Thyroid hormone modulates membrane currents in guinea-pig ventricular myocytes

Summary Thyroid hormones have been previously shown to alter cardiac electrophysiological and mechanical properties in humans and in experimental animals. To investigate electrophysiological mechanisms responsible for some of these alterations, we recorded action potentials and membrane currents from isolated ventricular myocytes obtained from euthyroid, hypothyroid and hyperthyroid guinea-pigs. Hyperthyroidism was induced by injecting 150 g/kg triiodothyronine for 8–11 days, and hypothyroidism was induced by propylthiouracil treatment for 35–45 days. We found that the slow inward current, was increased by hyperthyroidism and decreased by hypothyroidism: in euthyroid, hyperthyroid and hypothyroid myocytes peak slow inward current was (mean ± SEM) : –1.08 ± 0.06 nA, –1.83 ±0.18^a nA and –0.64 ± 0.07^a nA, respectively (a,p < 0.005). In addition, the membrane potential at which peak slow inward current occurred was modified by the thyroid state and in euthyroid, hyperthyroid and hypothyroid myocytes it was (mean ± SEM): 4.8 ± 0.7 mV, –1.8 ± 1.6^a mV and 11.0 ± 1.4^a mV, respectively.The outward rectifying current, was also affected by the thyroid state, and in euthyroid, hyperthyroid and hypothyroid myocytes, the amplitude atV _M = + 60 mV was (mean ± SEM): 0.51 ± 0.09 nA, 1.15 ± 0.08^a nA and 0.49 ± 0.05 nA,respectively. a,p < 0.001 compared to euthyroid myocytes. Intraperitoneal administration of a single dose of triiodothyronine to guinea-pigs, 2 h prior to the electrophysiological experiment, increased the slow inward current amplitude, as was seen with chronic hyperthyroidism, but had no significant effect on the outward current and on the action potential. Finally, action potential and membrane currents were not altered by superfusing euthyroid myocytes with 10^–6 mol/l triiodothyronine for 30 min, or by introducing 10^–7 mol/l triiodothyronine into the cell through the recording pipette.From these findings we conclude that cardiac mechanical performance may be modulated by thyroid hormones through their effect on the slow inward current, while shortening of action potential duration is brought about, at least in part, by increasing the amplitude of the outward rectifying current. Send offprint requests to O. Binah at the above address     

Effects of metoprolol on action potential and membrane currents in guinea-pig ventricular myocytes

Summary The effects of the beta-adrenoceptor antagonist metoprolol on action potentials and membrane currents were studied in single guinea-pig ventricular myocytes. The experiments were carried out using the nystatin-method of whole-cell technique. This method was used in order to prevent the run-down of the calcium current. Metoprolol at concentrations of 10–100 mol/l shortened action potential in a dose-dependent way. The drug only decreased resting membrane potential at a concentration of 100 mol/1 in two out of five cells. Under voltage-clamp conditions, metoprolol blocked the high threshold calcium current at concentrations of 30 and 100 mol/l to 82 ± 4% and 73 ± 5% from control, respectively. The drug decreased the inward rectifying potassium current in a concentration-dependent manner. This effect was evident for inward current at voltages negative to the apparent reversal potential and for outward current at voltages between –30 and –80 mV. This blocking effect on the inward rectifying potassium current can explain the effect on resting membrane potential. At voltages positive to –30 mV metoprolol increased a time-independent outward current. This metoprolol-enhanced outward current was blocked by barium and cesium. This result suggests that the metoprolol-enhanced current is carried by potassium. The current component enhanced by metoprolol was not sensitive to glibenclamide and tetraethylammonium applied externally, which suggests that the adenosine triphosphate-sensitive channel is not the target of metoprolol. The activation of this time-independent outward current by metoprolol and the blocking effects on the calcium current seem to explain the shortening in action potential induced by the drug. Send offprint requests to J. Sánchez-Chapula at the above address     

Effects of cibenzoline,a new class Ia antiarrhythmic drug,on various membrane ionic currents and action potentials of guinea-pig ventricular cells

We examined the effects of cibenzoline, a new class Ia antiarrhythmic drug, on various membrane ionic currents and action potentials of guinea-pig single ventricular cells, using patch clamp techniques in whole-cell configuration. Action potentials and the membrane currents were evoked at a clamping rate of 0.2 Hz, and all experiments were performed at 32–33°C.

DDPH对豚鼠心室乳头状肌细胞电压与频率依赖性的电生理作用

目的　观察１ （２,６ 二甲基苯氧基） ２ （３,４ 二甲氧基苯乙氨基） 丙烷盐酸盐〔１ （２,６ ｄｉｍｅｔｈｙｌｐｈｅｎｏｘｙ） ２ （３ ,４ ｄｉｍｅｔｈｏｘｙｐｈｅｎｙｌｅｔｈｙｌａｍｉｎｏ） ｐｒｏｐａｎｅ ｈｙｄｒｏｃｈｌｏｒｉｄｅ, ＤＤＰＨ〕对豚鼠右心室乳头状肌细胞电压与频率依赖性的电生理作用。方法　采用常规细胞内玻璃微电极技术,分别以０－０２Ｈｚ、２ Ｈｚ 的刺激频率,观察ＤＤＰＨ（１ ～５０ μｍｏｌ·Ｌ－１） 对豚鼠右心室乳头状肌细胞动作电位（ＡＰ） 各参数的影响,并采用０－１ ～３－３ Ｈｚ 的刺激频率和提高Ｋ＋ 浓度（２－７ ～１５ ｍｍｏｌ·Ｌ－ １）将静息膜电位除极化到不同的水平,观察了ＤＤＰＨ（１０μｍｏｌ·Ｌ－１）对豚鼠右心室乳头状肌细胞动作电位零相最大除极化速率（ Ｖｍａｘ）频率与电压依赖性的抑制作用。结果　ＤＤＰＨ以浓度依赖降低豚鼠右心室乳头状肌细胞动作电位振幅（ＡＰＡ） 、缩短动作电位复极５０ ％ 时程（ＡＰＤ５０） 、延长动作电位复极９０％ 时程（ＡＰＤ９０） 并抑制Ｖｍａｘ ,但不明显改变静息膜电位（ＲＰ）,其作用在刺激频率为２ Ｈｚ 时较０－０２ Ｈｚ 时更为明显。对Ｖｍａｘ 呈现出刺激频率     

Ionic basis for OPC-8212-induced increase in action potential duration in isolated rabbit, guinea pig and human ventricular myocytes

Changes in transmembrane ionic currents induced by OPC-8212 (3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1-piperazinyl]- 2(1H)-quinoline), a recently introduced positive inotropic agent which lengthens cardiac action potential duration, were examined using whole-cell voltage-clamp techniques in single rabbit, guinea pig and human ventricular myocytes. In rabbit, OPC-8212 (12 μmol/l) significantly increased membrane action potential duration measured at 90% of repolarization by an average of 88 ms (from 462 ± 25 to 550 ± 35 ms, n = 4; P < 0.05). In rabbit this increase in duration was not associated with significant changes in either the inward rectifier or transient outward K⁺ currents. The magnitude of the secondary inward current evoked from a holding potential of −50 mV was significantly increased by 97 ± 8% (n = 6; P < 0.01) while a demonstrable delayed rectifier outward current could not be identified in the rabbit myocytes examined at room temperature. In guinea pig ventricular myocytes, where the delayed rectifier was large, 12 μmol/l OPC-8212 significantly depressed the current by 58 ± 10% (n = 6; P < 0.01). The effects of OPC-8212 in human ventricular myocytes obtained from the explanted heart of a single patient having an idiopathic cardiomyopathy most closely resembled those observed in isolated rabbit ventricular myocytes. Thus, in rabbit and a few human ventricular myocytes examined at room temperature, OPC-8212 appeared to lengthen cardiac membrane action potential duration primarily by increasing the amplitude of the secondary inward current believed to primarily represent current through L-type Ca²⁺ channels. In guinea pig preparations, OPC-8212 also decreased the delayed rectifier outward K⁺ current which also would account for an increase in action potential duration. OPC-8212 could not be demonstrated to affect Na⁺ current inactivation in a manner similar to that produced by 1 mg/l veratrine, a recognized Na + channel agonist, which dramatically slowed this process.     

芍药苷对大鼠心肌细胞L钙通道的阻断作用

目的　在单个大鼠心肌细胞上 ,研究芍药苷对L型钙通道的阻断作用。方法　采用全细胞膜片钳技术。结果　芍药苷浓度依赖性阻断ICa ,L,其IC50 为 387μmol·L- 1 。应用 40 0 μmol·L- 1 芍药苷后 ,ICa,L最大电流幅度下降 51 % ,但I U曲线的形态和反转电位没有变化 ,激活曲线也没有明显的改变 ;失活曲线向较负电压的方向偏移约 8 3mV ,通道从失活中恢复的时间明显延长 ,由给药前的 (96± 1 7)ms增至 (1 85± 2 8)ms ;芍药苷的阻断作用没有显示出频率依赖性。结论　芍药苷对心肌细胞ICa ,L有阻断作用     

IHC-66对豚鼠心肌细胞电生理特性的影响

目的：为进一步研究ＩＨＣ－６６抗心律失常作用的电生理机制。方法：采用细胞内微电极技术，研究了ＩＨＣ－６６对豚鼠心肌细胞电活动的影响。结果：ＩＨＣ－６６可缩短心室肌细胞动作电位复极５０％时程（ＡＤＰ５０），大剂量时降低动作电位零相最大上升速率。但可延长心房肌细胞动作电位时程。对Ｃｄ２＋影响下的心室肌细胞电活动的作用较对Ｍｎ２＋影响下的电活动更为显著。但对乌头碱（ａｃｏｎｉｔｉｎｅ，ＡＣ）诱发的异常电活动无明显对抗作用。结论：ＩＨＣ－６６对豚鼠心肌细胞电生理的作用与其影响了Ｃａ２＋、Ｎａ＋、Ｋ＋的跨膜转运有关。     

藻酸双酯钠对豚鼠心室肌细胞动作电位及L-型钙电流的影响

目的　研究藻酸双酯钠 (PSS)对豚鼠心室肌细胞动作电位和L 型钙电流的影响。方法　采用全细胞膜片钳记录技术。结果　PSS明显缩短动作电位时程 (APD) ,5 0、10 0和 2 0 0mg·L-1浓度 ,分别使APD90 缩短 2 4 9% (P <0 0 5 ,n=6 )、37 7%和 4 7 7% (P <0 0 1,n =6 ) ,使APD50 缩短2 1 1% (P <0 0 5 ,n =6 )、4 5 0 %和 6 3 2 % (P <0 0 1,n =6 ) ,呈明显的剂量依赖关系。PSS浓度依赖性减小L 型钙电流 (Ica L) ,5 0、10 0和 2 0 0mg·L-1浓度 ,分别使其峰值降低34 3% (P <0 0 5 ,n =6 )、5 8 4 %和 74 9% (P <0 0 1,n =6 )。随着PSS浓度的增加 ,L 型钙电流的I U曲线逐渐上移 ,但其最大激活电压保持为 0mV不变。结论　PSS明显缩短APD ,抑制Ica L,且呈浓度依赖关系     

Effect of osmotic stress on spontaneous calcium sparks in rat ventricular myocytes

AIM: To study whether the volume of cardiomyocytes and their functions would change under severe pathological conditions or osmotic stress. To clarify the role of ryanodine receptors/calcium release channels (RyRs) in the functional change, the effect of osmotic stress on spontaneous Ca2+ sparks in rat ventricular myocytes was investigated. METHODS: A laser scanning confocal microscope was used to detect spontaneous Ca2+ sparks of intact or saponin permeabilized myocytes loaded with Fluo-4. High and low tonicity was obtained by adding sucrose and reducing NaCl concentration in the external medium, respectively. RESULTS: In intact myocytes the frequency of Ca2+ sparks was increased and decreased by hyperosmotic (1.5 T) and hyposmotic (0.6 T) exposure, respectively. In addition, hyperosmotic exposure increased the temporal parameters and decreased the spatial parameter of Ca2+ sparks, while opposite changes occurred with hyposmotic exposure. The spatio-temporal properties of Ca2+ sparks were slightly affected by altering [K+]i (50-200 mmol/L) in saponin permeabilized myocytes in the presence of 8% dextran. It was observed that the spatio-temporal parameters of the Ca2+ sparks in permeabilized myocytes were dose-dependently altered by dextran. The propagating velocity of Ca2+ waves in intact and permeabilized myocyte was also affected by osmotic pressure or dextran. CONCLUSION: The effect of osmotic stress on the frequency of spontaneous Ca2+ sparks might be ascribed to the change of myoplasmic Ca2+ and Ca2+ content in the sarcoplasmic reticulum, while the effect on the spatio-temporal properties is caused by the alteration of Ca2+ diffusion mainly resulting from the morphological change of the myocytes.     

碘化N-正丁基氟哌啶醇对大鼠心室肌细胞膜钾通道的影响

目的研究碘化N-正丁基氟哌啶醇(F2)对大鼠心室肌细胞膜瞬时外向钾通道的影响。方法采用酶急性消化法分离得到单个大鼠心室肌细胞,应用膜片钳全细胞记录技术观察F2对大鼠心室肌细胞膜瞬时外向钾电流(Ito)的影响。结果F2可剂量依赖地抑制Ito,IC50为0.28mmol·L-1。F2可使Ito的稳态失活曲线左移,半量膜电位Vmid变负,斜率因子S变大;但对激活曲线几乎无影响;对Ito灭活后再复活时程无影响。结论F2对心室肌膜Ito具有抑制作用。     

不同浓度瑞芬太尼对兔心室肌单相动作电位的电生理特性影响

目的：观察不同浓度瑞芬太尼对心室肌单相动作电位的影响。方法制备Langendorff离体兔心脏模型24个,K－H液灌注15 min后,随机分为4组,每组均6只,对照组继续灌注 K －H 液60 min;实验组分别灌注含12,25,50 ng? mL－1瑞芬太尼的K－H液60 min。平衡灌注15 min （T0）、继续灌注15,30,60 min （ T1～T3）时,记录心率、0相最大上升速率（ Vmax ）、单相动作电位振幅（ MAPA）及单相动作电位复极50％,90％的时程（ MAPD50,MAPD90）。结果与T0和对照组比较,3个浓度瑞芬太尼组在T1～T3时点,心率明显减慢,MAPD50、MAPD90明显延长（P＜0．05）。与T0和对照组及低浓度组比较,中、高2个浓度实验组在T1～T3时,Vmax明显减慢,同时MAPA明显降低（P＜0．05）。结论瑞芬太尼无浓度依赖性地减慢心率、延长动作电位时程;中、高2个浓度瑞芬太尼影响除极,可能是其引起心脏传导阻滞的机制之一。     

Oscillatory transient inward currents in ventricular myocytes of healthy versus myopathic Syrian hamster

The present experiments were performed in order to study abnormal action potential configuration and ion channel activity in ventricular myocytes obtained from 23 male myopathic Syrian hamsters (Biobreeders strain 14.6, 32-52 weeks old) compared with 10 age-matched healthy control hamsters (Biobreeders F1B) by means of whole-cell patch-clamp techniques. The results show that the myopathic myocytes had a longer action potential duration, a reduced transient outward K(+) current on depolarization and a smaller transient inward current on repolarization after prolonged depolarizing pulses (> 500 msec). However, the L-type Ca(2+) current and the inwardly rectifing K(+) current were not significantly different from those of healthy myocytes. The oscillatory transient inward currents could be diminished by treatment with ryanodine (0.01-1 micromol/L), a sarcoplasmic reticulum (SR) Ca(2+) release channel blocker, or with Na(+)-free superfusate. We conclude that the hereditary myopathic hamsters are less likely to develop delayed after depolarization-related transient inward currents and triggered arrhythmias owing to a smaller SR Ca(2+) content.     

Effects of 17β-estradiol on action potentials and ionic currents in male rat ventricular myocytes

This study describes electrophysiological effects of estrogens in isolated male rat ventricular myocytes. According to the literature these cells do not express the nuclear estrogen receptor. Action potentials or membrane currents were recorded in the whole-cell configuration with standard techniques. Action potential durations (APD) measured at a level of 0 mV (APD 0) and –70 mV (APD –70) were prolonged by 17β-estradiol (0.5 Hz stimulation frequency, 24–26° C). Threshold concentration was 1 μmol/l. At the highest concentration used (30 μmol/l) no saturation of the response was reached and APD 0 was 162% and APD –70 was 230% of the respective control. The resting potential remained unaffected in most cells. The prolongation induced by 17β-estradiol developed rapidly and reached a steady state 10 min after start of hormone superfusion. Effects of estrogen were completely reversible during 10–15 min wash-out with hormone-free solution. The extent of prolongation (10 μmol/l 17β-estradiol) was frequency dependent. Expressed as percentage of the respective control APD 0 (or APD –70) was 115% (188%) at 0.05 Hz, 118% (163%) at 0.5 Hz and 99% (129%) at 5 Hz stimulation frequency. The response was stereoselective, because 30 μmol/l 17α-estradiol did not prolong action potentials (APD 0: 101%, APD –70: 104% of the respective control, 0.5 Hz stimulation frequency). The endogenous estrogens estrone and estriol were less effective than 17β-estradiol. With 30 μmol/l estrone (0.5 Hz stimulation frequency) APD 0 was 103% and ADP-70 148% of control and with 30 μmol/l estriol APD 0 was 135% and APD –70 137% of control. The prolongation of action potentials can be explained by inhibition of transient outward current which, in rat ventricle, is composed of fast (i _to,f) and slowly (i _to,s) inactivating components. At 30 μmol/l 17β-estradiol i _to,f was reduced to 50% and i _to,s to 43% of their maximal amplitudes. The voltage sensor of i _to,f or i _to,s was hardly affected. Additionally, 17β-estradiol decreased the calcium current (i _Ca,L) to 76% (10 μmol/l) and 38% at 30 μmol/l. The inwardly rectifying potassium current (i _K1) was reduced partly with 30 μmol/l 17β-estradiol and its amplitude was 72% of control at –90 mV (inward current flow) and 65% at –40 mV (outward current flow). These results show that 17β-estradiol is active in cardiac cells which do not express the nuclear estrogen receptor. The hormone exerts class III activity and reduces calcium inward current. These effects, however, occur in vitro with concentrations above the physiological level and therefore may be without significance in vivo. Received: 6 May 1997 / Accepted: 18 October 1997     

Effects of prasugrel on membrane potential and contractile activity of rat ventricular myocytes

Background

Though prasugrel is one of the important P2Y₁₂ inhibitors currently in use for antiplatelet therapy, its potential effects on contractility and electrical activity of ventricular myocytes have not yet been investigated. Hence this study was designed to study the impact of prasugrel on contractile function and membrane potential of isolated ventricular myocytes.

Dog left ventricular midmyocardial myocytes for assessment of drug-induced delayed repolarization: short-term variability and proarrhythmic potential

Background and purpose:

Increased oxidative stress and up-regulation of matrix metalloproteinases (MMPs) may cause structural and functional vascular changes in renovascular hypertension. We examined whether treatment with spironolactone (SPRL), hydrochlorothiazide (HCTZ) or both drugs together modified hypertension-induced changes in arterial blood pressure, aortic remodelling, vascular reactivity, oxidative stress and MMP levels and activity, in a model of renovascular hypertension.

Experimental approach:

We used the two-kidney,one-clip (2K1C) model of hypertension in Wistar rats. Sham-operated or hypertensive rats were treated with vehicle, SPRL (25 mg·kg⁻¹·day⁻¹), HCTZ (20 mg·kg⁻¹·day⁻¹) or a combination for 8 weeks. Systolic blood pressure was monitored weekly. Aortic rings were isolated to assess endothelium-dependent and -independent relaxations. Morphometry of the vascular wall was carried out in sections of aorta. Aortic NADPH oxidase activity and superoxide production were evaluated. Formation of reactive oxygen species was measured in plasma as thiobarbituric acid-reactive substances. Aortic MMP-2 levels and activity were determined by gelatin and in situ zymography, fluorimetry and immunohistochemistry.

Key results:

Treatment with SPRL, HCTZ or the combination attenuated 2K1C-induced hypertension, and reversed the endothelial dysfunction in 2K1C rats. Both drugs or the combination reversed vascular aortic remodelling induced by hypertension, attenuated hypertension-induced increases in oxidative stress and reduced MMP-2 levels and activity.

Conclusions and implications:

SPRL or HCTZ, alone or combined, exerted antioxidant effects, and decreased renovascular hypertension-induced MMP-2 up-regulation, thus improving the vascular dysfunction and remodelling found in this model of hypertension.     

Effects of adenosine analogues on contractile response and CAMP content in guinea-pig isolated ventricular myocytes

Summary In the present study the effects of adenosine analogues were investigated on cAMP content and contractile response in guinea-pig ventricular myocytes. The adenosine analogues (–)-N⁶-phenylisopropyladenosine (R-PIA), 5-N-ethylcarboxamideadenosine (NECA) and (+)-N⁶-phenyl-isopropyladenosine (S-PIA) in the presence of 0.01 mol/l isoprenaline reduced contractile response concentration-dependently. R-PIA and NECA were about equipotent (IC₂₅: 0.01 mol/l and 0.039 mol/l respectively), while S-PIA was less potent (IC₂₅: 0.6 mol/l). Isoprenaline stimulated cAMP content was reduced by R-PIA (IC₂₅: 0.004 mol/l) and with lower potency by S-PIA (IC₂₅: 0.15 mol/l) but the extent of reduction of cAMP by R-PIA and S-PIA (to 55% and 64% respectively) was less than the reduction of contractile response (to 26% and 55% respectively). This suggests that the effects of R- and S-PIA on contractile response are only in part due to a reduction in cAMP content. In addition, NECA did not decrease cAMP content but decreased contractile response to the same extent as R-PIA. Similar results were obtained in the presence of the phosphodiesterase inhibitor Ro 20-1724. Time course studies revealed that the effects of R-PIA (1 mol/l) on cAMP content and contractile response coincided reaching steady state after 5 min and remained stable thereafter. The effects of NECA (1 µmol/l) on contractility also reached steady state within 5 min, whereas it did not change cAMP content. It is concluded that the reduction of contractility by adenosine analogues in the presence of isoprenaline can only in part be explained by a reduction of cAMP content. It is suggested that a cAMP-independent effect, possibly an activation of phosphatases, might be involved additionally.Abbreviations R-PIA (–)-N⁶-phenylisopropyladenosine - NECA 5-N-ethylcarboxamideadenosine - S-PIA (+)-N⁶-phenylisopropyl-adenosine - Ro 20-1724 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone Send offprint requests to J. Neumann at the above address     

卡维地洛对大鼠心室肌细胞钠通道的影响

目的研究卡维地洛对大鼠心室肌细胞膜钠通道的影响,在离子通道水平探讨卡维地洛的抗心律失常作用机制。方法用急性酶解法获得单个大鼠心室肌细胞,标准的全细胞膜片钳技术记录钠通道电流。结果①卡维地洛呈浓度依赖性抑制钠通道电流,IC50=(6.35±0.40)μmol·L-1。②10μmol·L-1卡维地洛能使心肌细胞钠通道电流-电压关系曲线明显上移,峰电流从(17.31±1.68)pA/pF减少至(6.58±1.35)pA/pF(n=8,P<0.05),激活电位、峰电位和翻转电位无明显改变。③卡维地洛能使钠通道电流失活曲线明显左移。④卡维地洛对钠通道电流的激活和复活曲线无明显影响。⑤卡维地洛呈频率依赖性地抑制钠通道电流。⑥1μmol·L-1卡维地洛能明显阻断10μmol·L-1异丙肾上腺素增加钠通道电流效应。结论卡维地洛能够抑制心肌细胞钠通道电流,呈浓度依赖性和频率依赖性。     

Effects of goniopora toxin on the action potential and membrane currents of guinea-pig single ventricular cells

Summary The effects of Goniopora toxin (GPT), a polypeptide isolated from a coral, Goniopora spp., on action potential and membrane currents were studied in single ventricular cells of the guinea-pig using the whole-cell clamp technique with a single patch electrode. GPT at a concentration of 10 nmol/l prolonged the duration of the action potential without significant change in the resting membrane potential and action potential amplitudes. This prolongation became more evident at lower stimulus frequencies and persisted after washing with toxin-free solution. Tetrodotoxin (TTX, 1 mol/l), but not Co²⁺ (2 mmol/1), abolished the prolonged action potential. Under voltage-clamp conditions, a sustained inward current, not present in the control, followed the transient inward current during depolarizing pulses in the GPT-treated cells. The current-voltage relationship for the sustained inward current was much the same as that for the fast sodium current reported in rat single ventricular cells (Brown et al. 1981). Both the sustained and transient currents were abolished by the shift of holding potential in the direction of depolarization and reappeared after repolarization; the reappearance of the sustained current was much slower than that of the transient current. TTX but not Co²⁺ abolished both the sustained and transient inward currents. Calcium current and time-independent current were not affected by GPT. Time-dependent outward current induced by large depolarizing pulses was attenuated by GPT. From these results, it is suggested that GPT predominantly acts on the sodium channels in cardiac muscle, to give rise to a sustained sodium current which is responsible for the prolongation of action potential, and that the sodium channels modified by this toxin may reveal slow inactivation and slow recovery from inactivation as compared with those of unmodified sodium channels. Send offprint requests to I. Muramatsu     

异丙酚和硫喷妥钠对大鼠心肌细胞钙离子移动的影响

目的　观察不同浓度异丙酚和硫喷妥钠对氯化钾、异丙肾上腺素和咖啡因诱发的大鼠心肌细胞钙离子移动的影响 ,探讨其心肌抑制作用的机制。方法　用Fluo 3AM钙荧光指示剂染色培养的大鼠心肌细胞 ,在激光共聚焦显微镜下动态观察用药前和使用临床麻醉诱导峰浓度和 5倍诱导峰浓度的异丙酚 (5 0、2 5 0 μmol·L-1)和硫喷妥钠 (10 0、5 0 0μmol·L-1)后 ,氯化钾、异丙肾上腺素和咖啡因诱发的细胞内钙离子荧光强度的变化。结果　诱导峰浓度的异丙酚和硫喷妥钠减弱氯化钾和异丙肾上腺素诱发的钙离子跨膜内流 ,细胞内钙荧光强度的峰值下降 ,与对照组相比差异有显著性 (P <0 0 5 ) ,硫喷妥钠的抑制作用较异丙酚大。 5倍诱导峰浓度的异丙酚和硫喷妥钠使钙离子跨膜内流进一步降低。但两种浓度的异丙酚和硫喷妥钠对咖啡因诱发的细胞肌浆网内储存钙的释放、钙荧光强度的升高均无影响 (P >0 0 5 )。结论　异丙酚和硫喷妥钠呈浓度依赖地抑制电压门控和受体门控性钙通道的开放 ,钙离子跨膜内流 ,降低兴奋收缩耦联时细胞内钙离子浓度 ,这可能是其心肌抑制作用的原因之一 ,而对肌浆网的功能无明显影响     

红花黄素对豚鼠单个心室肌细胞动作电位和钙电流的影响

目的　观察红花黄素对豚鼠单个心室肌细胞动作电位和钙电流的影响。方法　采用膜片钳全细胞式记录技术。结果　红花黄素（３３ μｇ· Ｌ－ １） 能延长单个心室肌细胞动作电位时程,由（３５９３３ ±２７１８） ｍｓ 延长至（４１３３３ ±６１８８）ｍｓ（ Ｐ＜ ００５ ,ｎ ＝ ６） 。同时增加内向 Ｌ 型钙电流的峰值,由（ － １０２１ ±７４） ｐ Ａ 至（ － １４３６ ±２１２） ｐ Ａ（ Ｐ＜ ００５ ,ｎ ＝ ７） 。结论　由于红花黄素具有上述电生理作用,可用于心力衰竭和快速性心律失常的治疗