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

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

V-1, a catecholamine biosynthesis regulatory protein,positively controls catecholamine secretion in PC12D cells

Stably transfected PC12D cell lines overexpressing a catecholamine biosynthesis regulatory protein, V-1, were used to examine the functional role of V-1 in catecholamine secretion. High K⁺-induced dopamine secretion in V-1 overexpressing clones was shown to be markedly potentiated compared with control clones carried with a vector alone. As assayed intracellular calcium concentration ([Ca²⁺]_i) using fura-PE3, V-1 overexpression was observed to enhance high K⁺-elicited [Ca²⁺]_i elevation. Electron microscopic analysis revealed an increase in dense-cored vesicle formation by V-1 overexpression. These results suggest that the enhancement of high K⁺-induced dopamine secretion by V-1 overexpression results from the potentiation of high K⁺-induced [Ca²⁺]_i elevation and the increase in the number of dense-cored vesicles.     

Calcium signaling and secretory responses in agonist-stimulated pituitary gonadotrophs

In cultured pituatary gonadotrophs, gonadotropin-releasing hormone (GnRH) caused dose-dependent and biphasic increases in cytoplasmic calcium concentration ([Ca²⁺]_i) and LH release. Both extra- and intracellular calcium pools participate in GnRH-induced elevation of [Ca²⁺]_i and LH secretion. The spike phase of the [Ca²⁺]_i response represents the primary signal derived predominantly from the rapid mobilization of intracellular Ca²⁺. In contrast, the prolonged phase of the Ca²⁺ signal depends exclusively on Ca²⁺ entry from the extracellular pool. The influx of Ca²⁺ occurs partially through dihydropyridine-sensitive calcium channels. Both [Ca²⁺]_i and LH responses to increasing concentrations of GnRH occur over very similar time scales, suggesting that increasing degrees of receptor occupancy are transduced into amplitude-modulated Ca²⁺ responses, which in turn activate exocytosis in a linear manner. However, several lines of evidence indicated the complexity over the relationship between Ca²⁺ signaling and LH exocytosis. In contrast to [Ca²⁺]_i measurements in cell suspension, single cell Ca²⁺ measurements revealed the existence of a more complicated pattern of Ca²⁺ response to GnRH, with a biphasic response to high agonist doses and prominent oscillatory responses to lower GnRH concentrations, with a log-linear correlation between GnRH dose and the frequency of Ca²⁺ spiking. In addition, analysis of the magnitudes of the magnitudes of the [Ca²⁺]_i and LH responses of gonadotrophs to a wide range of GnRH concentrations in the presence and absence of extracellular Ca²⁺, and to K⁺ and phorbol ester stimulation, showed non-linearity between these parameters with amplification of [Ca²⁺]_i-mediated exocytosis. Studies on cell depleted of protein kinase C under conditions that did not change the LH pool suggested the participation of protein kinase C in this amplication, especially during the plateau phase of the secretory response to GnRH.     

Muscarinic receptor-mediated intracellular Ca mobilization in embryonic chick heart cells

Activation of muscarinic receptors of heart cells elevates the intracellular Ca²⁺ concentration. The increase is considered to be due to influx of extracellular Ca²⁺. We show that intracellular Ca²⁺ mobilization is involved. Cell suspensions prepared from hearts of 6-day-old chick embryos were loaded with the fluorescent Ca²⁺ chelator chlortetracycline. Muscarinic stimulation induces a dose-dependent fluorescence decrease (ED₅₀=2.6 × 10⁻⁶ M) indicating intracellular Ca²⁺ mobilization.     

Oxidative stress increases potassium efflux from pancreatic islets by depletion of intracellular calcium stores

Oxidative stress to B-cells is thought to be of relevance in declining B-cell function and in the process of B-cell destruction. In other tissues including heart, brain and liver, oxidative stress has been shown to elevate the intracellular free calcium concentration and to provoke potassium efflux. We studied the effect of oxidative stress on Ca²⁺ and K⁺ (Rb⁺) outflow from pancreatic islets using the thiol oxidants DIP and BuOOH. Both compounds reversibly increased ⁸⁶Rb⁺ efflux in the presence of 3 and 16.7 mmol/l glucose. Stimulation of ⁸⁶Rb⁺ efflux was also evident in the absence of calcium. DIP evoked release of ⁴⁵Ca²⁺ from the pancreatic islets both in the presence or absence of extracellular calcium. Employing inhibitors of the calcium-activated potassium channel (K_Ca) and the high conductance K⁺-channel (BK_Ca), the effect of DIP on ⁸⁶Rb⁺ efflux was slightly diminished. Tolbutamide had no effect on ⁸⁶Rb⁺ efflux in the presence of DIP. On the other hand thapsigargin, a blocker of the Ca²⁺-ATPase of the endoplasmic reticulum, completely suppressed the DIP-mediated ⁸⁶Rb⁺ outflow. The data suggest that thiol oxidant-induced potassium efflux from pancreatic islets is mainly mediated through liberation of intracellular calcium and subsequent stimulation of calcium-activated potassium efflux.     

Simulation analysis of intracellular Na+ and Cl- homeostasis during beta 1-adrenergic stimulation of cardiac myocyte

To quantitatively understand intracellular Na⁺ and Cl⁻ homeostasis as well as roles of Na⁺/K⁺ pump and cystic fibrosis transmembrane conductance regulator Cl⁻ channel (I_CFTR) during the β1-adrenergic stimulation in cardiac myocyte, we constructed a computer model of β1-adrenergic signaling and implemented it into an excitation-contraction coupling model of the guinea-pig ventricular cell, which can reproduce membrane excitation, intracellular ion changes (Na⁺, K⁺, Ca²⁺ and Cl⁻), contraction, cell volume, and oxidative phosphorylation. An application of isoproterenol to the model cell resulted in the shortening of action potential duration (APD) after a transient prolongation, the increases in both Ca²⁺ transient and cell shortening, and the decreases in both Cl⁻ concentration and cell volume. These results are consistent with experimental data. Increasing the density of I_CFTR shortened APD and augmented the peak amplitudes of the L-type Ca²⁺ current (I_CaL) and the Ca²⁺ transient during the β1-adrenergic stimulation. This indirect inotropic effect was elucidated by the increase in the driving force of I_CaL via a decrease in plateau potential. Our model reproduced the experimental data demonstrating the decrease in intracellular Na⁺ during the β-adrenergic stimulation at 0 or 0.5 Hz electrical stimulation. The decrease is attributable to the increase in Na⁺ affinity of Na⁺/K⁺ pump by protein kinase A. However it was predicted that Na⁺ increases at higher beating rate because of larger Na⁺ influx through forward Na⁺/Ca²⁺ exchange. It was demonstrated that dynamic changes in Na⁺ and Cl⁻ fluxes remarkably affect the inotropic action of isoproterenol in the ventricular myocytes.     

Insulin secretion and intracellular Ca rises in monolayer cultures of neonatal rat β-cells

Glucose-induced insuline release, glucose-induced rises in intracellular free Ca²⁺ concentration ([Ca²⁺]_i), and voltage-dependent Ca²⁺ channel activity were assessed in monolayer cultures of β-vells 3–5 day-old rats. The glucose-stimulated insulin secretory responses and [Ca²⁺]_i rises were like those in adult rat β-cells rather than fetal rat β-cells. Voltage-dependent Ca²⁺ channel antagonists decreased glucose-induced insulin secretion, aborted the [Ca²⁺]₂ rise and, like deprivation of extracellular Ca²⁺, prevented the glucose-induced rise in [Ca²⁺]_i when added before the glucose challenge. The presence of nifedipine-sensitive, voltage-dependent Ca²⁺ channels was demonstrated directly by measuring Ca²⁺ currents using the whole-cell configuration of the patch-clamp technique and indirectly by measuring [Ca²⁺]₁ after membrane depolarization by 45 mMm K⁺ or 200 μM tolbutamide. Thus, in cultured β-cells of 3–5 day-old rats the coupling of glucose stimulation to Ca²⁺ influx is essentially mature, in contrast to what has been reported for fetal or very early neonatal cells.     

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

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

Voltage-dependent L-type Ca channels and a novel type of non-selective cation channel activated by cAMP-dependent phosphorylation in mesoderm-like (MES-1) cells

Undifferentiated P19 embryonal carcinoma cells (ECC P19), the P19-derived clonal cell lines END-2 (visceral endoderm-like), EPI-7 (epithelioid ectoderm-like), MES-1 (mesoderm-like) and a parietal yolk sac cell line (PYS-2) were used as cellular models to examine the functional expression of voltage-dependent Ca channels and other Ca-permeable cation channels at various stages of early embryonic development. Whole-cell currents were recorded by means of the patch clamp technique. Whereas more than 75% of MES-1 cells possessed Ca channel currents, neither P19, END-2, EPI-7 nor PYS-2 cells had detectable voltage-dependent inward currents. Ca channel currents of MES-1 cells were highly sensitive towards 1,4-dihydropyridines and blocked by cadmium. Adrenaline (10 μM) caused Ca channel stimulation in only 14% of MES-1 cells examined. However, in 62% of the cells adrenaline activated a linear current component which under physiological conditions reversed close to 0 mV. Removal of extracellular Na⁺ suppressed the adrenaline-induced inward current, while reducing extracellular Cl⁻ had no significant effect. These findings suggest that the adrenaline-induced current is carried through non-selective cation channels which were found to be permeable for Na⁺, K⁺, Cs⁺ å Ca²⁺. Remarkably, the intracellular signalling pathway for activation of the non-selective cation current involved the cascade of reactions leading to cAMP-dependent phosphorylation, a regulatory pathway well known for cardiac Ca channels. A possible functional role of adrenaline-induced non-selective cation currents and Ca channels in embryonal development is discussed.     

Characterization of Acetylcholine-induced Increases in Cytosolic Free Calcium Concentration in Individual Rat Pancreatic β-cells

The intracellular free Ca²⁺ ion concentration ([Ca²⁺]i) was measured using fura-2 microspec-trofluorimetry in individual rat pancreatic β-cells prepared by enzymatic digestion and fluorescence-activated cell sorting. The mean basal concentration of [Ca²⁺]i in β-cells in the presence of 4.4 mM glucose and 1.8 mM Ca²⁺ was 112±1.6 nM (n=207). The action of acetylcholine (ACh) was concentration-dependent, and raising the concentration resulted in [Ca²⁺]i spikes of increasing amplitude and duration in some, but not all of the β-cells. In addition, the β-cells demonstrated variable sensitivity to ACh. The increases in [Ca²⁺]i were rapid, transient and were blocked by atropine at 10^-6M. A brief exposure to 50 mM K⁺ resulted in a transient increase in [Ca²⁺]i similar to that induced by ACh, but resistant to atropine. A high concentration of ACh (100μL 10^-4M or 10^-3M) induced [Ca²⁺]i oscillations in 11 out of 57 β-cells in the presence of 4.4 mM glucose. Using calcium channel blockers and Ca²⁺ free medium, the source of the increase in [Ca²⁺]i was deduced to be from extracellular spaces. Changing the temperature from 22 to 37°C did not affect the action of ACh on [Ca²⁺]i. These data strongly suggest that ACh exerted a direct action on [Ca²⁺]i in normal rat pancreatic β-cells and support a role for Ca²⁺ as a second messenger in the action of ACh.     

Identification and characterization of functional subunits of Clostridium botulinum type A progenitor toxin involved in binding to intestinal microvilli and erythrocytes

Dimethyl adipimidate (DMA) reduces K⁺ loss from, and dehydration of, red cells containing haemoglobin S (HbS cells). Three membrane transporters may contribute to these processes: the deoxygenation-induced cation-selective channel (P_sickle), the Ca²⁺-activated K⁺ channel (or Gardos channel) and the K⁺-Cl⁻ cotransporter (KCC). We show that DMA inhibited all three pathways in deoxygenated HbS cells. The Gardos channel could be activated following Ca²⁺ loading. Considerable KCC activity was present in oxygenated HbS cells, showing a selective action of DMA on the transporter in deoxygenated cells. Inhibition of sickling correlated strongly with that of P_sickle and moderately with that of KCC activity. We conclude that DMA does not inhibit the K⁺ pathways directly, but acts mainly by preventing HbS polymerisation and sickling. These findings are relevant to the development of novel chemotherapeutic agents for amelioration of sickle cell disease.     

盐胁迫下外源褪黑素和Ca2+对甜瓜幼苗的缓解效应

以甜瓜品种‘羊角酥瓜’为试材,利用人工气候室控制环境条件(昼/夜25/18 ℃),研究盐胁迫条件下外源褪黑素(MT)和Ca²⁺对甜瓜幼苗根系和叶片中Cl^-、Na⁺、K⁺、Mg²⁺、Ca²⁺离子含量,Na⁺/K⁺、 Na⁺/Ca²⁺、Na⁺/Mg²⁺值,以及H⁺-ATP酶活性、渗透调节物质积累和细胞膜质过氧化的影响.结果表明: 与对照相比,盐胁迫处理显著抑制甜瓜幼苗生长,增加根系和叶片中Cl^-、Na⁺含量,降低K⁺、Mg²⁺、Ca²⁺含量.盐胁迫下,喷施外源MT或Ca²⁺处理均可以显著降低甜瓜根系和叶片中Cl^-、Na⁺含量,提高K⁺、Mg²⁺、Ca²⁺含量,植株体内Na⁺/K⁺、Na⁺/Ca²⁺和 Na⁺/Mg²⁺值下降;同时也提高了根系和叶片H⁺-ATP酶活性及叶片渗透调节物质的含量,降低盐胁迫对细胞膜的伤害,表现在甜瓜叶片相对电导率和丙二醛含量降低.总之,在盐胁迫条件下,外源MT、Ca²⁺单独和复配处理均可通过提高H⁺-ATP酶活性来降低盐害离子的含量,改善甜瓜幼苗中的离子平衡,同时增加渗透调节物质的含量,降低膜质过氧化水平,从而增强其对盐胁迫的适应性,其中MT和Ca²⁺复配处理时的效果更好.复配外施 MT 和Ca²⁺在诱导甜瓜幼苗提高耐盐方面具有协同增效作用.     

Contrasting effects of PACAP and carbachol on [Ca]i and inositol phosphates in human neuroblastoma NB-OK-1 cells

The effects of PACAPs on [Ca²⁺]_i were compared to those of carbachol in human neuroblastoma NB-OK-1 cells. PACAP(1–27) and PACAP(1–38) increased [Ca²⁺]_i in a biphasic manner: a transient rise and a secondary plateau. The transient phase reflected the mobilization of [Ca²⁺]_i pool(s) via the inositol phosphate pathway. The modest sustained plateau required extracellular Ca²⁺. Carbachol also increased [Ca²⁺]_i in a biphasic manner, but it mobilized intracellular Ca²⁺ pool(s) with a higher efficacy than PACAPs, then greatly increased Ca²⁺ entry, this being accompanied by a more marked and prolonged elevation of IP₃ and IP₄ than with PACAPs. It is likely that cAMP-mediated phosphorylations due to PACAPs facilitated desensitization at the PACAP receptor-phospholipase C level, so that there was less Ca²⁺ handling through PACAP receptors than with muscarinic M₁ receptors.     

Adipokinetic hormone-induced influx of extracellular calcium into insect fat body cells is mediated through depletion of intracellular calcium stores

Adipokinetic hormone I (AKH I) needs extracellular Ca²⁺ for its activating action on glycogen phosphorylase in locust fat body in vitro. TMB-8 reduces this AKH effect significantly, indicating that for a major part, hormone action also requires the mobilization of Ca²⁺ from intracellular stores. Using ⁴⁵Ca²⁺, AKH was shown to stimulate both the influx and the efflux of Ca²⁺. Thapsigargin also enhances the influx of extracellular Ca²⁺ into the fat body cells, indicating that the stimulating effect of AKH on Ca²⁺ influx may be mediated through depletion of intracellular Ca²⁺ stores as well. AKH is known to enhance cAMP levels in locust fat body. We show that elevation of cAMP with forskolin or theophylline leads to activation of glycogen phosphorylase, both in the presence and in the absence of extracellular Ca²⁺. The present data are discussed in an attempt to elucidate further the mechanism underlying transduction of the hormonal signal in locust fat body.     

盐胁迫下外源褪黑素和Ca2+对甜瓜幼苗的缓解效应

以甜瓜品种‘羊角酥瓜’为试材,利用人工气候室控制环境条件(昼/夜25/18 ℃),研究盐胁迫条件下外源褪黑素(MT)和Ca²⁺对甜瓜幼苗根系和叶片中Cl^-、Na⁺、K⁺、Mg²⁺、Ca²⁺离子含量,Na⁺/K⁺、 Na⁺/Ca²⁺、Na⁺/Mg²⁺值,以及H⁺-ATP酶活性、渗透调节物质积累和细胞膜质过氧化的影响.结果表明: 与对照相比,盐胁迫处理显著抑制甜瓜幼苗生长,增加根系和叶片中Cl^-、Na⁺含量,降低K⁺、Mg²⁺、Ca²⁺含量.盐胁迫下,喷施外源MT或Ca²⁺处理均可以显著降低甜瓜根系和叶片中Cl^-、Na⁺含量,提高K⁺、Mg²⁺、Ca²⁺含量,植株体内Na⁺/K⁺、Na⁺/Ca²⁺和 Na⁺/Mg²⁺值下降;同时也提高了根系和叶片H⁺-ATP酶活性及叶片渗透调节物质的含量,降低盐胁迫对细胞膜的伤害,表现在甜瓜叶片相对电导率和丙二醛含量降低.总之,在盐胁迫条件下,外源MT、Ca²⁺单独和复配处理均可通过提高H⁺-ATP酶活性来降低盐害离子的含量,改善甜瓜幼苗中的离子平衡,同时增加渗透调节物质的含量,降低膜质过氧化水平,从而增强其对盐胁迫的适应性,其中MT和Ca²⁺复配处理时的效果更好.复配外施 MT 和Ca²⁺在诱导甜瓜幼苗提高耐盐方面具有协同增效作用.     

伴生植物盐角草和星花碱蓬不同生育期几种阳离子含量变化分析

为了分析伴生植物盐角草和星花碱蓬在一个生育期内不同器官中Na⁺、K⁺、Ca²⁺及Mg²⁺含量的变化,利用火焰分光光度法和原子吸收法测定了两种植株不同发育时期的根、茎、叶中这4种阳离子含量。结果显示这两种植物根和茎中4种离子的积累趋势和叶器官中不同：Na⁺、K⁺元素含量随着植物的发育呈下降趋势,而在叶器官中则呈上升的趋势;两种植物根和茎及盐角草的叶器官中Ca²⁺、Mg²⁺元素的含量随生育期的发育呈由低到高再降低的趋势,而星花碱蓬的叶器官中这两种离子的含量则表现为由高到低再升高的趋势。两种植物除了Ca²⁺、Mg²⁺的积累趋势在两种植物间存在有差异外,其余指标的变化趋势基本一致。尤其对Na⁺的积累,两种植物Na⁺的含量占根部干重的3%左右,占茎部干重的5%左右,占叶部干重的10%左右。表明伴生植物盐角草和星花碱蓬具有改良盐碱地的良好潜能,可以作为改良盐碱地的优良种植资源。     

Inhibitors of Na+/Ca2+ exchanger prevent oxidant-induced intracellular Ca2+ increase and apoptosis in a human hepatoma cell line

Oxidative stress appears to be implicated in the pathogenesis of various diseases including hepatotoxicity. Although intracellular Ca²⁺ signals have been suggested to play a role in the oxidative damage of hepatocytes, the sources and effects of oxidant-induced intracellular Ca²⁺ increases are currently debatable. Thus, in this study we investigated the exact source and mechanism of oxidant-induced liver cell damage using HepG2 human hepatoma cells as a model liver cellular system. Treatment with 200 μM of tert-butyl hydroperoxide (tBOOH) induced a sustained increase in the level of intracellular reactive oxygen intermediates (ROI) and apoptosis, assessed by 2',7'-dichlorofluorescein fluorescence and flow cytometry, respectively. Antioxidants, N-acetyl cysteine (NAC) or N,N'-diphenyl-p-phenylenediamine significantly inhibited both the ROI generation and apoptosis. In addition, tBOOH induced a slow and sustained increase in intracellular Ca²⁺ concentration, which was completely prevented by the antioxidants. An intracellular Ca²⁺ chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid/cetoxymethyl ester significantly suppressed the tBOOH-induced apoptosis. These results imply that activation of an intracellular Ca²⁺ signal triggered by increased ROI may mediate the tBOOH-induced apoptosis. Both intracellular Ca²⁺ increase and induction of apoptosis were significantly inhibited by an extracellular Ca²⁺ chelator or Na⁺/Ca²⁺ exchanger blockers (bepridil and benzamil), whereas neither Ca²⁺ channel antagonists (verapamil and nifedipine) nor a nonselective cation channel blocker (flufenamic acid) had an effect. These results suggest that tBOOH may increase intracellular Ca²⁺ through the activation of reverse mode of Na⁺/Ca²⁺ exchanger. However, tBOOH decreased intracellular Na⁺ concentration, which was completely prevented by NAC. These results indicate that ROI generated by tBOOH may increase intracellular Ca²⁺ concentration by direct activation of the reverse mode of Na⁺/Ca>²⁺ exchanger, rather than indirect elevation of intracellular Na⁺ levels. Taken together, these results suggest that the oxidant, tBOOH induced apoptosis in human HepG2 cells and that intracellular Ca²⁺ may mediate this action of tBOOH. These results further suggest that Na⁺/Ca²⁺ exchanger may be a target for the management of oxidative hepatotoxicity.     

Depletion of intracellular calcium stores activates an outward potassium current in mast and RBL-1 cells that is correlated with CRAC channel activation

Highly Ca²⁺ selective Ca²⁺ channels activated by store depletion have been recently described in several cell types and have been termed CRAC channels (for calcium release-activated calcium). The present study shows that following store depletion in mast and RBL-1 cells, monovalent outward currents could be recorded if the internal solution contained K⁺ but not Cs⁺. The activation of the outward K⁺ current correlated with the activation of I_CRAC, in both time and amplitude, suggesting that the K⁺ current might be carried by CRAC channels. The amplitude of the outward current was increased if external Ca²⁺ was reduced or replaced by external Ba²⁺. The outward K⁺ conductance might have a physiological role in maintaining the driving force for Ca²⁺ entry during the activation of CRAC channels.     

Electrophysiology of mammalian Schwann cells

Schwann cells are the satellite cell of the peripheral nervous system, and they surround axons and motor nerve terminals. The review summarises evidence for the ion channels expressed by mammalian Schwann cells, their molecular nature and known or speculated functions. In addition, the recent evidence for gap junctions and cytoplasmic diffusion pathways within the myelin and the functional consequences of a lower-resistance myelin sheath are discussed.

The main types of ion channel expressed by Schwann cells are K⁺ channels, Cl⁻ channels, Na⁺ channels and Ca²⁺ channels. Each is represented by a variety of sub-types. The molecular and biophysical characteristics of the cation channels expressed by Schwann cells are closely similar or identical to those of channels expressed in peripheral axons and elsewhere. In addition, Schwann cells express P₂X ligand-gated ion channels. Possible in vivo roles for each ion channel type are discussed. Ion channel expression in culture could have a special function in driving or controlling cell proliferation and recent evidence indicates that some Ca²⁺ channel and Kir channel expression in culture is dependent upon the presence of neurones and local electrical activity.     

Fluctuations in the concentration of extracellular ATP synchronized with intracellular Ca(2+) oscillatory rhythm in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The intracellular concentration of free Ca²⁺ also changes rhythmically in association with the rhythmic contraction of myocytes (Ca²⁺ oscillation). Both the contraction and Ca²⁺ oscillatory rhythms are synchronized among myocytes, and intercellular communication via gap junctions has been considered primarily responsible for the synchronization. However, a recent study has demonstrated that intercellular communication via extracellular ATP-purinoceptor signaling is also involved in the intercellular synchronization of intracellular Ca²⁺ oscillation. In this study, we aim to elucidate whether the concentration of extracellular ATP changes cyclically and contributes to the intercellular synchronization of Ca²⁺ oscillation among myocytes. In almost all the cultured cardiac myocytes at four days in vitro (4 DIV), intracellular Ca²⁺ oscillations were synchronized with each other. The simultaneous measurement of the concentration of extracellular ATP and intracellular Ca²⁺ revealed the extracellular concentration of ATP actually oscillated concurrently with the intracellular Ca²⁺ oscillation. In addition, power spectrum and cross-correlation analyses suggested that the treatment of cultured cardiac myocytes with suramin, a blocker of P2 purinoceptors, resulted in the asynchronization of Ca²⁺ oscillatory rhythms among cardiac myocytes. Treatment with suramin also resulted in a significant decrease in the amplitudes of the cyclic changes in both intracellular Ca²⁺ and extracellular ATP. Taken together, the present study demonstrated the possibility that the concentration of extracellular ATP changes cyclically in association with intracellular Ca²⁺, contributing to the intercellular synchronization of Ca²⁺ oscillation among cultured cardiac myocytes.