channels (SK channels). Application of ATP or 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) increased the frequency of STOCs. The effects of 2-MeS-ATP persisted in the presence of charybdotoxin but were blocked by combination of ChTX (200 nM) and apamin (500 nM). 2-MeS-ATP did not increase STOCs in the presence of pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, a P(2) receptor blocker. Similarly, pretreatment of cells with U-73122 (1 microM), an inhibitor of phospholipase C (PLC), abolished the effects of 2-MeS-ATP. Xestospongin C, an inositol 1,4,5-trisphosphate (IP(3)) receptor blocker, attenuated STOCs, but these events were not affected by ryanodine. The data suggest that purinergic activation through P(2Y) receptors results in localized Ca(2+) release via PLC- and IP(3)-dependent mechanisms. Release of Ca(2+) is coupled to STOCs, which are composed of currents mediated by large-conductance Ca(2+)-activated K(+) channels and SK channels. The latter are thought to mediate hyperpolarization and relaxation responses of gastrointestinal muscles to inhibitory purinergic stimulation.

Focal sarcoplasmic reticulum calcium stores and diffuse inositol 1,4,5-trisphosphate and ryanodine receptors in human myometrium

Intracellular calcium stores of human uterine myocytes in primary and second passage cell culture were visualized using the low-affinity calcium-sensitive fluorescent dye, fluo-3FF. The calcium stores appeared as numerous small (0.2-0.5 microm diameter) focal fluorescences. The stores were not depleted by exposing the cells to oxytocin or ryanodine under standard conditions. The stores were rapidly depleted by oxytocin or ryanodine exposure when sarcoplasmic reticulum (SR) calcium re-uptake was inhibited by pretreatment with thapsigargin. Immunofluorescence experiments indicated that both ryanodine and inositol 1,4,5-trisphosphate (IP(3)) receptors were smoothly distributed throughout the SR, and neither receptor co-localized with the calcium stores. Since IP(3) and ryanodine calcium channels are tightly associated with their receptor, these results suggest that SR calcium release occurs via second messenger channels that are remote from the SR calcium stores. These observations are consistent only with a mechanism for release of calcium stores where the SR serves three functions: (1) as site of calcium storage, (2) as the structure that contains the IP(3)- and ryanodine receptors and their associated release channels, and (3) as a conduit between the calcium stores and the release channels.

Norepinephrine-induced Ca(2+) waves depend on InsP(3) and ryanodine receptor activation in vascular myocytes

In rat portal vein myocytes, Ca(2+) signals can be generated by inositol 1,4,5-trisphosphate (InsP(3))- and ryanodine-sensitive Ca(2+) release channels, which are located on the same intracellular store. Using a laser scanning confocal microscope associated with the patch-clamp technique, we showed that propagated Ca(2+) waves evoked by norepinephrine (in the continuous presence of oxodipine) were completely blocked after internal application of an anti-InsP(3) receptor antibody. These propagated Ca(2+) waves were also reduced by approximately 50% and transformed in homogenous Ca(2+) responses after application of an anti-ryanodine receptor antibody or ryanodine. All-or-none Ca(2+) waves obtained with increasing concentrations of norepinephrine were transformed in a dose-response relationship with a Hill coefficient close to unity after ryanodine receptor inhibition. Similar effects of the ryanodine receptor inhibition were observed on the norepinephrine- and ACh-induced Ca(2+) responses in non-voltage-clamped portal vein and duodenal myocytes and on the norepinephrine-induced contraction. Taken together, these results show that ryanodine-sensitive Ca(2+) release channels are responsible for the fast propagation of Ca(2+) responses evoked by various neurotransmitters producing InsP(3) in vascular and visceral myocytes.

Ca2+ spark as a regulator of ion channel activity

Ca2+ spark is a local and transient Ca2+ release from sarcoplasmic reticulum (SR) through the ryanodine receptor Ca2+-releasing channel (RyR). In cardiac myocytes, Ca2+ spark is an elementary unit of Ca2+-induced Ca2+ release (CICR) by opening of RyR(s) in junctional SR (jSR), which is triggered by Ca2+-influx through L-type Ca2+ channels to the narrow space between a transverse tubule and jSR. Ca2+ spark has, therefore, been described as the evidence for "the local control of excitation-contraction coupling". In contrast, Ca2+ sparks in smooth muscle have been reported in relation to Ca2+-dependent K+ (K(Ca)) channel activation and muscle relaxation. A spontaneous Ca2+ spark in a superficial area activates 10-100 K(Ca) channels nearby and induces membrane hyperpolarization, which reduces Ca2+ channel activity. In several types of smooth muscle cells, which have relatively high membrane excitability, an action potential (AP) elicits 5-20 Ca2+ hot spots (evoked sparks with long life) in the early stage via CICR in discrete superficial SR elements and activates K(Ca)-channel current highly responsible for AP repolarization and afterhyperpolarization. CICR available for contraction may occur more slowly by the propagation of CICR from superficial SR to deeper ones. The regulatory mechanism of ion channel activity on plasma membrane by superficial SR via Ca2+ spark generation in smooth muscle cells may be analogously common in several types of cells including neurons.

Characterization of elementary Ca2+ release signals in NGF-differentiated PC12 cells and hippocampal neurons

Elementary Ca2+ release signals in nerve growth factor- (NGF-) differentiated PC12 cells and hippocampal neurons, functionally analogous to the "Ca2+ sparks" and "Ca2+ puffs" identified in other cell types, were characterized by confocal microscopy. They either occurred spontaneously or could be activated by caffeine and metabotropic agonists. The release events were dissimilar to the sparks and puffs described so far, as many arose from clusters of both ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (InsP3Rs). Increasing either the stimulus strength or loading of the intracellular stores enhanced the frequency of and coupling between elementary release sites and evoked global Ca2+ signals. In the PC12 cells, the elementary Ca2+ release preferentially occurred around the branch points. Spatio-temporal recruitment of such elementary release events may regulate neuronal activities.