Calcium release from the sarcoplasmic reticulum

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.

Role of TRPC1 channel in skeletal muscle function

Skeletal muscle contraction is reputed not to depend on extracellular Ca2+. Indeed, stricto sensu, excitation-contraction coupling does not necessitate entry of Ca2+. However, we previously observed that, during sustained activity (repeated contractions), entry of Ca2+ is needed to maintain force production. In the present study, we evaluated the possible involvement of the canonical transient receptor potential (TRPC)1 ion channel in this entry of Ca2+ and investigated its possible role in muscle function. Patch-clamp experiments reveal the presence of a small-conductance channel (13 pS) that is completely lost in adult fibers from TRPC1(-/-) mice. The influx of Ca2+ through TRPC1 channels represents a minor part of the entry of Ca(2+) into muscle fibers at rest, and the activity of the channel is not store dependent. The lack of TRPC1 does not affect intracellular Ca2+ concentration ([Ca2+](i)) transients reached during a single isometric contraction. However, the involvement of TRPC1-related Ca2+ entry is clearly emphasized in muscle fatigue. Indeed, muscles from TRPC1(-/-) mice stimulated repeatedly progressively display lower [Ca2+](i) transients than those observed in TRPC1(+/+) fibers, and they also present an accentuated progressive loss of force. Interestingly, muscles from TRPC1(-/-) mice display a smaller fiber cross-sectional area, generate less force per cross-sectional area, and contain less myofibrillar proteins than their controls. They do not present other signs of myopathy. In agreement with in vitro experiments, TRPC1(-/-) mice present an important decrease of endurance of physical activity. We conclude that TRPC1 ion channels modulate the entry of Ca(2+) during repeated contractions and help muscles to maintain their force during sustained repeated contractions.

Generation and propagation of gastric slow waves

1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle. 2. At present, there is strong agreement that interstitial cells of Cajal (ICC) are the pacemaker cells that generate slow waves. What has been less clear is the relative role of primary types of ICC, including the network in the myenteric plexus (ICC-MY) and the intramuscular network (ICC-IM). It is concluded that both ICC-MY and ICC-IM are likely to serve a major role in slow wave generation and propagation. 3. There has been long-standing controversy as to how slow waves 'propagate' circumferentially and down the gastrointestinal tract. Two mechanisms have been proposed, one being action potential (AP)-like conduction and the other phase wave-based 'propagation' resulting from an interaction of coupled oscillators. Studies made on single bundle gastric strips indicate that both mechanisms apply with relative dominance depending on conditions; the phase wave mechanism is dominant under circumstances of rhythmically generating slow waves and the AP-like propagation is dominant when the system is perturbed. 4. The phase wave mechanism (termed Ca(2+) phase wave) uses cyclical Ca(2+) release as the oscillator, with coupling between oscillators mediated by several factors, including: (i) store-induced depolarization; (ii) resultant electrical current flow/depolarization through the pacemaker cell network; and (iii) depolarization-induced increase in excitability of downstream Ca(2+) stores. An analogy is provided by pendulums in an array coupled together by a network of springs. These, when randomly activated, entrain to swing at the same frequency but with a relative delay along the row giving the impression of a propagating wave. 5. The AP-like mechanism (termed voltage-accelerated Ca(2+) wave) propagates sequentially like a conducting AP. However, it is different in that it depends on regenerative store Ca(2+) release and resultant depolarization rather than regenerative activation of voltage-dependent channels in the cell membrane. 6. The applicability of these mechanisms to describing propagation in large intact gastrointestinal tissues, where voltage-dependent Ca(2+) entry is also likely to be functional, is discussed.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Synaptic activation and membrane potential changes modulate the frequency of spontaneous elementary Ca2+ release events in the dendrites of pyramidal neurons

In most neurons postsynaptic [Ca(2+)](i) changes result from synaptic activation opening voltage gated channels, ligand gated channels, or mobilizing Ca(2+) release from intracellular stores. In addition to these changes that result directly from stimulation we found that in pyramidal cells there are spontaneous, rapid, Ca(2+) release events, predominantly, but not exclusively localized at dendritic branch points. They are clearest on the main apical dendrite but also have been detected in the finer branches and in the soma. Typically they have a spatial extent at initiation of approximately 2 microm, a rise time of <15 ms, duration <100 ms, and amplitudes of 10-70% of that generated by a backpropagating action potential at the same location. These events are not caused by background electrical or synaptic activity. However, their rate can be increased by repetitive synaptic stimulation at moderate frequencies, mainly through metabotropic glutamate receptor mobilization of IP(3). In addition, their frequency can be modulated by changes in membrane potential in the subthreshold range, predominantly by affecting Ca(2+) entry through L-type channels. They resemble the elementary events ("sparks" and "puffs") mediated by IP(3) receptors and ryanodine receptors that have been described primarily in non-neuronal preparations. These spontaneous Ca(2+) release events may be the fundamental units underlying some postsynaptic signaling cascades in mature neurons.

DNA sequence of the skeletal muscle calcium release channel cDNA and verification of the Arg615----Cys615 mutation, associated with porcine malignant hyperthermia, in Norwegian landrace pigs

Porcine calcium release channel (CRC) cDNA from skeletal muscle has been cloned and sequenced. The deduced amino acid sequence showed 97% identity to the corresponding rabbit and human sequences. Using oligonucleotide primers based on the nucleotide sequence, CRC cDNA fragments from seven pigs representing HALNN, HALNn and HALnn genotypes have been amplified. Sequencing and restriction digestion of the amplified cDNA confirm that the reported C----T mutation, which gives rise to Arg615----Cys615 change in the calcium release channel, is associated with the halothane sensitive allele in Norwegian Landrace pigs. The mutation may alter the reactivity of a neighbouring serine residue which is potentially phosphorylated.

Action of praziquantel on Grillotia erinaceus metacestodes (Cestoda: Trypanorhyncha) in vitro

Isometric tension recordings and the single sucrose-gap technique were used to record mechanical and electrical activity of somatic muscle from G. erinaceus metacestodes in vitro. Praziquantel (≥ 10−6 M) raised baseline tension and largely abolished spontaneous fluctuations in tension. The effect of the drug was critically dependent on the presence and availability of calcium in the bathing medium. In calcium-free solution or in the presence of manganese or lanthanum (10 mm) praziquantel failed to induce a rise in baseline tension. Electrical recordings of somatic muscle activity showed the action of praziquantel to be accompanied by membrane depolarization and loss of slow depolarizations and spikes. Tegumental membrane potential, as measured by microelectrodes, was not affected by the drug. These results suggest that in vitro praziquantel causes contraction of the somatic muscle of G. erinaceus by membrane depolarization and influx of extracellular calcium.

Temporal switching and cell-to-cell variability in Ca2+ release activity in mammalian cells

Genetically identical cells in a uniform external environment can exhibit different phenotypes, which are often masked by conventional measurements that average over cell populations. Although most studies on this topic have used microorganisms, differentiated mammalian cells have rarely been explored. Here, we report that only approximately 40% of clonal human embryonic kidney 293 cells respond with an intracellular Ca²⁺ increase when ryanodine receptor Ca²⁺ release channels in the endoplasmic reticulum are maximally activated by caffeine. On the other hand, the expression levels of ryanodine receptor showed a unimodal distribution. We showed that the difference in the caffeine sensitivity depends on a critical balance between Ca²⁺ release and Ca²⁺ uptake activities, which is amplified by the regenerative nature of the Ca²⁺ release mechanism. Furthermore, individual cells switched between the caffeine-sensitive and caffeine-insensitive states with an average transition time of approximately 65 h, suggestive of temporal fluctuation in endogenous protein expression levels associated with caffeine response. These results suggest the significance of regenerative mechanisms that amplify protein expression noise and induce cell-to-cell phenotypic variation in mammalian cells.

Indeterminacy of spatiotemporal cardiac alternans

Cardiac alternans, a beat-to-beat alternation in action potential duration (at the cellular level) or in electrocardiogram morphology (at the whole heart level), is a marker of ventricular fibrillation, a fatal heart rhythm that kills hundreds of thousands of people in the United States each year. Investigating cardiac alternans may lead to a better understanding of the mechanisms of cardiac arrhythmias and eventually better algorithms for the prediction and prevention of such dreadful diseases. In paced cardiac tissue, alternans develops under increasingly shorter pacing period. Existing experimental and theoretical studies adopt the assumption that alternans in homogeneous cardiac tissue is exclusively determined by the pacing period. In contrast, we find that, when calcium-driven alternans develops in cardiac fibers, it may take different spatiotemporal patterns depending on the pacing history. Because there coexist multiple alternans solutions for a given pacing period, the alternans pattern on a fiber becomes unpredictable. Using numerical simulation and theoretical analysis, we show that the coexistence of multiple alternans patterns is induced by the interaction between electrotonic coupling and an instability in calcium cycling.

Effect of luminal Ca2+ on the stability of coupled gating between ryanodine receptors from the rat heart

AIM

Two or more RYR2 channels reconstituted into a bilayer lipid membrane (BLM) can open and close either independently (single gating) or simultaneously (coupled gating). The coupled gating phenomenon has been suggested as an attractive candidate for a termination mechanism of Ca2+ release from the sarcoplasmic reticulum, required for periodic contraction and relaxation of cardiac muscle.

METHODS

Using the method of reconstitution of a channel into the BLM, we investigated the potential effect of luminal Ca2+on the stability of the interaction between coupled RYR2 channels isolated from the rat heart. We introduced a new parameter - the coupling stability - for each detected simultaneous opening and closing and further averaged values for experiments performed under identical conditions.

RESULTS

We found that the coupling stability during simultaneous opening of RYR2 channels was significantly lower in comparison with the simultaneous closing under the same experimental conditions. Furthermore, high concentration of luminal Ca2+ (53 mmol L(-1)) as well as the absence of luminal Ca2+ noticeably destabilized functional coupling between coupled RYR2 channels during opening, in contrast to lower tested concentrations (8-20 mmol L(-1)).

CONCLUSIONS

We provide experimental evidence that the strength of interaction between coupled RYR2 channels depends on the functional state of the channels. Furthermore, we show, for the first time, the regulation role of luminal Ca2+ in the inter-RYR2 functional coupling in the rat heart.

Calcium and signal transduction

Cell signaling is an essential process in which a variety of external signals, defined as first messengers, are translated inside the cells into specific responses, which are mediated by a less numerous group of second messengers. The exchange of signals became a necessity when the transition from monocellular to pluricellular life brought with it the division of labor among the cells of the organisms: unicellular organisms do not depend on the mutual exchange of signals, as they essentially only compete with each other for nutrients. Calcium (Ca²⁺) was selected during evolution as second messenger, because its chemistry made it a much more flexible ligand than the other abundant cations in the primordial environment (Na⁺, K⁺, Mg²⁺ ). Ca²⁺ can accept binding sites of irregular geometries and is thus ideally suited to be a carrier of biological information. The Ca²⁺ signal has properties that set it apart from those of all other biological messengers: they will be reviewed in this contribution. Among them, the ambivalent character of the Ca²⁺ signal is the most important: while essential to the viability of the cells, it can also easily become a conveyor of doom.

Immuno-proteomic approach to excitation--contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins

A comprehensive understanding of excitation-contraction (E-C) coupling in skeletal and cardiac muscle requires that all the major components of the Ca(2+) release machinery be resolved. We utilized a unique immuno-proteomic approach to generate a monoclonal antibody library that targets proteins localized to the skeletal muscle triad junction, which provides a structural context to allow efficient E-C coupling. Screening of this library has identified several mitsugumins (MG); proteins that can be localized to the triad junction in mammalian skeletal muscle. Many of these proteins, including MG29 and junctophilin, are important components in maintaining the structural integrity of the triad junction. Other triad proteins, such as calumin, play a more direct role in regulation of muscle Ca(2+) homeostasis. We have recently identified a family of trimeric intracellular cation-selective (TRIC) channels that allow for K(+) movement into the endoplasmic or sarcoplasmic reticulum to counter a portion of the transient negative charge produced by Ca(2+) release into the cytosol. Further study of TRIC channel function and other novel mitsugumins will increase our understanding of E-C coupling and Ca(2+) homoeostasis in muscle physiology and pathophysiology.

Muscle-specific overexpression of IGF-I improves E-C coupling in skeletal muscle fibers from dystrophic mdx mice

Duchenne muscular dystrophy (DMD) is a lethal X-linked disease caused by the absence of functional dystrophin. Abnormal excitation-contraction (E-C) coupling has been reported in dystrophic muscle fibers from mdx mice, and alterations in E-C coupling components may occur as a direct result of dystrophin deficiency. We hypothesized that muscle-specific overexpression of insulin-growth factor-1 (IGF-I) would reduce E-C coupling failure in mdx muscle. Mechanically skinned extensor digitorum longus muscle fibers from mdx mice displayed a faster decline in depolarization-induced force responses (DIFR); however, there were no differences in sarcoplasmic reticulum (SR)-mediated Ca(2+) resequestration or in the properties of the contractile apparatus when compared with nondystrophic controls. The rate of DIFR decline was restored to control levels in fibers from transgenic mdx mice that overexpressed IGF-I in skeletal muscle (mdx/IGF-I mice). Dystrophic muscles have a lower transcript level of a specific dihydropyridine receptor (DHPR) isoform, and IGF-I-mediated changes in E-C coupling were associated with increased transcript levels of specific DHPR isoforms involved in Ca(2+) regulation. Importantly, IGF-I overexpression also increased the sensitivity of the contractile apparatus to Ca(2+). The results demonstrate that IGF-I can ameliorate fundamental aspects of E-C coupling failure in dystrophic muscle fibers and that these effects are important for the improvements in cellular function induced by this growth factor.

TRIC channels are essential for Ca2+ handling in intracellular stores

Cell signalling requires efficient Ca2+ mobilization from intracellular stores through Ca2+ release channels, as well as predicted counter-movement of ions across the sarcoplasmic/endoplasmic reticulum membrane to balance the transient negative potential generated by Ca2+ release. Ca2+ release channels were cloned more than 15 years ago, whereas the molecular identity of putative counter-ion channels remains unknown. Here we report two TRIC (trimeric intracellular cation) channel subtypes that are differentially expressed on intracellular stores in animal cell types. TRIC subtypes contain three proposed transmembrane segments, and form homo-trimers with a bullet-like structure. Electrophysiological measurements with purified TRIC preparations identify a monovalent cation-selective channel. In TRIC-knockout mice suffering embryonic cardiac failure, mutant cardiac myocytes show severe dysfunction in intracellular Ca2+ handling. The TRIC-deficient skeletal muscle sarcoplasmic reticulum shows reduced K+ permeability, as well as altered Ca2+ 'spark' signalling and voltage-induced Ca2+ release. Therefore, TRIC channels are likely to act as counter-ion channels that function in synchronization with Ca2+ release from intracellular stores.

Expansion of the calcium hypothesis of brain aging and Alzheimer's disease: minding the store

Evidence accumulated over more than two decades has implicated Ca²⁺ dysregulation in brain aging and Alzheimer's disease (AD), giving rise to the Ca²⁺ hypothesis of brain aging and dementia. Electrophysiological, imaging, and behavioral studies in hippocampal or cortical neurons of rodents and rabbits have revealed aging-related increases in the slow afterhyperpolarization, Ca²⁺ spikes and currents, Ca²⁺ transients, and L-type voltage-gated Ca²⁺ channel (L-VGCC) activity. Several of these changes have been associated with age-related deficits in learning or memory. Consequently, one version of the Ca²⁺ hypothesis has been that increased L-VGCC activity drives many of the other Ca²⁺-related biomarkers of hippocampal aging. In addition, other studies have reported aging- or AD model-related alterations in Ca²⁺ release from ryanodine receptors (RyR) on intracellular stores. The Ca²⁺-sensitive RyR channels amplify plasmalemmal Ca²⁺ influx by the mechanism of Ca²⁺-induced Ca²⁺ release (CICR). Considerable evidence indicates that a preferred functional link is present between L-VGCCs and RyRs which operate in series in heart and some brain cells. Here, we review studies implicating RyRs in altered Ca²⁺ regulation in cell toxicity, aging, and AD. A recent study from our laboratory showed that increased CICR plays a necessary role in the emergence of Ca²⁺-related biomarkers of aging. Consequently, we propose an expanded L-VGCC/Ca²⁺ hypothesis, in which aging/pathological changes occur in both L-type Ca²⁺ channels and RyRs, and interact to abnormally amplify Ca²⁺ transients. In turn, the increased transients result in dysregulation of multiple Ca²⁺-dependent processes and, through somewhat different pathways, in accelerated functional decline during aging and AD.

Effects of elevated physiological temperatures on sarcoplasmic reticulum function in mechanically skinned muscle fibers of the rat

Properties of the sarcoplasmic reticulum (SR) with respect to Ca(2+) loading and release were measured in mechanically skinned fiber preparations from isolated extensor digitorum longus (EDL) muscles of the rat that were either kept at room temperature (23 degrees C) or exposed to temperatures in the upper physiological range for mammalian skeletal muscle (30 min at 40 or 43 degrees C). The ability of the SR to accumulate Ca(2+) was significantly reduced by a factor of 1.9-2.1 after the temperature treatments due to a marked increase in SR Ca(2+) leak, which persisted for at least 3 h after treatment. Results with blockers of Ca(2+) release channels (ruthenium red) and SR Ca(2+) pumps [2,5-di(tert-butyl)-1,4-hydroquinone] indicate that the increased Ca(2+) leak was not through the SR Ca(2+) release channel or the SR Ca(2+) pump, although it is possible that the leak pathway was via oligomerized Ca(2+) pump molecules. No significant change in the maximum SR Ca(2+)-ATPase activity was observed after the temperature treatment, although there was a tendency for a decrease in the SR Ca(2+)-ATPase. The observed changes in SR properties were fully prevented by the superoxide (O(2)(*-)) scavenger Tiron (20 mM), indicating that the production of O(2)(*-) at elevated temperatures is responsible for the increase in SR Ca(2+) leak. Results show that physiologically relevant elevated temperatures 1) induce lasting changes in SR properties with respect to Ca(2+) handling that contribute to a marked increase in the SR Ca(2+) leak and, consequently, to the reduction in the average coupling ratio between Ca(2+) transport and SR Ca(2+)-ATPase and muscle performance, and 2) that these changes are mediated by temperature-induced O(2)(*-) production.

Postulated role of interdomain interaction between regions 1 and 2 within type 1 ryanodine receptor in the pathogenesis of porcine malignant hyperthermia

We have demonstrated recently that CICR (Ca2+-induced Ca2+ release) activity of RyR1 (ryanodine receptor 1) is held to a low level in mammalian skeletal muscle ('suppression' of the channel) and that this is largely caused by the interdomain interaction within RyR1 [Murayama, Oba, Kobayashi, Ikemoto and Ogawa (2005) Am. J. Physiol. Cell Physiol. 288, C1222-C1230]. To test the hypothesis that aberration of this suppression mechanism is involved in the development of channel dysfunctions in MH (malignant hyperthermia), we investigated properties of the RyR1 channels from normal and MHS (MH-susceptible) pig skeletal muscles with an Arg615-->Cys mutation using [3H]ryanodine binding, single-channel recordings and SR (sarcoplasmic reticulum) Ca2+ release. The RyR1 channels from MHS muscle (RyR1MHS) showed enhanced CICR activity compared with those from the normal muscle (RyR1N), although there was little or no difference in the sensitivity to several ligands tested (Ca2+, Mg2+ and adenine nucleotide), nor in the FKBP12 (FK506-binding protein 12) regulation. DP4, a domain peptide matching the Leu2442-Pro2477 region of RyR1 which was reported to activate the Ca2+ channel by weakening the interdomain interaction, activated the RyR1N channel in a concentration-dependent manner, and the highest activity of the affected channel reached a level comparable with that of the RyR1MHS channel with no added peptide. The addition of DP4 to the RyR1MHS channel produced virtually no further effect on the channel activity. These results suggest that stimulation of the RyR1MHS channel caused by affected inter-domain interaction between regions 1 and 2 is an underlying mechanism for dysfunction of Ca2+ homoeostasis seen in the MH phenotype.

Enhancement of Ca2+-induced Ca2+ release by cyclic ADP-ribose in frog motor nerve terminals

Ca2+-induced Ca2+ release (CICR) occurs via activation of ryanodine receptors (RyRs) in frog motor nerve terminals after RyRs are primed for activation by repetitive Ca2+ entries, thereby contributing to synaptic plasticity. To clarify how the mechanism of CICR becomes activable by repetitive Ca2+ entries, we studied effects of a RyR modulator, cyclic ADP-ribose (cADPr), on CICR by Ca2+ imaging techniques. Use-dependent binding of fluorescent ryanodine and its blockade by ryanodine revealed the existence of RyRs in the terminals. Repetition of tetani applied to the nerve produced repetitive rises in intracellular Ca2+ ([Ca2+]i) in the terminals. The amplitude of each rise slowly waxed and waned during the course of the stimulation. These slow rises and decays were blocked by ryanodine, indicating the priming, activation and inactivation of CICR. Uncaging of caged-cADPr loaded in the terminals increased the amplitude of short tetanus-induced rises in [Ca2+]i and the amplitude, time to peak and half decay time of the slow waxing and waning rises in [Ca2+]i evoked by repetitive tetani. A cADPr blocker, 8-amino-cADPr, loaded in the terminals decreased the slow waxing and waning component of rises and blocked all the actions of exogenous cADPr. It is concluded that cADPr enhances the priming and activation of CICR. The four-state model for RyRs suggests that cADPr inhibits the inactivation of CICR and increases the activation efficacy of RyR.

Mg2+ activates the ryanodine receptor type 2 (RyR2) at intermediate Ca2+ concentrations

To clarify whether activity of the ryanodine receptor type 2 (RyR2) is reduced in the sarcoplasmic reticulum (SR) of cardiac muscle, as is the case with the ryanodine receptor type 1 (RyR1), Ca(2+)-dependent [(3)H]ryanodine binding, a biochemical measure of Ca(2+)-induced Ca(2+) release (CICR), was determined using SR vesicle fractions isolated from rabbit and rat cardiac muscles. In the absence of an adenine nucleotide or caffeine, the rat SR showed a complicated Ca(2+) dependence, instead of the well-documented biphasic dependence of the rabbit SR. In the rat SR, [(3)H]ryanodine binding initially increased as [Ca(2+)] increased, with a plateau in the range of 10-100 microM Ca(2+), and thereafter further increased to an apparent peak around 1 mM Ca(2+), followed by a decrease. In the presence of these modulators, this complicated dependence prevailed, irrespective of the source. Addition of 0.3-1 mM Mg(2+) unexpectedly increased the binding two- to threefold and enhanced the affinity for [(3)H]ryanodine at 10-100 microM Ca(2+), resulting in the well-known biphasic dependence. In other words, the partial suppression of RyR2 is relieved by Mg(2+). Ca(2+) could be a substitute for Mg(2+). Mg(2+) also amplifies the responses of RyR2 to inhibitory and stimulatory modulators. This stimulating effect of Mg(2+) on RyR2 is entirely new, and is referred to as the third effect, in addition to the well-known dual inhibitory effects. This effect is critical to describe the role of RyR2 in excitation-contraction coupling of cardiac muscle, in view of the intracellular Mg(2+) concentration.

Calcium signals for egg activation in mammals

A dramatic increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) occurs in eggs at fertilization common to all animal species examined to date, and this serves as a pivotal signal for egg activation characterized by resumption of meiotic cell division and formation of the pronuclei. In mammalian eggs, repetitive [Ca(2+)](i) rises (Ca(2+) oscillations) each of which accompanies a propagating wave across the egg occur due to release of Ca(2+) from the endoplasmic reticulum mainly through type 1 inositol 1,4,5-trisphosphate (IP(3)) receptor. Ca(2+) oscillations are induced by a cytosolic sperm factor driven into the egg cytoplasm upon sperm-egg fusion. A current strong candidate of the sperm factor is a novel sperm-specific isozyme of phospholipase C (IP(3)-producing enzyme), PLCzeta. Recent extensive research has reveled characteristics of PLCzeta such as the Ca(2+) oscillation-inducing activity after injection of PLCzeta-encoding RNA or recombinant PLCzeta into mouse eggs, extremely high Ca(2+)-sensitivity of the enzymatic activity in vitro, and nuclear translocation ability possibly related to cell-cycle-dependent regulation of Ca(2+) oscillations. [Ca(2+)](i) rises cause successive activation of calmodulin-dependent kinase II and E3 ubiquitin ligase, lead to proteolysis of ubiquitinated cyclin B1 and inactivation of metaphase-promoting factor (Cdk1/cyclin B1 complex), and result in the release of eggs from meiotic arrest.

The emergence of a general theory of the initiation and strength of the heartbeat

Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.

Functional role of store-operated and stretch-activated channels in murine adult skeletal muscle fibres

In skeletal muscle, Ca(2+) is implicated in contraction, and in regulation of gene expression. An alteration of [Ca(2+)](i) homeostasis is responsible, at least partially, for the muscle degeneration that occurs after eccentric contractions in Duchenne muscular dystrophy, a disease characterized by the loss of the cytoskeletal protein dystrophin. Using patch clamp in the cell-attached configuration, we characterized the store-operated channels (SOCs) and the stretch-activated channels (SACs) present in isolated mouse skeletal muscle. SOCs were voltage independent, had a unitary conductance between 7 and 8 pS (110 mm Ca(2+) in the pipette), and their open probability increased when the sarcoplasmic reticulum was depleted by thapsigargin. These SOCs were identical to those previously described in the pathophysiology of Duchenne muscular dystrophy. Under the same experimental conditions, we detected a channel activity that was increased by applying a negative pressure to the patch electrode. The SACs responsible for this current had the same unitary conductance and current-voltage relationship as those observed for SOCs. SOCs and SACs had a similar sensitivity to pharmacological agents such as Gd(3+), SKF-96365, 2-aminoethoxydiphenyl borate and GsMTx4 toxin. Moreover, stimulation with IGF-1 increased the occurrence of the activity of both channel types. Together, these observations suggest that SOCs and SACs might belong to the same population or share common constituents. From a functional point of view, treatment of soleus muscle with SKF-96365 or GsMTx4 toxin increased its sensitivity to a fatigue protocol, suggesting that the influx of Ca(2+) that occurs through these channels during contraction is also involved in force maintaining during repeated stimulations.

Cyclic ADP-ribose requires FK506-binding protein to regulate intracellular Ca2+ dynamics and catecholamine release in acetylcholine-stimulated bovine adrenal chromaffin cells

The present study was undertaken to elucidate whether cyclic ADP-ribose (cADPR) mediates the amplification of Ca2+ signaling and catecholamine release via the involvement of FK506-binding proteins (FKBPs)/ryanodine receptor (RyR) in bovine adrenal chromaffin cells. cADPR induced Ca2+ release in digitonin-permeabilized chromaffin cells and this was blocked by FK506 and rapamycin, ligands for FKBPs; 8Br-cADPR, a competitive antagonist for cADPR; and antibody for FKBP12/12.6, while it was enhanced by cyclosporin A. Ryanodine-induced Ca2+ release was not affected by 8Br-cADPR and was remarkably enhanced by FK506, rapamycin, cyclosporin A, and cADPR. FK506 binds to FKBP12.6 and removes it from RyRs, but cADPR did not affect the binding between FKBP12.6 and RyR. In intact chromaffin cells, 8Br-cADPR, FK506, and rapamycin, but not cyclosporin A attenuated the sustained intracellular free Ca2+ concentration ([Ca2+]i) rise induced by acetylcholine (ACh). 8Br-cADPR, FK506, and SK&F 96365 reduced the Mn2+ entry stimulated with ACh only when Ca2+ was present in the extracellular medium. 8Br-cADPR, FK506, and rapamycin concentration-dependently inhibited the ACh-induced catecholamine (CA) release. Here, we present evidence that FKBP12.6 associated with RyR may be required for Ca2+ release induced by cADPR in bovine adrenal chromaffin cells. cADPR-mediated Ca2+ release from endoplasmic reticulum in ACh-stimulated chromaffin cells is coupled with Ca2+ influx through the plasma membrane which is essential for ACh-stimulated CA release.

The junctional SR protein JP-45 affects the functional expression of the voltage-dependent Ca2+ channel Cav1.1

JP-45, an integral protein of the junctional face membrane of the skeletal muscle sarcoplasmic reticulum (SR), colocalizes with its Ca2+ -release channel (the ryanodine receptor), and interacts with calsequestrin and the skeletal-muscle dihydropyridine receptor Cav1. We have identified the domains of JP-45 and the Cav1.1 involved in this interaction, and investigated the functional effect of JP-45. The cytoplasmic domain of JP-45, comprising residues 1-80, interacts with Cav1.1. JP-45 interacts with two distinct and functionally relevant domains of Cav1.1, the I-II loop and the C-terminal region. Interaction between JP-45 and the I-II loop occurs through the alpha-interacting domain in the I-II loop. beta1a, a Cav1 subunit, also interacts with the cytosolic domain of JP-45, and its presence drastically reduces the interaction between JP-45 and the I-II loop. The functional effect of JP-45 on Cav1.1 activity was assessed by investigating charge movement in differentiated C2C12 myotubes after overexpression or depletion of JP-45. Overexpression of JP-45 decreased peak charge-movement and shifted VQ1/2 to a more negative potential (-10 mV). JP-45 depletion decreased both the content of Cav1.1 and peak charge-movements. Our data demonstrate that JP-45 is an important protein for functional expression of voltage-dependent Ca2+ channels.

Functional properties of ryanodine receptors carrying three amino acid substitutions identified in patients affected by multi-minicore disease and central core disease, expressed in immortalized lymphocytes

More than 80 mutations in the skeletal muscle ryanodine receptor gene have been found to be associated with autosomal dominant forms of malignant hyperthermia and central core disease, and with recessive forms of multi-minicore disease. Studies on the functional effects of pathogenic dominant mutations have shown that they mostly affect intracellular Ca2+ homoeostasis, either by rendering the channel hypersensitive to activation (malignant hyperthermia) or by altering the amount of Ca2+ released subsequent to physiological or pharmacological activation (central core disease). In the present paper, we show, for the first time, data on the functional effect of two recently identified recessive ryanodine receptor 1 amino acid substitutions, P3527S and V4849I, as well as that of R999H, another substitution that was identified in two siblings that were affected by multi-minicore disease. We studied the intracellular Ca2+ homoeostasis of EBV (Epstein-Barr virus)-transformed lymphoblastoid cells from the affected patients, their healthy relatives and control individuals. Our results show that the P3527S substitution in the homozygous state affected the amount of Ca2+ released after pharmacological activation with 4-chloro-m-cresol and caffeine, but did not affect the size of the thapsigargin-sensitive Ca2+ stores. The other substitutions had no effect on either the size of the intracellular Ca2+ stores, or on the amount of Ca2+ released after ryanodine receptor activation; however, both the P3527S and V4849I substitutions had a small but significant effect on the resting Ca2+ concentration.

Probing a putative dantrolene-binding site on the cardiac ryanodine receptor

Dantrolene is an inhibitor of intracellular Ca2+ release from skeletal muscle SR (sarcoplasmic reticulum). Direct photoaffinity labelling experiments using [3H]azidodantrolene and synthetic domain peptides have demonstrated that this drug targets amino acids 590-609 [termed DP1 (domain peptide 1)] of RyR1 (ryanodine receptor 1), the skeletal muscle RyR isoform. Although the identical sequence exists in the cardiac isoform, RyR2 (residues 601-620), specific labelling of RyR2 by dantrolene has not been demonstrated, even though some functional studies show protective effects of dantrolene on heart function. Here we test whether dantrolene-active domains exist within RyR2 and if so, whether this domain can be modulated. We show that elongated DP1 sequences from RyR1 (DP1-2s; residues 590-628) and RyR2 (DP1-2c; residues 601-639) can be specifically photolabelled by [3H]azidodantrolene. Monoclonal anti-RyR1 antibody, whose epitope is the DP1 region, can recognize RyR1 but not RyR2 in Western blot and immunoprecipitation assays, yet it recognizes both DP1-2c and DP1-2s. This suggests that although the RyR2 sequence has an intrinsic capacity to bind dantrolene in vitro, this site may be poorly accessible in the native channel protein. To examine whether it is possible to modulate this site, we measured binding of [3H]dantrolene to cardiac SR as a function of free Ca2+. We found that > or =10 mM EGTA increased [3H]dantrolene binding to RyR2 by approximately 2-fold. The data suggest that the dantrolene-binding site on RyR2 is conformationally sensitive. This site may be a potential therapeutic target in cardiovascular diseases sensitive to dysfunctional intracellular Ca2+ release.

Thirty years of calcium signals at fertilization

It was discovered about 30 years ago that a dramatic increase in intracellular calcium ion concentration ([Ca(2+)](i)) occurs at fertilization and that this increase acts as the pivotal signal for egg activation. Later, the Ca(2+) signal at fertilization turned out to be ubiquitous among animal species. Extensive advance has been brought during these 30 years in research on spatiotemporal aspects and signaling mechanisms of the [Ca(2+)](i) increase, sperm factors that induce the Ca(2+) response, and cell cycle resumption caused by the [Ca(2+)](i) rise. I provide a historical account of these advances in mammals, sea urchins, and a few other models.

Muscle activation assessment: Effects of method, stimulus number, and joint angle

Activation capacity has traditionally been assessed using the interpolated twitch technique (ITT) and central activation ratio (CAR). However, the quantitative agreement of the two methods and the physiological mechanisms underpinning any possible differences have not been fully elucidated. The aim of this study was to compare and assess the sensitivity of the ITT and CAR to potential errors introduced by (1) evoking inadequate force, by manipulating the number of stimuli, and (2) neglecting differences in series elasticity between conditions, by manipulating joint angle. Ten subjects performed knee extension contractions at 30 degrees and 90 degrees knee-joint angles during which the ITT and CAR methods were applied using 1, 2, 4, and 8 electrical stimuli. Joint angle influenced the ITT outcome with higher values taken at 90 degrees (P < 0.05), while the number of stimuli influenced the CAR outcome with a higher number of stimuli yielding lower values (P < 0.05). For any given joint angle and stimulus number, the CAR method produced higher activation values than the ITT method by 8%-16%. Therefore, in the quantification of voluntary drive with the ITT and CAR methods consideration should be given not only to the number of stimuli applied but also to the effect of series elasticity due to joint-angle differences, since these factors may differently affect the outcome of the calculation, depending on the approach followed.

Type-3 ryanodine receptor involved in Ca2+-induced Ca2+ release and transmitter exocytosis at frog motor nerve terminals

Ca(2+)-induced Ca2+ release (CICR) occurs in frog motor nerve terminals after ryanodine receptors (RyRs) are primed for activation by conditioning large Ca2+ entry. We studied which type of RyR exists, whether CICR occurs without conditioning Ca2+ entry and how RyRs are primed. Immunohistochemistry revealed the existence of RyR3 in motor nerve terminals and axons and both RyR1 and RyR3 in muscle fibers. A blocker of RyR, 8-(N,N-diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) slightly decreased rises in intracellular Ca2+ ([Ca2+]i) induced by a short tetanus (50 Hz, 1-2s), but not after treatment with ryanodine. Repetitive tetani (50 Hz for 15s every 20s) produced repetitive rises in [Ca2+]i, whose amplitude overall waxed and waned. TMB-8 blocked the waxing and waning components. Ryanodine suppressed a slow increase in end-plate potentials (EPPs) induced by stimuli (33.3 Hz, 15s) in a low Ca2+, high Mg2+ solution. KN-62, a blocker of Ca(2+)/calmoduline-activated protein kinase II (CaMKII), slightly reduced short tetanus-induced rises in [Ca2+]i, but markedly the slow waxing and waning rises produced by repetitive tetani in both normal and low Ca2+, high Mg2+ solutions. Likewise, KN-62, but not KN-04, an inactive analog, suppressed slow increases in EPP amplitude and miniature EPP frequency during long tetanus. Thus, CICR normally occurs weakly via RyR3 activation by single impulse-induced Ca2+ entry in frog motor nerve terminals and greatly after the priming of RyR via CaMKII activation by conditioning Ca2+ entry, thus, facilitating transmitter exocytosis and its plasticity.

Acetylcholine stimulates cyclic ADP-ribose formation via M1 muscarinic receptors in rat superior cervical ganglion

The role of cyclic ADP-ribose (cADPR) as the downstream signal of neuronal muscarinic acetylcholine receptors (mAChRs) and the enzyme responsible for its synthesis, ADP-ribosyl cyclase, were examined in the rat superior cervical ganglion (SCG). Application of acetylcholine or other mAChR agonists increased the ADP-ribosyl cyclase activity by about 250-300% in crude membrane fractions from the SCG of 14-day-old rats. This effect was inhibited by atropine or by the M1-mAChR antagonist, pirenzepine, and was mimicked by GTP. These results indicate that the M1 mAChRs couple to the membrane-bound form of ADP-ribosyl cyclase and suggest that cADPR is a second messenger of M1 mAChR signaling in nervous tissue.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

Amplification of depolarization-induced and ryanodine-sensitive cytosolic Ca2+ elevation by synthetic carbocyclic analogs of cyclic ADP-ribose and their antagonistic effects in NG108-15 neuronal cells

We synthesized analogs modified in the ribose unit (ribose linked to N1 of adenine) of cyclic ADP-ribose (cADPR), a Ca2+-mobilizing second messenger. The biological activities of these analogs were determined in NG108-15 neuroblastoma x glioma hybrid cells that were pre-loaded with fura-2 acetoxymethylester and subjected to whole-cell patch-clamp. Application of the hydrolysis-resistant cyclic ADP-carbocyclic-ribose (cADPcR) through patch pipettes potentiated elevation of the cytoplasmic free Ca2+ concentration ([Ca2+]i) at the depolarized membrane potential. The increase in [Ca2+]i evoked upon sustained membrane depolarization was significantly larger in cADPcR-infused cells than in non-infused cells and its degree was equivalent to or significantly greater than that induced by cADPR or beta-NAD+. 8-Chloro-cADPcR and two inosine congeners (cyclic IDP-carbocyclic-ribose and 8-bromo-cyclic IDP-carbocyclic-ribose) did not induce effects similar to those of cADPcR or cADPR. Instead, 8-chloro-cADPcR together with cADPR or cADPcR caused inhibition of the depolarization-induced [Ca2+]i increase as compared with either cADPR or cADPcR alone. These results demonstrated that our cADPR analogs have agonistic or antagonistic effects on the depolarization-induced [Ca2+]i increase and suggested the presence of functional reciprocal coupling between ryanodine receptors and voltage-activated Ca2+ channels via cADPR in mammalian neuronal cells.

A probable role of dihydropyridine receptors in repression of Ca2+ sparks demonstrated in cultured mammalian muscle

To activate skeletal muscle contraction, action potentials must be sensed by dihydropyridine receptors (DHPRs) in the T tubule, which signal the Ca(2+) release channels or ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) to open. We demonstrate here an inhibitory effect of the T tubule on the production of sparks of Ca(2+) release. Murine primary cultures were confocally imaged for Ca(2+) detection and T tubule visualization. After 72 h of differentiation, T tubules extended from the periphery for less than one-third of the myotube radius. Spontaneous Ca(2+) sparks were found away from the region of cells where tubules were found. Immunostaining showed RyR1 and RyR3 isoforms in all areas, implying inhibition of both isoforms by a T tubule component. To test for a role of DHPRs in this inhibition, we imaged myotubes from dysgenic mice (mdg) that lack DHPRs. These exhibited T tubule development similar to that of normal myotubes, but produced few sparks, even in regions where tubules were absent. To increase spark frequency, a high-Ca(2+) saline with 1 mM caffeine was used. Wild-type cells in this saline plus 50 microM nifedipine retained the topographic suppression pattern of sparks, but dysgenic cells in high-Ca(2+) saline did not. Shifted excitation and emission ratios of indo-1 in the cytosol or mag-indo-1 in the SR were used to image [Ca(2+)] in these compartments. Under the conditions of interest, wild-type and mdg cells had similar levels of free [Ca(2+)] in cytosol and SR. These data suggest that DHPRs play a critical role in reducing the rate of spontaneous opening of Ca(2+) release channels and/or their susceptibility to Ca(2+)-induced activation, thereby suppressing the production of Ca(2+) sparks.

Calcium signalling: Past, present and future

Ca2+ is a universal second messenger controlling a wide variety of cellular reactions and adaptive responses. The initial appreciation of Ca2+ as a universal signalling molecule was based on the work of Sydney Ringer and Lewis Heilbrunn. More recent developments in this field were critically influenced by the invention of the patch clamp technique and the generation of fluorescent Ca2+ indicators. Currently the molecular Ca2+ signalling mechanisms are being worked out and we are beginning to assemble a reasonably complete picture of overall Ca2+ homeostasis. Furthermore, investigations of organellar Ca2+ homeostasis have added complexity to our understanding of Ca2+ signalling. The future of the Ca2+ signalling field lies with detailed investigations of the integrative function in vivo and clarification of the pathology associated with malfunctions of Ca2+ signalling cascades.

Spontaneous Propagating Calcium Waves Underpin Airway Peristalsis in Embryonic Rat Lung

Prenatal airways from diverse species exhibit spontaneous peristaltic contractions (airway peristalsis). These contractile waves appear coupled to and may function to regulate prenatal lung growth. They are unaffected by atropine or tetrodotoxin but abolished by nifedipine. Nevertheless, the mechanisms by which these contractile waves are generated, regulated, and propagated remain obscure. Using calcium imaging and whole embryonic lung organ culture, we demonstrate for the first time that peristalsis of the embryonic airway is driven by spontaneous, regenerative, temperature-sensitive calcium (Ca2+) waves. These Ca2+ waves propagate between individual airway smooth muscle cells coupled via gap junctions, are likely to be action potential-mediated, and are dependent on not only extracellular calcium entry via L-type voltage-gated channels but also intracellular Ca2+ stores. Thus, if airway peristalsis regulates lung growth, these findings mean that airway smooth muscle Ca2+ waves in turn regulate prenatal lung morphogenesis.

Inactivation of Ca2+ transients in amphibian and mammalian muscle fibres

MagFluo-4 fluorescence (Ca2+) transients associated with action potentials were measured in intact muscle fibres, manually dissected from toads ( Leptodactylus insularis ) or enzymatically dissociated from mice. In toads, the decay phase of the Ca2+ transients is described by a single exponential with a time constant ( tau ) of about 7 ms. In mice, a double exponential function with tau 's of 1.5 and 15.5 ms, respectively gives a better fit. In both species the amplitude of Ca2+ transients diminished during repetitive stimulation: in amphibian muscle fibres, the decrease was about 20% with 1 Hz stimulation and 55% at 10 Hz. In mammalian fibres, repetitive stimulation causes a less conspicuous decrease of the transient amplitude: 10% at 1 Hz and 15% at 10 Hz. During tetanic stimulation at 100 Hz the transient amplitude decays to 20% in toad fibres and 40% in mouse fibres. This decrease could be associated with the phenomenon of inactivation of Ca2+ release, described by other authors. Recovery from inactivation, studied by a double stimuli protocol also indicates that in toad fibres the ability to release Ca2+ is abolished to a greater extent than in mouse fibres. In fact the ratio between the amplitudes of the second and first transient, when they are separated by a 10 ms interval, is 0.29 for toad and 0.58 for mouse fibres. In toad fibres, recovery from inactivation, to about 80 % of the initial value, occurs with a tau of 32 ms at 22 degrees C; while in mouse fibres recovery from inactivation is almost complete and occurs with a tau of 36 ms under the same conditions. The results indicate that Ca2+ release in enzymatically dissociated mammalian muscle fibres inactivates to a smaller extent than in intact amphibian muscle fibres.

Involvement of ryanodine receptors in pacemaker Ca2+ oscillation in murine gastric ICC

Using a cell cluster preparation from the stomach smooth muscle tissue of mice, we measured intracellular Ca(2+) oscillations in interstitial cells of Cajal (ICCs) in the presence of nifedipine. Pacemaker [Ca(2+)](i) activity in ICCs was significantly suppressed by caffeine application and restored after washout. Application of either ryanodine or FK-506 terminated the pacemaker [Ca(2+)](i) activity irreversibly. Immunostaining of smooth muscle tissue showed that c-Kit-immunopositive cells (that form network-like structure cells in the myenteric plexus, equivalent to ICCs) clearly express ryanodine receptors (RyR). RT-PCR revealed that ICCs (identified with c-Kit-immunoreactivity) predominantly express type 3 RyR (RyR3). Furthermore, the FK-binding proteins 12 and 12.6, both of which would interact with RyR3, were detected. In conclusion, we provide first evidence for the essential contribution of RyR to generating pacemaker activity in gastric motility. Similar mechanisms might account for spontaneous rhythmicity seen in smooth muscle tissues distributed in the autonomic nervous system.

Effect of bupivacaine enantiomers on Ca2+ release from sarcoplasmic reticulum in skeletal muscle

Local anesthetics affect intracellular Ca2+ movement in the myocyte. The use of isomers may help to reveal specific mechanisms of action, such as receptor mediation. In the present study, we used skinned fibers from mammalian skeletal muscle to test whether bupivacaine enantiomers had different effects on Ca2+ release and uptake by the sarcoplasmic reticulum, and on the Ca2+ sensitivity of the contractile system. Ca2+-induced Ca2+ release was enhanced by S-bupivacaine 1 approximately 3 mM, but inhibited by R-bupivacaine 3 mM, remaining unaffected at lower doses. These enantiomers inhibited Ca2+ uptake to different degrees, with R-bupivacaine having a stronger effect. Ca2+ sensitivity of the contractile system was equally enhanced by R- and S-bupivacaine. These findings might help to explain the myoplasmic Ca2+ elevation induced by bupivacaine. The observed stereoselectivity suggests effects on specific proteins, the ryanodine Ry1 receptor and the Ca2+-ATPase pump, rather than non-specific increase in Ca2+ permeability.

Regulation of Ca2+ sparks by Ca2+ and Mg2+ in mammalian and amphibian muscle. An RyR isoform-specific role in excitation-contraction coupling?

Ca²⁺ and Mg²⁺ are important mediators and regulators of intracellular Ca²⁺ signaling in muscle. The effects of changes of cytosolic [Ca²⁺] or [Mg²⁺] on elementary Ca²⁺ release events were determined, as functions of concentration and time, in single fast-twitch permeabilized fibers of rat and frog. Ca²⁺ sparks were identified and their parameters measured in confocal images of fluo-4 fluorescence. Solutions with different [Ca²⁺] or [Mg²⁺] were rapidly exchanged while imaging. Faster and spatially homogeneous changes of [Ca²⁺] (reaching peaks >100 μM) were achieved by photolysing Ca NP-EGTA with laser flashes. In both species, incrementing cytosolic [Ca²⁺] caused a steady, nearly proportional increase in spark frequency, reversible upon [Ca²⁺] reduction. A greater change in spark frequency, usually transient, followed sudden increases in [Ca²⁺] after a lag of 100 ms or more. The nonlinearity, lag, and other features of this delayed effect suggest that it requires increase of [Ca²⁺] inside the SR. In the frog only, increases in cytosolic [Ca²⁺] often resulted, after a lag, in sparks that propagated transversally. An increase in [Mg²⁺] caused a fall of spark frequency, but with striking species differences. In the rat, but not the frog, sparks were observed at 4–40 mM [Mg²⁺]. Reducing [Mg²⁺] below 2 mM, which should enable the RyR channel's activation (CICR) site to bind Ca²⁺, caused progressive increase in spark frequency in the frog, but had no effect in the rat. Spark propagation and enhancement by sub-mM Mg²⁺ are hallmarks of CICR. Their absence in the rat suggests that CICR requires RyR3 para-junctional clusters, present only in the frog. The observed frequency of sparks corresponds to a channel open probability of 10⁻⁷ in the frog or 10⁻⁸ in the rat. Together with the failure of photorelease to induce activation directly, this indicates a basal inhibition of channels in situ. It is proposed that relief of this inhibition could be the mechanism by which increased SR load increases spark frequency.

Postulated role of interdomain interactions within the type 1 ryanodine receptor in the low gain of Ca2+-induced Ca2+ release activity of mammalian skeletal muscle sarcoplasmic reticulum

Ryanodine receptor (RyR) type 1 (RyR1) exhibits a markedly lower gain of Ca(2+)-induced Ca(2+) release (CICR) activity than RyR type 3 (RyR3) in the sarcoplasmic reticulum (SR) of mammalian skeletal muscle (selective stabilization of the RyR1 channel), and this reduction in the gain is largely eliminated using 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS). We have investigated whether the hypothesized interdomain interactions within RyR1 are involved in the selective stabilization of the channel using [(3)H]ryanodine binding, single-channel recordings, and Ca(2+) release from the SR vesicles. Like CHAPS, domain peptide 4 (DP4, a synthetic peptide corresponding to the Leu(2442)-Pro(2477) region of RyR1), which seems to destabilize the interdomain interactions, markedly stimulated RyR1 but not RyR3. Their activating effects were saturable and nonadditive. Dantrolene, a potent inhibitor of RyR1 used to treat malignant hyperthermia, reversed the effects of DP4 or CHAPS in an identical manner. These findings indicate that RyR1 is activated by DP4 and CHAPS through a common mechanism that is probably mediated by the interdomain interactions. DP4 greatly increased [(3)H]ryanodine binding to RyR1 with only minor alterations in the sensitivity to endogenous CICR modulators (Ca(2+), Mg(2+), and adenine nucleotide). However, DP4 sensitized RyR1 four- to six-fold to caffeine in the caffeine-induced Ca(2+) release. Thus the gain of CICR activity critically determines the magnitude and threshold of Ca(2+) release by drugs such as caffeine. These findings suggest that the low CICR gain of RyR1 is important in normal Ca(2+) handling in skeletal muscle and that perturbation of this state may result in muscle diseases such as malignant hyperthermia.