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1.
Using fura-2-acetoxymethyl ester (AM) fluorescence imaging and patch clamp techniques, we found that endothelin-1 (ET-1) significantly elevated the intracellular calcium level ([Ca2+]i) in a dose-dependent manner and activated the L-type Ca2+ channel in cardiomyocytes isolated from rats. The effect of ET-1 on [Ca2+]i elevation was abolished in the presence of the ETA receptor blocker BQ123, but was not affected by the ETB receptor blocker BQ788. ET-1-induced an increase in [Ca2+]i, which was inhibited 46.7% by pretreatment with a high concentration of ryanodine (10 μmol/L), a blocker of the ryanodine receptor. The ET-1-induced [Ca2+]i increase was also inhibited by the inhibitors of protein kinase A (PKA), protein kinase C (PKC) and angiotensin type 1 receptor (AT1 receptor). We found that ET-1 induced an enhancement of the amplitude of the whole cell L-type Ca2+ channel current and an increase of open-state probability (NPo) of an L-type single Ca2+ channel. BQ123 completely blocked the ET-1-induced increase in calcium channel open-state probability. In this study we demonstrated that ET-1 regulates calcium overload through a series of mechanisms that include L-type Ca2+ channel activation and Ca2+-induced Ca2+ release (CICR). ETA receptors, PKC, PKA and AT1 receptors may also contribute to this pathway. Supported by the National Natural Science Foundation of China (Grant No. 200830870910).  相似文献   

2.
Xenopus follicles are endowed with specific receptors for ATP, ACh, and AII, transmitters proposed as follicular modulators of gamete growth and maturation in several species. Here, we studied ion‐current responses elicited by stimulation of these receptors and their activation mechanisms using the voltage‐clamp technique. All agonists elicited Cl? currents that depended on coupling between oocyte and follicular cells and on an increase in intracellular Ca2+ concentration ([Ca2+]i), but they differed in their activation mechanisms and in the localization of the molecules involved. Both ATP and ACh generated fast Cl? (FCl) currents, while AII activated an oscillatory response; a robust Ca2+ influx linked specifically to FCl activation elicited an inward current (Iiw,Ca) which was carried mainly by Cl? ions, through channels with a sequence of permeability of SCN? > I? > Br? > Cl?. Like FCl, Iiw,Ca was not dependent on oocyte [Ca2+]i; instead both were eliminated by preventing [Ca2+]i increase in the follicular cells, and also by U73122 and 2‐APB, drugs that inhibit the phospolipase C (PLC) pathway. The results indicated that FCl and Iiw,Ca were produced by the expected, PLC‐stimulated Ca2+‐release and Ca2+‐influx, respectively, and by the opening of ICl(Ca) channels located in the follicular cells. Given their pharmacological characteristics and behavior in conditions of divalent cation deprivation, Ca2+‐influx appeared to be driven through store‐operated, calcium‐like channels. The AII response, which is also known to require PLC activation, did not activate Iiw,Ca and was strictly dependent on oocyte [Ca2+]i increase; thus, ATP and ACh receptors seem to be expressed in a population of follicular cells different from that expressing AII receptors, which were coupled to the oocyte through distinct gap‐junction channels. J. Cell. Physiol. 227: 3457–3470, 2012. © 2011 Wiley Periodicals, Inc.  相似文献   

3.
In cystic fibrosis, the mutation of the CFTR protein causes reduced transepithelial Cl secretion. As recently proposed, beside its role of Cl channel, CFTR may regulate the activity of other channels such as a Ca2+-activated Cl channel. Using a calcium imaging system, we show, in adenovirus-CFTR infected Chinese Hamster Ovary (CHO) cell monolayers, that CFTR can act as a regulator of intracellular [Ca2+] i ([Ca2+] i ), involving purino-receptors. Apical exposure to ATP or UTP produced an increase in ([Ca2+] i in noninfected CHO cell monolayers (CHO-WT), in CHO monolayers infected with an adenovirus-CFTR (CHO-CFTR) or infected with an adenovirus-LacZ (CHO-LacZ). The transient [Ca2+] i increase produced by ATP or UTP could be mimicked by activation of CFTR with forskolin (20 μm) in CHO-CFTR confluent monolayers. However, forskolin had no significant effect on [Ca2+] i in noninfected CHO-WT or in CHO-LacZ cells. Pretreatment with purino-receptor antagonists such as suramin (100 μm) or reactive blue-2. (100 μm), and with hexokinase (0.28 U/mg) inhibited the [Ca2+] i response to forskolin in CHO-CFTR infected cells. Taken together, our experiments provide evidence for purino-receptor activation by ATP released from the cell and regulation of [Ca2+] i by CFTR in CHO epithelial cell membranes. Received: 5 April 1999/Revised: 28 June 1999  相似文献   

4.
The calcium indicator fura-2 was used to study the effect of hypotonic solutions on the intracellular calcium concentration, [Ca2+] i , in a human osteoblast-like cell line. Decreasing the tonicity of the extracellular solution to 50% leads to an increase in [Ca2+] i from ∼150 nm up to 1.3 μm. This increase in [Ca2+] i was mainly due to an influx of extracellular Ca2+ since removing of extracellular Ca2+ reduced this increase to ∼250 nm. After cell swelling most of the cells were able to regulate their volume to the initial level within 800 sec. The whole-cell recording mode of the patch-clamp technique was also used to study the effect of an increase in [Ca2+] i on membrane currents in these cells. An increase in [Ca2+] i revealed two types of Ca2+-activated K+ channels, K(Ca) channels. Current through both channel types could not be observed below voltage of +80 mV with [Ca2+] i buffered to 100 nm or less. With patch-electrodes filled with solutions buffering [Ca2+] i to 10 μm both channels types could be readily observed. The activation of the first type was apparently voltage-independent since current could be observed over the entire voltage range used from −160 to +100 mV. In addition, the current was also blocked by charybdotoxin (CTX). The second type of K(Ca) channels in these cells could be activated with depolarizations more positive than −40 mV from a holding potential of −80 mV. This type was blocked by CTX and paxilline. Adding paxilline to the extracellular solution inhibited regulatory volume decrease (RVD), but could not abolish RVD. We conclude that two K(Ca) channel types exist in human osteoblasts, an intermediate conductance K(Ca) channel and a MaxiK-like K(Ca) channel. MaxiK channels might get activated either directly or by an increase in [Ca2+] i elicited through hypotonic solutions. In combination with the volume-regulated Cl conductance in the same cells this K+ channel seems to play a vital role in volume regulation in human osteoblasts. Received: 8 February 2000/Revised: 13 July 2000  相似文献   

5.
Ionic channels regulated by extracellular Ca2+ concentration ([Ca2+]0) were examined in freshly isolated rabbit osteoclasts. K+ current was suppressed by intracellular and extracellular Cs+ ions. In this condition, high [Ca2+]0 evoked an outwardly rectifying current with a reversal potential of about −25 mV. When the concentration of extracellular Cl ions was altered, the reversal potential of the outwardly rectifying current shifted as predicted by the Nernst equation. 4′,4-diisothiocyanostilbene-2′,2-disulphonic acid (DIDS) inhibited the outwardly rectifying current. These results indicated that this current was carried through Cl channels. Cd2+ or Ni2+ caused a transient activation of the Cl current in contrast to the sustained activation elicited by Ca2+. Intracellular 20 mM ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA) inhibited the divalent cation-induced Cl current. Either when the osmolarity of extracellular medium was increased, or when 100 μM cAMP was dissolved in the patch pipette solution, high [Ca2+]0 still elicited the Cl current, indicating that the divalent cation-induced Cl current was carried through Ca2+-activated Cl channels. Under perforated whole cell clamp extracellular divalent cations evoked the Cl current, indicating that the activation of Cl current did not arise from possible leakage of divalent cations from the extracellular medium under the whole cell clamp condition. This experiment further excluded a possible activation of volume-sensitive Cl channels under whole cell clamp. Intracellular application of guanosine 5′-O-(3-thiotriphosphate) (GTPγS) activated the Cl current and it was inhibited by intracellular 20 mM EGTA, suggesting that the activation of Cl current was mediated through a G protein, and that an increase in [Ca2+]i was critical for the activation of Cl channels. A protein phosphatase inhibitor, okadaic acid (100 nM), caused an irreversible activation of the Cl current, suggesting that protein phosphatase 1 or 2A was involved in the regulation of Ca2+-activated Cl channels. © 1996 Wiley-Liss, Inc.  相似文献   

6.
TRPC5 forms non-selective cation channels. Here we studied the role of internal Ca2+ in the activation of murine TRPC5 heterologously expressed in human embryonic kidney cells. Cell dialysis with various Ca2+ concentrations (Ca2+i) revealed a dose-dependent activation of TRPC5 channels by internal Ca2+ with EC50 of 635.1 and 358.2 nm at negative and positive membrane potentials, respectively. Stepwise increases of Ca2+i induced by photolysis of caged Ca2+ showed that the Ca2+ activation of TRPC5 channels follows a rapid exponential time course with a time constant of 8.6 ± 0.2 ms at Ca2+i below 10 μm, suggesting that the action of internal Ca2+ is a primary mechanism in the activation of TRPC5 channels. A second slow activation phase with a time to peak of 1.4 ± 0.1 s was also observed at Ca2+i above 10 μm. In support of a Ca2+-activation mechanism, the thapsigargin-induced release of Ca2+ from internal stores activated TRPC5 channels transiently, and the subsequent Ca2+ entry produced a sustained TRPC5 activation, which in turn supported a long-lasting membrane depolarization. By co-expressing STIM1 plus ORAI1 or the α1C and β2 subunits of L-type Ca2+ channels, we found that Ca2+ entry through either calcium-release-activated-calcium or voltage-dependent Ca2+ channels is sufficient for TRPC5 channel activation. The Ca2+ entry activated TRPC5 channels under buffering of internal Ca2+ with EGTA but not with BAPTA. Our data support the hypothesis that TRPC5 forms Ca2+-activated cation channels that are functionally coupled to Ca2+-selective ion channels through local Ca2+ increases beneath the plasma membrane.  相似文献   

7.
Rat melanotrophs express several types of voltage-gated and ligand-gated calcium channels, although mechanisms involved in the maintenance of the resting intracellular Ca2+ concentration ([Ca2+]i) remain unknown. We analyzed mechanisms regulating resting [Ca2+]i in dissociated rat melanotrophs by Ca2+-imaging and patch-clamp techniques. Treatment with antagonists of L-type, but not N- or P/Q-type voltage-gated Ca2+ channels (VGCCs) as well as removal of extracellular Ca2+ resulted in a rapid and reversible decrease in [Ca2+]i, indicating constitutive Ca2+ influx through L-type VGCCs. Reduction of extracellular Na+ concentration (replacement with NMDG+) similarly decreased resting [Ca2+]i. When cells were champed at –80 mV, decrease in the extracellular Na+ resulted in a positive shift of the holding current. In cell-attached voltage-clamp and whole-cell current-clamp configurations, the reduction of extracellular Na+ caused hyperpolarisation. The holding current shifted in negative direction when extracellular K+ concentration was increased from 5 mM to 50 mM in the presence of K+ channel blockers, Ba2+ and TEA, indicating cation nature of persistent conductance. RT-PCR analyses of pars intermedia tissues detected mRNAs of TRPV1, TRPV4, TRPC6, and TRPM3-5. The TRPV channel blocker, ruthenium red, shifted the holding current in positive direction, and significantly decreased the resting [Ca2+]i. These results indicate operation of a constitutive cation conductance sensitive to ruthenium red, which regulates resting membrane potential and [Ca2+]i in rat melanotrophs.  相似文献   

8.
The purpose of this study was to explore the effect of tamoxifen on cytosolic free Ca2+ concentrations ([Ca2+]i) and cell viability in OC2 human oral cancer cells. [Ca2+]i and cell viability were measured by using the fluorescent dyes fura-2 and WST-1, respectively. Tamoxifen at concentrations above 2 μM increased [Ca2+]i in a concentration-dependent manner. The Ca2+ signal was reduced partly by removing extracellular Ca2+. The tamoxifen-induced Ca2+ influx was sensitive to blockade of L-type Ca2+ channel blockers but insensitive to the estrogen receptor antagonist ICI 182,780 and protein kinase C modulators. In Ca2+-free medium, after pretreatment with 1 μM thapsigargin (an endoplasmic reticulum Ca2+ pump inhibitor), tamoxifen-induced [Ca2+]i rises were substantially inhibited; and conversely, tamoxifen pretreatment inhibited a part of thapsigargin-induced [Ca2+]i rises. Inhibition of phospholipase C with 2 μM U73122 did not change tamoxifen-induced [Ca2+]i rises. At concentrations between 10 and 50 μM tamoxifen killed cells in a concentration-dependent manner. The cytotoxic effect of 23 μM tamoxifen was not reversed by prechelating cytosolic Ca2+ with BAPTA. Collectively, in OC2 cells, tamoxifen induced [Ca2+]i rises, in a nongenomic manner, by causing Ca2+ release from the endoplasmic reticulum, and Ca2+ influx from L-type Ca2+ channels. Furthermore, tamoxifen-caused cytotoxicity was not via a preceding [Ca2+]i rise.  相似文献   

9.
We investigated the cytosolic free calcium concentration ([Ca2+]i) of leech Retzius neurons in situ while varying the extracellular Ca2+ concentration via the bathing solution ([Ca2+]B). Changing [Ca2+]B had only an effect on [Ca2+]i if the cells were depolarized by raising the extracellular K+ concentration. Surprisingly, raising [Ca2+]B from 2 to 10 mm caused a decrease in [Ca2+]i, and an increase was evoked by reducing [Ca2+]B to 0.1 mm. These changes were not due to shifts in membrane potential. At low [Ca2+]B moderate membrane depolarizations were sufficient to evoke a [Ca2+]i increase, while progressively larger depolarizations were necessary at higher [Ca2+]B. The changes in the relationship between [Ca2+]i and membrane potential upon varying [Ca2+]B could be reversed by changing extracellular pH. We conclude that [Ca2+]B affects [Ca2+]i by modulating Ca2+ influx through voltage-dependent Ca2+ channels via the electrochemical Ca2+ gradient and the surface potential at the extracellular side of the plasma membrane. These two parameters are affected in a counteracting way: Raising the extracellular Ca2+ concentration enhances the electrochemical Ca2+ gradient and hence Ca2+ influx, but it attenuates Ca2+ channel activity by shifting the extracellular surface potential to the positive direction, and vice versa. Received: 23 January 2001/Revised: 23 June 2001  相似文献   

10.
Rat sympathetic neurons undergo programmed cell death (PCD) in vitro and in vivo when they are deprived of nerve growth factor (NGF). Chronic depolarization of these neurons in cell culture with elevated concentrations of extracellular potassium ([K+]o) prevents this death. The effect of prolonged depolarization on neuronal survival is thought to be mediated by a rise of intracellular calcium concentration ([Ca2+]i) caused by Ca2+ influx through voltage-gated channels. In this report we investigate the effects of chronic treatment of rat sympathetic neurons with thapsigargin, an inhibitor of intracellular Ca2+ sequestration. In medium containing a normal concentration of extracellular Ca2+ ([Ca2+]o), thapsigargin caused a sustained rise of intracellular Ca2+ concentration and partially blocked death of NGF-deprived cells. Elevating [Ca2+]o in the presence of thapsigargin further increased [Ca2+]i, suggesting that the sustained rise of [Ca2+]i was caused by a thapsigargin-induced Ca2+ influx. This treatment potentiated the effect of thapsigargin on survival. The dihydropyridine Ca2+ channel antagonist, nifedipine, blocked both a sustained elevation of [Ca2+]i and enhanced survival caused by depolarization with elevated [K+]o, suggesting that these effects are mediated by Ca2+ influx through L-type channels. Nifedipine did not block the sustained rise of [Ca2+]i or enhanced survival caused by thapsigargin treatment, indicating that these effects were not mediated by influx of Ca2+ through L-type channels. These results provide additional evidence that increased [Ca2+]i can suppress neuronal PCD and identify a novel method for chronically raising neuronal [Ca2+]i for investigation of this and other Ca2+-dependent phenomena. © 1995 John Wiley & Sons, Inc.  相似文献   

11.
This study investigated the underlying mechanisms of oxytocin (OT)-induced increases in intracellular Ca2+ concentrations ([Ca2+]i) in acutely dispersed myometrial cells from prepartum sows. A dosedependent increase in [Ca2+]i was induced by OT (0.1 nM to 1 μM) in the presence and absence of extracellular Ca2+ ([Ca2+]e). [Ca2+]i was elevated by OT in a biphasic pattern, with a spike followed by a sustained plateau in the presence of [Ca2+]e. However, in the absence of [Ca2+]e, the [Ca2+]i response to OT became monophasic with a lower amplitude and no plateau, and this monophasic increase was abolished by pretreatment with ionomycin, a Ca2+ ionophore. Administration of OT (1 μM) for 15 sec increased inositol 1,4,5-trisphosphate (IP3) formation by 61%. Pretreatment with pertussis toxin (PTX, 1 μg/ml) for 2 hr failed to alter the OT-induced increase in [Ca2+]i and IP3 formation. U-73122 (30 nM to 3 μM), a phospholipase C (PLC) inhibitor, depressed the rise in [Ca2+]i by OT dose dependently. U-73122 (3 μM) also abolished the OT-induced IP3 formation. Thapsigargin (2 μM), an inhibitor of Ca2+-ATPase in the endoplasmic reticulum, did not increase [Ca2+]i. However, it did time-dependently inhibit the OT-induced increase in [Ca2+]i. Nimodipine (1 μM), a Voltage-dependent Ca2+ channel (VDCC) blocker, inhibited the OT-induced plateau by 26%. La3+ (1 μM), a nonspecific Ca2+ channel blocker, abrogated the OT-induced plateau. In whole-cell patch-clamp studies used to evaluate VDCC activities, OT (0.1 μM) increased Ca2+ Current (Ica) by 40% with no apparent changes in the current-voltage relationship. The OT-induced increase in Ica reached the maximum in 5 min, and the increase was abolished by nimodipine (1 μM). These results suggested that (1) activation of OT receptors in porcine myometrium evokes a cascade in the PTX-insensitive G-protein–PLC-IP3 signal transduction, resulting in an increase in [Ca2+]i; (2) the OT-induced increase in [Ca2+]i is characterized by a biphasic pattern, in which the spike is predominately contributed by the intracellular Ca2+ release from the IP3-sensitive pool, and to a lesser extent by Ca2+ influx, whereas the plateau is from increased Ca2+ influx; and (3) the influx is via VDCC and receptor-operated Ca2+ channels. © 1995 Wiley-Liss, Inc.  相似文献   

12.
Abstract

Protriptyline, a tricyclic anti-depressant, is used primarily to treat the combination of symptoms of anxiety and depression. However, the effect of protriptyline on prostate caner is unknown. This study examined whether the anti-depressant protriptyline altered Ca2+ movement and cell viability in PC3 human prostate cancer cells. The Ca2+-sensitive fluorescent dye fura-2 was used to measure [Ca2+]i. Protriptyline evoked [Ca2+]i rises concentration-dependently. The response was reduced by removing extracellular Ca2+. Protriptyline-evoked Ca2+ entry was inhibited by store-operated channel inhibitors (nifedipine, econazole and SKF96365), protein kinase C activator (phorbol 12-myristate 13 acetate, PMA) and protein kinase C inhibitor (GF109203X). Treatment with the endoplasmic reticulum Ca2+ pump inhibitor 2,5-di-tert-butylhydr-oquinone (BHQ) in Ca2+-free medium inhibited 60% of protriptyline-evoked [Ca2+]i rises. Conversely, treatment with protriptyline abolished BHQ-evoked [Ca2+]i rises. Inhibition of phospholipase C with U73122 suppressed 50% of protriptyline-evoked [Ca2+]i rises. At concentrations of 50–70?µM, protriptyline decreased cell viability in a concentration-dependent manner; which were not reversed by chelating cytosolic Ca2+ with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM). Collectively, in PC3 cells, protriptyline evoked [Ca2+]i rises by inducing phospholipase C-associated Ca2+ release from the endoplasmic reticulum and other stores, and Ca2+ influx via protein kinase C-sensitive store-operated Ca2+ channels. Protriptyline caused cell death that was independent of [Ca2+]i rises.  相似文献   

13.
Regulation of critical cellular functions, including Ca2+-dependent gene expression, is determined by the temporal and spatial aspects of agonist-induced Ca2+ signals. Stimulation of cells with physiological concentrations of agonists trigger increases [Ca2+]i due to intracellular Ca2+ release and Ca2+ influx. While Orai1-STIM1 channels account for agonist-stimulated [Ca2+]i increase as well as activation of NFAT in cells such as lymphocytes, RBL and mast cells, both Orai1-STIM1 and TRPC1-STIM1 channels contribute to [Ca2+]i increases in human submandibular gland (HSG) cells. However, only Orai1-mediated Ca2+ entry regulates the activation of NFAT in HSG cells. Since both TRPC1 and Orai1 are activated following internal Ca2+ store depletion in these cells, it is not clear how the cells decode individual Ca2+ signals generated by the two channels for the regulation of specific cellular functions. Here we have examined the contributions of Orai1 and TRPC1 to carbachol (CCh)-induced [Ca2+]i signals and activation of NFAT in single cells. We report that Orai1-mediated Ca2+ entry generates [Ca2+]i oscillations at different [CCh], ranging from very low to high. In contrast, TRPC1-mediated Ca2+ entry generates sustained [Ca2+]i elevation at high [CCh] and contributes to frequency of [Ca2+]i oscillations at lower [agonist]. More importantly, the two channels are coupled to activation of distinct Ca2+ dependent gene expression pathways, consistent with the different patterns of [Ca2+]i signals mediated by them. Nuclear translocation of NFAT and NFAT-dependent gene expression display “all-or-none” activation that is exclusively driven by local [Ca2+]i generated by Orai1, independent of global [Ca2+]i changes or TRPC1-mediated Ca2+ entry. In contrast, Ca2+ entry via TRPC1 primarily regulates NFκB-mediated gene expression. Together, these findings reveal that Orai1 and TRPC1 mediate distinct local and global Ca2+ signals following agonist stimulation of cells, which determine the functional specificity of the channels in activating different Ca2+-dependent gene expression pathways.  相似文献   

14.
Somatostatin (SST) inhibits Ca2+ entry into pancreatic B-cells via voltage-operated Ca2+ channels (VOCCs) of L-type, leading to the suppression of insulin secretion. Activation of R-type channels increases insulin secretion. However, the role of R-type Ca2+ channels (CaV2.3) in mediating the effects of SST on insulin secretion has not been so far investigated. Here, we identify the SST-receptor subtypes (SSTR) expressed on insulin-producing INS-1 cells by RT-PCR and by functional assays. The role of R-type channels in regulating [Ca2+]i in response to SST-treatment was detected by cell fluorescence imaging and patch-clamp technique. INS-1 expressed SSTR2 and SSTR3 and agonists (ag.) selective for these receptors reduced 10 nM exendin-4/20 mM glucose-stimulated insulin secretion. Surprisingly, SST and SST2-ag. transiently increased [Ca2+]i. Subsequently, these agonists led to a decrease in [Ca2+]i below the basal levels. In contrast, SST3-ag. failed to induce a transient peak of [Ca2+]i. Instead, a persistent minor suppression of [Ca2+]i was detected from 25 min. R-type channel blocker SNX-482 altered [Ca2+]i in SST- and SST2-ag.-treated cells. Notably, the inhibition of insulin secretion by SST and SST2-ag., but not SST3-ag. was attenuated by SNX-482. Taken together, SST and SSTR2 regulate [Ca2+]i and insulin secretion in INS-1 cells via R-type channels. In contrast, the R-type calcium channel does not mediate the effects of SST3-ag. on insulin secretion. We conclude that R-type channels play a major role in the inhibition of insulin secretion by somatostatin in INS-1 cells.  相似文献   

15.
The mechanisms of glucagon secretion and its suppression by glucose are presently unknown. This study investigates the relationship between intracellular calcium levels ([Ca2+]i) and hormone secretion under low and high glucose conditions. We examined the effects of modulating ion channel activities on [Ca2+]i and hormone secretion from ex vivo mouse pancreatic islets. Glucagon-secreting α-cells were unambiguously identified by cell specific expression of fluorescent proteins. We found that activation of L-type voltage-gated calcium channels is critical for α-cell calcium oscillations and glucagon secretion at low glucose levels. Calcium channel activation depends on KATP channel activity but not on tetrodotoxin-sensitive Na+ channels. The use of glucagon secretagogues reveals a positive correlation between α-cell [Ca2+]i and secretion at low glucose levels. Glucose elevation suppresses glucagon secretion even after treatment with secretagogues. Importantly, this inhibition is not mediated by KATP channel activity or reduction in α-cell [Ca2+]i. Our results demonstrate that glucose uncouples the positive relationship between [Ca2+]i and secretory activity. We conclude that glucose suppression of glucagon secretion is not mediated by inactivation of calcium channels, but instead, it requires a calcium-independent inhibitory pathway.  相似文献   

16.
Characteristics of the increasing effect for the concentration of intracellular calcium ions ([Ca2+]i) by high-KCl application were investigated in the neuroblastoma×glioma hybrid NG108-15 cell line (NG108-15 cells). The present study confirmed that the increasing effect of [Ca2+]i by high-KCl application in single NG108-15 cells, differentiated with dibutyryl cAMP (Bt2cAMP), was significantly enhanced, compared to undifferentiated cells. The following observations were made at first: (1) The response to high-KCl application, in both undifferentiated and differentiated cells, was significantly inhibited by calciseptine (CaS), an L-type Ca2+ channel blocker, but not by N-, P- and R-type Ca2+ channel blockers. The IC50 values for CaS in both undifferentiated and differentiated cell was almost identical. (2) The inhibitory effect of CaS was irreversible. (3) The increasing effect for [Ca2+]i by high-KCl application was completely dependent on the presence of extracellular calcium ions. (4) The increased [Ca2+]i by high-KCl application under a plateau concentration was quickly decreased to basal levels when the high-KCl solution was exchanged for a high-KCl solution containing EGTA (without CaCl2). Together, these results suggest that the enhancement of the response effect of [Ca2+]i by high-KCl application in differentiated single NG108-15 cells was mainly due to the quantitative increase of L-type voltage-sensitive calcium channels (VSCCs), which were irreversibly inhibited by CaS.  相似文献   

17.
A Ca2+-activated (I Cl,Ca) and a swelling-activated anion current (I Cl,vol) were investigated in Ehrlich ascites tumor cells using the whole cell patch clamp technique. Large, outwardly rectifying currents were activated by an increase in the free intracellular calcium concentration ([Ca2+] i ), or by hypotonic exposure of the cells, respectively. The reversal potential of both currents was dependent on the extracellular Cl concentration. I Cl,Ca current density increased with increasing [Ca2+] i , and this current was abolished by lowering [Ca2+] i to <1 nm using 1,2-bis-(o-aminophenoxy)ethane-N,N,N′,N′-tetra-acetic acid (BAPTA). In contrast, activation of I Cl,vol did not require an increase in [Ca2+] i . The kinetics of I Cl,Ca and I Cl,vol were different: at depolarized potentials, I Cl,Ca as activated in a [Ca2+] i - and voltage-dependent manner, while at hyperpolarized potentials, the current was deactivated. In contrast, I Cl,vol exhibited time- and voltage-dependent deactivation at depolarized potentials and reactivation at hyperpolarized potentials. The deactivation of I Cl,vol was dependent on the extracellular Mg2+ concentration. The anion permeability sequence for both currents was I > Cl > gluconate. I Cl,Ca was inhibited by niflumic acid (100 μm), 5-Nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 μm) and 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid (DIDS, 100 μm), niflumic acid being the most potent inhibitor. In contrast, I Cl,vol was unaffected by niflumic acid (100 μm), but abolished by tamoxifen (10 μm). Thus, in Ehrlich cells, separate chloride currents, I Cl,Ca and I Cl,vol, are activated by an increase in [Ca2+] i and by cell swelling, respectively. Received: 12 November 1997/Revised: 5 February 1998  相似文献   

18.
The rise in intracellular Ca2+ mediated by AMPA subtype of glutamate receptors has been implicated in the pathogenesis of motor neuron disease, but the exact route of Ca2+ entry into motor neurons is not clearly known. In the present study, we examined the role of voltage gated calcium channels (VGCCs) in AMPA induced Ca2+ influx and subsequent intracellular signaling events responsible for motor neuron degeneration. AMPA stimulation caused sodium influx in spinal neurons that would depolarize the plasma membrane. The AMPA induced [Ca2+]i rise in motor neurons as well as other spinal neurons was drastically reduced when extracellular sodium was replaced with NMDG, suggesting the involvement of voltage gated calcium channels. AMPA mediated rise in [Ca2+]i was significantly inhibited by L-type VGCC blocker nifedipine, whereas ω-agatoxin-IVA and ω-conotoxin-GVIA, specific blockers of P/Q type and N-type VGCC were not effective. 1-Napthyl-acetyl spermine (NAS), an antagonist of Ca2+ permeable AMPA receptors partially inhibited the AMPA induced [Ca2+]i rise but selectively in motor neurons. Measurement of AMPA induced currents in whole cell voltage clamp mode suggests that a moderate amount of Ca2+ influx occurs through Ca2+ permeable AMPA receptors in a subpopulation of motor neurons. The AMPA induced mitochondrial calcium loading [Ca2+]m, mitochondrial depolarization and neurotoxicity were also significantly reduced in presence of nifedipine. Activation of VGCCs by depolarizing concentration of KCl (30 mM) in extracellular medium increased the [Ca2+]i but no change was observed in mitochondrial Ca2+ and membrane potential. Our results demonstrate that a subpopulation of motor neurons express Ca2+ permeable AMPA receptors, however the larger part of Ca2+ influx occurs through L-type VGCCs subsequent to AMPA receptor activation and consequent mitochondrial dysfunction is the trigger for motor neuron degeneration. Nifedipine is an effective protective agent against AMPA induced mitochondrial stress and degeneration of motor neurons.  相似文献   

19.
Combined patch-clamp and Fura-2 measurements were performed on chinese hamster ovary (CHO) cells co-expressing two channel proteins involved in skeletal muscle excitation-contraction (E-C) coupling, the ryanodine receptor (RyR)-Ca2+ release channel (in the membrane of internal Ca2+ stores) and the dihydropyridine receptor (DHPR)-Ca2+ channel (in the plasma membrane). To ensure expression of functional L-type Ca2+ channels, we expressed α2, β, and γ DHPR subunits and a chimeric DHPR α1 subunit in which the putative cytoplasmic loop between repeats II and III is of skeletal origin and the remainder is cardiac. There was no clear indication of skeletal-type coupling between the DHPR and the RyR; depolarization failed to induce a Ca2+ transient (CaT) in the absence of extracellular Ca2+ ([Ca2+]o). However, in the presence of [Ca2+]o, depolarization evoked CaTs with a bell-shaped voltage dependence. About 30% of the cells tested exhibited two kinetic components: a fast transient increase in intracellular Ca2+ concentration ([Ca2+]i) (the first component; reaching 95% of its peak <0.6 s after depolarization) followed by a second increase in [Ca2+]i which lasted for 5–10 s (the second component). Our results suggest that the first component primarily reflected Ca2+ influx through Ca2+ channels, whereas the second component resulted from Ca2+ release through the RyR expressed in the membrane of internal Ca2+ stores. However, the onset and the rate of Ca2+ release appeared to be much slower than in native cardiac myocytes, despite a similar activation rate of Ca2+ current. These results suggest that the skeletal muscle RyR isoform supports Ca2+-induced Ca2+ release but that the distance between the DHPRs and the RyRs is, on average, much larger in the cotransfected CHO cells than in cardiac myocytes. We conclude that morphological properties of T-tubules and/or proteins other than the DHPR and the RyR are required for functional “close coupling” like that observed in skeletal or cardiac muscle. Nevertheless, some of our results imply that these two channels are potentially able to directly interact with each other.  相似文献   

20.
The presence and localization of voltage-gated Ca2+ channels of L-type were investigated in intestinal cells of the Atlantic cod. Enterocytes were loaded with the fluorescent Ca2+ probe, fure-2/AM and changes in intracellular Ca2+ concentrations ([Ca2+] i ) were measured, in cell suspensions, in the presence of high potassium levels (100 mm), BAY K-8644 (5 μm), nifedipine (5 μm) or ω-conotoxin (1 μm). L-type Ca2+ channels were visualized on intestinal sections using the fluorescent dihydropyridine (-)-STBodipy. Depolarization of the plasma membrane produced a rapid (within 5 sec) and transient (at basal levels after 21 sec) increase in [Ca2+] i . BAY K-8644 increased the [Ca2+] i by 7.2%. Cells in a Ca2+-free buffer increased [Ca2+] i after addition of 10 mm Ca2+, and this increase was abolished by nifedipine in both depolarizing and normal medium but not by ω-conotoxin. Single cell experiments using video microscopy revealed that enterocytes remained polarized several hours after preparation and that the Ca2+ entry and extrusion occurred at specific and different regions of the enterocyte outer membrane. Fluorescent staining of L-type Ca2+ channels in the intestinal mucosa showed the most intense staining at the brushborder membrane. These results demonstrate the presence of voltage gated L-type Ca2+ channels in enterocytes from the Atlantic cod. The channels are mainly located at the apical side of the cells, and there is a polarized uptake of Ca2+ into the enterocytes. This suggests that the L-type Ca2+ channels are involved in the transcellular Ca2+ entry into the enterocytes. Received: 21 August 1997/Revised: 15 April 1998  相似文献   

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