Depolarization of the membrane potential decreases the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores in rat thyroid FRTL-5 cells.

The aim of the present study was to investigate the effect of membrane depolarization on ATP-induced changes in intracellular Ca2+ ([Ca2+]i) and the refilling of intracellular Ca2+ stores in thyroid follicular FRTL-5 cells. Depolarizing the cells with 50 mM K+, an amount sufficient to almost totally depolarize the cells as determined by bisoxonal, significantly reduced the ATP-induced uptake of 45Ca2+. This effect was not dependent on an enhanced efflux of Ca2+, as no difference in the ATP-induced efflux of 45Ca2+ was obtained between control cells and depolarized cells. The ATP-induced transient increase in [Ca2+]i in Fura-2 loaded cells was not altered by depolarization, whereas the ATP-induced plateau in [Ca2+]i was decreased compared with control cells. Furthermore, in cells stimulated with ATP in a Ca(2+)-free buffer, readdition of Ca2+ after the termination of the ATP response induced a decreased response in [Ca2+]i in depolarized cells. Refilling of intracellular Ca2+ stores was investigated by first stimulating the cells with noradrenaline (NA). The effect of NA was then terminated with prazosin, and the cells restimulated with ATP. In cells depolarized with high K+, the response to ATP was decreased compared with that seen in control cells. The results thus suggest that both the ATP-induced influx of extracellular Ca2+ and the refilling of intracellular Ca2+ stores is decreased in depolarized FRTL-5 cells.     

Extracellular nucleotides mediate Ca2+ fluxes in J774 macrophages by two distinct mechanisms

We have studied the effects of extracellular nucleotides on the cytosolic free calcium concentration [( Ca2+]i) in J774 macrophages using quin2 and indo-1 as indicator dyes. Micromolar quantities of ATP induced a biphasic increase in [Ca2+]i: a rapid and transient increase (peak I) which was due to mobilization of Ca2+ from intracellular stores and a second more sustained elevation (peak II) due to influx of extracellular Ca2+. The sustained peak II elevation had two components, a "low threshold" (1 microM ATP) response which saturated at 10-50 microM ATP and a "high threshold" response, apparent at [ATP] greater than 100 microM. The latter component was not seen with nucleotides other than ATP and correlated with an ATP-induced generalized increase in plasma membrane permeability. A variant J774 cell line was isolated which does not demonstrate this ATP-induced increase in plasma membrane permeability; nevertheless, it demonstrated both the release of Ca2+ from intracellular stores and the low threshold component of the Ca2+ influx across the plasma membrane in response to nucleoside di- and triphosphates. Several lines of evidence indicate that the fully ionized (i.e. free acid) forms of nucleoside di- and triphosphates were the ligands that mediated these increases in [Ca2+]i. These data show that extracellular nucleotides mediate Ca2+ fluxes by two distinct mechanisms in J774 cells. In one, the rise in [Ca2+]i is due to release of Ca2+ from intracellular stores and Ca2+ influx across the plasma membrane. This response is elicited preferentially by the free acid forms of purine and pyrimidine nucleoside di- and triphosphates. In the other, the rise in [Ca2+]i reflects a more generalized increase in plasma membrane permeability and is elicited by ATP4- only.     

Phorbol ester-stimulated insulin secretion by RINm5F insulinoma cells is linked with membrane depolarization and an increase in cytosolic free Ca2+ concentration

In studying the regulation of insulin secretion by phorbol esters, we examined their effects on the cytosolic free Ca2+ concentration ([Ca2+]i), using the Ca2+ indicator fura-2 in the rat insulin-secreting beta-cell line RINm5F. [Ca2+]i was measured in parallel with the rate of insulin release. 50 nM 12-O-tetradecanoylphorbol-13-acetate (TPA), which may act via protein kinase C, stimulated insulin release and caused an increase in [Ca2+]i. Ca2+-free conditions eliminated the increase in [Ca2+]i and resulted in a reduced stimulation of insulin release by TPA. The Ca2+ channel blocker nitrendipine (300 nM) inhibited both the increase in [Ca2+]i and the increased rate of insulin secretion. Another phorbol ester, 4 beta-phorbol 12,13-didecanoate, which activates protein kinase C, also induced an increase in [Ca2+]i and in the rate of insulin release, while 4 alpha-phorbol 12,13-didecanoate, which fails to stimulate protein kinase C, was without effect. Further studies with bis-oxonol as an indicator of membrane potential showed that TPA depolarized the beta-cell plasma membrane. From these results, it is concluded that TPA depolarizes the plasma membrane, induces the opening of Ca2+ channels in the RINm5F beta-cell plasma membrane, increases [Ca2+]i, and results in insulin secretion. The action of TPA was next compared with that of a depolarizing concentration of KC1 (25 mM), which stimulates insulin secretion simply by opening Ca2+ channels. TPA consistently elicited less depolarization, a smaller rise of [Ca2+]i, but a greater release of insulin than KC1. Therefore an additional action of TPA is suggested, which potentiates the action of the elevated [Ca2+]i on insulin secretion.     

Extracellular ATP increases cytoplasmic free Ca2+ concentration in clonal insulin-producing RINm5F cells. A mechanism involving direct interaction with both release and refilling of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool.

Effects of extracellularly applied ATP (added as disodium salt) on stimulus-secretion coupling were investigated in clonal insulin-producing RINm5F cells. Cytoplasmic free Ca2+ concentration [( Ca2+]i), electrical activity, membrane potential, formation of InsP3 and insulin release were measured. Addition of ATP in a Ca2(+)-containing medium promoted a rapid rise in [Ca2+]i, which was followed by a slow decline towards the basal level. In a Ca2(+)-free medium, the ATP-induced increase in [Ca2+]i was smaller, but still enough to elicit insulin secretion. Upon normalization of the extracellular Ca2+ concentration, the response to ATP recovered instantaneously. The presence of glucose in the incubation medium was a prerequisite to obtain a pronounced effect of ATP in the absence of extracellular Ca2+. However, glucose did not enhance the response to ATP in a Ca2(+)-containing medium. The effect of ATP was dose-dependent, with a clearly detectable increase in [Ca2+]i at 1 microM and a maximal response being obtained at 200 microM-ATP. The response to ATP was unaffected by activating adenylate cyclase by forskolin, but was abolished by 10 nM of the phorbol ester phorbol 12-myristate 13-acetate. The effects of ATP on [Ca2+]i could not be accounted for by a generalized increase in plasma-membrane permeability, as evident from the failure of the nucleotide to increase the fluorescence of the nuclear stain ethidium bromide. After stimulation with ATP there was an increase in membrane potential, in both the absence and the presence of extracellular Ca2+. Blockage of the voltage-activated Ca2+ channals with D-600, in a Ca2(+)-containing medium, decreased the effect of ATP on [Ca2+]i slightly. Patch-clamp measurements using the cell-attached patch configuration revealed that the RINm5F cells produce spontaneous action potentials, the frequency of which increased markedly on addition of ATP. Whole-cell recordings demonstrated that the increase in spike frequency was not associated with the development of an inward current, but was rather accountable for by a decrease in the activity of the ATP-regulated K+ channels. Addition of 200 microM-ATP stimulated phospholipase C activity, as evident from the formation of InsP3, both in the absence and in the presence of extracellular Ca2+. Thus in the absence of extracellular Ca2+ the stimulatory effect of ATP on insulin release can be explained by InsP3-induced mobilization of intracellularly bound Ca2+. Hence, in the RINm5F cells extracellular ATP acts in a manner similar to other Ca2(+)-mobilizing agents.(ABSTRACT TRUNCATED AT 400 WORDS)     

Somatostatin inhibits insulin secretion by a G-protein-mediated decrease in Ca2+ entry through voltage-dependent Ca2+ channels in the beta cell.

We tested the hypothesis that somatostatin (SRIF) inhibits insulin secretion from an SV40 transformed hamster beta cell line (HIT cells) by an effect on the voltage-dependent Ca2+ channels and examined whether G-proteins were involved in the process. Ca2+ currents were recorded by the whole cell patch-clamp method, the free cytosolic calcium, [Ca2+]i, was monitored in HIT cells by fura-2, and cAMP and insulin secretion were measured by radioimmunoassay. SRIF decreased Ca2+ currents, [Ca2+]i, and basal insulin secretion in a dose-dependent manner over the range of 10(-12)-10(-7)M. The increase in [Ca2+]i and insulin secretion induced by either depolarization with K+ (15 mM) or by the Ca2+ channel agonist, Bay K 8644 (1 microM) was attenuated by SRIF in a dose-dependent manner over the same range of 10(-12)-10(-7) M. the half-maximal inhibitory concentrations (IC50) for SRIF inhibition of insulin secretion were 8.6 X 10(-12) M and 8.3 X 10(-11) M for K+ and Bay K 8644-stimulated secretion and 1 X 10(-10) M and 2.9 X 10(-10) M for the SRIF inhibition of the K+ and Bay K 8644-induced rise in [Ca2+]i, respectively. SRIF also attenuated the rise in [Ca2+]i induced by the cAMP-elevating agent, isobutylmethylxanthine (1 mM) in the presence of glucose. Bay K 8644, K+ and SRIF had no significant effects on cAMP levels and SRIF had no effects on adenylyl cyclase activity at concentrations lower than 1 microM. SRIF (100 nM) did not change K+ efflux (measured by 86Rb+) through ATP-sensitive K+ channels in HIT cells. SRIF (up to 1 microM) had no significant effect on membrane potential measured by bisoxonol fluorescence. Pretreatment of the HIT cells with pertussis toxin (0.1 microgram/ml) overnight abolished the effects of SRIF on Ca2+ currents, [Ca2+]i and insulin secretion implying a G-protein dependence in SRIF's actions. Thus, one mechanism by which SRIF decreases insulin secretion is by inhibiting Ca2+ influx through voltage-dependent Ca2+ channels, an action mediated through a pertussis toxin-sensitive G-protein.     

Differential regulation of thrombin- or ATP-induced mobilization of intracellular Ca2+ by prostacyclin receptor in mouse mastocytoma cells.

Thrombin induced an increase in [Ca2+]i in mouse mastocytoma P-815 cells. This increase was markedly reduced by prior exposure to pertussis toxin (PT) but not by removal of extracellular Ca2+, suggesting that thrombin stimulates phospholipase C via a PT-sensitive GTP-binding protein. ATP also induced an increase in [Ca2+]i. This increase was insensitive to PT but completely suppressed on removal of extracellular Ca2+, suggesting that ATP stimulates Ca2+ influx in a PT-insensitive manner. Iloprost, a stable prostacyclin analogue, increased the cellular cAMP level and dose-dependently inhibited the thrombin-induced increase in [Ca2+]i, whereas the ATP-induced increase in [Ca2+]i was markedly enhanced by iloprost. Cyclic AMP analogues, dibutyryl cAMP and 8-bromo cAMP, also inhibited the increase in [Ca2+]i induced by thrombin and promoted that by ATP, indicating that the inhibitory and stimulatory effects of iloprost are mediated by cAMP. These results suggest that the prostacyclin receptor differentially regulates two distinct Ca2+ mobilizing systems via cAMP in mastocytoma cells.     

Receptor-mediated calcium influx in PC12 cells. ATP and bradykinin activate two independent pathways

In the neurosecretory cell line PC12 the cytosolic free Ca2+ concentration, [Ca2+]i, and membrane potential were affected by both external ATP and the nonapeptide bradykinin, BK. The latter caused a rapid and large release of Ca2+ from intracellular stores (Ca2+ redistribution) and, in the presence of external Ca2+, a long lasting, but moderate Ca2+ influx, which was insensitive to dihydropyridine blockers. On the contrary, ATP evoked a [Ca2+]i rise which rapidly inactivated. At least three different mechanisms accounted for the ATP-induced increase in [Ca2+]i: less than 20% of the total response was due to intracellular Ca2+ redistribution, consistent with a small increase in inositol 1,4,5-trisphosphate level; the rest (over 80%) was equally accounted for by ATP-activated cation channels and voltage-gated Ca2+ channels. ATP and BK (the latter after K+ channel blockade) caused plasma membrane depolarization. With both agonists the inward current was carried by both Na+ and Ca2+, although the BK-activated current appeared to be more selective for Ca2+. Channels triggered by ATP and BK differed not only in their cation selectivity, but also in modulation by both [Ca2+]i and drugs such as the phorbol ester phorbol 12-myristate 13-acetate and the new antagonist of ligand-activated Ca2+ influx, SK&F 96365.     

Video imaging of cytosolic Ca2+ in pancreatic beta-cells stimulated by glucose, carbachol, and ATP.

In order to define the differences in the distribution of cytosolic free Ca2+ ([Ca2+]i) in pancreatic beta-cells stimulated with the fuel secretagogue glucose or the Ca(2+)-mobilizing agents carbachol and ATP, we applied digital video imaging to beta-cells loaded with fura-2.83% of the cells responded to glucose with an increase in [Ca2+]i after a latency of 117 +/- 24 s (mean +/- S.E., 85 cells). Of these cells, 16% showed slow wave oscillations (frequency 0.35/min). In order to assess the relationship between membrane potential and the distribution of the [Ca2+]i rise, digital image analysis and perforated patch-clamp methods were applied simultaneously. The system used allowed sufficient temporal resolution to visualize a subplasmalemmal Ca2+ transient due to a single glucose-induced action potential. Glucose could also elicit a slow depolarization which did not cause Ca2+ influx until the appearance of the first of a train of action potentials. [Ca2+]i rose progressively during spike firing. Inhibition of Ca2+ influx by EGTA abolished the glucose-induced rise in [Ca2+]i. In contrast, the peak amplitude of the [Ca2+]i response to carbachol was not significantly different in normal or in Ca(2+)-deprived medium. Occasionally, the increase of the [Ca2+]i rise was polarized to one area of the cell different from the subplasmalemmal rise caused by glucose. The amplitude of the response and the number of responding cells were significantly increased when carbachol was applied after the addition of high glucose (11.2 mM). ATP also raised [Ca2+]i and promoted both Ca2+ mobilization and Ca2+ influx. The intracellular distribution of [Ca2+]i was homogeneous during the onset of the response. A polarity in the [Ca2+]i distribution could be detected either in the descending phase of the peak or in subsequent peaks during [Ca2+]i oscillations caused by ATP. In the absence of extracellular Ca2+, the sequential application of ATP and carbachol revealed that carbachol was still able to raise [Ca2+]i after exhaustion of the ATP response. This may be due to desensitization to the former agonist, since the response occurred in the same area of the cell. These results reveal subtle differences in [Ca2+]i distribution following membrane depolarization with glucose or the application of Ca(2+)-mobilizing agonists.     

Activators of protein kinase C depolarize insulin-secreting cells by closing K+ channels.

Carbohydrate stimuli of insulin secretion depolarize the pancreatic B cell and the B-cell line RINm5F by inhibiting ATP-sensitive K+ channels. We examined the possibility that this effect is mediated by activation of protein kinase C. In RINm5F cells, the triose D-glyceraldehyde evoked a rapid increase of the mass of 1,2-diacylglycerol, the endogenous activator of protein kinase C. This effect is mainly due to de novo synthesis of the lipid from glycolytic intermediates, as glyceraldehyde carbon was incorporated into 1,2-diacylglycerol within 1 min of exposure to 14C-labelled glyceraldehyde. The effects of two exogenous activators of kinase C, 4-beta-12-phorbol-myristate 13-acetate (PMA) and 1,2-didecanoylglycerol (DC10) on single K+ channel currents were examined in RINm5F cell-attached membrane patches. Both PMA and DC10 depolarized the cells and decreased the open-state probability of the ATP-sensitive K+ channels. These actions were not due to changes in cellular ATP content, since PMA, like glyceraldehyde, failed to alter cellular ATP. As is the case for glyceraldehyde, PMA and DC10 raised cytosolic free Ca2+ [( Ca2+]i) and stimulated insulin secretion. Both of these effects are inhibited in the absence of external Ca2+. This, and the attenuation of the [Ca2+]i rise by verapamil, suggest that all three stimuli raise [Ca2+]i by promoting Ca2+ influx through voltage-gated channels in turn leading to insulin secretion. As the exogenous activators of protein kinase C mimic the effects of glyceraldehyde, it is proposed that the carbohydrate-mediated production of 1,2-diacylglycerol constitutes the link between metabolism and membrane depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that phorbol ester interferes with stimulated Ca2+ redistribution by activating Ca2+ efflux in neutrophil leucocytes.

The free calcium ion concentration, [Ca2+]i, in the cytoplasmic matrix of quin2-loaded neutrophil leucocytes increases rapidly after addition of concanavalin A. This increase is effectively abolished by a short (3 min) preincubation with 10 nM-TPA (12-O-tetradecanoylphorbol 13-acetate). TPA also inhibits a [Ca2+]i rise of similar magnitude induced by low concentrations (10 nM) of calcium ionophore A23187, suggesting that phorbol ester does not interfere with a physiological influx mechanism. To investigate the effects of TPA further, cells were depleted of Ca2+ during quin2 loading and then re-equilibrated with normal extracellular [Ca2+]. The return to a stable [Ca2+]i value was preceded by a transient overshoot in [Ca2+]i, implying delayed activation of an efflux mechanism by rising [Ca2+]i. TPA abolished the transient, suggesting preactivation by TPA of the efflux mechanism before Ca2+ influx. TPA also stimulates net Ca2+ efflux from neutrophils and neutrophil cytoplasts. These observations are consistent with the thesis that TPA stimulates a Ca2+-efflux mechanism in these cells.     

2-O-methyl PAF as a Ca2+ mobilizer in Madin Darby canine kidney cells

In Madin-Darby canine kidney (MDCK) cells, the effect of 2-O-methyl PAF, an inactive analogue of platelet activating factor (PAF), on intracellular Ca2+ concentration ([Ca2+]i) was measured by using the Ca2+-sensitive fluorescent dye fura-2. 2-O-methyl PAF (> or = 15 microM) caused a rapid rise of [Ca2+]i in a concentration-dependent manner. 2-O-methyl PAF-induced [Ca2+]i rise was partly reduced by removal of extracellular Ca2+. 2-O-methyl PAF-induced extracellular Ca2+ influx was also suggested by Mn2+ influx-induced fura-2 fluorescence quench. The 2-O-methyl PAF-induced Ca2+ influx was blocked by nifedipine, verapamil and diltiazem. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+]i rise, after which 2-O-methyl PAF failed to increase [Ca2+]i; also, pretreatment with 2-O-methyl PAF depleted thapsigargin-sensitive Ca2+ stores. U73122, an inhibitor of phospholipase C, abolished ATP (but not 2-O-methyl PAF)-induced [Ca2+]i rise. These findings suggest that 2-O-methyl PAF evokes a rapid increase in [Ca2+]i in renal tubular cells by stimulating both extracellular Ca2+ influx and intracellular Ca2+ release.     

Imaging of cytosolic Ca2+ transients arising from Ca2+ stores and Ca2+ channels in sympathetic neurons

Changes in cytosolic free Ca2+ concentration [( Ca2+]i) due to Ca2+ entry or Ca2+ release from internal stores were spatially resolved by digital imaging with the Ca2+ indicator fura-2 in frog sympathetic neurons. Electrical stimulation evoked a rise in [Ca2+]i spreading radially from the periphery to the center of the soma. Elevated [K+]o also increased [Ca2+]i, but only in the presence of external Ca2+, indicating that Ca2+ influx through Ca2+ channels is the primary event in the depolarization response. Ca2+ release or uptake from caffeine-sensitive internal stores was able to amplify or attenuate the effects of Ca2+ influx, to generate continued oscillations in [Ca2+]i, and to persistently elevate [Ca2+]i above basal levels after the stores had been Ca2(+)-loaded.     

Single islet beta-cell stimulation by nutrients: relationship between pyridine nucleotides, cytosolic Ca2+ and secretion.

It is generally believed that the initiation of insulin secretion by nutrient stimuli necessitates the generation of metabolic coupling factors, leading to membrane depolarization and the gating of voltage-sensitive Ca2+ channels. To establish this sequence of events, the kinetics of endogenous fluorescence of reduced pyridine nucleotides [NAD(P)H], reflecting nutrient metabolism, were compared to those of cytosolic calcium ([Ca2+]i) rises in single cultured rat islet beta-cells. In preliminary experiments, the loss of quinacrine fluorescence from prelabelled cells was used as an indicator of secretion. This dye is concentrated in the acidic insulin-containing secretory granules. Both glucose and 2-ketoisocaproate (KIC) raised [Ca2+]i in a dose-dependent manner. There was marked cellular heterogeneity in the [Ca2+]i response patterns. The two nutrient stimuli also increased NAD(P)H fluorescence, again showing cell-to-cell variations. In combined experiments, where the two parameters were measured in the same cell, the elevation of the NAD(P)H fluorescence preceded the rise in [Ca2+]i, confirming the statistical evaluation performed on separate cells. The application of two consecutive glucose challenges revealed coordinated changes in [Ca2+]i and NAD(P)H fluorescence. Finally, quinacrine secretion was stimulated by two nutrients with onset times similar to those recorded for [Ca2+]i elevations. These results clearly demonstrate that increased metabolism occurs during the lag period preceding Ca2+ influx via voltage-sensitive Ca2+ channels, a prerequisite for the triggering of insulin secretion by nutrient stimuli.     

Agonists that suppress M-current elicit phosphoinositide turnover and Ca2+ transients, but these events do not explain M-current suppression

The hypothesis that acetylcholine, substance P, and LHRH suppress M-current by activating phospholipase C was tested. Each agonist caused turnover of phosphoinositide, as measured by release of inositol phosphates, and a modest transient rise in intracellular free Ca2+ ([ Ca2+]i), as determined with fura-2. Active phorbol esters depressed M-current only 50% and did not prevent further suppression by LHRH. M-current, its control by agonists, and its depression by phorbol esters were not affected by adding inositol trisphosphate or Ca2+ buffers with high or low Ca2+ to the whole-cell, voltage-clamp pipette. We conclude that phospholipase C activation does occur but does not mediate the suppression of M-current by agonists. Caffeine produced large [Ca2+]i transients and acted as an agonist to suppress M-current.     

Role of GPR40 in fatty acid action on the beta cell line INS-1E

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.     

Inositol tetrakisphosphate isomers and elevation of cytosolic Ca2+ in vasopressin-stimulated insulin-secreting RINm5F cells.

Signal generation during the stimulation of insulin secretion by arginine vasopressin (AVP) was investigated in RINm5F cells. AVP (0.1 microM) caused a biphasic cytosolic Ca2+ ([Ca2+]i) rise, namely a rapid transient marked elevation after stimulation followed by a series of oscillations. In the absence of extracellular Ca2+, the sustained oscillations were abolished, while the initial [Ca2+]i transient was only partly decreased, indicating that the former are due to Ca2+ influx and the latter due mainly to mobilization from internal Ca2+ stores. AVP also evoked a transient depolarization of the average membrane potential. AVP-induced Ca2+ influx during the sustained phase, which was strictly dependent on receptor occupancy, was attenuated by membrane hyperpolarization with diazoxide. However, blockade of Ca2+ channels of the L- or T-type was ineffective. AVP stimulated production of diacylglycerol and inositol phosphates; for the latter both [3H] inositol labeling and mass determinations were performed. A transient increase in Ins(1,4,5)P3 was followed by a marked enhancement of Ins(1,3,4,5)P4 (8-fold) peaking at 15 s and gradually returning to basal values. Ins(1,3,4,6)P4 and Ins(3,4,5,6)P4 exhibited the most long-lasting augmentation (4- and 1.7-fold, respectively), and therefore correlated best with the period of sustained [Ca2+]i oscillations. InsP5 and InsP6 were not elevated. The effects of AVP, including the stimulation of insulin secretion from perifused cells, were obliterated by a V1 receptor antagonist. In conclusion, AVP induces protracted [Ca2+]i elevation in RINm5F cells which is associated with long-lasting increases in InsP4 isomers. The accumulation of InsP4 isomers reflects receptor occupancy and accelerated metabolism of the inositol phosphates. Activation of second messenger-operated Ca2+ channels is not necessarily implicated because of the attenuating effect of membrane hyperpolarization.     

Paroxetine-induced Ca2+ movement and death in OC2 human oral cancer cells

The effect of the antidepressant paroxetine on cytosolic free Ca2+ concentrations ([Ca2+]i) in OC2 human oral cancer cells is unclear. This study explored whether paroxetine changed basal [Ca2+]i levels in suspended OC2 cells by using fura-2 as a Ca2+-sensitive fluorescent dye. Paroxetine at concentrations between 100-1,000 microM increased [Ca2+]i in a concentration-dependent manner. The Ca2+ signal was reduced by 50% by removing extracellular Ca2+. Paroxetine-induced Ca2+ influx was inhibited by the store-operated Ca2+ channel blockers nifedipine, econazole and SK&F96365, and protein kinase C modulators. In Ca2+-free medium, pretreatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin abolished paroxetine-induced [Ca2+]i rise. Inhibition of phospholipase C with U73122 did not alter paroxetine-induced [Ca2+]i rise. Paroxetine at 10-50 microM induced cell death in a concentration-dependent manner. The death was not reversed when cytosolic Ca2+ was chelated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. Propidium iodide staining suggests that apoptosis plays a role in the death. Collectively, in OC2 cells, paroxetine induced [Ca2+]i rise by causing phospholipase C-independent Ca2+ release from the endoplasmic reticulum and Ca2+ influx via store-operated Ca2+ channels in a manner regulated by protein kinase C and phospholipase A2. Paroxetine (up to 50 microM) induced cell death in a Ca2+-independent manner.     

Effect of the antidepressant sertraline on Ca2+fluxes in Madin-Darby canine renal tubular cells

The effect of the antidepressant sertraline on cytosolic-free Ca2+ concentrations ([Ca2+]i) in Madin Darby canine kidney (MDCK) cells is unclear. This study explored whether sertraline changed basal [Ca2+]i levels in suspended MDCK cells by using fura-2 as a Ca2+-sensitive fluorescent dye. Sertraline at concentrations between 1and 100 μM increased [Ca2+]i in a concentration-dependent manner. The Ca2+ signal was reduced partly by removing extracellular Ca2+ implicating Ca2+ entry and release both contributed to the [Ca2+]i rise. Sertraline induced Mn2+ influx, leading to quench of fura-2 fluorescence, suggesting Ca2+ influx. This Ca2+ influx was inhibited by suppression of phospholiapase A2 but not by store-operated Ca2+ channel blockers and protein kinase C/A modulators. In Ca2+-free medium, pretreatment with the endoplasmic reticulum Ca2+ pump inhibitors nearly abolished sertraline-induced Ca2+ release. Conversely, pretreatment with sertraline partly reduced inhibitor-induced [Ca2+]i rise, suggesting that sertraline released Ca2+ from endoplasmic reticulum. Inhibition of phospholipase C did not much alter sertraline-induced [Ca2+]i rise. Collectively, in MDCK cells, sertraline induced [Ca2+]i rises by causing phospholipase C-independent Ca2+ release from the endoplasmic reticulum and Ca2+ influx via phospholipase A2-sensitive Ca2+ channels.     

Effect of nortriptyline on Ca2+ handling in SIRC rabbit corneal epithelial cells

To explore the effect of nortriptyline, a tricyclic antidepressant, on cytosolic free Ca2+ concentrations ([Ca2+]i) in corneal epithelial cells, [Ca2+]i levels in suspended SIRC rabbit corneal epithelial cells were measured by using fura-2 as a Ca2+-sensitive fluorescent dye. Nortriptyline at concentrations between 20-200 microM increased [Ca2+]i in a concentration-dependent manner. The Ca2+ signal was reduced partly by removing extracellular Ca2+. Nortriptyline-induced Ca2+ influx was inhibited by the store-operated Ca2+ channel blockers econazole and SK&F96365, the phospholipase A2 inhibitor aristolochic acid, and alteration of activity of protein kinase C. In Ca2+-free medium, 200 microM nortriptyline pretreatment greatly inhibited the rise of [Ca2+]i induced by the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin. Conversely, pretreatment with thapsigargin or 2,5-di-tert-butylhydroquinone (BHQ; another endoplasmic reticulum Ca2+ pump inhibitor) nearly abolished nortriptyline-induced [Ca2+]i rise. Inhibition of phospholipase C with U73122 decreased nortriptyline-induced [Ca2+]i rise by 75%. Taken together, nortriptyline induced [Ca2+]i rises in SIRC cells by causing phospholipase C-dependent Ca2+ release from the endoplasmic reticulum and Ca2+ influx via store-operated Ca2+ channels.     

Subcellular mechanism of desensitization of the ATP-induced Ca2+ mobilization in human umbilical vein endothelial cells: role of intracellular Ca2+ stores

Confluent monolayers of human umbilical vein endothelial cells subcultured on glass coverslips were loaded with the fluorescent Ca2+ indicator, fura-2. Changes in fura-2 fluorescence were detected by means of a fluorescence spectrophotometer. Both ATP and ADP (0.3-100 microM) caused a concentration-dependent transient peak response of the intracellular free calcium concentration ([Ca2+]i), followed by a lower sustained response. AMP and adenosine did not induce detectable changes in [Ca2+]i. The sustained response to ATP was abolished by superfusion with the Ca2(+)-free solution (with 1 mM EGTA), while the transient peak response was uninfluenced. The transient peak response to ATP (30 microM) was inhibited by pre-exposure to ATP in a graded manner depending on the concentration of ATP. The response to ATP recovered after washout for 20 min with the solution containing Ca2+, but not with the Ca2(+)-free solution. The transient peak response to ATP was markedly reduced by preceding exposure to histamine, while the response to histamine was not influenced by pre-exposure to ATP. These findings indicate that depletion and refilling of the ATP-sensitive intracellular Ca2+ store may be responsible for the desensitization and recovery of the ATP-induced [Ca2+]i response. The pharmacological characteristics of the ATP-sensitive intracellular Ca2+ store seem different from those of the histamine-sensitive store.