Carbachol triggers RyR-dependent Ca(2+) release via activation of IP(3) receptors in isolated rat gastric myocytes

Knockdown of Amyloid Precursor Protein Increases Ion Channel Expression and Alters Ca2+ Signaling Pathways

Although the physiological role of the full-length Amyloid Precursor Protein (APP) and its proteolytic fragments remains unclear, they are definitively crucial for normal synaptic function. Herein, we report that the downregulation of APP in SH-SY5Y cells, using short hairpin RNA (shRNA), alters the expression pattern of several ion channels and signaling proteins that are involved in synaptic and Ca²⁺ signaling. Specifically, the levels of GluR2 and GluR4 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid glutamate receptors (AMPAR) were significantly increased with APP knockdown. Similarly, the expression of the majority of endoplasmic reticulum (ER) residing proteins, such as the ER Ca²⁺ channels IP₃R (Inositol 1,4,5-triphosphate Receptor) and RyR (Ryanodine Receptor), the Ca²⁺ pump SERCA2 (Sarco/endoplasmic reticulum Ca²⁺ ATPase 2) and the ER Ca²⁺ sensor STIM1 (Stromal Interaction Molecule 1) was upregulated. A shift towards the upregulation of p-AKT, p-PP2A, and p-CaMKIV and the downregulation of p-GSK, p-ERK1/2, p-CaMKII, and p-CREB was observed, interconnecting Ca²⁺ signal transduction from the plasma membrane and ER to the nucleus. Interestingly, we detected reduced responses to several physiological stimuli, with the most prominent being the ineffectiveness of SH-SY5Y/APP- cells to mobilize Ca²⁺ from the ER upon carbachol-induced Ca²⁺ release through IP₃Rs and RyRs. Our data further support an emerging yet perplexing role of APP within a functional molecular network of membrane and cytoplasmic proteins implicated in Ca²⁺ signaling.

Neuronal microRNAs safeguard ER Ca2+ homeostasis and attenuate the unfolded protein response upon stress

Ca2+ is a critical mediator of neurotransmitter release, synaptic plasticity, and gene expression, but also excitotoxicity. Ca2+ signaling and homeostasis are coordinated by an intricate network of channels, pumps, and calcium-binding proteins, which must be rapidly regulated at all expression levels. Τhe role of neuronal miRNAs in regulating ryanodine receptors (RyRs) and inositol 1,4,5-triphosphate receptors (IP3Rs) was investigated to understand the underlying mechanisms that modulate ER Ca2+ release. RyRs and IP3Rs are critical in mounting and propagating cytosolic Ca2+ signals by functionally linking the ER Ca2+ content, while excessive ER Ca2+ release via these receptors is central to the pathophysiology of a wide range of neurological diseases. Herein, two brain-restricted microRNAs, miR-124-3p and miR-153-3p, were found to bind to RyR1-3 and IP3R3 3'UTRs, and suppress their expression at both the mRNA and protein level. Ca2+ imaging studies revealed that overexpression of these miRNAs reduced ER Ca2+ release upon RyR/IP3R activation, but had no effect on [Ca2+]i under resting conditions. Interestingly, treatments that cause excessive ER Ca2+ release decreased expression of these miRNAs and increased expression of their target ER Ca2+ channels, indicating interdependence of miRNAs, RyRs, and IP3Rs in Ca2+ homeostasis. Furthermore, by maintaining the ER Ca2+ content, miR-124 and miR-153 reduced cytosolic Ca2+ overload and preserved protein-folding capacity by attenuating PERK signaling. Overall, this study shows that miR-124-3p and miR-153-3p fine-tune ER Ca2+ homeostasis and alleviate ER stress responses.

Physiological Overview of the Potential Link between the UPS and Ca2+ Signaling

The ubiquitin–proteasome system (UPS) is the main proteolytic pathway by which damaged target proteins are degraded after ubiquitination and the recruit of ubiquitinated proteins, thus regulating diverse physiological functions and the maintenance in various tissues and cells. Ca²⁺ signaling is raised by oxidative or ER stress. Although the basic function of the UPS has been extensively elucidated and has been continued to define its mechanism, the precise relationship between the UPS and Ca²⁺ signaling remains unclear. In the present review, we describe the relationship between the UPS and Ca²⁺ signaling, including Ca²⁺-associated proteins, to understand the end point of oxidative stress. The UPS modulates Ca²⁺ signaling via the degradation of Ca²⁺-related proteins, including Ca²⁺ channels and transporters. Conversely, the modulation of UPS is driven by increases in the intracellular Ca²⁺ concentration. The multifaceted relationship between the UPS and Ca²⁺ plays critical roles in different tissue systems. Thus, we highlight the potential crosstalk between the UPS and Ca²⁺ signaling by providing an overview of the UPS in different organ systems and illuminating the relationship between the UPS and autophagy.

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.

Relaxant effects of a hydroalcoholic extract of Ruta graveolens on isolated rat tracheal rings

BACKGROUND

Ruta graveolens L. (R. graveolens) is a medicinal plant employed in non-traditional medicines that has various therapeutic properties, including anthelmintic, and vasodilatory actions, among others. We evaluated the trachea-relaxant effects of hydroalcoholic extract of R. graveolens against potassium chloride (KCl)- and carbachol-induced contraction of rat tracheal rings in an isolated organ bath.

RESULTS

The results showed that the airway smooth muscle contraction induced by the depolarizing agent (KCl) and cholinergic agonist (carbachol) was markedly reduced by R. graveolens in a concentration-dependent manner, with maximum values of 109 ± 7.9 % and 118 ± 2.6 %, respectively (changes in tension expressed as positive percentages of change in proportion to maximum contraction), at the concentration of 45 μg/mL (half-maximal inhibitory concentration IC50: 35.5 μg/mL and 27.8 μg/mL for KCl- and carbachol-induced contraction, respectively). Additionally, the presence of R. graveolens produced rightward parallel displacement of carbachol dose-response curves and reduced over 35 % of the maximum smooth muscle contraction.

CONCLUSIONS

The hydroalcoholic extract of R. graveolens exhibited relaxant activity on rat tracheal rings. The results suggest that the trachea-relaxant effect is mediated by a non-competitive antagonistic mechanism. More detailed studies are needed to identify the target of the inhibition, and to determine more precisely the pharmacological mechanisms involved in the observed biological effects.

Mechanisms of intracellular calcium homeostasis in developing and mature bovine corpora lutea

Although calcium (Ca(2+)) is accepted as an intracellular mediator of prostaglandin F2 alpha (PGF2alpha) actions on luteal cells, studies defining mechanisms of Ca(2+) homeostasis in bovine corpora lutea (CL) are lacking. The increase in intracellular Ca(2+) concentration ([Ca(2+)]i) induced by PGF2alpha in steroidogenic cells from mature CL is greater than in those isolated from developing CL. Our hypothesis is that differences in signal transduction associated with developing and mature CL contribute to the increased efficacy of PGF2alpha to induce a Ca(2+) signal capable of inducing regression in mature CL. To test this hypothesis, major genes participating in Ca(2+) homeostasis in the bovine CL were identified, and expression of mRNA, protein, or activity, in the case of phospholipase Cbeta (PLCbeta), in developing and mature bovine CL was compared. In addition, we examined the contribution of external and internal Ca(2+) to the PGF2alpha stimulated rise in [Ca(2+)]i in LLCs isolated from developing and mature bovine CL. Three differences were identified in mechanisms of calcium homeostasis between developing and mature CL, which could account for the lesser increase in [Ca(2+)]i in response to PGF2alpha in developing than in mature CL. First, there were lower concentrations of inositol 1,4,5-trisphosphate (IP3) after similar PGF2alpha challenge, indicating reduced phospholipase C beta (PLCbeta) activity, in developing than mature CL. Second, there was an increased expression of sorcin (SRI) in developing than in mature CL. This cytoplasmic Ca(2+) binding protein modulates the endoplasmic reticulum (ER) Ca(2+) release channel, ryanodine receptor (RyR), to be in the closed configuration. Third, there was greater expression of ATP2A2 or SERCA, which causes calcium reuptake into the ER, in developing than in mature CL. Developmental differences in expression detected in whole CL were confirmed by Western blots using protein samples from steroidogenic cells isolated from developing and mature CL. Localization of these genes in steroidogenic luteal cells was confirmed by immunohistochemistry. Therefore, it is concluded that the cellular mechanisms that allow PGF2alpha to induce a calcium signal of greater magnitude in mature than in developing CL involve 1) greater PLCbeta activity with enhanced generation of IP3, 2) an enhanced Ca(2+) release from the ER via unrestrained RYR2 due to a decrease in SRI expression, and 3) a reduction in calcium reuptake to the ER due to lower expression of ATP2A2. Accordingly, the increase in [Ca(2+)]i induced by PGF2alpha in mature large steroidogenic cells had less dependency from extracellular calcium than in those isolated from immature CL.

Involvement of presenilin holoprotein upregulation in calcium dyshomeostasis of Alzheimer's disease

Mutations in presenilins (PS1 and PS2) account for the vast majority of early onset familial Alzheimer's disease cases. Beside the well investigated role of presenilins as the catalytic unit in γ-secretase complex, their involvement in regulation of intracellular calcium homeostasis has recently come into more focus of Alzheimer's disease research. Here we report that the overexpression of PS1 full-length holoprotein forms, in particular familial Alzheimer's disease-causing forms of PS1, result in significantly attenuated calcium release from thapsigargin- and bradykinin-sensitive stores. Interestingly, treatment of HEK293 cells with γ-secretase inhibitors also leads to decreased amount of calcium release from endoplasmic reticulum (ER) accompanying elevated PS1 holoprotein levels. Similarly, the knockdown of PEN-2 which is associated with deficient PS1 endoproteolysis and accumulation of its holoprotein form also leads to decreased ER calcium release. Notably, we detected enhanced PS1 holoprotein levels also in postmortem brains of patients carrying familial Alzheimer's disease PS1 mutations. Taken together, the conditions in which the amount of full length PS1 holoprotein is increased result in reduction of calcium release from ER. Based on these results, we propose that the disturbed ER calcium homeostasis mediated by the elevation of PS1 holoprotein levels may be a contributing factor to the pathogenesis of Alzheimer's disease.

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.

Endothelin 1 stimulates Ca2+-sparks and oscillations in retinal arteriolar myocytes via IP3R and RyR-dependent Ca2+ release

PURPOSE

To investigate endothelin 1 (Et1)-dependent Ca(2+)-signaling at the cellular and subcellular levels in retinal arteriolar myocytes.

METHODS

Et1 responses were imaged from Fluo-4-loaded smooth muscle in isolated segments of rat retinal arteriole using confocal laser microscopy.

RESULTS

Basal [Ca(2+)](i), subcellular Ca(2+)-sparks, and cellular Ca(2+)-oscillations were all increased during exposure to Et1 (10 nM). Ca(2+)-spark frequency was also increased by 90% by 10 nM Et1. The increase in oscillation frequency was concentration dependent and was inhibited by the EtA receptor (Et(A)R) blocker BQ123 but not by the EtB receptor antagonist BQ788. Stimulation of Ca(2+)-oscillations by Et1 was inhibited by a phospholipase C blocker (U73122; 10 μM), two inhibitors of inositol 1,4,5-trisphosphate receptors (IP(3)Rs), xestospongin C (10 μM), 2-aminoethoxydiphenyl borate (100 μM), and tetracaine (100 μM), a blocker of ryanodine receptors (RyRs).

CONCLUSIONS

Et1 stimulates Ca(2+)-sparks and oscillations through Et(A)Rs. The underlying mechanism involves the activation of phospholipase C and both IP(3)Rs and RyRs, suggesting crosstalk between these Ca(2+)-release channels. These findings suggest that phasic Ca(2+)-oscillations play an important role in the smooth muscle response to Et1 within the retinal microvasculature and support an excitatory, proconstrictor role for Ca(2+)-sparks in these vessels.

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.

Ryanodine receptors selectively contribute to the formation of taste-evoked calcium signals in mouse taste cells

The peripheral taste system uses multiple signaling pathways to transduce a stimulus into an output signal that activates afferent neurons. All of these signaling pathways depend on transient increases in intracellular calcium, but current understanding of these calcium signals is not well developed. Using molecular and physiological techniques, this study establishes that ryanodine receptors (RyRs), specifically isoform 1, are expressed in taste cells and that their physiological function differs among cell types employing different signaling pathways. RyR1 contributes to some taste-evoked signals that rely on calcium release from internal stores but can also supplement the calcium signal that is initiated by opening voltage-gated calcium channels. In taste cells expressing both signaling pathways, RyR1 contributes to the depolarization-induced calcium signal but not to the calcium signal that depends on calcium release from stores. These data suggest that RyR1 is an important regulator of calcium signaling and that its physiological role in taste cells is dictated by the nature of the calcium signaling mechanisms expressed.

Agonist-evoked Ca(2+) wave progression requires Ca(2+) and IP(3)

Smooth muscle responds to IP(3)-generating agonists by producing Ca(2+) waves. Here, the mechanism of wave progression has been investigated in voltage-clamped single smooth muscle cells using localized photolysis of caged IP(3) and the caged Ca(2+) buffer diazo-2. Waves, evoked by the IP(3)-generating agonist carbachol (CCh), initiated as a uniform rise in cytoplasmic Ca(2+) concentration ([Ca(2+)](c)) over a single though substantial length (approximately 30 microm) of the cell. During regenerative propagation, the wave-front was about 1/3 the length (approximately 9 microm) of the initiation site. The wave-front progressed at a relatively constant velocity although amplitude varied through the cell; differences in sensitivity to IP(3) may explain the amplitude changes. Ca(2+) was required for IP(3)-mediated wave progression to occur. Increasing the Ca(2+) buffer capacity in a small (2 microm) region immediately in front of a CCh-evoked Ca(2+) wave halted progression at the site. However, the wave front does not progress by Ca(2+)-dependent positive feedback alone. In support, colliding [Ca(2+)](c) increases from locally released IP(3) did not annihilate but approximately doubled in amplitude. This result suggests that local IP(3)-evoked [Ca(2+)](c) increases diffused passively. Failure of local increases in IP(3) to evoke waves appears to arise from the restricted nature of the IP(3) increase. When IP(3) was elevated throughout the cell, a localized increase in Ca(2+) now propagated as a wave. Together, these results suggest that waves initiate over a surprisingly large length of the cell and that both IP(3) and Ca(2+) are required for active propagation of the wave front to occur.

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.

cAMP/PKA-dependent increases in Ca Sparks, oscillations and SR Ca stores in retinal arteriolar myocytes after exposure to vasopressin

PURPOSE

To investigate the effects of arginine vasopressin (AVP) on Ca(2+) sparks and oscillations and on sarcoplasmic reticulum (SR) Ca(2+) content in retinal arteriolar myocytes.

METHODS

Fluo-4-loaded smooth muscle in intact segments of freshly isolated porcine retinal arteriole was imaged by confocal laser microscopy. SR Ca(2+) store content was assessed by recording caffeine-induced Ca(2+) transients with microfluorimetry and fura-2.

RESULTS

The frequencies of Ca(2+) sparks and oscillations were increased both during exposure to, and 10 minutes after washout of AVP (10 nM). Caffeine transients were increased in amplitude 10 and 90 minutes after a 3-minute application of AVP. Both AVP-induced Ca(2+) transients and the enhancement of caffeine responses after AVP washout were inhibited by SR 49059, a V(1a) receptor blocker. Forskolin, an activator of adenylyl cyclase, also persistently enhanced caffeine transients. Rp-8-HA-cAMPS, a membrane-permeant PKA inhibitor, prevented enhancement of caffeine transients by both AVP and forskolin. Forskolin, but not AVP, produced a reversible, Rp-8-HA-cAMPS insensitive reduction in basal [Ca(2+)](i).

CONCLUSIONS

AVP activates a cAMP/PKA-dependent pathway via V(1a) receptors in retinal arteriolar smooth muscle. This effect persistently increases SR Ca(2+) loading, upregulating Ca(2+) sparks and oscillations, and may favor prolonged agonist activity despite receptor desensitization.

The contribution of inositol 1,4,5-trisphosphate and ryanodine receptors to agonist-induced Ca(2+) signaling of airway smooth muscle cells

The relative contribution of inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) and ryanodine receptors (RyRs) to agonist-induced Ca(2+) signaling in mouse airway smooth muscle cells (SMCs) was investigated in lung slices with phase-contrast or laser scanning microscopy. At room temperature (RT), methacholine (MCh) or 5-hydroxytryptamine (5-HT) induced Ca(2+) oscillations and an associated contraction in small airway SMCs. The subsequent exposure to an IP(3)R antagonist, 2-aminoethoxydiphenyl borate (2-APB), inhibited the Ca(2+) oscillations and induced airway relaxation in a concentration-dependent manner. 2-APB also inhibited Ca(2+) waves generated by the photolytic release of IP(3). However, the RyR antagonist ryanodine had no significant effect, at any concentration, on airway contraction or agonist- or IP(3)-induced Ca(2+) oscillations or Ca(2+) wave propagation. By contrast, a second RyR antagonist, tetracaine, relaxed agonist-contracted airways and inhibited agonist-induced Ca(2+) oscillations in a concentration-dependent manner. However, tetracaine did not affect IP(3)-induced Ca(2+) release or wave propagation nor the Ca(2+) content of SMC Ca(2+) stores as evaluated by Ca(2+)-release induced by caffeine. Conversely, both ryanodine and tetracaine completely blocked agonist-independent slow Ca(2+) oscillations induced by KCl. The inhibitory effects of 2-APB and absence of an effect of ryanodine on MCh-induced airway contraction or Ca(2+) oscillations of SMCs were also observed at 37 degrees C. In Ca(2+)-permeable SMCs, tetracaine inhibited agonist-induced contraction without affecting intracellular Ca(2+) levels indicating that relaxation also resulted from a reduction in Ca(2+) sensitivity. These results indicate that agonist-induced Ca(2+) oscillations in mouse small airway SMCs are primary mediated via IP(3)Rs and that tetracaine induces relaxation by both decreasing Ca(2+) sensitivity and inhibiting agonist-induced Ca(2+) oscillations via an IP(3)-dependent mechanism.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Cholinergic-induced Ca2+ signaling in interstitial cells of Cajal from the guinea pig bladder

Acetylcholine released from parasympathetic excitatory nerves activates contraction in detrusor smooth muscle. Immunohistochemical labeling of guinea pig detrusor with anti-c-Kit and anti-VAChT demonstrated a close structural relationship between interstitial cells of Cajal (ICC) and cholinergic nerves. The ability of guinea pig bladder detrusor ICC to respond to the acetylcholine analog, carbachol, was investigated in enzymatically dissociated cells, loaded with the Ca²⁺ indicator fluo 4AM. ICC fired Ca²⁺ transients in response to stimulation by carbachol (1/10 μM). Their pharmacology was consistent with carbachol-induced contractions in strips of detrusor which were inhibited by 4-DAMP (1 μM), an M₃ receptor antagonist, but not by the M₂ receptor antagonist methoctramine (1 μM). The source of Ca²⁺ underlying the carbachol transients in isolated ICC was investigated using agents to interfere with influx or release from intracellular stores. Nifedipine (1 μM) or Ni²⁺ (30–100 μM) to block Ca²⁺ channels or the removal of external Ca²⁺ reduced the amplitude of the carbachol transients. Application of ryanodine (30 μM) or tetracaine (100 μM) abolished the transients. The phospholipase C inhibitor, U-73122 (2.5 μM), significantly reduced the responses. 2-Aminoethoxydiethylborate (30 μM) caused a significant reduction and Xestospongin C (1 μM) was more effective, almost abolishing the responses. Intact in situ preparations of guinea pig bladder loaded with a Ca²⁺ indicator showed distinctively different patterns of spontaneous Ca²⁺ events in smooth muscle cells and ICC. Both cell types responded to carbachol by an increase in frequency of these events. In conclusion, guinea pig bladder detrusor ICC, both as isolated cells and within whole tissue preparations, respond to cholinergic stimulation by firing Ca²⁺ transients.

Modification of smooth muscle Ca2+-sparks by tetracaine: evidence for sequential RyR activation

Spontaneous Ca(2+)-sparks were imaged using confocal line scans of fluo-4 loaded myocytes in retinal arterioles. Tetracaine produced concentration-dependent decreases in spark frequency, and modified the spatiotemporal characteristics of residual sparks. Tetracaine (10 microM) reduced the rate of rise but prolonged the average rise time so that average spark amplitude was unaltered. The mean half-time of spark decay was also unaffected, suggesting that spark termination, although delayed, remained well synchronized. Sparks spread transversely across the myocytes in these vessels, and the speed of spread within individual sparks was slowed by approximately 60% in 10 microM tetracaine, as expected if the spark was propagated across the cell but the average P(o) for RyRs was reduced. Staining of isolated vessels with BODIPY-ryanodine and di-4-ANEPPS showed that RyRs were located both peripherally, adjacent to the plasma membrane, and in transverse extensions of the SR from one side of the cell to the other. Immuno-labelling of retinal flat mounts demonstrated the presence RyR(2) in arteriole smooth muscle but not RyR(1). We conclude that Ca(2+)-sparks in smooth muscle can result from sequential activation of RyRs distributed over an area of several microm(2), rather than from tightly clustered channels as in striated muscle.

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.

Complex effects of ryanodine on the sarcoplasmic reticulum Ca2+ levels in smooth muscle cells

We have studied the effects of ryanodine and inhibition of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) with thapsigargin, on both [Ca(2+)](i) and the sarcoplasmic reticulum (SR) Ca(2+) level during caffeine-induced Ca(2+) release in single smooth muscle cells. Incubation with 10 microM ryanodine did not inhibit the first caffeine-induced [Ca(2+)](i) response, although it abolished the [Ca(2+)](i) response to a second application of caffeine. To assess whether ryanodine was inducing a permanent depletion of the internal Ca(2+) stores, we measured the SR Ca(2+) level with Mag-Fura-2. The magnitude of the caffeine-induced reduction in the SR Ca(2+) level was not augmented by incubating cells with 1 microM ryanodine. Moreover, on removal of caffeine, the SR Ca(2+) levels partially recovered in 61% of the cells due to the activity of thapsigargin-sensitive SERCA pumps. Unexpectedly, 10 microM ryanodine instead of inducing complete depletion of SR Ca(2+) stores markedly reduced the caffeine-induced SR Ca(2+) response. It was necessary to previously inhibit SERCA pumps with thapsigargin for ryanodine to be able to induce caffeine-triggered permanent depletion of SR Ca(2+) stores. These data suggest that the effect of ryanodine on smooth muscle SR Ca(2+) stores was markedly affected by the activity of SERCA pumps. Our data highlight the importance of directly measuring SR Ca(2+) levels to determine the effect of ryanodine on the internal Ca(2+) stores.

IP3-mediated Ca2+ increases do not involve the ryanodine receptor, but ryanodine receptor antagonists reduce IP3-mediated Ca2+ increases in guinea-pig colonic smooth muscle cells

Smooth muscle responds to IP3-generating (sarcolemma acting) neurotransmitters and hormones by releasing Ca2+ from the sarcoplasmic reticulum (SR) via IP3 receptors (IP3Rs). This release may propagate as Ca2+ waves. The Ca2+ signal emanating from IP3 generation may be amplified by its activating further Ca2+ release from ryanodine receptors (RyRs) in the process of Ca2+-induced Ca2+ release (CICR). Evidence for this proposal has relied largely on the use of blocking drugs such as ryanodine, tetracaine and dantrolene, reportedly specific inhibitors of RyRs. Here we have examined whether or not Ca2+ released via IP3Rs subsequently activates RyRs. In addition, the specificity of the blocking agents has been assessed by determining the extent of their ability to block IP3-mediated Ca2+ release under conditions in which RyRs were not activated. IP3-evoked Ca2+ release and Ca2+ waves did not require or activate RyRs. However, the RyR blocking drugs inhibited IP3-mediated Ca2+ signals at concentrations thought to be selective for RyRs. In single colonic smooth muscle cells, voltage clamped in the whole cell configuration, carbachol (CCh) evoked propagating Ca2+ waves which were not inhibited by ryanodine when the sarcolemma potential was -70 mV. At -20 mV, at which potential the SR Ca2+ content was increased and RyRs activated, ryanodine inhibited the Ca2+ waves. Photolysed caged IP3 increased [Ca2+]c; ryanodine, by itself, did not reduce the IP3-evoked [Ca2+]c increase when the sarcolemma potential was maintained at -70 mV. However, after activation of RyRs by caffeine, in the continued presence of ryanodine, the IP3-evoked [Ca2+]c increase was inhibited. In other experiments, RyRs were activated (as evidenced by the occurrence of spontaneous transient outward currents) by depolarizing the sarcolemma to -20 mV and again ryanodine was effective in inhibiting IP3-evoked Ca2+ increase. Thus while ineffective by itself, ryanodine inhibited IP3-evoked Ca2+ increases, presumably by causing persistent opening of the channel and depleting the SR of Ca2+, after RyRs were activated. These experiments establish that IP3-evoked Ca2+ release and Ca2+ waves do not activate RyRs; had they done so ryanodine would have inhibited the Ca2+ increase. However, under conditions where ryanodine was ineffective against the IP3-evoked Ca2+ transient (i.e. when RyRs were not activated, e.g. at a membrane potential of -70 mV) tetracaine and dantrolene each blocked IP3-evoked Ca2+ increases. The results show that although IP3-mediated Ca2+ release does not activate RyRs, RyR blockers can inhibit IP3-mediated Ca2+ signals.

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.

Interplay between ryanodine and IP3 receptors in ATP-stimulated mouse luteinized-granulosa cells

In mouse luteinized-granulosa cells (MGLC), ATP induces an increase in intracellular Ca2+ concentration by stimulating phospholipase C (PLC) associated with purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent release of Ca2+ from intracellular stores. In this study, we examined the cross-talk between the ryanodine receptors (RyR) and IP3 receptors (IP3R) in response to ATP in MGLC. Specifically, the effect of RyR modulators on ATP response was examined. The results showed that ATP-induced intracellular calcium elevation was abolished by inhibitors of the RyR, such as dantrolene (25 microM) and ryanodine (80 microM). When the MGLC were stimulated with activators of RyR, 2 microM ryanodine and 10 mM caffeine, the ATP-elicited response was decreased. These actions were independent of IP3 production stimulated by ATP. Hence, ATP-induced intracellular Ca2+ mobilization involves the coordinated action of both types of calcium release channels (CRCs). Using fluorescent probes, it was shown that IP3R is uniformly distributed throughout the cell; in contrast, RyR is mainly found around the nuclei. It is concluded that the IP3R and the RyR are functionally associated, and both play a role in the pattern of Ca2+ increase observed during purinergic stimulation of MGLC. This coupling may provide a highly efficient amplification mechanism for ATP stimulation of Ca2+ mobilization.

Suppression of Ca2+ influx by unfractionated heparin in non-excitable intact cells via multiple mechanisms

Effect of unfractionated heparin (UFH), described as a cell-impermeant IP3 receptor antagonist, was studied on the capacitive Ca(2+) entry in non-permeabilized, intact cells, measuring the intracellular Ca(2+) levels using fluorescence microplate technique. Ca(2+) influx induced via Ca(2+) mobilization by histamine in Hela cells or evoked by store depletion with thapsigargin in RBL-2H3 cells was dose-dependently suppressed by UFH added either before or after the stimuli. UFH also prevented the spontaneous Ba(2+) entry indicating that the non-capacitive Ca(2+) channels may also be affected. In addition, UFH caused a significant and dose-dependent delay in Ca(2+), and other bivalent cation inflow after treatment of the cells with Triton X-100, but it did not diminish the amount of these cations indicating that UFH did not act simply as a cation chelator, but modulated the capacitive Ca(2+) entry possibly via store operated Ca(2+) channels (SOCCs). Inhibitory activities of UFH and 2-aminoethyl diphenyl borate on the capacitive Ca(2+) influx was found reversible, but the time courses of their actions were dissimilar suggesting distinct modes of action. It was also demonstrated using a fluorescence potentiometric dye that UFH had a considerable hyperpolarizing effect and could alter the changes of membrane potential during Ca(2+) influx after store depletion by thapsigargin. We presume that the hyperpolarizing property of this agent might contribute to the suppression of Ca(2+) influx. We concluded that UFH can negatively modulate SOCCs and also other non-capacitive Ca(2+) channels and these activities might also account for its multiple biological effects.

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.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

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.

Calcium mobilization is required for peroxynitrite-mediated enhancement of spontaneous transient outward currents in arteriolar smooth muscle cells

Transiently local release of Ca(2+) from the sarcoplasmic reticulum (SR) activates nearby Ca(2+)-activated K(+) channels to produce spontaneous transient outward currents (STOCs) in smooth muscle cells. The purpose of the present study was to investigate the possible effect of peroxynitrite (ONOO(-)) on STOCs in mesenteric arteriolar smooth muscle cells (ASMCs) and decide whether Ca(2+) mobilization was involved in STOCs alteration by ONOO(-). STOCs were recorded and characterized using the perforated whole-cell patch-clamp configuration. The results demonstrated that STOCs activity was greatly suppressed by removal of extracellular Ca(2+); by addition of nifedipine, a specific inhibitor of L-type voltage-gated Ca(2+) channels (VGCCs); or by addition of ryanodine, a SR ryanodine receptors (RyRs) blocker. In contrast, both caffeine, a RyR activator, and 2-aminoethoxydiphenylborate (2-APB), a membrane-permeable inositol 1,4,5-trisphosphate receptors, (IP3R) antagonist, increased STOCs activity. 3-morpholinosydnonimine (SIN-1), an ONOO(-) donor, at concentrations of 20-200 microM, induced a dose-dependent enhancement of STOCs in ASMCs and led to conspicuous increases in STOCs frequency and amplitude, which were prevented by prior exposure to low external Ca(2+) (200 nM), ryanodine (10 microM), or nifedipine (10 microM). In contrast, caffeine (0.5 mM) did not further stimulate STOCs in ASMCs preincubated with SIN-1, and pretreatment with 2-APB (50 microM) had little effect on ONOO(-) -induced STOCs activation. These findings suggest that complex Ca(2+)-mobilizing pathways, including external Ca2+ influx through VGCCs activation and subsequent internal Ca(2+) release through RyRs but not IP3Rs, are involved in ONOO(-)mediated STOCs enhancement in ASMCs.

Functional architecture of the SR calcium store in uterine smooth muscle

Sarcoplasmic reticulum (SR) is abundant in uterine smooth muscle cells. The functional role of this organelle in the regulation of uterine myocytes is not fully understood. The data available in the literature suggest that SR plays a dual role: as a source of calcium and as a calcium sink shaping calcium transients produced by membrane depolarisation and uterotonic agonists. Advances in digital imaging techniques including confocal microscopy of isolated living cells, and the development of methods for direct measurement of intraluminal calcium, has triggered a substantial increase in the number of publications elucidating the role of intracellular stores in calcium signalling. In this paper we review the literature and our own work on the SR calcium store in uterine smooth muscle cells.

Comparison of U46619-, endothelin-1- or phenylephrine-induced changes in cellular Ca2+ profiles and Ca2+ sensitisation of constriction of pressurised rat resistance arteries

1. In pressurised rat mesenteric small arteries (50 mmHg), we examined the effects of stimulation with U46619, endothelin-1 (ET-1) or phenylephrine (PE) on changes in vessel diameter, global [Ca(2+)](i), individual smooth muscle cell [Ca(2+)](i) and Ca(2+)-sensitisation of contraction. 2. U46619 or ET-1 gave tonic diameter reductions, whereas PE-stimulated vessels gave tonic contractions or initial vasoconstrictions followed by diameter oscillations. Global [Ca(2+)](i) changes were transient for each agonist, with tonic constrictions being accompanied by maintained submaximal global [Ca(2+)](i) levels. 3. U46619, ET-1 or PE tonic constrictions were accompanied by apparently asynchronous [Ca(2+)](i) waves in individual smooth muscle cells of the vessel wall, as examined by confocal fluorescent microscopy. In vessels exhibiting vasomotion to PE, some apparent synchrony of activation of individual cells was evident; however, this was incomplete with many cells responding out of phase with their neighbours. 4. In alpha-toxin-permeabilised preparations, agonist-induced Ca(2+)-sensitisation of constriction at submaximal Ca(2+) (pCa6.7) in the presence of GTP was greater with U46619 or ET than PE. 5. We conclude that, in pressurised mesenteric arteries, (i) a general feature of receptor-coupled constriction is the generation of periodic smooth muscle [Ca(2+)](i) waves; (ii) complete synchrony of Ca(2+) oscillations between smooth muscle cells is not a prerequisite for receptor-coupled vasomotion; (iii) varied Ca(2+)-sensitising actions of agonists may partly determine tonic or phasic vessel responses to different stimuli.

Dysregulated ryanodine receptors mediate cellular toxicity: restoration of normal phenotype by FKBP12.6

Ca2+ homeostasis is a vital cellular control mechanism in which Ca2+ release from intracellular stores plays a central role. Ryanodine receptor (RyR)-mediated Ca2+ release is a key modulator of Ca2+ homeostasis, and the defective regulation of RyR is pathogenic. However, the molecular events underlying RyR-mediated pathology remain undefined. Cells stably expressing recombinant human RyR2 (Chinese hamster ovary cells, CHOhRyR2) had similar resting cytoplasmic Ca2+ levels ([Ca2+]c) to wild-type CHO cells (CHOWT) but exhibited increased cytoplasmic Ca2+ flux associated with decreased cell viability and proliferation. Intracellular Ca2+ flux increased with human RyR2 (hRyR2) expression levels and determined the extent of phenotypic modulation. Co-expression of FKBP12.6, but not FKBP12, or incubation of cells with ryanodine suppressed intracellular Ca2+ flux and restored normal cell viability and proliferation. Restoration of normal phenotype was independent of the status of resting [Ca2+]c or ER Ca2+ load. Heparin inhibition of endogenous inositol trisphosphate receptors (IP3R) had little effect on intracellular Ca2+ handling or viability. However, purinergic stimulation of endogenous IP3R resulted in apoptotic cell death mediated by hRyR2 suggesting functional interaction occurred between IP3R and hRyR2 Ca2+ release channels. These data demonstrate that defective regulation of RyR causes altered cellular phenotype via profound perturbations in intracellular Ca2+ signaling and highlight a key modulatory role of FKBP12.6 in hRyR2 Ca2+ channel function.