肥大和衰竭心肌L型Ca2+通道研究进展

心肌细胞 L 型钙电流 （ IC a· L）触发了 Ca2 +诱导的 Ca2 +释放 （ CICR） ,调节着细胞内瞬时 Ca2 +动力学 ,从而在兴奋—收缩耦联过程中起关键性作用。ICa· L密度和通道功能的改变参与了心力衰竭的发生。作者综述了 :1L型 Ca2 +通道的结构和生理作用 ;2肥大和衰竭的心肌 ICa· L变化及其频率依赖性调节机制 ;3 Ca2 + 通道和心力衰竭的治疗关系 ;4L型 Ca2 +通道未来研究方向     

瞬时感受器电位超家族与血管平滑肌功能

血管平滑肌的功能主要是通过多种机制激活Ca2 内流使胞内Ca2 浓度持续升高来实现的。目前的研究表明这种Ca2 内流与一系列瞬时感受器电位超家族成员有关,瞬时感受器电位超家族主要成员有:瞬时感受器电位C1、C3~6、V1、V2、V4、M4、M7和P2。文章概述瞬时感受器电位在血管平滑肌功能方面的作用。     

低分化鼻咽癌细胞的自发氯电流特性

目的：研究自发的低分化鼻咽癌细胞（ CNE-2Z）氯电流特征。方法采用全细胞膜片钳记录自发的CNE-2Z细胞氯电流,通过去除电极内液中的ATP、细胞外灌流氯离子通道阻断剂或高渗液等方法观察并分析电流的特性。结果细胞处于等渗环境中可自发激活一个具有明显外向优势的电流,该电流没有明显的时间依赖性失活,电流的翻转电位为（-7．6±0．9） mV,接近氯离子平衡电位（-0．9 mV）。该电流的激活依赖于细胞内ATP。氯通道阻断剂ATP可抑制该电流,且其对外向电流的抑制作用大于内向电流。细胞外灌流高渗液对该电流有明显抑制作用。结论 CNE-2Z细胞处于等渗环境中可自发激活一个电流特征与容积敏感性氯通道相似的氯通道。     

硫化氢对大鼠肝星状细胞增殖及Ca2+浓度的影响

目的 探讨硫化氢(H2S)对大鼠原代肝星状细胞(HsC)增殖及Ca2+浓度的影响及其作用机制.方法 大鼠肝星状原代细胞作为研究对象,将过氧化氢(H2O2)作用于大鼠原代HSC制造肝纤维化的氧化应激模型,用钙离子荧光探针Fluo-3/AM负载细胞,并在此基础上应用不同剂量的NaSH(H2S供体)和KATP通道抑制剂(格列本脲)对各组细胞进行干预,用激光扫描共聚焦显微镜(LSCM)和CCK-8的方法分别检测不同刺激条件对细胞内Ca2+浓度改变及细胞增殖情况的影响.结果 低浓度H2S(100μmo/L NaSH)明显降低HSC细胞内Ca2+浓度(P＜0.05),抑制细胞增殖;K离子通道阻断剂——格列本脲可阻断H2S的作用.高浓度H2S(1mmol/L NaSH)使HSC细胞内Ca2+浓度增加,促进细胞增殖.结论 低浓度H2S通过激活HSC细胞KATP通道,降低细胞内Ca2+浓度,从而抑制细胞增殖;高浓度H2S使HSC细胞内Ca2+浓度增加,促进细胞增殖.     

肺动脉平滑肌细胞上的钙离子通道研究进展

钙离子(Ca2+)为肺动脉平滑肌细胞(PASMC)内至关重要的第二信史,其细胞内浓度的精细变化直接受到多种Ca2+通道的调控.按照细胞内Ca2+的来源,位于细胞膜上,调控细胞外Ca2+进入细胞的通道称为钙内流通道,位于肌质网上调控内质网/肌质网内钙库的Ca2+释放的通道称为钙释放通道.根据Ca2+通道激活方式的不同,C...     

心脏氯通道与心律失常关系的研究进展

心肌细胞膜上存在囊性纤维化跨膜转运调节体氯通道(CFTR)、Ca2+激活的氯通道、CIC-2和CIC-3氯通道,与心脏电活动密切相关。心肌缺血等病理状态下,激活CFTR氯通道可缩短动作电位时程(APD)和有效不应期,促进折返形成;钙超载时激活Ca2+激活的氯通道可以促进致心律失常的瞬间内向电流(ITI)的形成,产生延迟后除极并诱发触发活动;CIC-2氯通道可能在缺血和酸中毒时引起细胞自律性增加;CIC-3通道的激活可能在细胞低渗状态时导致APD缩短。     

074 雌激素作用于血管平滑肌的"大钾离子通道"

[英]/Silberberg SD…//Science．-1999，285．-1859～1860 女性在绝经期前心血管病的发病率明显低于男性，这在很大程度上是血液中雌激素的作用。一方面，雌激素与血管内皮细胞和血管平滑肌细胞内的某种受体结合，结合后的复合物改变基因的表达，其结果是保护血管免受损伤和减少动脉粥样硬化的发生；另一方面，雌激素直接作用于血管壁，迅速引起血管扩张。 血管平滑肌细胞膜上有一种可由Ca2+激活的大容量传导K+通道(又称为大K+通道)。它与骨胳肌细胞膜上的K+通道不同，具有很强的传导性，并且二者结构也有差异。它除具有与骨胳肌相同的电压依赖性K+通道(亚单位α)外，还具有肌胳肌K+通道缺乏的亚单位β。亚单位β具有与Ca2+结合部位，所以这种大K+通道受电压和细胞内Ca2+浓度双重调控。亚单位β对大K+通道具有重要影响，血管平滑肌细胞内很低Ca2+浓度即可激活大K+通道。Valverde等发现雌激素直接激活这种大K+通道，使Ca2+通道关闭，Ca2+内流停止，血管平滑肌松弛，从而发挥快速调节血管张力的作用，但雌激素不能增强仅由亚单位α构成的K+通道的活性。还发现与血清白蛋白结合的雌激素虽不能穿越细胞膜，仍可激活大K+通道，由此证实雌激素在细胞外发挥作用。雌激素可激活存在于人工重构细胞膜上的大K+通道，说明大K+通道是雌激素的受体。用荧光标记雌激素并与人类胚胎肾脏细胞结合，结果同时具有亚单位α、β的大K+通道发出的荧光远比仅具有亚单位α的普通K+通道明亮。上述研究证实，雌激素与大K+通道的外置位点(即β亚单位)结合，只有当亚单位β存在时才能明显提高通道活性。 血管平滑肌细胞膜上跨膜电压改变的程度决定着平滑肌细胞收缩的程度。当膜内电位升高(去极化)时，电压依赖性的Ca2+通道激活，Ca2+内流，平滑肌细胞收缩。这一过程能够被选择性K+通道的开放所逆转。肌细胞内K+外流的增加使细胞内电压负值上升，使Ca2+通道关闭，肌细胞松弛。大K+通道的开放能够抑制Ca2+通道，并受雌激素的影响。这一发现对于合理设计防治心血管疾病的新药奠定了基础。 (李玉阳摘 赵子彦校)     

apoE4慢性作用对神经元细胞内静息[Ca^2＋]i的影响

目的 观察载脂蛋白E(apoE)对神经元细胞内游离[Ca2+]i的影响.方法 采用激光共聚焦显微镜(LSCM)和Fluo-3/AM荧光探针标记技术,从单个活细胞水平检测apoE不同亚型对原代培养皮层神经元细胞内钙信号的影响,通过使用NMDA受体阻断剂MK-801,观察NMDA受体是否介导了apoE可能导致的胞内钙变化.结果 ApoE4慢性作用可以浓度依赖性地升高神经元胞内静息[Ca2+]i,而apoE3则无影响;MK-801预处理可减弱apoE4引起的胞内静息[Ca2+]i升高,但不能完全阻断该效应.结论 ApoE慢性作用对神经元细胞内静息[Ca2+]的影响具有亚型特异性,表现为apoE4对胞内静息[Ca2+]i的破坏作用,且NMDA受体的激活可能参与了apoE4所致的胞内钙升高.     

梗阻性不稳定膀胱逼尿肌细胞内Ca2+及N型Ca2+通道的变化及意义

目的对Ca2 和N型Ca2 通道在兔不全梗阻性不稳定膀胱逼尿肌细胞中的变化进行研究。方法成年同龄雄性新西兰白兔30只,随机分两组,15只中膀胱出口不全梗阻8w尿流动力学证实为不稳定膀胱者为实验组,15只为对照组(成年雄性新西兰白兔仅仅手术游离兔膀胱颈而不做结扎梗阻为对照组)。采用急性酶法分离及传代培养的方法获得单个膀胱逼尿肌细胞,采用激光共聚焦显微镜观察模型膀胱逼尿肌细胞静态Ca2 浓度;运用共聚焦显微镜及免疫组织化学方法观察N型Ca2 通道在梗阻性不稳定膀胱逼尿肌细胞中的变化。结果静息状态下不全梗阻性不稳定膀胱组逼尿肌细胞内游离钙离子浓度明显增高(Ca2 超负荷);共聚焦显微镜及免疫组化均证实不稳定膀胱逼尿肌细胞膜之N钙离子通道数量明显增多。结论膀胱逼尿肌细胞Ca2 及其N型Ca2 通道病理性改变是出现不稳定膀胱的重要因素。     

心肌细胞Ca~(2 )信号与心肌肥厚研究新进展

心肌细胞内 Ca2 信号形成涉及胞外 Ca2 的内流及内贮 Ca2 的释放。Ca2 信号以多种形式表现如钙微粒、钙闪烁、钙波、钙振荡、全细胞性瞬时性钙增高等。Ca2 信号主要是通过钙振荡或钙跃升的频率来编码外来信息 ,钙峰的幅度、持续时间及 Ca2 信号的空间位置均参与对外来信息的编码。异常的 Ca2 信号编码与心肌肥厚关系密切 ,肥厚的心肌钙转运异常导致 Ca2 超载加重 ,使心功能恶化 ,最终导致心律失常。     

Ca2+ signaling, TRP channels, and endothelial permeability

Increased endothelial permeability is the hallmark of inflammatory vascular edema. Inflammatory mediators that bind to heptahelical G protein-coupled receptors trigger increased endothelial permeability by increasing the intracellular Ca2+ concentration ([Ca2+]i). The rise in [Ca2+]i activates key signaling pathways that mediate cytoskeletal reorganization (through myosin-light-chain-dependent contraction) and the disassembly of VE-cadherin at the adherens junctions. The Ca2+-dependent protein kinase C (PKC) isoform PKCalpha plays a crucial role in initiating endothelial cell contraction and disassembly of VE-cadherin junctions. The increase in [Ca2+]i induced by inflammatory agonists such as thrombin and histamine is achieved by the generation of inositol 1,4,5-trisphosphate (IP3), activation of IP3-receptors, release of stored intracellular Ca2+, and Ca2+ entry through plasma membrane channels. IP3-sensitive Ca2+-store depletion activates plasma membrane cation channels (i.e., store-operated cation channels [SOCs] or Ca2+ release-activated channels [CRACs]) to cause Ca2+ influx into endothelial cells. Recent studies have identified members of Drosophila transient receptor potential (TRP) gene family of channels that encode functional SOCs in endothelial cells. These studies also suggest that the canonical TRPC homologue TRPC1 is the predominant isoform expressed in human vascular endothelial cells, and is the essential component of the SOC in this cell type. Further, evidence suggests that the inflammatory cytokine tumor necrosis factor-alpha can induce the expression of TRPC1 in human vascular endothelial cells signaling via the nuclear factor-kappaB pathway. Increased expression of TRPC1 augments Ca2+ influx via SOCs and potentiates the thrombin-induced increase in permeability in human vascular endothelial cells. Deletion of the canonical TRPC homologue in mouse, TRPC4, caused impairment in store-operated Ca2+ current and Ca2+-store release-activated Ca2+ influx in aortic and lung endothelial cells. In TRPC4 knockout (TRPC4-/-) mice, acetylcholine-induced endothelium-dependent smooth muscle relaxation was drastically reduced. In addition, TRPC4-/- mouse-lung endothelial cells exhibited lack of actin-stress fiber formation and cell retraction in response to thrombin activation of protease-activated receptor-1 (PAR-1) in endothelial cells. The increase in lung microvascular permeability in response to PAR-1 activation was inhibited in TRPC4-/- mice. These results indicate that endothelial TRP channels such as TRPC1 and TRPC4 play an important role in signaling agonist-induced increases in endothelial permeability.     

Activation of nicotinic receptors triggers exocytosis from bovine chromaffin cells in the absence of membrane depolarization.

The traditional function of neurotransmitter-gated ion channels is to induce rapid changes in electrical activity. Channels that are Ca(2+)-permeable, such as N-methyl-D-aspartate receptors at depolarized membrane potentials, can have a broader repertoire of consequences, including changes in synaptic efficacy, developmental plasticity, and excitotoxicity. Neuronal nicotinic receptors for acetylcholine (nAChRs) are usually less Ca(2+)-permeable than N-methyl-D-aspartate receptors but have a significant Ca2+ permeability, which is greater at negative potentials. Here we report that in neuroendocrine cells, activation of nAChRs can trigger exocytosis at hyperpolarized potentials. We used whole-cell patch-clamp recordings to record currents and the capacitance detection technique to monitor exocytosis in isolated bovine chromaffin cells. Stimulation of nAChRs at hyperpolarized potentials (-60 or -90 mV) evokes a large current and a maximal capacitance increase corresponding to the fusion of approximately 200 large dense-core vesicles. The amount of exocytosis is controlled both by the Ca2+ influx through nAChRs and by a contribution from thapsigargin-sensitive Ca2+ sequestering stores. This is a form of neurotransmitter action in which activation of nAChRs triggers secretion through an additional coupling pathway that coexists with classical voltage-dependent Ca2+ entry.     

IP3 constricts cerebral arteries via IP3 receptor-mediated TRPC3 channel activation and independently of sarcoplasmic reticulum Ca2+ release

Vasoconstrictors that bind to phospholipase C-coupled receptors elevate inositol-1,4,5-trisphosphate (IP(3)). IP(3) is generally considered to elevate intracellular Ca(2+) concentration ([Ca(2+)](i)) in arterial myocytes and induce vasoconstriction via a single mechanism: by activating sarcoplasmic reticulum (SR)-localized IP(3) receptors, leading to intracellular Ca(2+) release. We show that IP(3) also stimulates vasoconstriction via a SR Ca(2+) release-independent mechanism. In isolated cerebral artery myocytes and arteries in which SR Ca(2+) was depleted to abolish Ca(2+) release (measured using D1ER, a fluorescence resonance energy transfer-based SR Ca(2+) indicator), IP(3) activated 15 pS sarcolemmal cation channels, generated a whole-cell cation current (I(Cat)) caused by Na(+) influx, induced membrane depolarization, elevated [Ca(2+)](i), and stimulated vasoconstriction. The IP(3)-induced I(Cat) and [Ca(2+)](i) elevation were attenuated by cation channel (Gd(3+), 2-APB) and IP(3) receptor (xestospongin C, heparin, 2-APB) blockers. TRPC3 (canonical transient receptor potential 3) channel knockdown with short hairpin RNA and diltiazem and nimodipine, voltage-dependent Ca(2+) channel blockers, reduced the SR Ca(2+) release-independent, IP(3)-induced [Ca(2+)](i) elevation and vasoconstriction. In pressurized arteries, SR Ca(2+) depletion did not alter IP(3)-induced constriction at 20 mm Hg but reduced IP(3)-induced constriction by approximately 39% at 60 mm Hg. [Ca(2+)](i) elevations and constrictions induced by endothelin-1, a phospholipase C-coupled receptor agonist, were both attenuated by TRPC3 knockdown and xestospongin C in SR Ca(2+)-depleted arteries. In summary, we describe a novel mechanism of IP(3)-induced vasoconstriction that does not occur as a result of SR Ca(2+) release but because of IP(3) receptor-dependent I(Cat) activation that requires TRPC3 channels. The resulting membrane depolarization activates voltage-dependent Ca(2+) channels, leading to a myocyte [Ca(2+)](i) elevation, and vasoconstriction.     

Modulation of Ca(2+) entry by polypeptides of the inositol 1,4, 5-trisphosphate receptor (IP3R) that bind transient receptor potential (TRP): evidence for roles of TRP and IP3R in store depletion-activated Ca(2+) entry

Homologues of Drosophilia transient receptor potential (TRP) have been proposed to be unitary subunits of plasma membrane ion channels that are activated as a consequence of active or passive depletion of Ca(2+) stores. In agreement with this hypothesis, cells expressing TRPs display novel Ca(2+)-permeable cation channels that can be activated by the inositol 1,4,5-trisphosphate receptor (IP3R) protein. Expression of TRPs alters cells in many ways, including up-regulation of IP3Rs not coded for by TRP genes, and proof that TRP forms channels of these and other cells is still missing. Here, we document physical interaction of TRP and IP3R by coimmunoprecipitation and glutathione S-transferase-pulldown experiments and identify two regions of IP3R, F2q and F2g, that interact with one region of TRP, C7. These interacting regions were expressed in cells with an unmodified complement of TRPs and IP3Rs to study their effect on agonist- as well as store depletion-induced Ca(2+) entry and to test for a role of their respective binding partners in Ca(2+) entry. C7 and an F2q-containing fragment of IP3R decreased both forms of Ca(2+) entry. In contrast, F2g enhanced the two forms of Ca(2+) entry. We conclude that store depletion-activated Ca(2+) entry occurs through channels that have TRPs as one of their normal structural components, and that these channels are directly activated by IP3Rs. IP3Rs, therefore, have the dual role of releasing Ca(2+) from stores and activating Ca(2+) influx in response to either increasing IP3 or decreasing luminal Ca(2+).     

Dual modulation of K channels by thyrotropin-releasing hormone in clonal pituitary cells.

Transmembrane electrical activity in pituitary tumor cells can be altered by substances that either stimulate or inhibit their secretory activity. Using patch recording techniques, we have measured the resting membrane potentials, action potentials, transmembrane macroscopic ionic currents, and single Ca2+-activated K channel currents of GH3 and GH4/C1 rat pituitary tumor cells in response to thyrotropin-releasing hormone (TRH). TRH, which stimulates prolactin secretion, causes a transient hyperpolarization of the membrane potential followed by a period of elevated action potential frequency. In single cells voltage clamped and internally dialyzed with solutions containing K+, TRH application results in a transient increase in Ca2+-activated K currents and a more protracted decrease in voltage-dependent K currents. However, in cells internally dialyzed with K+-free solutions, TRH produces no changes in inward Ca2+ or Ba2+ currents through voltage-dependent Ca channels. The time courses of the effects on Ca2+-activated and voltage-dependent K currents correlate with the phases of hyperpolarization and hyperexcitability, respectively. During application of TRH to whole cells, single Ca2+-activated K channel activity increases in cell-attached patches not directly exposed to TRH. In contrast, TRH applied directly to excised membrane patches produces no change in single Ca2+-activated K channel behavior. We conclude that TRH (i) triggers intracellular Ca2+ release, which opens Ca2+-activated K channels, (ii) depresses voltage-dependent K channels during the hyperexcitable phase, which further elevated intracellular Ca2+, and (iii) does not directly modulate Ca channel activity.     

Spontaneous electrical activity of interstitial cells of Cajal isolated from canine proximal colon.

Interstitial cells of Cajal (ICC) have been suggested as pacemaker cells in the gastrointestinal tract. A method was developed to isolate ICC from the slow-wave pacemaker region of the canine proximal colon. These cells were identified under phase-contrast microscopy, and their identity was verified by comparing their ultrastructure with the morphology of ICC in situ. Patch-clamp experiments demonstrated that these cells are excitable; voltage-dependent inward and outward currents were elicited by depolarization. Inward current transients were identified as calcium currents. A portion of the outward current appears to be due to Ca2+-activated K channels commonly expressed in these cells. ICC were also spontaneously active, generating electrical depolarizations similar in waveform to slow-wave events of intact colonic muscles. These findings are consistent with the hypothesis that ICC initiate rhythmicity in the colon.     

Evidence for the expression of transient receptor potential proteins in guinea pig airway smooth muscle cells

OBJECTIVE: The present study investigates the expression of transient receptor potential (TRPC) proteins in airway smooth muscle (ASM) cells in order to determine whether these proteins may be candidate molecular counterparts of plasma membrane Ca2+-permeable channels involved in the contraction of ASM. METHODS: Expression of TRPC mRNA was detected using specific primers and RT-PCR. Expression of the TRPC1, TRPC3 and TRPC6 proteins was detected using antibodies in immunoprecipitation and Western blot. RESULTS: Guinea pig ASM cells exhibited thapsigargin- and acetylcholine-initiated Ca2+ inflow but none by 1-oleoyl-2-acetyl-sn-glycerol. mRNA encoding each of the TRPC1 to TRPC6 proteins was detected in ASM cells. mRNA encoding TRPC1, TRPC3, TRPC4 and TRPC6 was detected in ASM cells at a concentration approximately equivalent to that in guinea pig brain. mRNA encoding TRPC2 and TRPC5 was more abundant in ASM cells than in brain. The TRPC1 protein, but not the TRPC3 or TRPC6 proteins, was detected in extracts of ASM cells, while all three proteins were detected in brain. CONCLUSION: The results provide evidence for a low level of expression of the TRPC1 to TRPC6 proteins in ASM cells. These proteins may function as store-operated Ca2+ and/or second messenger-activated non-selective cation channels in ASM cells.     

Calcium influx through hyperpolarization-activated cation channels (I(h) channels) contributes to activity-evoked neuronal secretion

The hyperpolarization-activated cation channels (I(h)) play a distinct role in rhythmic activities in a variety of tissues, including neurons and cardiac cells. In the present study, we investigated whether Ca(2+) can permeate through the hyperpolarization-activated pacemaker channels (HCN) expressed in HEK293 cells and I(h) channels in dorsal root ganglion (DRG) neurons. Using combined measurements of whole-cell currents and fura-2 Ca(2+) imaging, we found that there is a Ca(2+) influx in proportion to I(h) induced by hyperpolarization in HEK293 cells. The I(h) channel blockers Cs(+) and ZD7288 inhibit both HCN current and Ca(2+) influx. Measurements of the fractional Ca(2+) current showed that it constitutes 0.60 +/- 0.02% of the net inward current through HCN4 at -120 mV. This fractional current is similar to that of the low Ca(2+)-permeable AMPA-R (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) channels in Purkinje neurons. In DRG neurons, activation of I(h) for 30 s also resulted in a Ca(2+) influx and an elevated action potential-induced secretion, as assayed by the increase in membrane capacitance. These results suggest a functional significance for I(h) channels in modulating neuronal secretion by permitting Ca(2+) influx at negative membrane potentials.     

Diphenylhydantoin blocks cardiac calcium channels and binds to the dihydropyridine receptor

Diphenylhydantoin was studied for its effects on Ca currents in single isolated guinea pig ventricular cells. The whole-cell patch-clamp technique was used, and Ca currents were studied after suppressing Na and K currents. At low frequencies (0.1 Hz) and negative holding potentials (-50 mV), diphenylhydantoin produced a concentration-dependent decrease in Ca currents without any significant change in the current-voltage relations. Half blocking effect occurred at 2 X 10(-4) M. The effects of diphenylhydantoin on Ca currents were dependent upon the holding potential. Inactivation curves for Ca currents were shifted to more negative potentials by the drug. The recovery of Ca currents from inactivation was prolonged by diphenylhydantoin, and the repriming of the current displayed an additional component, attributed to slow release of the drug from the channels. The voltage-dependent block was attributed to preferred binding by the inactivated channel state. Diphenylhydantoin also blocked specific [3H]-nitrendipine binding to guinea pig ventricular membrane preparations. The inhibition of [3H]-nitrendipine binding by diphenylhydantoin was competitive. Diphenylhydantoin also blocks cardiac Na channels in a voltage-dependent manner. We suggest that diphenylhydantoin binding sites exist on both Ca and Na channels.     

Repetitive increases in cytosolic Ca2+ of guard cells by abscisic acid activation of nonselective Ca2+ permeable channels.

Many signal-transduction processes in higher plant cells have been suggested to be triggered by signal-induced opening of Ca2+ channels in the plasma membrane. However, direct evidence for activation of plasma-membrane Ca2+ channels by physiological signals in higher plants has not yet been obtained. In this context, several lines of evidence suggest that Ca2+ flux into the cytosol of guard cells is a major factor in the induction of stomatal closing by abscisic acid (ABA). ABA closes stomatal pores, thereby reducing transpirational loss of water by plants under drought conditions. To directly investigate initial events in ABA-induced signal transduction in guard cells, we devised an experimental approach that allows simultaneous photometric measurements of cytosolic Ca2+ and patch-clamp recordings of ion currents across the plasma membrane of single Vicia faba guard cells. Using this approach, we found that the resting cytosolic Ca2+ concentration was 0.19 +/- 0.09 microM (n = 19). In responsive guard cells, external exposure to ABA produced transient repetitive increases in the cytosolic free Ca2+ concentration. These Ca2+ transients were accompanied by concomitantly occurring increases in an inward-directed ion current. Depolarization of the membrane terminated both repetitive elevations in cytosolic Ca2+ and inward-directed ion currents, suggesting that ABA-mediated Ca2+ transients were produced by passive influx of Ca2+ from the extracellular space through Ca2(+)-permeable channels. Detailed voltage-clamp measurements revealed that ABA-activated ion currents could be reversed by depolarizations more positive than -10 mV. Interestingly, reversal potentials of ABA-induced currents show that these currents are not highly Ca2(+)-selective, thereby permitting permeation of both Ca2+ and K+. These results provide direct evidence for ABA activation of Ca2(+)-permeable ion channels in the plasma membrane of guard cells. ABA-activated ion channels allow repetitive elevations in the cytosolic Ca2+ concentration, which, in turn, can modulate cellular responses promoting stomatal closure.