Effects of tetracaine and procaine on skinned muscle fibres depend on free calcium

Intestinal enteroendocrine cells rely on ryanodine and IP3 calcium store receptors for mechanotransduction

Enteroendocrine cells (EECs) are specialized sensors of luminal forces and chemicals in the gastrointestinal (GI) epithelium that respond to stimulation with a release of signalling molecules such as serotonin (5-HT). For mechanosensitive EECs, force activates Piezo2 channels, which generate a very rapidly activating and inactivating (∼10 ms) cationic (Na+ , K+ , Ca2+ ) receptor current. Piezo2 receptor currents lead to a large and persistent increase in intracellular calcium (Ca2+ ) that lasts many seconds to sometimes minutes, suggesting signal amplification. However, intracellular calcium dynamics in EEC mechanotransduction remain poorly understood. The aim of this study was to determine the role of Ca2+ stores in EEC mechanotransduction. Mechanical stimulation of a human EEC cell model (QGP-1) resulted in a rapid increase in cytoplasmic Ca2+ and a slower decrease in ER stores Ca2+ , suggesting the involvement of intracellular Ca2+ stores. Comparing murine primary colonic EECs with colonocytes showed expression of intercellular Ca2+ store receptors, a similar expression of IP3 receptors, but a >30-fold enriched expression of Ryr3 in EECs. In mechanically stimulated primary EECs, Ca2+ responses decreased dramatically by emptying stores and pharmacologically blocking IP3 and RyR1/3 receptors. RyR3 genetic knockdown by siRNA led to a significant decrease in mechanosensitive Ca2+ responses and 5-HT release. In tissue, pressure-induced increase in the Ussing short circuit current was significantly decreased by ryanodine receptor blockade. Our data show that mechanosensitive EECs use intracellular Ca2+ stores to amplify mechanically induced Ca2+ entry, with RyR3 receptors selectively expressed in EECs and involved in Ca2+ signalling, 5-HT release and epithelial secretion. KEY POINTS: A population of enteroendocrine cells (EECs) are specialized mechanosensors of the gastrointestinal (GI) epithelium that respond to mechanical stimulation with the release of important signalling molecules such as serotonin. Mechanical activation of these EECs leads to an increase in intracellular calcium (Ca2+ ) with a longer duration than the stimulus, suggesting intracellular Ca2+ signal amplification. In this study, we profiled the expression of intracellular Ca2+ store receptors and found an enriched expression of the intracellular Ca2+ receptor Ryr3, which contributed to the mechanically evoked increases in intracellular calcium, 5-HT release and epithelial secretion. Our data suggest that mechanosensitive EECs rely on intracellular Ca2+ stores and are selective in their use of Ryr3 for amplification of intracellular Ca2+ . This work advances our understanding of EEC mechanotransduction and may provide novel diagnostic and therapeutic targets for GI motility disorders.

IL-1α reversibly inhibits skeletal muscle ryanodine receptor. a novel mechanism for critical illness myopathy?

Critical illness myopathies in patients with sepsis or sustained mechanical ventilation prolong intensive care treatment and threaten both patients and health budgets; no specific therapy is available. Underlying pathophysiological mechanisms are still patchy. We characterized IL-1α action on muscle performance in "skinned" muscle fibers using force transducers and confocal Ca(2+) fluorescence microscopy for force/Ca(2+) transients and Ca(2+) sparks. Association of IL-1α with sarcoplasmic reticulum (SR) release channel, ryanodine receptor (RyR) 1, was investigated with coimmunoprecipitation and confocal immunofluorescence colocalization. Membrane integrity was studied in single, intact fibers challenged with IL-1α. IL-1α reversibly stabilized Mg(2+) inhibition of Ca(2+) release. Low Mg(2+)-induced force and Ca(2+) transients were reversibly abolished by IL-1α. At normal Mg(2+), IL-1α reversibly increased caffeine-induced force and Ca(2+) transients. IL-1α reduced SR Ca(2+) leak via RyR1, as judged by (1) increased SR Ca(2+) retention, (2) increased IL-1α force transients being reproduced by 25 μM tetracaine, and (3) reduced Ca(2+) spark frequencies by IL-1α or tetracaine. Coimmunoprecipitation confirmed RyR1/IL-1 association. RyR1/IL-1 immunofluorescence patterns perfectly colocalized. Long-term, 8-hour IL-1α challenge of intact muscle fibers compromised membrane integrity in approximately 50% of fibers, and confirmed intracellular IL-1α deposition. IL-1α exerts a novel, specific, and reversible interaction mechanism with the skeletal muscle RyR1 macromolecular release complex without the need to act via its membrane IL-1 receptor, as IL-1R membrane expression levels were not detectable in Western blots or immunostaining of single fibers. We present a potential explanation of how the inflammatory mediator, IL-1α, may contribute to muscle weakness in critical illness.

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.

Myotoxizität von Lokalanästhetika

Intramuscular injections of local anaesthetic agents regularly result in reversible muscle damage, with a dose-dependent extent of the lesions. All local anaesthetic agents that have been examined are myotoxic, whereby procaine produces the least and bupivacaine the most severe muscle injury. The histological pattern and the time course of skeletal muscle injury appear relatively uniform: hypercontracted myofibrils become evident directly after injection, followed by lytic degeneration of striated muscle sarcoplasmic reticulum myocyte edema and necrosis. Intriguingly, in most cases myoblasts, basal laminae and connective tissue elements remain intact which subsequently ensures complete muscular regeneration. Subcellular pathomechanisms of local anaesthetic myotoxicity are still not understood in detail. Increased intracellular Ca(2+) levels are suggested to be the most important element in myocyte injury, since denervation, inhibition of sarcolemmal Na(+) channels and direct toxic effects on myofibrils have been excluded as sites of action. Although experimental myotoxic effects are impressively intense and reproducible, only few case reports of myotoxic complications in patients after local anaesthetic administration have been published. In particular, the occurrence of clinically relevant myopathy and myonecrosis has been described after continuous peripheral blockades, infiltration of wound margins, trigger point injections, peribulbar and retrobulbar blocks.

Different signaling pathways in bovine sperm regulate capacitation and hyperactivation

Hyperactivated sperm motility is characterized by high-amplitude and asymmetrical flagellar beating that assists sperm in penetrating the oocyte zona pellucida. Other functional changes in sperm, such as activation of motility and capacitation, involve cross talk between the cAMP/PKA and tyrosine kinase/phosphatase signaling pathways. Our objective was to determine the role of the cAMP/protein kinase A (PKA) signaling pathway in hyperactivation. Western blot analyses of detergent extracts of whole sperm and flagella were performed using antiphosphotyrosine antibody. Bull sperm capacitated by 10 microg/ml heparin and/or 1 mM dibutyryl-cAMP plus 100 microM 3-isobutyl-1-methylxanthine exhibited increased protein tyrosine phosphorylation without becoming hyperactivated. Procaine (5 mM) or caffeine (10 mM) immediately induced hyperactivation in nearly 100% of motile sperm but did not increase protein tyrosine phosphorylation. After 4 h of incubation with caffeine, sperm expressed capacitation-associated protein tyrosine phosphorylation but hyperactivation was significantly reduced. Sperm initially hyperactivated by procaine or caffeine remained hyperactivated for at least 4 h in the presence of Rp-cAMPS (cAMP antagonist) or PKA inhibitors H-89 or H-8. Pretreatment with inhibitors also failed to block induction of hyperactivation; however, the inhibitors did block protein tyrosine phosphorylation when sperm were incubated with capacitating agents, thereby verifying inhibition of the cAMP/PKA pathway. While induction of hyperactivation did not depend on cAMP/PKA, it did require extracellular Ca(2+). These findings indicate that hyperactivation is mediated by a Ca(2+) signaling pathway that is separate or divergent from the pathway associated with acquisition of acrosomal responsiveness and does not involve protein tyrosine phosphorylation downstream of the actions of procaine or caffeine.

Cold-induced calcium elevation triggers DNA fragmentation in immature pig oocytes

Fluo-4 loaded immature oocytes were cooled from 30 degrees C to various lower temperatures between 20 and 10 degrees C and changes in intracellular calcium (Ca(2+)) levels were measured. Pig oocytes cooled to 14 degrees C exhibited a clear biphasic Ca(2+) rise. Lower temperatures produced similar responses, while higher temperatures did not exert any effect. The Ca(2+) response appeared to rely on ryanodine dependent stores as removal of extracellular Ca(2+) and intracytoplasmic injection of heparin did not modify cold-induced Ca(2+) elevation, while procaine or ruthenium red virtually eliminated the response. Confocal analysis of subcellular Ca(2+) distribution during cooling revealed that the ion rises sharply within the nucleus. As Ca(2+) imbalance may activate nuclear endonucleases, DNA integrity of cooled pig oocytes was evaluated by TUNEL and comet assays. Most cooled oocytes showed clear signs of DNA fragmentation. Oocytes injected with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetracetic acid tetrapotassium salt (BAPTA), a Ca(2+) chelator, maintained their DNA integrity thus confirming that intracellular Ca(2+) is involved in triggering DNA fragmentation. The protective effect exerted by ruthenium red and/or procaine further confirmed this hypothesis. These data show that a moderate and transient cooling is sufficient to cause an intracellular Ca(2+) rise that leads to DNA damage. The addition of inhibitors of ryanodine dependent Ca(2+) stores may represent a valuable protective treatment to reduce chilling injuries.

The effect of isoflurane on the release of [(3)H]-acetylcholine from rat brain cortical slices

Volatile general anaesthetics are believed to affect synaptic transmission, but their actions in the central nervous system (CNS) remains unclear. Acetylcholine (ACh) is one of the most important neurotransmitter in the CNS and thus, it is possible that its release could be one of the targets for volatile anaesthetic action. However, the effects of these agents on the release of ACh are not yet fully understood. Rat brain cortical slices were loaded with [(3)H]-choline in order to study the effect of isoflurane on the release of [(3)H]-ACh from this preparation. Isoflurane (28, 43, 54, 95 and 182 nM) significantly increased the basal release of [(3)H]-ACh. This effect was independent of the extracellular sodium and calcium concentration but was decreased by tetracaine and dantrolene, inhibitors of Ca(2+-)release from intracellular stores. These findings indicate that isoflurane may cause a Ca(2+-)release from internal stores that increases [(3)H]-ACh release in rat brain cortical slices.

Halothane enhances exocytosis of [3H]-acetylcholine without increasing calcium influx in rat brain cortical slices

1. The effect of halothane on the release of [3H]-acetylcholine ([3H]-ACh) in rat brain cortical slices was investigated. 2. Halothane (0.018 mM) did not significantly affect the basal and the electrical field stimulation induced release of [3H]-ACh. However, halothane (0.063 mM) significantly increased the basal release of [3H]-ACh and this effect was additive with the electrical field stimulation induced release of [3H]-ACh. 3. The release of [3H]-ACh induced by 0.063 mM halothane was independent of the extracellular sodium and calcium ion concentration and was decreased by tetracaine, an inhibitor of Ca(2+)-release from intracellular stores or dantrolene, an inhibitor of Ca(2+)-release from ryanodine-sensitive stores 4. Using 2-(4-phenylpiperidino)-cyclohexanol (vesamicol), a drug that blocks the storage of ACh in synaptic vesicles, we investigated whether exocytosis of this neurotransmitter is involved in the effect of halothane. Vesamicol significantly decreased the release of [3H]-ACh evoked by halothane. 5. It is suggested that halothane may cause a Ca2+ release from intracellular stores that increases [3H]-ACh exocytosis in rat brain cortical slices.

The structure, function, and cellular regulation of ryanodine-sensitive Ca2+ release channels

The fundamental biological process of Ca2+ signaling is known to be important in most eukaryotic cells, and inositol 1,2,5-trisphosphate and ryanodine receptors, intracellular Ca2+ release channels encoded by two distantly related gene families, are central to this phenomenon. Ryanodine receptors in the sarcoplasmic reticulum of skeletal and cardiac muscle have a predominant role in excitation-contraction coupling, but the channels are also present in the endoplasmic reticulum of noncontractile tissues including the central nervous system and the immune system. In all, three highly homologous ryanodine receptor isoforms have been identified, all very large proteins which assemble as (homo)tetramers of approximately 2 MDa. They contain large cytoplasmically disposed regulatory domains and are always associated with other structural or regulatory proteins, including calmodulin and immunophilins, which can have marked effects on channel function. The type 1 isoform in skeletal muscle is electromechanically coupled to surface membrane voltage sensors, whereas the remaining isoforms appear to be activated solely by endogenous cytoplasmic second messengers or other ligands, including Ca2+ itself ("Ca(2+)-induced Ca2+ release"). This review concentrates on ryanodine receptor structure-function relationships as probed by a variety of methods and on the molecular mechanisms of channel modulation at the cellular level (including evidence for the regulation of gene expression and transcription). It also touches on the relevance of ryanodine receptors to complex cellular functions and disease.

Inner sarcolemmal leaflet Ca(2+) binding: its role in cardiac Na/Ca exchange

A recently completed model of Ca concentration and movements in the cardiac cell diadic cleft space predicts that removal or neutralization of inner sarcolemmal (SL) leaflet anionic Ca-binding sites at the sarcolemmal border of this space will greatly diminish Na/Ca exchange-mediated Ca efflux. The present study tests this prediction using the local anesthetic dibucaine as a probe. It is shown, in isolated SL, that dibucaine competitively displaces Ca specifically from anionic phospholipid headgroups. Dibucaine also displaces Ca from the SL when applied to intact cells. It does not affect the content or release of Ca from sarcoplasmic reticulum (SR) in these cells. This eliminates a primary effect on SR Ca as a contributing factor to dibucaine's effect on Na/Ca exchange-mediated Ca efflux. Measurement of this efflux from whole cells shows a highly significant reduction of 58% (p < 0.001) by 0.5 mM dibucaine. The inhibiting effect of dibucaine on Na/Ca exchange-mediated Ca efflux can be significantly reversed by augmentation of Ca release from SR by caffeine at the time of activation of Na/Ca exchange. This supports the contention that the dibucaine-SL interaction is a competitive one vis-a-vis Ca. The results are supportive of the model in which inner SL leaflet Ca-binding sites account for the delay of Ca diffusion from the diadic cleft, thereby prolonging the time for which [Ca] remains elevated in the cleft. The prolonged increased [Ca] significantly enhances the ability of Na/Ca exchange to remove Ca from the cell during the excitation-contraction cycle.

Electrophysiologic effects of cocaine on subendocardial Purkinje fibers surviving 1 day of myocardial infarction

INTRODUCTION

Cocaine has been shown to have broad cardiovascular effects that could be life threatening. Most of the reported electrophysiologic effects of cocaine have been studied in normal but not infarcted myocardium.

METHODS AND RESULTS

Using microelectrode techniques, we investigated the electrophysiologic effects of cocaine on endocardial canine Purkinje fibers that survived 1 day of myocardial infarction. In quiescent infarcted preparations, stimulated trains were followed by subthreshold delayed afterdepolarizations (DADs), in the presence of propranolol (1 microM). Cocaine (10 microM) decreased the amplitude of DADs from 6.1 +/- 1.8 mV to 3.0 +/- 1.3 mV (P < 0.05, n = 6). When stimulated preparations (n = 23) showing no triggered activity during control (+propranolol) were superfused with a low concentration of caffeine (1 mM) or high extracellular Ca2+ (8.1 mM), triggered activity was induced. Subsequent cocaine (10 microM) superfusion prevented the induction of caffeine- and high Ca(2+)-induced triggered activity. Cocaine's effects were reversible upon washout. In preparations that showed triggered activity during control conditions (+propranolol), the mean cycle length of triggered activity was 755 +/- 45 msec. Cocaine (10 microM) superfusion lengthened the cycle length to 1030 +/- 141 msec and terminated triggered activity with a subthreshold DAD (n = 12). In addition, cocaine and ryanodine (10 microM) suppressed triggered activity in a similar manner when tested in the same preparations (n = 4). During control conditions, cocaine did not cause any significant change on the rate of rise of action potential upstroke (from 55.6 +/- 24.3 to 54.5 +/- 28.6 V/sec, n = 8) and maximum diastolic potential (from -58.4 +/- 4.3 to -56.6 +/- 6.5 mV, n = 8). In the absence of propranolol, 50 microM but not 10 microM cocaine induced early afterdepolarizations in 62% of the preparations exhibiting triggered activity during control conditions.

CONCLUSION

The results suggest that cocaine modulates DADs and triggered activity in infarcted endocardial fibers via direct inhibition of cyclic release of Ca2+ from sarcoplasmic reticulum (SR) independently from a local anesthetic or sympathomimetic effect. This SR inhibition could account for the myocardial depressant effect of cocaine. However, while cocaine suppressed DADs, its induction of EADs can precipitate malignant ventricular arrhythmias in the setting of cocaine overdose and infarction.

Voltage control of calcium transients elicited by caffeine and tetracaine in cultured rat muscle cells

Cultured hind limb skeletal muscle cells from newborn rats were used to study the effect of caffeine and tetracaine upon intracellular Ca2+ release under voltage or current clamp conditions. Free [Ca2+]i was measured using the fluorescent calcium-sensitive dye Fluo-3. A field containing one or several myotubes was observed with a video camera and image analysis of fluorescence changes was performed. Addition of 100-500 microM tetracaine to the external saline elicited strong fluorescence responses in non-clamped cells, but significantly lower responses in cells clamped at -90 mV. At the same time, tetracaine inhibited voltage induced calcium release. Voltage and tetracaine modulation over the action of caffeine (500 microM) was also observed. Pretreatment of cells with 10 microM nifedipine abolished the caffeine induced fluorescence response in non-clamped cells. These findings suggest that, in cultured muscle cells, calcium release through the caffeine and tetracaine sensitive pathways is controlled by both membrane potential and the dihydropyridine receptor.

Intracellular Ca2+ transients induced by high external K+ and tetracaine in cultured rat myotubes

Cultured myotubes from rat neonatal skeletal muscle were used to measure intracellular Ca2+ concentration ([Ca2+]i) and membrane potentials (Vm) using the Indo-1 microfluorimetry method and the nystatin perforated membrane patch technique, respectively. Sudden increases in external [K+]o from 5 mM to either 22, 42 or 84 mM elicited transient elevations in [Ca2+]i from a resting level of 106.2 +/- 10.3 nM (n = 41) to peak values of 297, 409 and 454 nM, respectively. Vm changes induced by elevated [K+]o followed the Nernst equation for [K+]o. The complex Ca2+ release response induced by elevated [K+]o can be described by a minimal model involving two components with different kinetics. This analysis revealed that the extent of the Ca2+ release by the fast component bears a sigmoidal relationship with Vm (midpoint at -47.5 mV and an effective valence of 4). Furthermore, while the fast transitory component was rather insensitive to [Ca2+]o and nifedipine, the slow component was profoundly inhibited by the dihydropyridine (10 microM) both in normal and in a Ca2+ deficient medium. Tetracaine (0.05 to 2 mM), a blocker of the charge movement associated with excitation-contraction (E-C) coupling, elicited a fast elevation in [Ca2+]i followed by a rise at a constant rate to levels as high as 1-2 microM, and the changes in [Ca2+]i were readily reversible. Simultaneous measurements of Vm and [Ca2+]i suggest that the fast component is coupled to the rapid depolarization of the membrane induced by the anesthetic. We concluded that tetracaine triggers the release of Ca2+ from internal stores by at least two different mechanisms, one of which is associated with the depolarizing effects of the drug.

The interaction of local anesthetics with the ryanodine receptor of the sarcoplasmic reticulum

The effects of various local anesthetics (LAs) on the skeletal muscle ryanodine receptor were tested. The LAs were divided into three categories according to their effects on the binding of ryanodine to the junctional sarcoplasmic reticulum membranes. Ryanodine binding was assayed in the presence of 0.2 M NaCl and 10 microM CaCl2. Tetracaine and dibucaine inhibit the binding with half-maximal inhibition (CI50) of 0.12 and 0.25 mM, respectively, while inhibition by benzocaine and procaine occurs with CI50 of about 10-fold higher. Lidocaine, its analogue QX-314, and prilocaine, on the other hand, stimulate the binding up to fourfold with half-maximal stimulation occurring with about 2 mM of the drugs. Lidocaine increases both the receptor affinity for ryanodine by about fivefold and the rate of ryanodine association with its binding site by about 10-fold. Tetracaine interacts with the ryanodine receptor in a non-competitive fashion with respect to ryanodine but it competes with lidocaine for its binding site, suggesting the existence of a single site for the inhibitory and stimulatory LA. The LAs also interact with the purified ryanodine receptor and produce effects similar to those with the membrane-bound receptor. Tetracaine and dibucaine inhibit binding of the photoreactive ATP analogue; [alpha-32P]benzoyl-benzoyl ATP (BzATP) to the ATP regulatory site of the ryanodine receptor, and high concentrations of ATP decrease the degree of ryanodine binding inhibition by tetracaine, indicating the relationship between the receptor conformations stabilized by ATP and LAs. Based on a structure-activity relationship, a model for the LA site of interaction in the ryanodine receptor is suggested.

Procaine effects on single sarcoplasmic reticulum Ca2+ release channels

The effects of the Ca(2+)-induced Ca2+ release blocker procaine on individual sarcoplasmic reticulum Ca2+ release channels have been examined in planar lipid bilayers. Procaine did not reduce the single channel conductance nor appreciably shorten the mean open times of the channel; rather, it increased the longest closed time. These results indicated that procaine interacted selectively with a closed state of the channel rather than with an open state. Gating of the sarcoplasmic reticulum Ca2+ release channel was described by a modified scheme of Ashley and Williams (1990. J. Gen. Physiol. 95:981-1005), including an additional long-lived closed state. Computer simulations determined that procaine was more likely to interact with this long-lived Ca(2+)-bound closed state of the channel rather than with other states of the channel. Simulations with the same model were also able to reproduce a prominent Ca(2+)-sensitive transition between "random" and "bursting" forms of gating of the channel, variations of which may account for "gearshift" behavior reported in studies with this and other single channels.

Effects of local anesthetics on single channel behavior of skeletal muscle calcium release channel

The effects of the two local anesthetics tetracaine and procaine and a quaternary amine derivative of lidocaine, QX314, on sarcoplasmic reticulum (SR) Ca2+ release have been examined by incorporating the purified rabbit skeletal muscle Ca2+ release channel complex into planar lipid bilayers. Recordings of potassium ion currents through single channels showed that Ca(2+)- and ATP-gated channel activity was reduced by the addition of the tertiary amines tetracaine and procaine to the cis (cytoplasmic side of SR membrane) or trans (SR lumenal) side of the bilayer. Channel open probability was lowered twofold at tetracaine and procaine concentrations of approximately 150 microM and 4 mM, respectively. Hill coefficients of 2.0 and greater indicated that the two drugs inhibited channel activity by binding to two or more cooperatively interacting sites. Unitary conductance of the K(+)-conducting channel was not changed by 1 mM tetracaine in the cis and trans chambers. In contrast, cis millimolar concentrations of the quaternary amine QX314 induced a fast blocking effect at positive holding potentials without an apparent change in channel open probability. A voltage-dependent block was observed at high concentrations (millimolar) of tetracaine, procaine, and QX314 in the presence of 2 microM ryanodine which induced the formation of a long open subconductance. Vesicle-45Ca2+ ion flux measurements also indicated an inhibition of the SR Ca2+ release channel by tetracaine and procaine. These results indicate that local anesthetics bind to two or more cooperatively interacting high-affinity regulatory sites of the Ca2+ release channel in or close to the SR membrane. Voltage-dependent blockade of the channel by QX314 in the absence of ryanodine, and by QX314, procaine and tetracaine in the presence of ryanodine, indicated one low-affinity site within the conduction pathway of the channel. Our results further suggest that tetracaine and procaine may primarily inhibit excitation-contraction coupling in skeletal muscle by binding to the high-affinity, regulatory sites of the SR Ca2+ release channel.

Different localization of inositol 1,4,5-trisphosphate and ryanodine binding sites in rat liver

The distribution of inositol 1,4,5-trisphosphate and ryanodine binding sites between plasma membrane, microsomal, and mitochondrial fractions of rat liver were compared. IP3 bound mostly to the plasma membrane fraction (Kd = 6 nM; Bmax = 802 fmol/mg protein). Some IP3 binding sites were also present in the microsomal and mitochondrial fractions (Kd = 2.5 and 2.9 nM; Bmax = 35 and 23 fmol/mg protein respectively). The possibility that these binding sites are due to contamination of the fractions with plasma membrane cannot be excluded. Binding of IP3 to the plasma membrane was inhibited by heparin but not by either caffeine or tetracaine. High-affinity ryanodine binding sites were present mostly in the microsomal fraction (Kd = 13 nM; Bmax = 301 fmol/mg protein). Lower affinity binding sites were also found to be present in the mitochondrial and plasma membrane fractions. Binding of ryanodine to the microsomal fraction was inhibited by both caffeine and tetracaine but not by heparin. These data demonstrate that IP3 and ryanodine binding sites are present in different cellular compartments in the liver. These differences in the localization of the binding sites might be indicative of their functional differences.

Effects of procaine and caffeine on calcium release from the sarcoplasmic reticulum in frog skeletal muscle

1. Resting myoplasmic free [Ca2+] and [Ca2+] transients (delta [Ca2+]) were measured in single voltage-clamped frog skeletal muscle fibres in the presence and absence of procaine, caffeine or procaine plus caffeine using Fura-2 fluorescence and antipyrylazo III (Ap III) absorbance signals. The rate of release (Rrel) of calcium from the sarcoplasmic reticulum (SR) was calculated from the calcium transients and corrected for the relatively small decline due to depletion of calcium from the SR. 2. Procaine (1 mM) reversibly suppressed delta [Ca2+] and the corresponding Rrel by about 40% for 60-100 ms depolarizing steps to -40 to +20 mV. Procaine had little effect on either the waveform or voltage dependence of the Rrel records. 3. [Ca2+] transients calculated from Fura-2 fluorescence changes in the presence or absence of procaine had similar time courses and amplitudes as those calculated from the Ap III absorbance changes suggesting that 1 mM-procaine did not interfere with the ability of Ap III or Fura-2 to monitor delta [Ca2+]. 4. Although 1 mM-procaine depressed Rrel it had no effect on intramembrane charge movements (IQ) calculated from membrane currents recorded simultaneously with delta [Ca2+]. 5. Procaine (1 mM) reversibly inhibited the potentiating effect of 0.5 mM-caffeine on delta [Ca2+]. The amplitude and waveform of the Rrel records were similar in control fibres and in the presence of 1 mM-procaine plus 0.5 mM-caffeine. 6. In the presence of 0.5 mM-caffeine delta [Ca2+] after 10-20 ms voltage steps exhibited an increase in the time to peak and a slower decay time course compared with caffeine-free controls, suggestive of significant calcium-induced calcium release in the presence of caffeine. These effects of caffeine were completely and reversibly blocked by 1 mM-procaine. 7. In the absence of caffeine, 1 mM-procaine caused a small decrease in time to peak of delta [Ca2+] after 10-30 ms duration voltage steps compared to the bracketing control and wash runs without procaine. Rrel turned off faster after 10 ms pulses in procaine than in the absence of procaine, but the turn-off of release was about equally fast with or without procaine after pulses of 20 ms or longer. The effect of procaine after 10 ms pulses in the absence of caffeine may indicate suppression of a component of calcium-induced calcium release in control that inactivates during the pulse.(ABSTRACT TRUNCATED AT 400 WORDS)

Auranofin dissociates chemotactic peptide-induced generation of inositol 1,4,5-trisphosphate from the subsequent mobilization of intracellular calcium in intact human neutrophils

Auranofin, an antiarthritic gold compound, modulates a number of chemotactic factor-induced inflammatory responses in human neutrophils. In order to unravel the mechanism involved, the present study investigated the effects of auranofin on early signal transduction events in these cells. Auranofin did not affect the chemotactic peptide (fMetLeuPhe)-induced formation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), neither in the presence nor in the absence of extracellular calcium ions. In contrast, there was a progressive inhibition by auranofin on the fMet-Leu-Phe-induced mobilization of intracellular calcium. This demonstrates that auranofin can dissociate the generation of Ins(1,4,5)P3 from the subsequent release of intracellular calcium, perhaps by interfering with the intracellular binding of Ins(1,4,5)P3 to its receptor. In experiments performed in electro-permeabilized cells, however, a relatively high concentration of the drug failed to abolish the specific binding of Ins(1,4,5)P3. In addition, in the same system, auranofin also failed to abolish the Ins(1,4,5)P3-induced release of Ca2+. Consequently, auranofin-mediated dissociation of fMLP-induced Ins(1,4,5)P3 formation and intracellular calcium release can not be explained merely by an antagonistic effect of auranofin on the Ins(1,4,5)P3 receptor. Instead the interaction between auranofin and the plasma membrane seems to be an initial and important part of the mechanism by which this drug interferes with the transduction signalling system.

The heavy metal ions Ag+ and Hg2+ trigger calcium release from cardiac sarcoplasmic reticulum

Heavy metal ions have been shown to induce Ca2+ release from skeletal sarcoplasmic reticulum (SR) by binding to free sulfhydryl groups on a Ca2+ channel protein and are now examined in cardiac SR. Ag+ and Hg2+ (at 10-25 microM) induced Ca2+ release from isolated canine cardiac SR vesicles whereas Ni2+, Cd2+, and Cu2+ had no effect at up to 200 microM. Ag(+)-induced Ca2+ release was measured in the presence of modulators of SR Ca2+ release was compared to Ca2(+)-induced Ca2+ release and was found to have the following characteristics. (i) Ag(+)-induced Ca2+ release was dependent on free [Mg2+], such that rates of efflux from actively loaded SR vesicles increased by 40% in 0.2 to 1.0 mM Mg2+ and decreased by 50% from 1.0 to 10.0 mM Mg2+. (ii) Ruthenium red (2-20 microM) and tetracaine (0.2-1.0 mM), known inhibitors of SR Ca2+ release, inhibited Ag(+)-induced Ca2+ release. (iii) Adenine nucleotides such as cAMP (0.25-2.0 mM) enhanced Ca2(+)-induced Ca2+ release, and stimulated Ag(+)-induced Ca2+ release. (iv) Low Ag+ to SR protein ratios (5-50 nmol Ag+/mg protein) stimulated Ca2(+)-dependent ATPase activity in Triton X-100-uncoupled SR vesicles. (v) At higher ratios of Ag+ to SR proteins (50-250 nmol Ag+/mg protein), the rate of Ca2+ efflux declined and Ca2(+)-dependent ATPase activity decreased gradually, up to a maximum of 50% inhibition. (vi) Ag+ stimulated Ca2+ efflux from passively loaded SR vesicles (i.e., in the absence of ATP and functional Ca2+ pumps), indicating a site of action distinct from the SR Ca2+ pump. Thus, at low Ag+ to SR protein ratios, Ag+ is very selective for the Ca2+ release channel. At higher ratios, this selectivity declines as Ag+ also inhibits the activity of Ca2+,Mg2(+)-ATPase pumps. Ag+ most likely binds to one or more sulfhydryl sites "on" or "adjacent" to the physiological Ca2+ release channel in cardiac SR to induce Ca2+ release.