Na+−K+ pump activity and the glucose-stimulated Ca2+-sensitive K+ permeability in the pancreatic B-cell

A rise in the extracellular concentration of glucose from an intermediate to a high value changes the burst pattern of electrical activity of the pancreatic B-cell into a continuous firing, and yet activates the B-cell Ca2+-sensitive K+ permeability. The hypothesis that glucose exerts such effects by inhibiting the Na+, K+-ATPase was investigated. Ouabain (1 mM) mimicked the effect of 16.7 mM glucose in stimulating 86Rb, 45Ca outflow and insulin release from perifused rat pancreatic islets first exposed to 8.3 mM glucose. The stimulation by ouabain of 86Rb outflow was reduced in the absence of extracellular Ca2+ and almost completely abolished in the presence of quinine, and inhibitor of the Ca2+-sensitive K+ permeability. In the presence of ouabain, a rise in the glucose concentration from 8.3 to 16.7 mM failed to stimulate 86Rb outflow. However, the rise in the glucose concentration failed to inhibit 86Rb influx in islet cells, while ouabain dramatically reduced 86Rb influx whether in the presence of 8.3 or 16.7 mM glucose. These findings do not suggest that inhibition of the B-cell Na+, K+-ATPase represents the mechanism by which glucose in high concentration stimulates 86Rb outflow and induces continuous electrical activity in the B-cell.     

K+稳态与植物耐盐性的关系

文章对K^ 在植物体内的生理作用、分布、吸收机制及其调控,以及在耐盐性中与质膜、液泡膜质子泵的关系作了概述。     

植物K+通道AKT1的研究进展

钾(K)是植物生长发育必需的大量营养元素之一, 主要通过根细胞的K⁺通道及转运蛋白介导吸收。AKT1是Shaker型K⁺通道家族的重要成员, 在植物根吸收K⁺和体内跨膜转运中发挥重要作用。该文综述了植物AKT1的分子结构、组织特异性表达、调控机制及生物学功能等方面的研究进展, 并对该通道今后的研究方向进行了展望。     

NaCl胁迫对阿月浑子各器官Na+、K+吸收的影响

在温室盆栽条件下,用NaCl处理新疆长果阿月浑子与美国品种Kerman,处理浓度为50、150、250和500 mmol·L-1,盐处理后5、10、20 d分别取叶、茎和根,测定各器官中Na+、K+含量及Na+/K+变化规律.研究结果表明,随着土壤盐浓度的增加,长果阿月浑子和Kerman的叶、茎和根中Na+离子含量在盐处理5、10、20 d后每个处理都表现出升高趋势;而两个品种的叶、根和茎中的K+含量在盐处理后其变化规律都不稳定.长果阿月浑子叶片、茎和根系中Na+/K+的比值在盐处理5 d和10 d都升高.Kerman的叶和根系中Na+/K+比值在处理5、10、20 d后升高;茎中Na+/K+处理10 d后升高,但20 d后没有明显变化.实验结果表明NaCl胁迫后,长果阿月浑子的叶片中Na+含量增加幅度均大于Kerman,Kerman根和茎中的Na+含量增加幅度大于长果阿月浑子;NaCl胁迫5 d和10 d后,长果阿月浑子叶片中的Na+/K+的比值高于Kerman,Kerman茎和根部中的Na+/K+的比值高于长果阿月浑子,表明了长果阿月浑子和Kerman都具有一定的耐盐性,但Kerman的耐盐性强于长果阿月浑子.由于叶片和根中Na+和Na+/K+比值的变化很稳定,可以确定为阿月浑子主要抗盐指标,茎中Na+/K+的变化规律较稳定,可以作为抗盐性参考指标.     

Fluoxetine Inhibits K+ Transport Pathways (K+ Efflux, Na+-K+-2Cl− Cotransport, and Na+ Pump) Underlying Volume Regulation in Corneal Endothelial Cells

We have studied regulatory volume responses of cultured bovine corneal endothelial cells (CBCEC) using light scattering. We assessed the contributions of fluoxetine (Prozac) and bumetanide-sensitive membrane ion transport pathways to such responses by determining K⁺ efflux and influx. Cells swollen by a 20% hypo-osmotic solution underwent a regulatory volume decrease (RVD) response, which after 6 min restored relative cell volume by 98%. Fluoxetine inhibited RVD recovery; 20 μm by 26%, and 50 μm totally. Fluoxetine had a triphasic effect on K⁺ efflux; from 20 to 100 μm it inhibited efflux 2-fold, whereas at higher concentrations the efflux first increased to 1.5-fold above the control value, and then decreased again. Cells shrunk by a 20% hyperosmotic solution underwent a regulatory volume increase (RVI) which also after 6 min restored the cell volume by 99%. Fluoxetine inhibited RVI; 20 μm by 25%, and 50 μm completely. Bumetanide (1 μm) inhibited RVI by 43%. In a Cl⁻-free medium, fluoxetine (50–500 μm) progressively inhibited bumetanide-insensitive K⁺ influx. The inhibitions of RVI and K⁺ influx induced by fluoxetine 20 to 50 μm were similar to those induced by 1 μm bumetanide and by Cl⁻-free medium. A computer simulation suggests that fluoxetine can interact with the selectivity filter of K⁺ channels. The data suggest that CBCEC can mediate RVD and RVI in part through increases in K⁺ efflux and Na-K-2Cl cotransport (NKCC) activity. Interestingly, the data also suggest that fluoxetine at 20 to 50 μm inhibits NKCC, and at 100–1000 μm inhibits the Na⁺ pump. One possible explanation for these findings is that fluoxetine could interact with K⁺-selective sites in K⁺ channels, the NKC cotransporter and the Na⁺ pump.     

KcsA通道对Na+、K+及Rb+离子选择性的统计热力学研究

钾离了的通透率至少比钠离子的通透率大10000倍,这个问题至今没有很好地解决,为了存分子水平阐释钾离子通道的选择性机制,以KcsA钾通道X射线衍射结构为基础,采用密度泛函理论计算了不同离子在离子通道中的位能．计算结果表明,Rb^ 离子具有与K^ 离子相类似的位能曲线,但是其在通透过程遇到的位垒要比K^ 离子的位垒高,因而所对应的通透率也就小十钾离子的通透率,而钠离子的的通透率仅仅足钾离子通透率的0．0067％．文中所涉及的系统仪仅包含269个原子,而用分子动力学虽然也可以得到相近的结果,但是它的系统火小为41000个原子．     

上皮细胞K+通道的生理学意义与临床应用:跨上皮细胞转运的驱动力生成和K+循环

K^＋通道在上皮细胞内以极化的方式表达,形成一个庞大的膜蛋白家族。出于对主要依赖Na^＋-K^＋-ATPase而维持的细胞内跨膜K^＋梯度的考虑,K^＋通道在跨上皮细胞转运中的主要作用为：膜电位生成和K^＋循环。本文以肾近端小管和胃壁上皮细胞转运为例简要阐述了K^＋通道的作用。在这两个组织中,K^＋通道活性限速跨上皮细胞转运,调节细胞体积。近年来,药理学工具和转基因动物的实验证实了对K^＋通道的原先认知,并将研究深入到分子水平。K^＋通道的分子结构挑战高亲和力药物分子的设计,及其多组织同时表达的两个典型特征阻碍了高活性、组织特异性小分子治疗的进展。然而,抑制K^＋通道能阻断胃酸分泌等病理生理机制的深入研究,促进K^＋通道药物用于胃病治疗和作为肾脏转运抑制剂用于肾脏相关疾病治疗。     

船与桥——K+进出细胞各行其道

1 问题提出 2005人教版普通高中课程标准实验教科书<生物①>第71页介绍物质通过主动运输的方式进出细胞时举例:人的红细胞中K 的浓度比血浆高30倍,Na 的浓度却只有血浆1/6.     

浅述几类K+通道的研究新进展

近些年来,随着生物技术的革新,有关Ｋ＾＋通道的分型、生理调控功能及其分子结构特征、所涉及的遗传或非遗传性通道疾病、特异或非特异性配体及其在通道上的靶结合位点等方面的知识已获得了长足的推进。本文将对几类Ｋ＾＋通道的基因分类及功能特征等方面的研究新进展作一简要的介绍。     

The α1 Na+−K+ pump of the Dahl salt-sensitive rat exhibits altered Na+ modulation of K+ transport in red blood cells

The properties of the α1 Na⁺-K⁺ pump were compared in Dahl salt-sensitive (DS) and salt-resistant (DR) strains by measuring ouabain-sensitive luxes (mmol/liter cell x hr = FU, Mean ± se) in red blood cells (RBCs) and varying internal ( _i ) and external ( _o ) Na⁺ and K⁺ concentrations. Kinetic parameters of several modes of operation, i.e., Na⁺/ K⁺, K⁺/K⁺, Na⁺/Na⁺ exchanges, were characterized and analyzed for curve-fitting using the Enzfitter computer program. In unidirectional flux studies (n=12 rats of each strain) into fresh cells incubated in 140 mm Na⁺ + 5 mm K⁺, ouabain-sensitive K⁺ influx was substantially lower in the DS than in DR RBCs, while ouabain-sensitive Na⁺ efflux and Na _i were similar in both strains. Thus, the coupling ratio between unidirectional Na⁺∶K⁺ fluxes was significantly higher in DS than in DR cells at similar RBC Na⁺ content. In the presence of 140 mm Na _o , activation of ouabain-sensitive K⁺ influx by K _o had a lower K _m and V _max in DS as estimated by the Garay equation (N=2.70 ± 0.33, K _m 0.74 ± 0.09 mm; V _max 2.87 ± 0.09 FU) than in DR rats (N=1.23 ± 0.36, K _m 2.31 ± 0.16 mm; v _max 5.70 ± 0.52 FU). However, the two kinetic parameters were similar following Na _o removal. The activation of ouabain-sensitive K⁺ influx by Na _i had significantly lower V _max in DS (9.3 ± 0.4 FU) than in DR (14.5 ± 0.6 FU) RBCs but similar K _m. These data suggest that the low K⁺ influx in DS cells is caused by a defect in modulation by Na _o and Na _i . Na⁺ efflux showed no differences in Na _i activation or trans effects by Na _o and K _o , thus accounting for the different Na⁺∶K⁺ coupling ratio in the Dahl strains. Further evidence for the differences in the coupling of ouabain-sensitive fluxes was found in studies of net Na⁺ and K⁺ fluxes, where the net ouabain-sensitive Na⁺ losses showed similar magnitudes in the two Dahl strains while the net ouabainsensitive K⁺ gains were significantly greater in the DR than the DS RBCs. Ouabain-sensitive Na⁺ influx and K⁺ efflux were also measured in these rat RBCs. The inhibition of ouabain-sensitive Na⁺ influx by K _o was fully competitive for the DS but not for the DR pumps. Thus, for DR pumps, K _o could activate higher K⁺ influx in DR pumps without a complete inhibition of ouabain-sensitive Na⁺ influx. This behavior is consistent with K _o interaction with distinct Na⁺ and K⁺ transport sites. In addition, the inhibition of K⁺ efflux by Na, was different between Dahl strains. Ouabain-sensitive K⁺ efflux at Na _i level of 4.6 mmol/liter cell, was significantly higher in DS (3.86 ± 0.67 FU) than in DR (0.86 ± 0.14 FU) due to a threefold higher K₅₀ for Na _i -inhibition 9.66 ± 0.41 vs. 3.09 ± 0.11 mmol/liter cell. This finding indicates that Na⁺ modulation of K⁺ transport is altered at both sides of the membrane. The dissociation of Na⁺ modulatory sites of K⁺ transport from Na⁺ transport sites observed in RBCs of Dahl strains suggests that K⁺ transport by the Na⁺-K⁺ pump is controlled by Na⁺ allosteric sites different from the Na⁺ transport sites. The alterations in K⁺ transport may be related to the amino acid substitution (Leu/Gln₂₇₆) reported for the cDNA of the α1 subunit of the Na⁺-K⁺ pump in the DS strain or to post-translational modifications during RBC maturation. These studies were supported by the following grants: NIH (HL-35664, HL-42120, HL-18318, HL-39267, HL-01967). J.R.R. is a Ford Foundation Predoctoral Fellow. A preliminary report of this work was presented at the International Conference on the Na⁺-K⁺ pump and 44th Annual Meeting of the Society of General Physiologists held at Woods Hole, MA, September 5–9, 1990, and published as an abstract in the J. Gen. Physiol. 96:70a, 1990.     

微动脉平滑肌细胞K+通道的研究进展

K+通道维持着血管平滑肌细胞的静息膜电位.目前发现血管微动脉平滑肌细胞上主要表达内向整流型K+通道、ATP敏感型K+通道、电压依赖型K+通道和大电导钙激活型K+通道等四种K+通道.本文对微动脉平滑肌细胞K+通道最新进展做一综述.     

高等植物K+吸收转运蛋白及其分子调节

K 在植物的生命活动中发挥着十分重要的作用。植物对K 的吸收,可分为高亲和吸收与低亲和吸收。在分子水平上,高亲和吸收主要由KUP/HAK/KT及HKT家族的K 转运蛋白来承担;而Shaker、KCO等家族的K 通道蛋白,则主要在植物的低亲和吸收中发挥重要作用。AKT1、HAK5及其在植物中的同源基因在高等植物K 吸收转运中占有举足轻重的地位。KUP/HAK/KT家族基因的调节,主要是转录水平的调节,而K 通道蛋白的调节则可能主要是一种翻译后调节。植物的蛋白激酶通过磷酸化K 通道蛋白来调节通道的活性,从而改变K 的吸收特性。本文综述了高等植物K 吸收运转及调节的分子机制研究方面的最新进展,并对研究的前景进行了展望。     

Is Na + K ATPase a Myelin-Associated Enzyme?

The Na + K ATPase activity associated with purified myelin has been investigated. On the basis of marker enzyme studies, the Na + K ATPase activity of myelin was higher than could be accounted for by microsomal contamination. Fractions prepared from white matter-enriched areas of rat brain showed a threefold enrichment in Na + K ATPase activity in myelin as compared with the white matter homogenate. The ATPase activity in myelin was stimulated fourfold by treatment with sodium deoxycholate, but the activity in the whole brain homogenate and the microsomal fraction was only doubled. This discontinuity temperature for Na + K ATPase activity was significantly higher for the myelin fraction (29 degrees C) than for the microsomal fraction (21 degrees C), but the energies of activation, both above and below the discontinuity temperature, were the same for both fractions, Myelin Na + K ATPase had a lower affinity for strophanthidin than the microsomal enzyme, but both fractions were inhibited to the same extent by 10-3 M-strophanthidin. The evidence thus indicated that much of the ATPase activity of myelin is not the result of microsomal contamination. Although the possibility of axolemmal contamination cannot be ruled out conclusively, indirect evidence suggest that this is not a significant factor and that Na + K ATPase may be a myelin-associated enzyme.     

Hydrochlorothiazide action on the apical Cl−, Ca2+ and K+ conductances in rabbit gallbladder epithelium. Presence of an apamin-sensitive,Ca2+-activated K+ conductance

In the rabbit gallbladder epithelium, hydrochlorothiazide (HCTZ) was shown to inhibit the transepithelial NaCl transport and the apical Na⁺-Cl^– symport, to depolarize the apical membrane potential and to enhance the cell-to-lumen Cl^– backflux (radiochemically measured), this increase being SITS-sensitive. To better investigate the causes of the depolarization and the Cl^– backflux increase, cells were punctured with conventional microelectrodes on the luminal side (incubation in bicarbonate-free saline at 27°C) and the apical membrane potential (V _m) was studied either with prolonged single impalements or with a set of short multiple impalements. The maximal depolarization was of 3–4 mV and was reached with 2.5 × 10^–4 m HCTZ. It was significantly enhanced by reducing luminal Cl^– concentration to 30 mm; it was abolished by SCN^–, furosemide, SITS; it was insensitive to DPC. SITS converted the depolarization into a hyperpolarization of about 4 mV; this latter was apamin, nifedipine and verapamil sensitive. It was concluded that HCTZ concomitantly opens apical Cl^– and (probably) Ca²⁺ conductances and, indirectly, a Ca²⁺-sensitive, apamin inhibitable K⁺ conductance: since the intracellular Cl^– activity is maintained above the value predicted at the electrochemical equilibrium, the opening of the apical Cl^– conductance depolarizes V _mand enhances Cl^– backflux. In the presence of apamin or verapamil, to avoid the hyperpolarizing effects due to HCTZ, the depolarization elicited by this drug was fully developed (7–10 mV) and proved to be Ca²⁺ insensitive. On this basis and measuring the transepithelial resistance and the apical/basolateral resistance ratio, the Cl^– conductance opened by HCTZ has been estimated and the Cl^– backflux increase calculated: it proved to be in the order of that observed radiochemically. The importance of this Cl^– leak to the lumen in the overall inhibition of the transepithelial NaCl transport by HCTZ has been evaluated.This research was supported by Ministero dell'Università e della Ricerca Scientifica e Tecnologica, Rome, Italy. We are very grateful to prof. G. Meyer and dr. G. Bottà for helpful discussion and criticism.     

Ca2+和K+对拟南芥幼苗镉毒害的缓解作用

该文探讨了外源钙(Ca)或钾(K)处理对不同程度的镉(Cd)胁迫(0–80 μmol·L^–1)下拟南芥(Arabidopsis thaliana)幼苗的生长和生理特性的影响。综合Ca和K对不同浓度Cd胁迫下拟南芥幼苗生长、根长以及生物量的影响情况, 表明各浓度Cd胁迫下外源Ca²⁺的最适缓解浓度均为10 mmol·L^–1; 而K⁺的最适缓解浓度在低浓度(20和40 μmol·L^–1)和高浓度(60和80 μmol·L^–1)Cd胁迫下分别为10 mmol·L^–1和20 mmol·L^–1。在低浓度Cd胁迫下, 添加适宜浓度的Ca²⁺或K⁺后幼苗可溶性蛋白和丙二醛(MDA)含量以及超氧化物歧化酶(SOD)活性相比未添加Ca和K的对照组无显著变化, 而过氧化物酶(POD)活性和总酚、类黄酮、花色素苷, 酸溶性硫醇化合物、谷胱甘肽(GSH)、植物螯合肽(PCs)的含量均下降; 高浓度Cd处理下, 添加适宜浓度的Ca²⁺或K⁺后幼苗的SOD活性升高, POD活性降低, 可溶性蛋白、MDA、总酚、类黄酮、花色素苷、酸溶性硫醇化合物、GSH以及PCs的含量也均低于对照组。在各浓度Cd胁迫下, 添加外源Ca或K均使拟南芥幼苗根部细胞DNA损伤减弱, 表现为TT嘧啶二聚体的累积量显著减少(P<0.05)。以上结果表明, 在Cd胁迫(尤其是高浓度Cd胁迫)下, 外源Ca或K通过调节酚类、金属螯合物质的代谢水平以及提高拟南芥的抗氧化能力来缓解Cd对拟南芥幼苗的毒害效应, 缓解细胞DNA损伤。该研究结果不仅能够为深入探讨Ca和K对缓解重金属毒害的分子机理提供实验依据, 而且为Ca和K应用于重金属污染的防治提供参考。     

Na+K+-ATP酶抑制引起的细胞凋亡和杂合性细胞死亡

Na^ -K^ -ATP酶也称Na^ 泵或Na^ -K^ 泵,是哺乳类细胞膜进行离子转运的跨膜载体蛋白。其基本作用是维持细胞膜内外Na^ -K^ 电化学梯度的平衡。近来研究表明,Na^ -K^ -ATP酶在细胞死亡中起重要作用,细胞K 缺失导致凋亡,在某些类型的细胞中,同一细胞兼具细胞肿胀、细胞器溶解等坏死特征和染色质凝集、DNA梯带、caspase级联反应等凋亡特征,呈现一种特殊的细胞死亡形式,即杂合性细胞死亡。     

Does sumoylation control K2P1/TWIK1 background K+ channels?

A novel model for the regulation of cell excitability has recently been proposed. It originates from the observation that the background K(+) channel K2P1 (TWIK1) may be silenced by sumoylation in Xenopus oocytes and that inactivation of the putative sumoylation site (mutation K274E) gives rise to robust current expression in transfected COS-7 cells. Here, we show that only the mutation K274E, and not K274R, is associated with an increase of K2P1 current density, suggesting a charge effect of K274E. Furthermore, we failed to observe any band shift by western blot analysis that would confirm an eventual sumoylation of K2P1 in COS-7 cells and oocytes.     

Na+和Ca2+对拟南芥根原生质体质膜内向K+通道电流的影响

以拟南芥(Arabidopsis thaliana Columbia)根为材料,利用膜片钳技术测定其根细胞原生质体质膜内向K^ 电流,并对Na^ 对其K^ 电流的影响进行了初步研究,发现Na^2 可明显抑制拟南芥根细胞原生质体的内向K^ 电流,外施Ca^2 可缓解Na^ 对内向K^ 电流的抑制．说明Ca^2 参与了质膜上K^ 通道对K^ ／Na^ 的选择性吸收的调节,从而使植物适应盐胁迫．