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1.
To examine the effects of the overexpression of sarcoplasmic reticulum (SR) CaATPase on function of the SR and Ca2+homeostasis, we measured [Ca2+]itransients (fluo-3), and L-type Ca2+currents (ICa,L), Na/Ca exchanger currents (INa/Ca), and SR Ca2+content with voltage clamp in ventricular myocytes isolated from wild type (WT) mice and transgenic (SRTG) mice. The amplitude of [Ca2+]itransients was insignificantly increased in SRTG myocytes, while the diastolic [Ca2+]itended to be lower. The initial and terminal declines of [Ca2+]itransients were significantly accelerated in SRTG myocytes, implying a functional upregulation of the SR CaATPase. We examined the functional contribution of only the SR CaATPase to the initial and the terminal phase of the decline of [Ca2+]i, by abruptly inhibiting Na/Ca exchange with a rapid switcher device. The rate of [Ca2+] decline mediated by the SR CaATPase was increased by 40% in SRTG compared with WT myocytes. The function of the L-type Ca2+channel was unchanged in SRTG myocytes, while INa/Ca density was slightly (10%) decreased. Measured SR Ca2+content was significantly increased by 29% in SRTG myocytes. Thus, overexpression of SR CaATPase markedly accelerates the decline of [Ca2+]itransients, and induces an increase in SR Ca2+content, with some downregulation of the Na/Ca exchanger.  相似文献   

2.
Reactive oxygen species (ROS) and intracellular Ca2+ overload play key roles in myocardial ischemia-reperfusion (IR) injury but the relationships among ROS, Ca2+ overload and LV mechanical dysfunction remain unclear. We tested the hypothesis that H2O2 impairs LV function by causing Ca2+ overload by increasing late sodium current (INa), similar to Sea Anemone Toxin II (ATX-II). Diastolic and systolic Ca2+ concentrations (d[Ca2+]i and s[Ca2+]i) were measured by indo-1 fluorescence simultaneously with LV work in isolated working rat hearts. H2O2 (100 μM, 30 min) increased d[Ca2+]i and s[Ca2+]i. LV work increased transiently then declined to 32% of baseline before recovering to 70%. ATX-II (12 nM, 30 min) caused greater increases in d[Ca2+]i and s[Ca2+]i. LV work increased transiently before declining gradually to 17%. Ouabain (80 μM) exerted similar effects to ATX-II. Late INa inhibitors, lidocaine (10 μM) or R56865 (2 μM), reduced effects of ATX-II on [Ca2+]i and LV function, but did not alter effects of H2O2. The antioxidant, N-(2-mercaptopropionyl)glycine (MPG, 1 mM) prevented H2O2-induced LV dysfunction, but did not alter [Ca2+]i. Paradoxically, further increases in [Ca2+]i by ATX-II or ouabain, given 10 min after H2O2, improved function. The failure of late INa inhibitors to prevent H2O2-induced LV dysfunction, and the ability of MPG to prevent H2O2-induced LV dysfunction independent of changes in [Ca2+]i indicate that impaired contractility is not due to Ca2+ overload. The ability of further increases in [Ca2+]i to reverse H2O2-induced LV dysfunction suggests that Ca2+ desensitization is the predominant mechanism of ROS-induced contractile dysfunction.  相似文献   

3.
Growth hormone release in goldfish is partly dependent on voltage-sensitive Ca2+ channels but somatotrope electrophysiological events affecting such channel activities have not been elucidated in this system. The electrophysiological properties of goldfish somatotropes in primary culture were studied using the whole-cell and amphotericin B-perforated patch-clamp techniques. Intracellular Ca2+ concentration ([Ca2+]i) of identified somatotropes was measured using Fura-2/AM dye. Goldfish somatotropes had an average resting membrane potential of −78.4 ± 4.6 mV and membrane input resistance of 6.2 ± 0.2 GΩ. Voltage steps from a holding potential of −90 mV elicited a non-inactivating outward current and transient inward currents at potentials more positive than 0 and −30 mV, respectively. Isolated current recordings indicate the presence of 4-aminopyridine- and tetraethylammonium (TEA)-sensitive K+, tetrodotoxin (TTX)-sensitive Na+, and nifedipine (L-type)- and ω-conotoxin GVIA (N-type)-sensitive Ca2+ channels. Goldfish somatotropes rarely fire action potentials (APs) spontaneously, but single APs can be induced at the start of a depolarizing current step; this single AP was abolished by TTX and significantly reduced by nifedipine and ω-conotoxin GVIA. TEA increased AP duration and triggered repetitive AP firing resulting in an increase in [Ca2+]i, whereas TTX, nifedipine and ω-conotoxin GVIA inhibited TEA-induced [Ca2+]i pulses. These results indicate that in goldfish somatotropes, TEA-sensitive K+ channels regulate excitability while TTX-sensitive Na+ channels together with N- and L-type Ca channels mediates the depolarization phase of APs. Opening of voltage-sensitive Ca2+ channels during AP firing leads to increases in [Ca2+]i.  相似文献   

4.
This study was designed to examine the effect of infrasound exposure (5 Hz at 130 dB) on whole-cell L-type Ca2+ currents (WLCC) in rat ventricular myocytes and the underlying mechanism(s) involved. Thirty-two adult Sprague-Dawley rats were randomly assigned to infrasound exposure and control groups. [Ca2+]i, WLCC, mRNA expression of the a1c subunit of L-type Ca2+ channels (LCC), and SERCA2 protein were examined on day 1, 7, and 14 after initiation of infrasound exposure. Fluo-3/AM fluorescence and the laser scanning confocal microscope techniques were used to measure [Ca2+]i in freshly isolated ventricular myocytes. The Ca2+ fluorescence intensity (FI), denoting [Ca2+]i in cardiomyocytes, was significantly elevated in a time-dependent manner in the exposure groups. There was a significant increase in WLCC in the 1-day group and a further significant increase in the 7- and 14-day groups. LCC mRNA expression measured by RT-PCR revealed a significant rise in the 1-day group and a significant additional rise in the 7- and 14-day groups compared with control group. SERCA2 expression was significantly upregulated in the 1-day group followed by an overt decrease in the 7- and 14-day groups. Prolonged exposure of infrasound altered WLCC in rat cardiomyocytes by shifting the steady-state inactivation curves to the right (more depolarized direction) without altering the slope and biophysical properties of I Ca,L. Taken together, our data suggest that changes in [Ca2+]I levels as well as expression of LCC and SERCA2 may contribute to the infrasound exposure-elicited cardiac response. Zhaohui Pei and Zhiqiang Zhuang contributed equally to this work.  相似文献   

5.
The vasodilating mechanisms of the K+ channel openers—cromakalim, pinacidil, nicorandil, KRN2391, and Ki4032—were examined by measurement of the cytoplasmic Ca2+ concentration ([Ca2+]i) using the fura-2 method in canine or porcine coronary arterial smooth muscle. The five K+ channel openers all produced a reduction of [Ca2+]i in 5 and 30 mM KCl physiological salt solution (PSS), the effects of which were antagonized by tetrabutylammonium (TBA) or glibenclamide, but failed to affect [Ca2+]i in 45 and 90 mM MCl-PSS. Cromakalim and Ki4032 only partially inhibited the 30 mM KCl-induced contractures, whereas pinacidil, nicorandil, and KRN2391 nearly abolished contractions produced by high KCl-PSS. The increased [Ca2+]i and force produced by a thromboxane A2 analogue, U46619, were inhibited by K+ channel openers and verapamil. In the absence of extracellular Ca2+, U46619 induced a transient increase in [Ca2+]i with a contraction, which is effectively inhibited by cromakalim and Ki4032. Their inhibitory effects were blocked by TBA and counteracted by 20 mM KCl-induced depolarization. Cromakalim and Ki4032 did not affect caffeine-induced Ca2+ release. Cromakalim reduced U46619-induced IP3 production and TBA blocked this inhibitory effect. Thus, cromakalim and Ki4032 are more specific K+ channel openers than pinacidil, nicorandil, and KRN2391. The vasodilation related with a reduction of [Ca2+]i produced by K+ channel openers is due to the hyperpolarization of the plasma membrane resulting in not only the closure of voltage-dependent Ca2+ channels but also inhibition of the production of IP3 and Ca2+ release from intracellular stores related to stimulation of the thromboxane A2 receptor.  相似文献   

6.
Aims/hypothesis Islets or beta cells from Sur1–/– mice were used to determine whether changes in plasma membrane potential (Vm) remain coupled to changes in cytosolic Ca2+ ([Ca2+]i) in the absence of KATP channels and thus provide a triggering signal for insulin secretion. The study also sought to elucidate whether [Ca2+]i influences oscillations in Vm in sur1–/– beta cells.Methods Plasma membrane potential and ion currents were measured with microelectrodes and the patch–clamp technique. [Ca2+]i was monitored with the fluorescent dye fura-2. Insulin secretion from isolated islets was determined by static incubations.Results Membrane depolarisation of Sur1–/– islets by arginine or increased extracellular K+, elevated [Ca2+]i and augmented insulin secretion. Oligomycin completely abolished glucose-stimulated insulin release from Sur1–/– islets. Oscillations in Vm were influenced by [Ca2+]i as follows: (1) elevation of extracellular Ca2+ lengthened phases of membrane hyperpolarisation; (2) simulating a burst of action potentials induced a Ca2+-dependent outward current that was augmented by increased Ca2+ influx through L-type Ca2+ channels; (3) Ca2+ depletion of intracellular stores by cyclopiazonic acid increased the burst frequency in Sur1–/– islets, elevating [Ca2+]i and insulin secretion; (4) store depletion activated a Ca2+ influx that was not inhibitable by the L-type Ca2+ channel blocker D600.Conclusions/interpretation Although Vm is largely uncoupled from glucose metabolism in the absence of KATP channels, increased electrical activity leads to elevations of [Ca2+]i that are sufficient to stimulate insulin secretion. In Sur1–/– beta cells, [Ca2+]i exerts feedback mechanisms on Vm by activating a hyperpolarising outward current and by depolarising Vm via store-operated ion channels.  相似文献   

7.
The extracellular matrix (ECM) protein-integrin-cytoskeleton axis plays a central role as a mechanotransducing protein assemblage in many cell types. However, how the process of mechanotransduction and the mechanically generated signals arising from this axis affect myofilament function in cardiac muscle are not completely understood. We hypothesize that ECM proteins can regulate cardiac function through integrin binding, and thereby alter the intracellular calcium concentration ([Ca2+]i) and/or modulate myofilament activation processes. Force measurements made in mouse papillary muscle demonstrated that in the presence of the soluble form of the ECM protein, fibronectin (FN), active force was increased significantly by 40% at 1 Hz, 54% at 2 Hz, 35% at 5 Hz and 16% at 9 Hz stimulation frequencies. Furthermore, increased active force in the presence of FN was associated with 12-33% increase in [Ca2+]i and 20-50% increase in active force per unit Ca2+. A function blocking antibody for α5 integrin prevented the effects of the FN on the changes in force and [Ca2+]i, whereas a function blocking α3 integrin antibody did not reverse the effects of FN. The effects of FN were reversed by an L-type Ca2+ channel blocker, verapamil or PKA inhibitor. Freshly isolated cardiomyocytes exhibited a 39% increase in contraction force and a 36% increase in L-type Ca2+ current in the presence of FN. Fibers treated with FN showed a significant increase in the phosphorylation of phospholamban; however, the phosphorylation of troponin I was unchanged. These results demonstrate that FN acts via α5β1 integrin to increase force production in myocardium and that this effect is partly mediated by increases in [Ca2+]i and Ca2+ sensitivity, PKA activation and phosphorylation of phospholamban.  相似文献   

8.
Alcohol-related acute pancreatitis can be mediated by a combination of alcohol and fatty acids (fatty acid ethyl esters) and is initiated by a sustained elevation of the Ca2+ concentration inside pancreatic acinar cells ([Ca2+]i), due to excessive release of Ca2+ stored inside the cells followed by Ca2+ entry from the interstitial fluid. The sustained [Ca2+]i elevation activates intracellular digestive proenzymes resulting in necrosis and inflammation. We tested the hypothesis that pharmacological blockade of store-operated or Ca2+ release-activated Ca2+ channels (CRAC) would prevent sustained elevation of [Ca2+]i and therefore protease activation and necrosis. In isolated mouse pancreatic acinar cells, CRAC channels were activated by blocking Ca2+ ATPase pumps in the endoplasmic reticulum with thapsigargin in the absence of external Ca2+. Ca2+ entry then occurred upon admission of Ca2+ to the extracellular solution. The CRAC channel blocker developed by GlaxoSmithKline, GSK-7975A, inhibited store-operated Ca2+ entry in a concentration-dependent manner within the range of 1 to 50 μM (IC50 = 3.4 μM), but had little or no effect on the physiological Ca2+ spiking evoked by acetylcholine or cholecystokinin. Palmitoleic acid ethyl ester (100 μM), an important mediator of alcohol-related pancreatitis, evoked a sustained elevation of [Ca2+]i, which was markedly reduced by CRAC blockade. Importantly, the palmitoleic acid ethyl ester-induced trypsin and protease activity as well as necrosis were almost abolished by blocking CRAC channels. There is currently no specific treatment of pancreatitis, but our data show that pharmacological CRAC blockade is highly effective against toxic [Ca2+]i elevation, necrosis, and trypsin/protease activity and therefore has potential to effectively treat pancreatitis.  相似文献   

9.
The present study was conducted to investigate the effects of the diabetic condition on the Ca2+ mobilization and glutamate release in cerebral nerve terminals (synaptosomes). Diabetes was induced in male mice by intraperitoneal injection of streptozotocin. Cytosolic free Ca2+ concentration ([Ca2+]i) and glutamate release in synaptosomes were determined using fura-2 and enzyme-linked fluorometric assay, respectively. Diabetes significantly enhanced the ability of the depolarizing agents K+ and 4-aminopyridine (4-AP) to increase [Ca2+]i. In addition, diabetes significantly enhanced K+- and 4-AP-evoked Ca2+-dependent glutamate release. The pretreatment of synaptosomes with a combination of ω-agatoxin IVA (a P-type Ca2+ channel blocker) and ω-conotoxin GVIA (an N-type Ca2+ channel blocker) inhibited K+- or 4-AP-induced increases in [Ca2+]i and Ca2+-dependent glutamate release in synaptosomes from the control and diabetic mice to a similar extent, respectively. These results indicate that diabetes enhances a K+- or 4-AP-evoked Ca2+-dependent glutamate release by increasing [Ca2+]i via stimulation of Ca2+ entry through both P- and N-type Ca2+ channels.  相似文献   

10.
Summary Plasma insulin levels in healthy subjects oscillate and non-insulin-dependent diabetic patients display an irregular pattern of such oscillations. Since an increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in the pancreatic beta cell is the major stimulus for insulin release, this study was undertaken to investigate the dynamics of electrical activity, [Ca2+]i-changes and insulin release, in stimulated islets from subjects of varying glucose tolerance. In four patients it was possible to investigate more than one of these three parameters. Stimulation of pancreatic islets with glucose and tolbutamide sometimes resulted in the appearance of oscillations in [Ca2+]i, lasting 2–3 min. Such oscillations were observed even in some islets from patients with impaired glucose tolerance. In one islet from a diabetic patient there was no response to glucose, whereas that islet displayed [Ca2+]i-oscillations in response to tolbutamide, suggesting that sulphonylurea treatment can mimic the complex pattern of glucose-induced [Ca2+]i-oscillations. We also, for the first time, made patch-clamp recordings of membrane currents in beta-cells in situ in the islet. Stimulation with glucose and tolbutamide resulted in depolarization and appearance of action potentials. The islet preparations responded to stimulation with a number of different secretagogues with release of insulin. The present study shows that human islets can respond to stimulation with glucose and sulphonylurea with oscillations in [Ca2+]i, which is the signal probably underlying the oscillations in plasma insulin levels observed in healthy subjects. Interestingly, even subjects with impaired glucose tolerance had islets that responded with oscillations in [Ca2+]i upon glucose stimulation, although it is not known to what extent the response of these islets was representative of most islets in these patients.Abbreviations [Ca2+]i Cytoplasmic free Ca2+ - NIDDM non-insulin-dependent diabetes mellitus - DMSO dimethylsulphoxide - PC pancreatic cancer  相似文献   

11.
ATP depletion due to ischemia or metabolic inhibition (MI) causes Na+and Ca2+accumulation in myocytes, which may be in part due to opening of connexin-43 hemichannels. Halothane (H) has been shown to reduce conductance of connexin-43 hemichannels and to protect the heart against ischemic injury. We therefore investigated the effect of halothane on [Ca2+]iand [Na+]iin myocytes during MI. Isolated rabbit left ventricular myocytes were loaded with 4μ m fluo-3 AM for 30 min, or with 5 μ m sodium green AM for 60 min at 37°C. After washing, the myocytes were exposed to: (1) Normal HEPES solution; (2) MI solution (2 m NaCN, 20 m 2-deoxy- -glucose and 0-glucose); or (3) MI+H (0.95 m , 4.7 m ) for 60 min. Propidium iodide (PI, 25 μ m) was added to all samples before data acquisition. The fluorescence intensity was measured by flow cytometry with 488 nm excitation and 530 nm emission for fluo-3 or sodium green, and 670 nm for PI. The [Ca2+]iand [Na+]iwere then calculated by calibration. In some experiments, the effect of 10 μ m tetrodotoxin (TTX) and 20 μ m nifedipine (NIF) were studied. Metabolic inhibition for 60 min caused a significant increase in [Ca2+]iand [Na+]iin myocytes when compared to controls, which was significantly reduced by halothane in a dose-dependent fashion. In the presence of TTX and NIF, halothane also significantly reduced the rise in the [Ca2+]iand [Na+]iin myocytes subjected to MI. 1-heptanol, another gap junction blocker, had similar effects. Thus, halothane reduced [Ca2+]iand [Na+]ioverload produced by MI in myocytes. This effect is not solely due to block of voltage-gated Na+and Ca2+channels, and is likely mediated by inhibiting the opening of connexin-43 hemichannels.  相似文献   

12.
13.
This article describes studies on the glucose-induced responses of intracellular Ca2+ concentration ([Ca2+]i), insulin release, and redistribution of calbindin-D28k, a calcium-binding regulatory protein, in β-cells of pancreatic islets of calbindin-D28k knockout (KO) and wild-type mice (C57BL6) as well as in βHC-13 control cells and βHC-13 CaBP40 cells (β-cell line overexpressing calbindin-D28k). Upon increasing the glucose concentration from 2.8 to 30 mM, islets of KO mice showed a significantly greater increase in [Ca2+]i (mean increase in [Ca2+]i, i.e., Δ[Ca2+], was 296 nM) compared with wild-type mice (Δ[Ca2+]i=97 nM). βHC-13 CaBP40 cells showed little change in [Ca2+]i upon elevation of glucose from 5.5 to 32.7 mM, whereas βHC-13 control cells exhibited significant increases in [Ca2+]i (Δ[Ca2+]i=510 nM). Similarly, upon addition of 30 mM glucose, the rate of insulin release increased from 25.2 (basal rate) to 145.2 pg/mL/min in βHC-13 control cells, whereas in βHC-13 CaBP40 cells the rate of insulin release was only 27.5 pg/mL/min in high glucose. Thus, levels of calbindin-D28k in β-cells affect both [Ca2+]i and insulin secretion in response to glucose. The three-dimensional reconstruct of confocal immunofluorescent images showed that glucose caused redistribution of calbindin-D28k resulting in co-localization in the region of L-type voltage-dependent calcium channels (VDCC). This colocalization may be an important regulatory function concerning Ca2+ influx via L-type VDCC and exocytosis of insulin granules.  相似文献   

14.
The Ca2+-dependent facilitation (CDF) of L-type Ca2+ channels, a major mechanism for force-frequency relationship of cardiac contraction, is mediated by Ca2+/CaM-dependent kinase II (CaMKII). Recently, CaMKII was shown to be activated by methionine oxidation. We investigated whether oxidation-dependent CaMKII activation is involved in the regulation of L-type Ca2+ currents (ICa,L) by H2O2 and whether Ca2+ is required in this process. Using patch clamp, ICa,L was measured in rat ventricular myocytes. H2O2 induced an increase in ICa,L amplitude and slowed inactivation of ICa,L. This oxidation-dependent facilitation (ODF) of ICa,L was abolished by a CaMKII blocker KN-93, but not by its inactive analog KN-92, indicating that CaMKII is involved in ODF. ODF was not affected by replacement of external Ca2+ with Ba2+ or presence of EGTA in the internal solutions. However, ODF was abolished by adding BAPTA to the internal solution or by depleting sarcoplasmic reticulum (SR) Ca2+ stores using caffeine and thapsigargin. Alkaline phosphatase, β-iminoadenosine 5′-triphosphate (AMP-PNP), an autophosphorylation inhibitor autocamtide-2-related inhibitory peptide (AIP), or a catalytic domain blocker (CaM-KIINtide) did not affect ODF. In conclusion, oxidation-dependent facilitation of L-type Ca2+ channels is mediated by oxidation-dependent CaMKII activation, in which local Ca2+ increases induced by SR Ca2+ release is required.  相似文献   

15.
Cardiac glycosides, which inhibit the plasma membrane Na+ pump, are one of the four categories of drug recommended for routine use to treat heart failure, yet their therapeutic window is limited by toxic effects. Elevated cytoplasmic Na+ ([Na+]i) compromises mitochondrial energetics and redox balance by blunting mitochondrial Ca2+ ([Ca2+]m) accumulation, and this impairment can be prevented by enhancing [Ca2+]m. Here, we investigate whether this effect underlies the toxicity and arrhythmogenic effects of cardiac glycosides and if these effects can be prevented by suppressing mitochondrial Ca2+ efflux, via inhibition of the mitochondrial Na+/Ca2+ exchanger (mNCE). In isolated cardiomyocytes, ouabain elevated [Na+]i in a dose-dependent way, blunted [Ca2+]m accumulation, decreased the NADH/NAD + redox potential, and increased reactive oxygen species (ROS). Concomitant treatment with the mNCE inhibitor CGP-37157 ameliorated these effects. CGP-37157 also attenuated ouabain-induced cellular Ca2+ overload and prevented delayed afterdepolarizations (DADs). In isolated perfused hearts, ouabain's positive effects on contractility and respiration were markedly potentiated by CGP-37157, as were those mediated by β-adrenergic stimulation. Furthermore, CGP-37157 inhibited the arrhythmogenic effects of ouabain in both isolated perfused hearts and in vivo. The findings reveal the mechanism behind cardiac glycoside toxicity and show that improving mitochondrial Ca2+ retention by mNCE inhibition can mitigate these effects, particularly with respect to the suppression of Ca2+-triggered arrhythmias, while enhancing positive inotropic actions. These results suggest a novel strategy for the treatment of heart failure.  相似文献   

16.
Summary In rat cardiac myocytes, calcium efflux by Na+/Ca2+-exchange is expected only during ventricular systole following initial action potential repolarization. In contrast, in guinea-pigs, calcium influx via Na+/Ca2+-exchange is expected only during the initial portion of the action potential. Thus electrical stimulation is expected to result in reduced intracellular calcium ([Ca2+]i) in rat and an increase in guinea pig. We tested this hypothesis by measuring total cellular calcium ([Ca]tot) using45Ca following stimulation of isolated rat and guinea-pig ventricular myocytes. Many studies have also emphasized that the rate and the direction of Na+/Ca2+-exchange across the sarcolemma are in part dependent on the magnitude of the transsarcolemmal sodium gradient. Thus, increasing intracellular sodium ([Na+]i) is expected to result in an increased [Ca2+]i. This hypothesis was also tested by measuring [Ca]tot following veratrine administration. Enzymatically isolated rat and guinea-pig ventricular myocytes were divided into two groups; non-stimulated and stimulated (1 Hz). The concentration-dependent effects of veratrine (1,10,100 g/ml) on [Ca]tot were determined in both these groups. In the absence of veratrine, non-stimulated rat myocytes had a significantly higher [Ca]tot than did stimulated ones. Non-stimulated guinea-pig myocytes had a significantly lower [Ca]tot when compared with stimulated ones Veratrine increased [Ca]tot in both species in a concentration-dependent fashion. In addition, following veratrine the difference between [Ca]tot in non-stimulated and stimulated rat myocytes was no longer significant. These results support those of others who have demonstrated that stimulation is associated with a gain of cellular calcium in both rabbit and guinea-pig ventricle and a calcium loss in rat ventricle. In addition, the increase in [Ca]tot following veratrine was similar to the results of others that have shown an increase in [Ca2+]i following exposure to veratrum alkaloids. Thus, we have demonstrated, using a technique limited to determination of [Ca]tot, that this parameter may reflect changes produced by alteration of Na+/Ca2+-exchange.  相似文献   

17.
Sudden cardiac death remains one of the most prevalent modes of death and is mainly caused by ventricular fibrillation (VF) in the setting of acute ischemia resulting from coronary thrombi. Animal experiments have shown that platelet activation may increase susceptibility of ischemic myocardium to VF, but the mechanism is unknown. In the present study, we evaluated the effects of activated blood platelet products (ABPPs) on electrophysiological properties and intracellular Ca2+ (Ca2+i) homeostasis. Platelets were collected from healthy volunteers. After activation, their secreted ABPPs were added to superfusion solutions. Rabbit ventricular myocytes were freshly isolated, and membrane potentials and Ca2+i were recorded using patch-clamp methodology and indo-1 fluorescence measurements, respectively. ABPPs prolonged action potential duration and induced early and delayed afterdepolarizations. ABPPs increased L-type Ca2+ current (ICa,L) density, but left densities of sodium current, inward rectifier K+ current, transient outward K+ current, and rapid component of the delayed rectifier K+ current unchanged. ABPPs did not affect kinetics or (in)activation properties of membrane currents. ABPPs increased systolic Ca2+i, Ca2+i transient amplitude, and sarcoplasmic reticulum Ca2+ content. ABPPs did not affect the Na+− Ca2+ exchange current (INCX) in Ca2+-buffered conditions. Products secreted from activated human platelets induce changes in ICa,L and Ca2+i, which result in action potential prolongation and the occurrence of early and delayed afterdepolarizations in rabbit myocytes. These changes may trigger and support reentrant arrhythmias in ischemia models of coronary thrombosis.  相似文献   

18.
The effects of gonadotropin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) on cytosolic free calcium concentration ([Ca2+]i) were investigated in 20 human nonfunctioning pituitary adenomas. We divided these tumors into three classes according to their response pattern to hypothalamic peptides. In type I adenomas (8 out of 20 adenomas), GnRH and GAP mobilized intracellular calcium ions stored in a thapsigargin (TG)-sensitive store. For the same concentration of agonist, two distinct patterns of GnRH-GAP-induced Ca2+ mobilization were observed (1) sinusoidal oscillations, and (2) monophasic transient. The latter is followed by a protein kinase C (PKC)-dependent increase in calcium influx through L-type channels. In type II adenomas (7 out of 20 adenomas), GnRH and GAP only stimulate calcium influx through dihydropyridine-sensitive Ca2+ channels by a PKC-dependent mechanism. TG (1 μM) did not affect [Ca2+]i in these cells, suggesting that they do not possess TG-sensitive Ca2+ pools. All the effects of GnRH and GAP were blocked by an inhibitor of phospholipase C (PLC), suggesting that they were owing to the activation of the phosphoinositide turnover. Type I and type II adenoma cells showed spontaneous Ca2+ oscillations that were blocked by dihydropyridines and inhibition of PKC activity. GnRH and GAP had no effect on the [Ca2+]i of type III adenoma cells that were also characterized by a low resting [Ca2+]i and by the absence of spontaneous Ca2+ fluctuations. K+-induced depolarization provoked a reduced Ca2+ influx, whereas TG had no effect on the [Ca2+]i of type III adenoma cells. The variety of [Ca2+]i response patterns makes these cells a good cell model for studying calcium homeostasis in pituitary cells.  相似文献   

19.
Reduction in [Ca2+]o prolongs the AP in ventricular cardiomyocytes and the QTc interval in patients. Although this phenomenon is relevant to arrhythmogenesis in the clinical setting, its mechanisms are counterintuitive and incompletely understood. To evaluate in silico the mechanisms of APD modulation by [Ca2+]o in human cardiomyocytes. We implemented the Ten Tusscher-Noble-Noble-Panfilov model of the human ventricular myocyte and modified the formulations of the rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs) and L-type Ca2+ current (ICaL) to incorporate their known sensitivity to intra- or extracellular Ca2+. Simulations were run with the original and modified models at variable [Ca2+]o in the clinically relevant 1 to 3 mM range. The original model responds with APD shortening to decrease in [Ca2+]o, i.e. opposite to the experimental observations. Incorporation of Ca2+ dependency of K+ currents cannot reproduce the inverse relation between APD and [Ca2+]o. Only when ICaL inactivation process was modified, by enhancing its dependency on Ca2+, simulations predict APD prolongation at lower [Ca2+]o. Although Ca2+-dependent ICaL inactivation is the primary mechanism, secondary changes in electrogenic Ca2+ transport (by Na+/Ca2+ exchanger and plasmalemmal Ca2+-ATPase) contribute to the reversal of APD dependency on [Ca2+]o. This theoretical investigation points to Ca2+-dependent inactivation of ICaL as a mechanism primarily responsible for the dependency of APD on [Ca2+]o. The modifications implemented here make the model more suitable to analyze repolarization mechanisms when Ca2+ levels are altered.  相似文献   

20.
Melastatin-related transient receptor potential channel 2 (TRPM2) is a Ca2+-permeable, nonselective cation channel that is involved in oxidative stress-induced cell death and inflammation processes. Although TRPM2 can be activated by ADP-ribose (ADPR) in vitro, it was unknown how TRPM2 is gated in vivo. Moreover, several alternative spliced isoforms of TRPM2 identified recently are insensitive to ADPR, and their gating mechanisms remain unclear. Here, we report that intracellular Ca2+ ([Ca2+]i) can activate TRPM2 as well as its spliced isoforms. We demonstrate that TRPM2 mutants with disrupted ADPR-binding sites can be activated readily by [Ca2+]i, indicating that [Ca2+]i gating of TRPM2 is independent of ADPR. The mechanism by which [Ca2+]i activates TRPM2 is via a calmodulin (CaM)-binding domain in the N terminus of TRPM2. Whereas Ca2+-mediated TRPM2 activation is independent of ADPR and ADPR-binding sites, both [Ca2+]i and the CaM-binding motif are required for ADPR-mediated TRPM2 gating. Importantly, we demonstrate that intracellular Ca2+ release activates both recombinant and endogenous TRPM2 in intact cells. Moreover, receptor activation-induced Ca2+ release is capable of activating TRPM2. These results indicate that [Ca2+]i is a key activator of TRPM2 and the only known activator of the spliced isoforms of TRPM2. Our findings suggest that [Ca2+]i-mediated activation of TRPM2 and its alternative spliced isoforms may represent a major gating mechanism in vivo, therefore conferring important physiological and pathological functions of TRPM2 and its spliced isoforms in response to elevation of [Ca2+]i.  相似文献   

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