肥厚心肌细胞Na+/Ca2+交换活性对β-肾上腺素能药物的反应性降低

目的探讨肥厚心肌细胞Na+/Ca2+交换对β-类肾上腺素能药物刺激的反应及发生这种改变的可能机制.方法采用胶原酶消化的高血压大鼠单个心室肌细胞及全细胞膜片钳技术,记录Na+/Ca2+交换电流并观察药物对它的影响.结果 (1)异丙肾上腺素可浓度依赖性地增加正常及肥厚大鼠心室肌细胞的Na+/Ca2+交换电流,但其对肥厚心室肌细胞Na+/Ca2+交换电流的增强作用要弱于正常心室肌细胞(P<0.05).(2)cAMP可浓度依赖性地增加正常及肥厚大鼠心室肌细胞的Na+/Ca2+交换电流,其对正常及肥厚大鼠心室肌细胞Na+/Ca2+交换电流的增强作用无差别(P>0.05).结论高血压大鼠心室肌细胞Na+/Ca2+交换对β-类肾上腺素能药物的反应性降低,可能与肥厚心肌细胞的β-受体数目及功能、G蛋白及腺苷酸环化酶的活性改变等环节有关,此种反应可能是肥厚心肌收缩及舒张功能障碍的机制之一.     

Na+/Ca2+交换体α-2(807-844)抗体的制备及其对大鼠心肌细胞Na+/Ca2+交换电流的抑制作用

目的 在制备和纯化抗心肌Na+/Ca2+交换体α-2(807- 844)肽段抗体的基础上,观察抗体对大鼠心肌细胞Na+/Ca2+交换电流及L型钙电流(ICa)、瞬时外向钾电流(Ito)和内向整流钾电流(Ik1)的影响.方法 利用人工合成的α-2(807-844)多肽免疫兔制备抗α-2(807- 844)抗体,抗血清经Protein A亲和柱纯化,利用全细胞膜片钳技术观察纯化后抗体对心肌细胞Na+/Ca2+交换电流(INa/Ca)、L型钙电流(ICa)、瞬时外向钾电流(Ito)和内向整流钾电流(Ik1)的影响.结果 经主动免疫,抗Na+/Ca2+交换体α- 2(840-877)抗血清效价达1∶243000,纯化后抗α-2(840- 877)抗体浓度为7.21 mg/mL.纯化后的抗体在SDS - PAGE时出现清晰的2条带,其分子量分别为25 kD和50 kD左右.在1 nmol/L～104 nmol/L浓度范围内,抗α-2抗体对成年大鼠心肌细胞外向和内向Na+/Ca2+变换电流均表现为剂量依赖性的抑制作用；对Ito和Ik1则均未见明显影响.此外,104 nmol/L该抗体对L型钙电流亦具有抑制作用.通过氨基酸序列比对发现,α- 2(807 - 844)肽段与L型钙通道第2结构域孔环(697～730位氨基酸)序列相似度为23.7％,可能是抗体对L型钙通道出现交叉反应的原因.结论 在1 nmol/L～103 nmol/L浓度范围内,抗心肌Na+/Ca2+变换体α-2(807- 844)抗体可特异性抑制大鼠心肌Na+/Ca2+变换活动；对L型钙通道电流、瞬时外向钾通道电流和内向整流钾通道电流均未见明显影响.     

普鲁卡因胺、利多卡因和普罗帕酮对豚鼠心室肌细胞Na^＋／Ca^2＋交换电流的抑制作用

目的利用膜片钳技术观察Ⅰ类抗心律失常药物普鲁卡因胺、利多卡因、普罗帕酮对Na+/Ca2+交换电流的直接作用.方法采用胶原酶消化的成年大鼠单个心室肌细胞及全细胞膜片钳技术,记录Na+/Ca2+交换电流并观察药物对它的影响.结果3种药物对Na+/Ca2+交换电流的抑制均呈剂量依赖性,但抑制程度不同,其中普鲁卡因胺抑制作用最强.50、100μM的普鲁卡因胺分别使外向Na+/Ca2+交换电流从对照值(181±22)pA降低至(125±19)、(109±20)pA,内向电流由对照值(172±18)pA分别降低至(137±13)、(121±12)pA;50、100μM的利多卡因使外向电流从对照值(170±15)pA分别降低至(139±15)、(127±10)pA,内向电流由对照值(165±15)p/A分别降至(142±16)、(129±20)pA;50、100μM的普罗帕酮使外向电流由对照值(160±23)pA分别降至(130±27)、(112±26)pA,内向电流由对照值(169±13)pA分别降至(143±13)、(134±14)pA.普鲁卡因胺、普罗帕酮对外向电流的抑制大于内向电流,而利多卡因对内、外向电流的抑制差异无显著性.结论Ⅰ类抗心律失常药物对心室肌细胞Na+/Ca2+交换电流具有直接抑制作用,且抑制程度不同.     

缺氧预处理对缺氧复氧乳鼠心室肌细胞游离钙的影响

观察缺氧预处理对缺氧复氧乳鼠心室肌细胞游离钙的影响。建立培养乳鼠心肌细胞缺氧/复氧损伤模型。设正常对照组(A组)、缺氧复氧组(B组)、缺氧预适应组(C组)。经Flou-3/AM负载染色后,采用流式细胞分析技术,测定细胞内钙离子浓度;利用膜片钳技术,观察L型钙通道和钠钙交换电流的变化。结果:①与A组比较,B组可显著增加细胞内的游离钙离子浓度(P<0.01);L型钙电流密度明显下降,I-V曲线上移,半数失活电压(V1/2)减小,ICa,L失活曲线明显左移,而Na+/Ca2+交换电流显著增加。②与B组比较,C组可减轻缺氧再灌注时[Ca2+]i增加(P<0.01);可减轻再灌注对L型钙电流的抑制,使I-V曲线下移程度减轻,V1/2增加及稳态失活曲线的右移;减少Na+/Ca2+交换电流的增加;③与A组比较,C组钙内流和Na+/Ca2+交换电流有轻度增加(P<0.05)。结论:缺氧预处理使缺氧/复氧造成的[Ca2+]i增高的程度减轻,是通过抑制Na+/Ca2+交换电流的增加实现的。     

心肌缺血-再灌注损伤钙超载及其防治策略

缺血-再灌注(I-R)可引起肌膜损伤、Na+/Ca2+交换逆转以及肌(内)质网钙泵(SERCA)含量或活性下降从而导致心肌钙超载.钙超载诱导心肌Calpain活化与线粒体膜通透性转换孔(mPTP)开放进而引起心肌细胞收缩功能障碍、凋亡甚至坏死.通过抑制Na+/Ca2+交换蛋白、Na+/H+交换蛋白、Calpain活性、...     

Na^＋／H^＋交换、Na^＋／K^＋／2Cl^－转运体抑制剂对缺血大鼠心脏保护作用的研究

目的研究Na+/H+交换抑制剂阿米洛利(Amiloride)、Na+/K+/2Cl-协同转运抑制剂呋塞米(Furosemide)对长时间低温保存下离体大鼠心脏的保护作用.方法建立Langendorff及工作心脏灌注模型.实验组用St.Thomas-2(STH-2)液+阿米洛利+呋塞米停搏并保存其中,对照组只用STH-2停搏、保存.置7 C环境5 h后恢复灌流.观察血流动力学、心肌酶学及超微结构的变化.结果实验组冠状动脉流量(CAF)、左心室收缩压(LVSP)、左心室压力变化速率(±dp/dt)的恢复率均优于对照组(P＜0.01);肌酸磷酸激酶(CPK)、乳酸脱氢酶(LDH)漏出量明显少于对照组(P＜0.05);心肌组织中Na+-K+-ATP酶、Ca2+-ATP酶和Ca2+-Mg2+-ATP酶活力均显著高于对照组(P＜0.01);实验组的心肌细胞超微结构得到较好的保护.结论Na+/H+交换、Na+/K+/2Cl-协同转运抑制剂对缺血大鼠心肌具有明显保护作用.     

地高辛增加心肌收缩力治疗心衰的细胞学机制(96)

<正>有强心作用的地高辛,其药理作用的主要机制是抑制心肌细胞膜上的钾钠ATP酶,进而抑制心肌细胞膜上钠泵的运转,结果使钠泵将心肌细胞外K+打进细胞内的作用,以及将细胞内的Na+打到细胞外的作用减弱,使心肌细胞内的Na+浓度轻度升高。心肌细胞内Na+浓度的这种轻度升高将增加心肌细胞膜上钠钙交换体的活性,使较多的Na+经该交换体打到细胞外,同时交换回更多的Ca2+进入心肌细胞内。     

Na~+/Ca~(2+)交换体重构与心力衰竭

心力衰竭 (简称心衰 )时存在着明确的Na+ /Ca2 + 交换体重构 ,主要表现为Na+ /Ca2 + 交换体的表达及功能上调 ,INa+ /Ca2 + 增强 ,并被认为是改善舒张功能的代偿性改变 ,但同时却与收缩功能的损害及心衰时室性心律失常的发生密切相关 ,预防或逆转Na+ /Ca2 + 交换体重构应成为心衰的治疗目标之一。     

缺血预适应与心肌细胞内钙离子的变化

缺血预适应作为心脏内源性的保护机理之一涉及细胞内多种复杂的变化。其中细胞Ca2 + 在缺血预处理期间短暂轻度升高或以启动细胞内第二信使系统 ,改变细胞内各种Ca2 + 的转运 ,减轻缺血再灌注期细胞内Ca2 + 的升高 ,产生对心脏的保护作用。在缺血预适应期及缺血再灌注期 ,细胞内Ca2 + 的变化涉及细胞膜L Ca2 + 型通道、细胞内肌浆网、线粒体及细胞膜Na+ /Ca2 + 交换等方面的变化     

心脏自律性发生和调节的离子流研究进展

起搏细胞自律性的发生和调节与超极化激活的阳离子流、电压依赖性Ca2+电流、电压依赖性Na+电流、电压依赖性K+电流、乙酰胆碱依赖性K+电流和ATP依赖性K+电流、Na+-Ca2+交换电流、Na+-K+泵电流密切相关,这些离子流产生的生物电活动是促使心脏自律性发生和维持的基础。     

Cardiac submembrane [Na+] transients sensed by Na+-Ca2+ exchange current

Na+ influx via INa during cardiac action potentials can raise bulk [Na+]i by 10 to 15 micromol/L. However, larger rises in submembrane [Na+] ([Na+]sm) local to Na+-Ca2+ exchangers (NCX) could enhance Ca2+ influx via NCX (and Ca2+-induced Ca2+ release). We tested whether INa could increase [Na+]sm, using NCX current (INCX) as a biosensor in rabbit ventricular myocytes (with [Ca2+]i buffered, [Na+]i=10 mmol/L, and other currents blocked). We measured INCX as early as 5 ms after INa. Prior INa activation did not affect INCX at physiological membrane potentials (Em=-100 to +50 mV), but for Em >+50 mV (where INCX is especially sensitive to [Na+]i), INCX shifted outward. At 5 ms and +100 mV, INa shifted INCX outward by 0.23 A/F (corresponding to Delta[Na+]sm=0.24 mmol/L). The effect of INa dissipated with a time constant of approximately 15 ms. Thus, the impact of INa on NCX is almost undetectable at physiological Em and short lived. This suggests that INa effects on excitation-contraction coupling (via outward INCX) are minimal and limited to early during the action potential. However, local Delta[Na+]sm during INa may be 60 times higher than bulk Delta[Na+]i.     

代谢性抑制预处理中Na+/Ca2+交换体反向转运的激活触发预处理后的心肌保护作用

目的研究大鼠心室肌细胞在代谢性抑制预处理中钠/钙交换体(NCX)反向转运的活性,以及NCX反向转运抑制剂是否可以阻止代谢性抑制预处理后的心肌保护作用。方法酶解法分离制备钙耐受心肌细胞,用Fura2/AM负载,采用双激发荧光光电倍增系统(IonOptixPhotometrySystem)检测钙信号,用单心肌细胞动缘探测技术观察心肌细胞收缩/舒张功能,台盼蓝染色法检测细胞存活率。结果在代谢性抑制预处理30min时,NCX反向转运被激活。NCX反向转运抑制剂KBR7943(0.5μmol/L)可以抑制代谢性抑制预处理对心肌细胞收缩功能和细胞存活率的作用。NCX激动剂E4031(1μmol/L)可以模拟代谢性抑制预处理后对心肌细胞收缩功能的保护作用,这一作用也可被KBR7943阻断。结论代谢性抑制预处理中,NCX反向转运的激活触发了代谢性抑制预处理后的心肌保护作用;NCX的抑制剂可以阻止代谢性抑制预处理后的心肌保护作用。     

Inhibition by KB-r7943 of the reverse mode of the Na+/Ca2+ exchanger reduces Ca2+ overload in ischemic-reperfused rat hearts.

Ca2+ influx via the Na+/Ca2+ exchanger (NCX) may lead to Ca2+ overload and myocardial injury in ischemia-reperfusion. Direct evidence that increased cytoplasmic Ca2+ concentration ([Ca2+]i) is mediated by the reverse mode of the NCX is limited, so in the present study the [Ca2+]i dynamics and left ventricular pressure were monitored in perfused beating hearts. The effects of KB-R7943 (KBR), a selective inhibitor of the NCX in the reverse mode, were analyzed during low-Na+ exposure and ischemia-reperfusion. Hearts from Sprague-Dawley rats were retrogradely perfused and loaded with 4 micromol/L fura-2 to measure the fluorescence ratio as an index of [Ca2+]i. To evaluate KBR effects on the reverse mode exchanger, the increase in [Ca2+]i induced by low-Na+ exposure (Na+: 30 mmol/L, 10 mmol/L caffeine pre-treatment) was measured with and without 10 micromol/L KBR (n=5). In another series, the hearts were subjected to 10 min of low-flow ischemia with pacing, followed by reperfusion in the absence (n=6) or in the presence of 10 micromol/L KBR (n=6). Background autofluorescence was subtracted to estimate the ratio in the ischemia-reperfusion protocol. KBR significantly suppressed the increase in [Ca2+]i induced by low-Na+ (40.2 +/- 11.2% of control condition, p=0.014), as well as on increase in diastolic [Ca2+]i during ischemia (% increase from pre-ischemia in [Ca2+]i at 10 min: KBR, 17.9 +/- 6.4%; no KBR, 44.4 +/- 7.7%; p=0.024). After reperfusion, diastolic [Ca2+]i normalized more rapidly in KBR-treated hearts (% increase at 1 min: KBR, 4.5 +/- 7.0%; no KBR, 39.8 +/- 12.2%; p=0.03). Treatment with KBR also accelerated recovery of the rate-pressure product on reperfusion (1 min: KBR, 8,944 +/- 1,554 min(-1) mmHg; no KBR, 4,970 +/- 1,325; p<0.05). Thus, inhibition of the reverse mode exchanger by KBR reduced ischemic Ca2+ overload and possibly improved functional myocardial recovery during reperfusion in a whole heart model.     

Role of sodium-calcium exchanger in modulating the action potential of ventricular myocytes from normal and failing hearts

Increased Na+-Ca2+ exchange (NCX) activity in heart failure and hypertrophy may compensate for depressed sarcoplasmic reticular Ca2+ uptake, provide inotropic support through reverse-mode Ca2+ entry, and/or deplete intracellular Ca2+ stores. NCX is electrogenic and depends on Na+ and Ca2+ transmembrane gradients, making it difficult to predict its effect on the action potential (AP). Here, we examine the effect of [Na+]i on the AP in myocytes from normal and pacing-induced failing canine hearts and estimate the direction of the NCX driving force using simultaneously recorded APs and Ca2+ transients. AP duration shortened with increasing [Na+]i and was correlated with a shift in the reversal point of the NCX driving force. At [Na+]i > or =10 mmol/L, outward NCX current during the plateau facilitated repolarization, whereas at 5 mmol/L [Na+]i, NCX had a depolarizing effect, confirmed by partially inhibiting NCX with exchange inhibitory peptide. Exchange inhibitory peptide shortened the AP duration at 5 mmol/L [Na+]i and prolonged it at [Na+]i > or =10 mmol/L. With K+ currents blocked, total membrane current was outward during the late plateau of an AP clamp at 10 mmol/L [Na+]i and became inward close to the predicted reversal point for the NCX driving force. The results were reproduced using a computer model. These results indicate that NCX plays an important role in shaping the AP of the canine myocyte, helping it to repolarize at high [Na+]i, especially in the failing heart, but contributing a depolarizing, potentially arrhythmogenic, influence at low [Na+]i.     

Staurosporine诱导心肌细胞氯通道电流的电生理学特性

目的 探讨staurosporine（STS）诱导乳鼠心肌细胞凋亡的早期,凋亡性容积减少（AVD）发生时,是否有氯通道电流的产生及其电生理学特性。方法 分别采用低渗组的灌流液和含STS的等渗组灌流液处理原代培养的SD乳鼠心肌细胞,以膜片钳全细胞记录法记录电流。结果 ①以低渗灌流液处理的心肌细胞时,可记录到氯通道电流。该电流呈现类似容积敏感性氯通道电流(volume-sensitive chloride channel current,ICl,Vol)的电生理学特性:即外向整流性、高电位刺激下的时间依赖性失活及对氯通道阻断剂4,4′-异二硫氮氐2,2′-二磺酸(DIDS)的敏感性。②以含4 μmol/L STS的等渗液灌流心肌细胞时,也可记录到类似低渗诱导产生的氯通道电流,具有ICl,Vol的电生理学特性,且用氯通道阻断剂500 μmol/L DIDS后,在+40 mV、+60 mV、+80 mV及+100 mV时,能够明显电压依赖性地阻断该电流。结论 首次应用STS在培养乳鼠心肌细胞中记录到类似容积敏感性氯通道电流。     

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.     

Autocrine stimulation of cardiac Na(+)-Ca(2+) exchanger currents by endogenous endothelin released by angiotensin II

The goal of the present study was to evaluate the effects of Ang II on the current produced by the Na(+)-Ca(2+) exchanger (I(NCX)) working in the reverse mode and the possible autocrine role played by the release of endothelin (ET) in these actions. I(NCX) was studied in isolation in cat cardiac myocytes. Angiotensin II (Ang II) (100 nmol/L) increased I(NCX) at potentials higher than 0 mV (at +60 mV: 2.07 +/- 0.22 pA/pF in control versus 2.73 +/- 0.22 pA/pF in Ang II, n=9; P<0.05). The increase in I(NCX) induced by Ang II was prevented by the treatment of the cells with the unspecific blocker of the ET receptors, TAK 044 (1 micromol/L) (at +60 mV: 2.15 +/- 0.27 pA/pF in control versus 2.01+/- 0.26 pA/pF in Ang II, n=5, NS). These results show, for the first time, that the effect of Ang II on I(NCX) is the result of the autocrine actions of ET released by the octapeptide.     

Calcium currents in GH3 cultured pituitary cells under whole-cell voltage-clamp: inhibition by voltage-dependent potassium currents.

To isolate inward Ca2+ currents in GH3 rat pituitary cells, an inward Na+ current as well as two outward K+ currents, a transient voltage-dependent current (IKV) and a slowly rising Ca2+-activated current (IKCa), must be suppressed. Blockage of these outward currents, usually achieved by replacement of intracellular K+ with Cs+, reveals sustained inward currents. Selective blockage of either K+ current can be accomplished in the presence of intracellular K+ by use of quaternary ammonium ions. When IKCa and Na+ currents are blocked, the net current elicited by stepping the membrane potential (Vm) from -60 to 0 mV is inward first, becomes outward and peaks in 10-30 msec, and finally becomes inward again. Under this condition, in which both IKV and Ca2+ currents should be present throughout the duration of the voltage step, the Ca2+ current was not detected at the time of peak outward current. That is, plots of peak outward current vs. Vm are monotonic and are not modified by nisoldipine or low external Ca2+ as would be expected if Ca2+ currents were present. However, similar plots at times other than at peak current are not monotonic and are altered by nisoldipine or low Ca2+ (i.e., inward currents decrease and plots become monotonic). When K+ channels are first inactivated by holding Vm at -30 mV, a sustained Ca2+ current is always observed upon stepping Vm to 0 mV. Furthermore, substitution of Ba2+ for Ca2+ causes blockage of IKV and inhibition of this current results in inward Ba2+ currents with square wave kinetics. These data indicate that the Ca2+ current is completely inhibited at peak outward IKV and that Ca2+ conductance is progressively disinhibited as the transient K+ current declines due to channel inactivation. This suggests that in GH3 cells Ca2+ channels are regulated by IKV.     

氟桂利嗪对原代培养大鼠皮质神经元钠离子通道电流的影响

目的探讨氟桂利嗪对大鼠皮质神经元电压门控钠离子通道电流的影响及机制。方法选择孕17¹8d SD胎鼠培养皮质神经元,应用全细胞记录式膜片钳技术记录神经元钠离子通道电流,先加入0.1、1.0、10.0和100.0μmol/L氟桂利嗪刺激,后续实验使用1μmol/L氟桂利嗪刺激,比较加药前后钠电流相关曲线的变化。结果 0.1、1.0、10.0和100.0μmol/L氟桂利嗪对峰值电流抑制率分别为(14.70±4.39)%,(43.48±3.21)%,(61.91±7.41)%和(77.01±5.80)%,差异有统计学意义(P<0.05),氟桂利嗪达到半数最大抑制率时的药物浓度为0.94μmol/L。与加药前比较,加药后钠电流失活曲线向超极化方向移动了17.03mV;钠通道复活时间常数由(7.51±0.05)ms增至(19.46±0.03)ms,差异有统计学意义(P<0.05);10Hz、50Hz脉冲频率下,加药后钠电流较加药前减少幅度分别为[(21.63±7.36)%至(9.44±5.13)%]和[(45.83±10.04)%至(12.56±6.72)%,P<0.05]。结论氟桂利嗪对皮质神经元钠电流的抑制效应,或能解释其预防偏头痛发作的另一可能机制。     

Cytosolic free magnesium modulates Na/Ca exchange currents in pig myocytes

OBJECTIVE: Cardiac Na/Ca exchanger (NCX) protein is up-regulated and intracellular free magnesium ([Mg(2+)](i)) is significantly reduced in experimental heart failure. We asked whether changes in [Mg(2+)](i) in a physiologically relevant range could alter the I(NCX). METHODS: The nickel-sensitive current was measured in voltage-clamped myocytes (Yorkshire pig; left ventricular) exposed to ramp pulses at 37 degrees C in Tyrode's solution containing ouabain, nifedipine and +/- Ni(2+) (5 mmol/l). The intracellular free [Ca(2+)] and [Mg(2+)] concentrations were set at 50 nmol/l and 1.25 mmol/l (HiMg) or 0.13 mmol/l (LoMg), respectively, through pipette dialysis. RESULTS: Reducing [Mg(2+)](i) resulted in a significant increase in both outward and inward Ni-sensitive current without a shift in the reversal potential. This effect was not due to the inadvertent reduction of intracellular free [ATP] secondary to binding of ATP to Mg(2+); reducing intracellular [ATP] in LoMg cells from 1.35 mmol/l to 0.18 mmol/l did not affect I(NCX). The intracellular free [Ca(2+)] was raised from 50 to 200 nmol/l, resulting in augmented inward and outward current due to calcium activation. HiMg attenuated both inward and outward currents significantly compared to LoMg, suggesting that [Mg(2+)](i) competes with [Ca(2+)](i) at the allosteric regulatory site. CONCLUSION: Cytosolic free magnesium modulates the I(NCX) over a physiologic range independent of [ATP](i). Reduced [Mg(2+)](i) in heart failure could contribute to altered calcium regulation of the NCX, contributing to the altered heart failure phenotype through enhanced NCX activity.