Localisation, function and composition of primary Ca(2+) spark discharge region in isolated smooth muscle cells from guinea-pig mesenteric arteries

Smooth muscle cells (SMCs) contain numerous calcium release domains, grouped into regions discharging as a single unit. Laser scanning confocal microscopy, voltage clamp and immunocytochemistry of single SMCs from small mesenteric arteries of guinea-pig were used to study the localisation, function and macromolecular composition of such calcium discharge regions (CDRs). Use of the Ca(2+)-sensitive fluorescent dye fluo-3 or fluo-4 with BODIPY TR-X ryanodine (BTR), a fluorescent derivative of ryanodine, showed spontaneous Ca(2+) sparks originating from regions stained by BTR, located immediately under the plasma membrane, in the arch formed by the sarcoplasmic reticulum surrounding the nucleus. Membrane depolarisation or application of noradrenaline or alpha,beta-methylene ATP, a P2X purinoceptor agonist, elicited Ca(2+) sparks from the same, spontaneous Ca(2+) spark-discharging region. The most active (primary) CDR accounted for nearly 60% of spontaneous transient outward currents at -40 mV and these were of significantly higher amplitude than the ones discharged by secondary CDRs. Immunocytochemical staining for type 1 IP(3) receptors, BK(Ca) channels, P2X(1) purinoceptors or alpha(1) adrenoceptors revealed their juxtaposition with BTR staining at the location typical of the primary CDR. These data suggest the existence of a primary calcium discharge region in SMCs; its position can be predicted from the cell's structure, it acts as a key region for the regulation of membrane potential via Ca(2+) sparks and is a potential link between the external, neurohumoral and the cell's internal, calcium signalling system.

Protein 4.1N does not interact with the inositol 1,4,5-trisphosphate receptor in an epithelial cell line

Cytosolic Ca2+ regulates a variety of cell functions, and the spatial patterns of Ca2+ signals are responsible in part for the versatility of this second messenger. The subcellular distribution of the inositol 1,4,5-trisphosphate receptor (IP3R) is thought to regulate Ca2+-signaling patterns but little is known about how the distribution of the IP3R itself is regulated. Here we examined the relationship between the IP3R and the cytoskeletal linker protein 4.1N in the polarized WIF-B cell line because protein 4.1N regulates targeting of the type I IP3R in neurons, but WIF-B cells do not express this cytoskeletal protein. WIF-B cells expressed all three isoforms of the IP3R, and each isoform was distributed throughout the cell. These cells did not express the ryanodine receptor. Photorelease of microinjected, caged IP3 induced a rapid rise in cytosolic Ca2+, but the increase began uniformly throughout the cell rather than at a specific initiation site. Expression of protein 4.1N was not associated with redistribution of the IP3R or changes in Ca2+-signaling patterns. These findings are consistent with the hypothesis that the subcellular distribution of IP3R isoforms regulates the formation of Ca2+ waves, and the finding that interactions between protein 4.1N and the IP3R vary among cell types may provide an additional, tissue-specific mechanism to shape the pattern of Ca2+ waves.

Acetylcholine and calcium signalling regulates muscle fibre formation in the zebrafish embryo

Nerve activity is known to be an important regulator of muscle phenotype in the adult, but its contribution to muscle development during embryogenesis remains unresolved. We used the zebrafish embryo and in vivo imaging approaches to address the role of activity-generated signals, acetylcholine and intracellular calcium, in vertebrate slow muscle development. We show that acetylcholine drives initial muscle contraction and embryonic movement via release of intracellular calcium from ryanodine receptors. Inhibition of this activity-dependent pathway at the level of the acetylcholine receptor or ryanodine receptor did not disrupt slow fibre number, elongation or migration but affected myofibril organisation. In mutants lacking functional acetylcholine receptors myofibre length increased and sarcomere length decreased significantly. We propose that calcium is acting via the cytoskeleton to regulate myofibril organisation. Within a myofibre, sarcomere length and number are the key parameters regulating force generation; hence our findings imply a critical role for nerve-mediated calcium signals in the formation of physiologically functional muscle units during development.

Depletion of FKBP does not affect the interaction between isolated ryanodine receptors

The ryanodine receptors/calcium release channels (RyRs) usually form two dimensional regular lattices in the endoplasmic/sarcoplasmic reticulum membranes. The native RyR is associated with many auxiliary proteins, including FKBP. It has been indicated that FKBP may play a role in the intermolecular interaction and coupled gating of neighboring RyRs. However, a more recent study shows that FKBP12 is not involved in the physical linkage between neighboring RyR1s. In the present work, the effect of FKBP12 on the interaction between RyR1s isolated from rabbit skeletal muscle was investigated in an aqueous medium with photon correlation spectroscopy. We found that the depletion of FKBP12 did not affect the oligomerization of RyR1s in the medium containing different [KCl] or under different channel functional states. No evidence is obtained for the involvement of FKBP12 in the intermolecular interaction between RyR1s.

The beta 1a subunit is essential for the assembly of dihydropyridine-receptor arrays in skeletal muscle

Homozygous zebrafish of the mutant relaxed (red(ts25)) are paralyzed and die within days after hatching. A significant reduction of intramembrane charge movements and the lack of depolarization-induced but not caffeine-induced Ca(2+) transients suggested a defect in the skeletal muscle dihydropyridine receptor (DHPR). Sequencing of DHPR cDNAs indicated that the alpha(1S) subunit is normal, whereas the beta(1a) subunit harbors a single point mutation resulting in a premature stop. Quantitative RT-PCR revealed that the mutated gene is transcribed, but Western blot analysis and immunocytochemistry demonstrated the complete loss of the beta(1a) protein in mutant muscle. Thus, the immotile zebrafish relaxed is a beta(1a)-null mutant. Interestingly, immunocytochemistry showed correct triad targeting of the alpha(1S) subunit in the absence of beta(1a). Freeze-fracture analysis of the DHPR clusters in relaxed myotubes revealed an approximately 2-fold reduction in cluster size with a normal density of DHPR particles within the clusters. Most importantly, DHPR particles in the junctional membranes of the immotile zebrafish mutant relaxed entirely lacked the normal arrangement in arrays of tetrads. Thus, our data indicate that the lack of the beta(1a) subunit does not prevent triad targeting of the DHPR alpha(1S) subunit but precludes the skeletal muscle-specific arrangement of DHPR particles opposite the ryanodine receptor (RyR1). This defect properly explains the complete deficiency of skeletal muscle excitation-contraction coupling in beta(1)-null model organisms.

Evolution of skeletal type e-c coupling: a novel means of controlling calcium delivery

The functional separation between skeletal and cardiac muscles, which occurs at the threshold between vertebrates and invertebrates, involves the evolution of separate contractile and control proteins for the two types of striated muscles, as well as separate mechanisms of contractile activation. The functional link between electrical excitation of the surface membrane and activation of the contractile material (known as excitation–contraction [e–c] coupling) requires the interaction between a voltage sensor in the surface membrane, the dihydropyridine receptor (DHPR), and a calcium release channel in the sarcoplasmic reticulum, the ryanodine receptor (RyR). Skeletal and cardiac muscles have different isoforms of the two proteins and present two structurally and functionally distinct modes of interaction.

We use structural clues to trace the evolution of the dichotomy from a single, generic type of e–c coupling to a diversified system involving a novel mechanism for skeletal muscle activation. Our results show that a significant structural transition marks the protochordate to the Craniate evolutionary step, with the appearance of skeletal muscle–specific RyR and DHPR isoforms.

The alpha1S N-terminus is not essential for bi-directional coupling with RyR1

The dihydropyridine receptor (DHPR) alpha(1S) II-III loop has been shown to be critical for excitation-contraction (EC) coupling in skeletal muscle, but the importance of other cytoplasmic regions, especially the N-terminus (residues 1-51), remains unclear. In this study, we found that deletion of alpha(1S) residues 2-37 (weakly conserved with N-termini of other L-type Ca(2+) channels) had little effect on the ability of alpha(1S) to serve as a Ca(2+) channel or voltage sensor for EC coupling. Strikingly, deletion of 10 additional residues, which are conserved in L-type channels, resulted in ablation of DHPR function. Specifically, confocal microscopy and measurement of charge movement showed that removal of residues 2-47 resulted in a failure of sarcolemmal insertion. Our results indicate that the weakly conserved, distal alpha(1S) N-terminus is not critical for EC coupling or function as a Ca(2+) channel. However, integrity of the proximal alpha(1S) N-terminus is necessary for sarcolemmal expression of the DHPR.

Adaptive expression pattern of different proteins involved in cellular calcium homeostasis in denervated rat vas deferens

The activity and protein expression of plasma membrane and sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPases and ryanodine receptors were investigated in surgically denervated rat vas deferens. The function of thapsigargin-sensitive but not thapsigargin-resistant (Ca2+-Mg2+)ATPase (from sarco(endo)plasmic reticulum and plasma membrane, respectively), evidenced by enzyme activity and Ca2+ uptake experiments, was significantly depressed by 30-50% when compared to innervated vas. Western blots showed that such reduction in sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPase performance was accompanied by a decrement of similar magnitude in sarco(endo)plasmic reticulum (Ca2+-Mg2+)ATPase type 2 protein expression, without any significant change in plasma membrane (Ca2+-Mg2+)ATPase expression. Finally, [3H]ryanodine binding revealed that the density of ryanodine binding sites was reduced by 45% after denervation without modification in affinity. The present findings demonstrate that sarco(endo)plasmic reticulum proteins involved in intracellular calcium homeostasis are clearly down-regulated and brings further evidence of a modified calcium translocation in denervated rat vas deferens.

Integrin stimulation induces calcium signalling in rat cardiomyocytes by a NO-dependent mechanism

The myocardial stretch-induced increase in intracellular [Ca(2+)] ([Ca(2+)](i)) is considered to be caused by integrin stimulation. Myocardial stretch is also associated with increased nitric oxide (NO) formation. We hypothesised that NO is implicated in calcium signalling following integrin stimulation. Integrins of neonatal rat cardiomyocytes were stimulated with a pentapeptide containing the Arg-Gly-Asp (RGD) sequence. [Ca(2+)](i) was measured with Fura2, [NO](i) was measured with DAF2 and phosphorylation of focal adhesion kinase (FAK) was monitored with immunofluorescence techniques. Integrin stimulation increased both [NO](i) and [Ca(2+)](i), the latter response being inhibited by ryanodine receptor-2 (RyR2) blockers and by N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NOS, but resistant to GdCl(3), diltiazem and wortmannin. Integrin-induced intracellular Ca(2+) release thus appears to be independent of the influx of extracellular Ca(2+) and phosphatidylinositol-3 kinase activity. In addition, integrin stimulation induced phosphorylation of FAK. Our results provide evidence for an integrin-induced Ca(2+) release from RyR2 which is mediated by NO formation, probably via FAK-induced NOS activation.

Heat- and anesthesia-induced malignant hyperthermia in an RyR1 knock-in mouse

Malignant hyperthermia (MH) is a life-threatening disorder characterized by skeletal muscle rigidity and elevated body temperature in response to halogenated anesthetics such as isoflurane or halothane. Mutation of tyrosine 522 of RyR1 (the predominant skeletal muscle calcium release channel) to serine has been associated with human malignant hyperthermia. In the present study, mice created harboring this mutation were found to represent the first murine model of human malignant hyperthermia. Mice homozygous for the Y522S mutation exhibit skeletal defects and die during embryonic development or soon after birth. Heterozygous mice, which correspond to the human occurrence of this mutation, are MH susceptible, experiencing whole body contractions and elevated core temperatures in response to isoflurane exposure or heat stress. Skeletal muscles from heterozygous mice exhibit increased susceptibility to caffeine- and heat-induced contractures in vitro. In addition, the heterozygous expression of the mutation results in enhanced RyR1 sensitivity to activation by temperature, caffeine, and voltage but not uncompensated sarcoplasmic reticulum calcium leak or store depletion. We conclude that the heterozygous expression of the Y522S mutation confers susceptibility to both heat- and anesthetic-induced MH responses.

Spatial segregation of neuronal calcium signals encodes different forms of LTP in rat hippocampus

Calcium regulates numerous processes in the brain. How one signal can coordinate so many diverse actions, even within the same neurone, is the subject of intense investigation. Here we have used two-photon calcium imaging to determine the mechanism that enables calcium to selectively and appropriately induce different forms of long-term potentiation (LTP) in rat hippocampus. Short-lasting LTP (LTP 1) required activation of ryanodine receptors (RyRs), which selectively increased calcium in synaptic spines. LTP of intermediate duration (LTP 2) was dependent on activation of inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) and subsequent calcium release specifically in dendrites. Long-lasting LTP (LTP 3) was selectively dependent on L-type voltage-dependent calcium channels (L-VDCCs), which generated somatic calcium influx. Activation of NMDA receptors was necessary, but not sufficient, for the generation of appropriate calcium signals in spines and dendrites, and the induction of LTP 1 and LTP 2. These results suggest that the selective induction of different forms of LTP is achieved via spatial segregation of functionally distinct calcium signals.

Overexpression of junctin causes adaptive changes in cardiac myocyte Ca(2+) signaling

In cardiac muscle, junctin forms a quaternary protein complex with the ryanodine receptor (RyR), calsequestrin, and triadin 1 at the luminal face of the junctional sarcoplasmic reticulum (jSR). By binding directly the RyR and calsequestrin, junctin may mediate the Ca(2+)-dependent regulatory interactions between both proteins. To gain more insight into the underlying mechanisms of impaired contractile relaxation in transgenic mice with cardiac-specific overexpression of junctin (TG), we studied cellular Ca(2+) handling in these mice. We found that the SR Ca(2+) load was reduced by 22% in cardiomyocytes from TG mice. Consistent with this, the frequency of Ca(2+) sparks was diminished by 32%. The decay of spontaneous Ca(2+) sparks was prolonged by 117% in TG. This finding was associated with a lower Na(+)-Ca(2+) exchanger (NCX) protein expression (by 67%) and a higher basal RyR phosphorylation at Ser(2809) (by 64%) in TG. The shortening- and Delta[Ca](i)-frequency relationships (0.5-4 Hz) were flat in TG compared to wild-type (WT) which exhibited a positive staircase for both parameters. Furthermore, increasing stimulation frequencies hastened the time of relaxation and the decay of [Ca](i) by a higher percentage in TG. We conclude that the impaired relaxation in TG may result from a reduced NCX expression and/or a higher SR Ca(2+) leak. The altered shortening-frequency relationship in TG seems to be a consequence of an impaired excitation-contraction coupling with depressed SR Ca(2+) release at higher rates of stimulation. Our data suggest that the more prominent frequency-dependent hastening of relaxation in TG results from a stimulation of SR Ca(2+) transport reflected by corresponding changes of [Ca](i).

Effects of diabetes on ryanodine receptor Ca release channel (RyR2) and Ca2+ homeostasis in rat heart

The defects identified in the mechanical activity of the hearts from type 1 diabetic animals include alteration of Ca2+ signaling via changes in critical processes that regulate intracellular Ca2+ concentration. These defects result partially from a dysfunction of cardiac ryanodine receptor calcium release channel (RyR2). The present study was designed to determine whether the properties of the Ca2+ sparks might provide insight into the role of RyR2 in the altered Ca2+ signaling in cardiomyocytes from diabetic animals when they were analyzed together with Ca2+ transients. Basal Ca2+ level as well as Ca2+-spark frequency of cardiomyoctes isolated from 5-week streptozotocin (STZ)-induced diabetic rats significantly increased with respect to aged-matched control rats. Ca2+ transients exhibited significantly reduced amplitude and prolonged time courses as well as depressed Ca2+ loading of sarcoplasmic reticulum in diabetic rats. Spatio-temporal properties of the Ca2+ sparks in cardiomyocytes isolated from diabetic rats were also significantly altered to being almost parallel to the changes of Ca2+ transients. In addition, RyR2 from diabetic rat hearts were hyperphosphorylated and protein levels of both RyR2 and FKBP12.6 depleted. These data show that STZ-induced diabetic rat hearts exhibit altered local Ca2+ signaling with increased basal Ca2+ level.

Models of the inositol trisphosphate receptor

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We review deterministic and stochastic mathematical models of the IPR, from the earliest ones of the 1970s and 1980s, to the most recent. The effects of IPR stochasticity on Ca2+ dynamics are briefly discussed.

Selective expression of the type 3 isoform of ryanodine receptor Ca2+ release channel (RyR3) in a subset of slow fibers in diaphragm and cephalic muscles of adult rabbits

The expression pattern of the RyR3 isoform of Ca2+ release channels was analysed by Western blot in neonatal and adult rabbit skeletal muscles. The results obtained show that the expression of the RyR3 isoform is developmentally regulated. In fact, RyR3 expression was detected in all muscles analysed at 2 and 15 days after birth while, in adult animals, it was restricted to a subset of muscles that includes diaphragm, masseter, pterygoideus, digastricus, and tongue. Interestingly, all of these muscles share a common embryonic origin being derived from the somitomeres or from the cephalic region of the embryo. Immunofluorescence analysis of rabbit skeletal muscle cross-sections showed that RyR3 staining was detected in all fibers of neonatal muscles. In contrast, in those adult muscles expressing RyR3 only a fraction of fibers was labelled. Staining of these muscles with antibodies against fast and slow myosins revealed a close correlation between expression of RyR3 and fibers expressing slow myosin isoform.

Ca-dependent reduction of I in rat ventricular cells: A novel paradigm for arrhythmia in heart failure?

OBJECTIVES

We investigated the inward rectifier potassium current (I(K1)), which can be blocked by intracellular Ca(2+), in heart failure (HF).

METHODS

We used the whole-cell patch-clamp technique to record I(K1) from single rat ventricular myocytes in voltage-clamp conditions. Fluorescence measurements of diastolic Ca(2+) were performed with Indo-1 AM. HF was examined 8 weeks after myocardial infarction (coronary artery ligation).

RESULTS

I(K1) was reduced and diastolic Ca(2+) was increased in HF cells. The reduction of I(K1) was attenuated when EGTA was elevated from 0.5 to 10 mM in the patch pipette and prevented with high BAPTA (20 mM). Ryanodine (100 nM) and FK506 (10 microM), both of which promote spontaneous SR Ca(2+) release from ryanodine receptor (RyR2) during diastole, reproduced the effect of HF on I(K1) in normal cells but had no effect in HF cells. The effects of ryanodine and FK506 were not additive and were prevented by BAPTA. Rapamycin (10 microM), which removes FKBP binding proteins from RyR2 with no effect on calcineurin, mimicked the effect of FK506 on I(K1). Cyclosporine A (10 microM), which inhibits calcineurin via cyclophilins, had no effect. In both HF cells and normal cells treated by FK506, the protein kinase C (PKC) inhibitor staurosporine totally restored the inward component of I(K1), but only partially restored its outward component at potentials corresponding to the late repolarizing phase of the action potential (-80 to -40 mV).

CONCLUSIONS

I(K1) is reduced by elevated diastolic Ca(2+)in HF, which involves in parallel PKC-dependent and PKC-independent mechanisms. This regulation provides a novel paradigm for Ca(2+)-dependent modulation of membrane potential in HF. Since enhanced RyR2-mediated Ca(2+)release also reduces I(K1), this paradigm might be relevant for arrhythmias related to acquired or inherited RyR2 dysfunction.

A Photoactivable Probe for Calcium Binding Proteins

Ca2+ as a signaling molecule carries information pivotal to cell life and death via its reversible interaction with a specific site in a protein. Although numerous Ca2+-dependent activities are known, the proteins responsible for some of these activities remain unidentified. We synthesized and characterized a photoreactive reagent, azido ruthenium (AzRu), which interacts specifically with Ca2+ binding proteins and strongly inhibits their Ca2+-dependent activities, regardless of their catalytic mechanisms or functional state as purified proteins, embedded in the membrane or in intact cells. As expected from a Ca2+ binding protein-specific reagent, AzRu had no effect on Ca2+-independent and Mg2+-dependent activities. Az103Ru covalently bound, and specifically labeled, known Ca2+ binding proteins. AzRu is a photoreactive reagent that provides an approach for identification of Ca2+ binding proteins, characterization of their binding sites, and exploration of new Ca2+-dependent processes.

Effects of pO2 on the activation of skeletal muscle ryanodine receptors by NO: A cautionary note

Eu et al., reported that O2 dynamically controls the redox state of 6-8 out of 50 thiols per skeletal ryanodine receptor (RyR1) subunit and thereby tunes the response of Ca2+-release channels to authentic nitric oxide (NO) [J.P. Eu, J. Sun, L. Xu, J.S. Stamler, G. Meissner, The skeletal muscle calcium release channel: coupled O2 sensor and NO signaling functions, Cell 102 (2000) 499-509]. A role for O2 was based on the observation that RyR1 can be activated by submicromolar NO at physiological ( approximately 10 mmHg) but not ambient (approximately 150 mmHg) pO2. At ambient pO2, these critical thiols were oxidized but incubation at low pO2 reset the redox state of these thiols, closed RyR1 channels and made these thiols available for nitrosation by low NO concentrations. Eu et al., postulated the existence of a redox/O2sensor that couples channel activity to NO and pO2 and explained that "the nature of the 'redox/O2 sensor' that couples channel activity to intracellular redox chemistry is a mystery". Here, we re-examined the effect of pO2 on RyR1 and find that incubation of RyR1 at low pO2 did not alter channel activity and NO (0.5-50 microM) failed to activate RyR1 despite a wide range of pO2 pre-incubation conditions. We show that low levels of NO do not activate RyR1, do not reverse the inhibition of RyR1 by calmodulin (CaM) even at physiological pO2. Similarly, the pre-incubation of SR vesicles in low pO2 (for 10-80 min) did not inhibit channel activity or sensitization of RyR1 to NO. We discuss the significance of these findings and propose that caution should be taken when considering a role for pO2 and nitrosation by NO as mechanisms that tune RyRs in striated muscles.

Second messenger function and the structure-activity relationship of cyclic adenosine diphosphoribose (cADPR)

Cyclic ADP-ribose (cADPR) is a Ca2+ mobilizing second messenger found in various cell types, tissues and organisms. Receptor-mediated formation of cADPR may proceed via transmembrane shuttling of the substrate NAD and involvement of the ectoenzyme CD38, or via so far unidentified ADP-ribosyl cyclases located within the cytosol or in internal membranes. cADPR activates intracellular Ca2+ release via type 2 and 3 ryanodine receptors. The exact molecular mechanism, however, remains to be elucidated. Possibilities are the direct binding of cADPR to the ryanodine receptor or binding via a separate cADPR binding protein. In addition to Ca2+ release, cADPR also evokes Ca2+ entry. The underlying mechanism(s) may comprise activation of capacitative Ca2+ entry and/or activation of the cation channel TRPM2 in conjunction with adenosine diphosphoribose. The development of novel cADPR analogues revealed new insights into the structure-activity relationship. Substitution of either the northern ribose or both the northern and southern ribose resulted in much simpler molecules, which still retained significant biological activity.

Toward a molecular understanding of the structure-function of ryanodine receptor Ca2+ release channels: perspectives from recombinant expression systems

Identification of the genetic basis of human diseases linked to dysfunctional free calcium (Ca2+) signaling has triggered an explosion of interest in the functional characterization of the molecular components regulating intracellular Ca2+ homeostasis. There is a growing appreciation of the central role of intracellular ryanodine-sensitive Ca2+ release channel (RyR) regulation in skeletal and cardiac muscle pathologies, including malignant hyperthermia, heart failure, and sudden cardiac death. The use of cloned RyR isoforms and recombinant expression techniques has greatly facilitated the elucidation of the molecular basis of RyR Ca2+ release functionality. This review will focus on the recombinant techniques used in the functional characterization of recombinant RyR isoforms and the insights that these approaches have yielded in unraveling the mechanistic basis of RyR channel functionality.

Hypoxia stimulates Ca2+ release from intracellular stores in astrocytes via cyclic ADP ribose-mediated activation of ryanodine receptors

The ability of O(2) levels to regulate Ca(2+) signalling in non-excitable cells is poorly understood, yet crucial to our understanding of Ca(2+)-dependent cell functions in physiological and pathological situations. Here, we demonstrate that hypoxia mobilizes Ca(2+) from an intracellular pool in primary cultures of cortical astrocytes. This pool can also be mobilized by bradykinin, which acts via phospholipase C and inositol trisphosphate production. By contrast, hypoxic Ca(2+) mobilization utilizes ryanodine receptors, which appear to be either present on the same intracellular pool, or on a separate but functionally coupled pool. Hypoxic activation of ryanodine receptors requires formation of cyclic ADP ribose, since hypoxic Ca(2+) mobilization was fully prevented by nicotinamide (which inhibits ADP ribosyl cyclase) or by 8-Br-cADP ribose, an antagonist of cyclic ADP ribose. Our results demonstrate for the first time the involvement of cyclic ADP ribose in hypoxic modulation of Ca(2+) signalling in the central nervous system, and suggest that this modulator of ryanodine receptors may play a key role in the function of astrocytes under conditions of fluctuating O(2) levels.

Induction of intracellular calcium elevation by Δ9-tetrahydrocannabinol in T cells involves TRPC1 channels

We have reported previously that Delta9-tetrahydrocannabinol (Delta9-THC) treatment of resting human and murine splenic T cells robustly elevated intracellular calcium ([Ca2+]i). The objective of the present investigation was to examine the putative role of [Ca2+]i store depletion and store-operated calcium (SOC) and receptor-operated cation (ROC) channels in the mechanism by which Delta9-THC increases [Ca2+]i in the cannabinoid-2 receptor-expressing human peripheral blood-acute lymphoid leukemia (HPB-ALL) human T cell line. By using the smooth endoplasmic reticulum Ca2+-ATPase pump inhibitor, thapsigargin, and the ryanodine receptor antagonist, 8-bromo-cyclic adenosine diphosphate ribose, we demonstrate that the Delta9-THC-mediated elevation in [Ca2+]i occurs independently of [Ca2+]i store depletion. Furthermore, the ROC channel inhibitor, SK&F 96365 was more efficacious at attenuating the Delta9-THC-mediated elevation in [Ca2+]i than SOC channel inhibitors, 2-aminoethoxydiphenyl borate and La3+. Recently, several members of the transient receptor potential canonical (TRPC) channel subfamily have been suggested to operate as SOC or ROC channels. In the present studies, treatment of HPB-ALL cells with 1-oleoyl-2-acetyl-sn-glycerol (OAG), a cell-permeant analog of diacylglycerol (DAG), which gates several members of the TRPC channel subfamily, rapidly elevated [Ca2+]i, as well as prevented a subsequent, additive elevation in [Ca2+]i by Delta9-THC, independent of protein kinase C. Reverse transcriptase-polymerase chain reaction analysis for TRPC1-7 showed that HPB-ALL cells express detectable mRNA levels of only TRPC1. Finally, small interference RNA knockdown of TRPC1 attenuated the Delta9-THC-mediated elevation of [Ca2+]i. Collectively, these results suggest that Delta9-THC-induced elevation in [Ca2+]i is attributable entirely to extracellular calcium influx, which is independent of [Ca2+]i store depletion, and is mediated, at least partially, through the DAG-sensitive TRPC1 channels.

Origin of spontaneous rhythmicity in smooth muscle

Rhythmic electrical activity is a feature of most smooth muscles but the mechanical consequences can vary from regular rapid phasic contractions to sustained contracture. For many years it was thought that spontaneous electrical activity originated in smooth muscle cells but recently it has become apparent that there are specialized pacemaker cells in many organs that are morphologically and functionally distinct from smooth muscle and that the former cells are the source of spontaneous electrical activity. Such a pacemaker function is well documented for the ICC of the gastrointestinal tract but evidence is accumulating that ICC-like cells play a similar role in other types of smooth muscle. We have recently shown that there are specialized pacemaking cells in the rabbit urethra which are spontaneously active when freshly isolated, readily distinguishable from smooth muscle cells under bright field illumination and relatively easy to study using patch-clamp and confocal imaging techniques. Recent results suggest that calcium oscillations in isolated rabbit urethral interstitial cells are initiated by calcium release from ryanodine sensitive intracellular stores, that oscillation frequency is very sensitive to the external calcium concentration and that conversion of the primary oscillation to a propagated calcium wave depends upon IP3-induced calcium release.

Two rings of negative charges in the cytosolic vestibule of type-1 ryanodine receptor modulate ion fluxes

The tetrameric ryanodine receptor calcium release channels (RyRs) are cation-selective channels that have pore architecture similar to that of K+ channels. We recently identified, in close proximity to the selectivity filter motif GGGIG, a conserved lumenal DE motif that has a critical role in RyR ion permeation and selectivity. Here, we substituted three aspartate residues (D4938, D4945, D4953) with asparagine and four glutamate residues (E4942, E4948, E4952, E4955) with glutamine hypothesized to line the cytosolic vestibule of the skeletal muscle RyR (RyR1). Mutant single channel properties were determined using the planar lipid bilayer method. Two mutants (D4938N, D4945N) showed a reduced K+ ion conductance, with D4938N also exhibiting a reduced selectivity for Ca2+ compared to K+. The cytosolic location of D4938 and D4945 was confirmed using the polycation neomycin. Both D4938N and D4945N exhibited an attenuated block by neomycin to a greater extent from the cytosolic than lumenal side. By comparison, charge neutralization of lumenal loop residues (D4899Q, E4900N) eliminated the block from the lumenal but not the cytosolic side. The results suggest that, in addition to negatively charged residues on the lumenal side, rings of four negative charges formed by D4938 and D4945 in the cytosolic vestibule determine RyR ion fluxes.

Ryanodine receptor binding to FKBP12 is modulated by channel activation state

Ryanodine receptor (RyR) Ca2+ release channels undergo a conformational change between the open and closed states. Its protein modulator, FK506 binding protein 12 (FKBP12), stabilises the channel gating between the four subunits that surround a central Ca2+-conducting pore. To understand the interdependence of RyR and FKBP12 binding, physiological and pharmacological agents were used to modulate the RyR open/closed state. ELISA sandwich binding assays showed that FKBP12 binding was dependent on the free Ca2+ and was lower at 1-10 μM free Ca2+ compared with 1 mM EGTA and 1 mM Ca2+, and this effect was enhanced by the inclusion of 1 mM ATP. Ruthenium red increased the binding of FKBP12. [3H]Ryanodine binding confirmed that 1 mM EGTA, 1 mM Ca2+ and 1 μM ruthenium red closed the channel, whereas 1 μM free Ca2+, 1 μM free Ca2+ + 1 mM ATP, or 10 mM caffeine opened it. These binding conditions were used in surface plasmon resonance studies to measure equilibrium binding kinetics. The affinity constant KA was significantly greater for the closed than the open channel, a change mediated by a decreased dissociation rate constant, kd. The results show that surface plasmon resonance is a powerful technique that can measure differences in RyR1 equilibrium binding kinetics with FKBP12.