Cyclic ADP-ribose induced Ca release in rabbit skeletal muscle sarcoplasmic reticulum

The Ca²⁺-mobilizing metabolite cyclic ADP-ribose (cADPR) has been shown to release Ca²⁺ from ryanodine-sensitive stores in many cells. We show that this metabolite at a concentration of 17μM, but not its precursor β-NAD⁺ nor non-cyclic ADPR at the same concentration, is active in releasing Ca²⁺ from rabbit skeletal muscle sarcoplasmic reticulum. The release was not sensitive to Ruthenium red (1μM) nor to the ryanodine receptor-specific scorpion toxin Buthotus₁-1 (10 μM). In planar bilayer single channel recordings, concentrations up to 50μM cADPR did not increase the open probability of Ruthenium red and toxin-sensitive Ca²⁺ release channels. Thus Ca²⁺ release induced by cADPR in skeletal muscle sarcoplasmic reticulum may not involve opening of ryanodine receptors.     

Ca2+ sparks in skeletal muscle

The study of Ca²⁺ sparks has led to extensive new information regarding the gating of the Ca²⁺ release channels underlying these events in skeletal, cardiac and smooth muscle cells, as well as the possible roles of these local Ca²⁺ release events in muscle function. Here we review basic procedures for studying Ca²⁺sparks in skeletal muscle, primarily from frog, as well as the basic results concerning the properties of these events, their pattern and frequency of occurrence during fiber depolarization and the mechanisms underlying their termination. Finally, we also consider the contribution of different ryanodine receptor (RyR) isoforms to Ca²⁺ sparks and the number of RyR Ca²⁺ release channels that may contribute to the generation of a Ca²⁺ spark. Over the decade since their discovery, Ca²⁺ sparks have provided a wealth of information concerning the function of Ca²⁺ release channels within their intracellular environment.     

Ins(1,4,5)P3 induces Ca2+ release from brain microsomes loaded either by the Ca2+ ATPase or by the Na+/Ca2+ exchanger.

In this study we investigated the release of Ca²⁺ in brain microsomes after Ca²⁺ loading by the Ca²⁺-ATPase or by the Na⁺/Ca²⁺ exchanger. The results show that in microsomes loaded with Ca²⁺ by the Ca²⁺-ATPase, Ins(1,4,5)P₃ (5 μM) release 21±2% of the total Ca²⁺ accumulated, and that in the microsomes loaded with Ca²⁺ by the Na²⁺/Ca²⁺ exchanger, Ins(1,4,5)P₃ released 28±3% of the total Ca²⁺ accumulated. These results suggest that receptors of Ins(1,4,5)P₃ may be co-localized with the Na²⁺/Ca²⁺ exchanger in the endoplasmic reticulum membrane or that there are Ins(1,4,5)P₃ receptors in the plasma membrane where the Na²⁺/Ca²⁺ exchanger is normally present, or both. We also found that Ins(1,4,5)P₃ inhibited the Ca²⁺-ATPase by 33.7%, but that it had no significant effect on the Na²⁺/Ca²⁺ exchanger.     

The function of Ca channel subtypes in exocytotic secretion: new perspectives from synaptic and non-synaptic release

By mediating the Ca²⁺ influx that triggers exocytotic fusion, Ca²⁺ channels play a central role in a wide range of secretory processes. Ca²⁺ channels consist of a complex of protein subunits, including an ₁ subunit that constitutes the voltage-dependent Ca²⁺-selective membrane pore, and a group of auxiliary subunits, including β, γ, and ₂–δ subunits, which modulate channel properties such as inactivation and channel targeting. Subtypes of Ca²⁺ channels are constituted by different combinations of ₁ subunits (of which 10 have been identified) and auxiliary subunits, particularly β (of which 4 have been identified). Activity-secretion coupling is determined not only by the biophysical properties of the channels involved, but also by the relationship between channels and the exocytotic apparatus, which may differ between fast and slow types of secretion. Colocalization of Ca²⁺ channels at sites of fast release may depend on biochemical interactions between channels and exocytotic proteins. The aim of this article is to review recent work on Ca²⁺ channel structure and function in exocytotic secretion. We discuss Ca²⁺ channel involvement in selected types of secretion, including central neurotransmission, endocrine and neuroendocrine secretion, and transmission at graded potential synapses. Several different Ca²⁺ channel subtypes are involved in these types of secretion, and their function is likely to involve a variety of relationships with the exocytotic apparatus. Elucidating the relationship between Ca²⁺ channel structure and function is central to our understanding of the fundamental process of exocytotic secretion.     

The Ca 2+ leak paradox and rogue ryanodine receptors: SR Ca 2+ efflux theory and practice

Ca²⁺ efflux from the sarcoplasmic reticulum (SR) is routed primarily through SR Ca²⁺ release channels (ryanodine receptors, RyRs). When clusters of RyRs are activated by trigger Ca²⁺ influx through L-type Ca²⁺ channels (dihydropyridine receptors, DHPR), Ca²⁺ sparks are observed. Close spatial coupling between DHPRs and RyR clusters and the relative insensitivity of RyRs to be triggered by Ca²⁺ together ensure the stability of this positive-feedback system of Ca²⁺ amplification. Despite evidence from single channel RyR gating experiments that phosphorylation of RyRs by protein kinase A (PKA) or calcium-calmodulin dependent protein kinase II (CAMK II) causes an increase in the sensitivity of the RyR to be triggered by [Ca²⁺]_i there is little clear evidence to date showing an increase in Ca²⁺ spark rate. Indeed, there is some evidence that the SR Ca²⁺ content may be decreased in hyperadrenergic disease states. The question is whether or not these observations are compatible with each other and with the development of arrhythmogenic extrasystoles that can occur under these conditions. Furthermore, the appearance of an increase in the SR Ca²⁺ “leak” under these conditions is perplexing. These and related complexities are analyzed and discussed in this report. Using simple mathematical modeling discussed in the context of recent experimental findings, a possible resolution to this paradox is proposed. The resolution depends upon two features of SR function that have not been confirmed directly but are broadly consistent with several lines of indirect evidence: (1) the existence of unclustered or “rogue” RyRs that may respond differently to local [Ca²⁺]_i in diastole and during the [Ca²⁺]_i transient; and (2) a decrease in cooperative or coupled gating between clustered RyRs in response to physiologic phosphorylation or hyper-phosphorylation of RyRs in disease states such as heart failure. Taken together, these two features may provide a framework that allows for an improved understanding of cardiac Ca²⁺ signaling.     

Vascular physiology of a Ca2+ mobilizing second messenger - cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a novel Ca²⁺ mobilizing second messenger, which is capable of inducing Ca²⁺ release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors (RyR) in vascular cells. This signaling nucleotide has also been reported to participate in generation or modulation of intracellular Ca²⁺ sparks ²⁺waves or oscillations, Ca²⁺-induced Ca²⁺ release (CICR) and spontaneous transient outward currents (STOCs) in vascular smooth muscle cells (VSMCs). With respect to the role of cADPR-mediated signaling in mediation of vascular responses to different stimuli, there is accumulating evidence showing that cADPR is importantly involved in the Ca²⁺ response of vascular endothelial cells (ECs) and VSMCs to various chemical factors such as vasoactive agonists acetylcholine, oxotemorine, endothelin, and physical stimuli such as stretch, electrical depolarization and sheer stress. This cADPR-RyR-mediated Ca²⁺ signaling is now recognized as a fundamental mechanism regulating vascular function. Here we reviewed the literature regarding this cADPR signaling pathway in vascular cells with a major focus on the production of cADPR and its physiological roles in the control of vascular tone and vasomotor response. We also summarized some publish results that unveil the underlying mechanisms mediating the actions of cADPR in vascular cells. Given the importance of Ca²⁺ in the regulation of vascular function, the results summarized in this brief review will provide new insights into vascular physiology and circulatory regulation.     

Cyclic ADP-ribose and inositol 1,4,5-trisphosphate mobilizes Ca from distinct intracellular pools in permeabilized lacrimal acinar cells

In permeabilized lacrimal acinar cells, cyclic ADP-ribose (cADP-ribose) and inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) release Ca²⁺ in a dose dependent manner from distinct thapsigargin-sensitive Ca²⁺ pools. Ryanodine specifically blocks the Ca²⁺ response to cADP-ribose, whereas heparin strongly reduces the response to Ins(1,4,5)P₃ application. GTP causes a rapid Ca²⁺ release by a ryanodine- and heparin-insensitive mechanism and potentiates Ins(1,4,5)P₃ but not cADP-ribose evoked Ca²⁺ release. It is estimated that cADP-ribose can release 16 μmol Ca²⁺/I cells, whereas Ins(1,4,5)P₃ can mobilize 55 μmol Ca²⁺/I cells. The results suggest that cADP-ribose and Ins(1,4,5)P₃ release Ca²⁺ from distinct internal stores and that a third Ca²⁺ pool exists which can selectively interact with the Ins(1,4,5)P₃-sensitive Ca²⁺ store by a GTP-mediated process.     

Neurotensin elevates cytosolic calcium in small cell lung cancer cells

The ability of neurotensin (NT) to elevate cytosolic Ca²⁺ in small cell lung cancer (SCLC) cells was investigated using the fluorescent Ca²⁺ indicator Fura 2-AM. Using SCLC cell line NCI-H345, NT elevated cytosolic Ca²⁺ levels in a concentration-dependent manner. Using a 10 nM dose, NT and C-terminal fragments such as NT(8–13) but not N-terminal fragments such as NT(1–8) elevated the cytosolic Ca²⁺ levels. Because EGTA (5 mM) did not affect the NT response, NT may cause release of Ca²⁺ from intracellular stores. These data indicate that SCLC NT receptors may use Ca²⁺ as a second messenger.     

Fluoxetine-induced inhibition of synaptosomal [H] 5-HT release: Possible CA-channel inhibition

Fluoxetine, a selective 5-HT uptake inhibitor, inhibited 15 mM K⁺-induced [³H] 5-HT release from rat spinal cord and cortical synaptosomes at concentrations > 0.5 uM. This effect reflected a property shared by another selective 5-HT uptake inhibitor paroxetine but not by less selective uptake inhibitors such as amitriptyline, desipramine, imipramine or nortriptyline. Inhibition of release by fluoxetine was inversely related to both the concentration of K⁺ used to depolarize the synaptosomes and the concentration of external Ca²⁺. Experiments aimed at determining a mechanism of action revealed that fluoxetine did not inhibit voltage-independent release of [³H] 5-HT release induced by the Ca²⁺-ionophore A 23187 or Ca²⁺-independent release induced by fenfluramine. Moreover the 5-HT autoreceptor antagonist methiothepin did not reverse the inhibitory actions of fluoxetine on K⁺-induced release. Further studies examined the effects of fluoxetine on voltage-dependent Ca²⁺ channels and Ca²⁺ entry. Whereas fluoxetine and paroxetine inhibited binding of [³H] nitrendipine to the dihydropyridine-sensitive L-type Ca²⁺ channel, the less selective uptake inhibitors did not alter binding. The dihydropyridine antagonist nimodipine partially blocked fluoxetine-induced inhibition of release. Moreover enhanced K⁺-stimulated release due to the dihydropyridine agonist Bay K 8644 was reversed by fluoxetine. Fluoxetine also inhibited the K⁺-induced increase in intracellular free Ca²⁺ in fura-2 loaded synaptosomes. These data are consistent with the suggestion that fluoxetine inhibits K⁺-induced [³H] 5-HT release by antagonizing voltage-dependent Ca²⁺ entry into nerve terminals.     

Mechanism of the activation of arachidonic acid release by oxysterols in NRK 49F cells: Role of calcium

We previously demonstrated that oxysterols added to the culture medium of NRK 49F cells labelled with [¹⁴C] arachidonic acid potentiated arachidonic acid (AA) release and prostaglandin (PG) E₂ biosynthesis induced by the activation of these cells with fetal calf serum (FCS). In the absence of FCS, oxysterols had no effect on AA release. As phospholipase (Plase) A₂ activity is Ca²⁺-dependent, we investigated whether oxysterol potentiating effect on AA release was related to an effect of these compounds on cell Ca²⁺ concentration. In this paper, we show that the intensity of potentiation by oxysterol varies with the external cell Ca²⁺ concentration; when external Ca²⁺ is chelated by EGTA, the oxysterol effect persists, though it is decreased. The Ca²⁺ channel inhibitor nifedipine does not decrease the potentiating effect of 25-OH cholesterol, indicating that, if oxysterol favours Ca²⁺ entry into the cell, the nifedipine inhibited channel is not involved. At the usual concentration (5 μm/ml), oxysterols are not able to increase, mimmediately or after a short time of contact (90 min) the concentration of intracellular free Ca²⁺ ([Ca²⁺])_i measured by fluorescence of Quinn-2; at very high concentration of oxysterol (25 μm/ml), [Ca²⁺]_i only slightly increases (+30%). The liberation of AA induced by cell activation with the Ca²⁺ ionophore ionomycin is also potentiated by 25-OH cholesterol. All these observations are not in favour of a proper effect o oxysterols on cell Ca²⁺ level.     

Effects of cholinergic agonists on diacylglycerol and intracellular calcium levels in pancreatic β-cells

We have studied the effects of cholinegic agonists on the rates of insulin release and the concentrations of diacylglycerol (DAG) and intracellular free Ca²⁺ ([Ca²⁺]_i) in the β-cell line MIN6. Insulin secretion was stimulated by glucose, by glibenclamide and by bombesin. In the presence of glucose, both acetylcholine (ACh) and carbachol (CCh) produced a sustained increase in the rate of insulin release which was blocked by EGTA or verapamil. The DAG content of MIN6 β-cells was not affected by glucose. Both CCh and ACh evoked an increase in DAG which was maximal after 5 min and returned to basal after 30 min; EGTA abolished the cholinergic-induced increased in DAG. ACh caused a transient rise in [Ca²⁺]_i which was abolished by omission of Ca²⁺ or by addition of devapamil. Thus, cholinergic stimulation of β-cell insulin release is associated with changes in both [Ca²⁺]_i and DAG. The latter change persists longer than the former and activation of protein kinase C and sensitization of the secretory process to Ca²⁺ may underlie the prolonged effects of cholinergic agonists on insulin release. However, a secretory response to CCh was still evident after both [Ca²⁺]_i and DAG had returned to control values suggesting that additional mechanisms may be involved.     

Modulation of calcium-evoked [3H]noradrenaline release from permeabilized cerebrocortical synaptosomes by the MARCKS protein, calmodulin and the actin cytoskeleton

In order to examine intracellular modulation of CNS catecholamine release, cerebrocortical synaptosomes were prelabeled with [³H]noradrenaline and permeabilized with streptolysin-O in the absence or presence of Ca²⁺. Plasma membrane permeabilization allowed efflux of cytosol and left a compartmentalized pool of [³H]noradrenaline intact, approximately 10% of which was released by addition of 10⁻⁵ M Ca²⁺. Addition of activators or inhibitors of protein kinase C, as well as inhibitors of Ca²⁺-calmodulin kinase II or calcineurin, failed to change Ca²⁺-induced noradrenaline release. Evoked release from permeabilized synaptosomes deficient in the vesicle-associated phosphoprotein synapsin I was also unchanged. In contrast, addition of a synthetic ‘active domain’ peptide from the myristoylated, alanine-rich C-kinase substrate (MARCKS) protein increased, while addition of calmodulin decreased Ca²⁺-induced release from the permeabilized synaptosomes, the latter effect being reversed by a peptide inhibitor of calcineurin. Moreover, addition of the actin-destabilizing agent DNase I, as well as antibodies to MARCKS, appeared to increase spontaneous, Ca²⁺-independent release from noradrenergic vesicles. These results indicate that the MARCKS protein may modulate release from permeabilized noradrenergic synaptosomes, possibly by modulating calmodulin levels and/or the actin cytoskeleton.     

2, 5-Di(Tert-Butyl)-1, 4-Benzohydroquinone —A Novel Mobilizer Of The Inositol 1,4,5-Trisphosphate-Sensitive Ca2+ Pool

Isolated hepatocytes and the isolated perfused rat liver have been used to study the alterations of cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) produced by 2,5-di(tert-butyl)-l.4-benzohydroquinone (tBuBHQ), a potent inhibitor of hepatic microsomal Ca²⁺ sequestration (Moore. G.A., McConkey. D.J., Kass, G.E.N., OBrien, P.J. and Orrenius, S. FEBS Lett.,224, 331-336). (1987). Addition of tBuBHQ to isolated hepatocytes caused a rapid increase in [Ca²⁺]_i which was similar in magnitude to the [Ca²⁺]_i elevation induced by the Ca²⁺ mobilizing hormone, vasopressin. In contrast with vasopressin which caused a Ca²⁺ transient, tBuBHQ elevated [Ca²⁺]_i to a new steady state that was maintained for up to 15-20min. When vasopressin was administered during the tBuBHQ-induced period of elevated [Ca²⁺]_i. [Ca²⁺]_i, rapidly returned to basal levels. Similarly, if vasopressin was administered just prior to tBuBHQ, the resultant tBuBHQ-dependent change in [Ca²⁺]_i was transient. and not sustained. The hydroquinone mobilized the same intracellular Ca²⁺ pool as inositol 1,4,5-trisphosphate. but tBuBHQ did not produce any detectable inositol polyphosphate accumulation. IBuBHQ stimulated glucose release from perifused hepatocytes. mimicking the effect of vasopressin. In the perfused liver, tBuBHQ infusion produced a single, slow and prolonged release of Ca²⁺ into the perfusate and inhibition of subsequent vasopressin-induced Ca²⁺ effluxes. Inhibition of the response to vasopressin was reversed over time, and closely correlated with the extent of inhibition of both Ca²⁺ sequestration and (Ca²⁺-Mg²⁺)-ATPase activity in microsomes isolated from the isolated perfused liver. The present results are consistent with tBuBHQ inhibiting ATP-dependent Ca²⁺ sequestration by a direct effect on the endoplasmic reticular Ca²⁺ pump, which results in net Ca²⁺ release and elevation of [Ca²⁺]_i. Furthermore. vasopressin appears to stimulate active removal of increased [Ca²⁺] from the hepatocyte cytosol by a mechanism which does not depend on reuptake of Ca²⁺ into the endoplasmic reticulum

2,5-Di(tert-butyl) -l,4-benmhydroquinone. calcium. hepatocytes. perfused liver, endoplasmic reticulum     

神经元钙信号系统异常与阿尔茨海默病的“钙假说”

钙信号是细胞调节各项生命活动的重要机制。神经元通过胞外钙离子(calcium ion, Ca²⁺)内流、内质网Ca²⁺释放以及Ca²⁺释放介导的Ca²⁺内流等方式产生具有时空特异性的钙信号,用于调控多种生物学过程,例如动作电位的调节、神经递质的释放、轴突的生长以及突触可塑性等。神经元胞内Ca²⁺浓度因受到细胞精确调控而处于动态平衡之中。若钙信号失调导致平衡被打破,则会造成神经元功能异常甚至死亡。近年来多项研究表明,钙稳态失衡与神经退行性疾病,例如阿尔茨海默病等的产生和发展密切相关,由此发展出关于阿尔茨海默病的钙假说。该假说认为,神经元钙稳态调节机制的持续性改变是神经元功能失常、大脑产生慢性疾病的重要因素。阿尔茨海默病发生发展过程中,神经元胞浆钙水平异常增高,致使多种钙依赖性酶的活性异常,进而影响基因转录。虽然内质网钙稳态的变化目前仍存在一定的争议,但较为确定的是线粒体中存在着钙超载的现象,导致氧化磷酸化反应下调,活性氧的产量增加,进而引发细胞凋亡。本文主要介绍了神经元钙信号系统及其功能,简要梳理了阿尔茨海默病钙假说的相关研究,并对后续研究进行了展望。     

Calcium-dependent mitochondrial formation of species promoting strand scission of genomic DNA in U937 cells exposed to tert-butylhydroperoxide: the role of arachidonic acid

Treatment of U937 cells with a sublethal concentration of tert-butylhydroperoxide generates DNA single strand breakage in U937 cells and this response is increased by caffeine, ATP, pyruvate or antimycin A. As we previously reported (Guidarelli, Clementi, Brambilla and Cantoni, (1997) Biochem. J. 328, 801-806), the enhancing effects of antimycin A are mediated by inhibition of complex III and the ensuing formation of superoxides and hydrogen peroxide in a reaction in which ubisemiquinone serves as an electron donor. Active electron transport was required in pyruvate-supplemented cells since the increased genotoxic response occurred as a consequence of enforced mitochondrial Ca²⁺ accumulation, a process driven by the increased electrochemical gradient. The enhancing effects of caffeine or ATP were also the consequence of mitochondrial Ca²⁺ accumulation but these responses were independent on electron transport. The increased formation of DNA lesions resulting from exposure to tert-butylhydroperoxide associated with the Ca²⁺-mobilizing agents or the respiratory substrate was mediated by arachidonic acid generated by Ca²⁺-dependent activation of phospholipase A₂. Melittin, a potent phospholipase A₂ activator, and reagent arachidonic acid mimicked the effects of caffeine, ATP or pyruvate on the tert-butylhydroperoxide-induced DNA single strand breakage.     

油桃花芽破眠过程中H2O2代谢与Ca2+转运的关系

利用化学测定法分析高温、单氰胺和TDZ 3种破眠处理对“曙光”油桃休眠花芽H₂O₂代谢的主要影响,利用非损伤微测技术检测H₂O₂对休眠芽Ca²⁺转运的影响,研究H₂O₂在芽休眠解除过程中的调控作用.结果表明： 在深休眠时期,高温和单氰胺处理均能诱导芽内H₂O₂含量升高和过氧化氢酶(CAT)活性降低,并具有显著的破眠作用;TDZ对H₂O₂含量及CAT、过氧化物酶(POD)活性影响不大,破眠效果较差.休眠花芽原基组织钙通道活跃,对外源Ca²⁺呈吸收状态.外源H₂O₂可诱导休眠花芽原基组织Ca²⁺转运发生变化,低浓度H₂O₂降低Ca²⁺吸收速率,高浓度H₂O₂使组织对Ca²⁺的转运由吸收转变为释放.这表明休眠芽内H₂O₂信号和Ca²⁺信号相关联,通过诱导H₂O₂积累调控Ca²⁺信号可能在高温和单氰胺打破休眠的信号转导过程中起重要作用.     

Ca entry through the store-mediated pathway directly activates only the K current but the subsequent Ca release from the store activates both K and Cl currents in submandibular gland acinar cells of the rat

The store-mediated Ca²⁺ entry was detected in single and cluster of rat submandibular acinar cells by measuring the Ca²⁺ activated ionic membrane currents. In the cells where intracellular Ca²⁺ was partly depleted by stimulation with submaximal concentration of acetylcholine (ACh) under a Ca²⁺-free extracellular condition, an employment of external Ca²⁺ in the absence of ACh caused a sustained increase of the K⁺ current without affecting the Cl⁻ current. A renewed ACh challenge without external Ca²⁺ caused repetitive spikes of both K⁺ and Cl⁻ currents due to the Ca²⁺ release. SK & F 96365 inhibited the generation of the sustained K⁺ current and refilling of the Ca²⁺ store following the Ca²⁺ readmission. It is suggested that the Ca²⁺ enters the cell through the store-mediated pathway near the K⁺ channels and is taken up by the store. Thus, only Ca²⁺ released from the store can activate both the K⁺ and Cl⁻ currents.     

The function of ATP in Ca uptake by rat brain mitochondria

Ca²⁺ uptake by rat brain mitochondria was studied under different experimental conditions. The most rapid uptake of Ca²⁺ occurred in the presence of ATP, succinate and P_i. ATP alone also supported Ca²⁺ uptake. In contrast, no Ca²⁺ uptake occurred with succinate and P_i when no ATP was added. Oligomycin and atractylate completely inhibited ATP-supported Ca²⁺ uptake but produced only a partial inhibition of Ca²⁺ transport in the presence of ATP, succinate and P_i. ATP plays a dual role in its action on brain mitochondria; it can support Ca²⁺ uptake by itself and it serves a function in allowing respiration-dependent Ca²⁺ uptake to proceed. The latter role of ATP does not involve transfer of energy from the nucleotide.     

Release of endogenous GABA can occur through Ca-dependent and Ca-independent processes

Release of γ-aminobutyric acid (GABA) can be elicited by electrical field stimulation even in the absence of external Ca²⁺. Indeed, the release of GABA under such conditions is even higher than in the presence of Ca²⁺. To investigate the underlying mechanism of this phenomenon, the release of endogenous GABA from rat striatal slices was measured by high performance liquid chromatography with electro- chemical detection. Electrical stimulation at 2 Hz for 3 min elevated GABA efflux by 4.5-fold. Withdrawing external Ca²⁺ and adding 1 mM EGTA produced a small, transient increase in the basal efflux of GABA and increased electrically-evoked overflow 3-fold. Tetrodotoxin (5 μM) did not affect basal efflux in either normal or Ca²⁺-free conditions, but abolished electrically-evoked release. In the presence of normal Ca²⁺, nipecotic acid (1 mM) enhanced both spontaneous efflux and evoked overflow. Nipecotic acid also increased spontaneous release when external Ca²⁺ was removed. However, in the absence of Ca²⁺, nipecotic acid failed to increase electrically evoked GABA overflow. These results suggest that there exists a Ca²⁺-independent process for GABA release via the same carrier system that is utilized for high affinity GABA uptake.     

Arrhythmogenic Ca(2+) release from cardiac myofilaments

We investigated the initiation of Ca²⁺waves underlying triggered propagated contractions (TPCs) occurring in rat cardiac trabeculae under conditions that simulate the functional non-uniformity caused by mechanical or ischemic local damage of the myocardium. A mechanical discontinuity along the trabeculae was created by exposing the preparation to a small constant flow jet of solution with a composition that reduces excitation–contraction coupling in myocytes within that segment. Force was measured and sarcomere length as well as [Ca²⁺]_i were measured regionally. When the jet-contained Caffeine, BDM or Low-[Ca²⁺], muscle-twitch force decreased and the sarcomeres in the exposed segment were stretched by shortening of the normal regions outside the jet. During relaxation the sarcomeres in the exposed segment shortened rapidly. Short trains of stimulation at 2.5 Hz reproducibly caused Ca²⁺-waves to rise from the borders exposed to the jet. Ca²⁺-waves started during force relaxation of the last stimulated twitch and propagated into segments both inside and outside of the jet. Arrhythmias, in the form of non-driven rhythmic activity, were triggered when the amplitude of the Ca²⁺-wave increased by raising [Ca²⁺]_o. The arrhythmias disappeared when the muscle uniformity was restored by turning the jet off. We have used the four state model of the cardiac cross bridge (Xb) with feedback of force development to Ca²⁺ binding by Troponin-C (TnC) and observed that the force–Ca²⁺ relationship as well as the force–sarcomere length relationship and the time course of the force and Ca²⁺ transients in cardiac muscle can be reproduced faithfully by a single effect of force on deformation of the TnC·Ca complex and thereby on the dissociation rate of Ca²⁺. Importantly, this feedback predicts that rapid decline of force in the activated sarcomere causes release of Ca²⁺ from TnC.Ca²⁺,which is sufficient to initiate arrhythmogenic Ca²⁺ release from the sarcoplasmic reticulum. These results show that non-uniform contraction can cause Ca²⁺-waves underlying TPCs, and suggest that Ca²⁺ dissociated from myofilaments plays an important role in the initiation of arrhythmogenic Ca²⁺-waves.