Hikaru Tanaka

Publications of Hikaru Tanaka

• Electrophysiological Effects of the Class Ic Antiarrhythmic Drug Pilsicainide on the Guinea-Pig Pulmonary Vein Myocardium.

  Authors: Akira Takahara, Kiyoshi Takeda, Yayoi Tsuneoka, Mihoko Hagiwara, Iyuki Namekata, Hikaru Tanaka

  Journal of pharmacological sciences. 03/2012;

  The pulmonary vein is known as an important source of ectopic beats, initiating frequent paroxysms of atrial fibrillation. We compared effects of the class Ic antiarrhythmic drug pilsicainide on theThe pulmonary vein is known as an important source of ectopic beats, initiating frequent paroxysms of atrial fibrillation. We compared effects of the class Ic antiarrhythmic drug pilsicainide on the electrophysiological parameters in the isolated pulmonary vein preparation from guinea pigs with those in the left atrium. Three pairs of bipolar electrodes were attached to the left atrium, pulmonary vein, and junctional region of the left atrium and pulmonary vein to measure intra-atrial and intra-pulmonary vein conduction velocity and effective refractory period. Pilsicainide (10 µM) decreased the conduction velocity in the pulmonary vein as well as the left atrium, whose effect on the pulmonary vein was relatively greater than that on the left atrium. The drug prolonged the effective refractory period in the pulmonary vein as well as the left atrium, and the effect of the drug on the pulmonary vein was less than that on the left atrium. The currently observed electrophysiological property of pilsicainide suggests that its effects on reentry within the pulmonary vein are estimated to be weaker than within the left atrium, which may be one of the key considerations for understanding its antiarrhythmic mechanisms in the atrium and pulmonary vein.

• Electrophysiological and pharmacological characteristics of triggered activity elicited in guinea-pig pulmonary vein myocardium.

  Authors: Akira Takahara, Takahiko Sugimoto, Takuma Kitamura, Kiyoshi Takeda, Yayoi Tsuneoka, Iyuki Namekata, Hikaru Tanaka

  Journal of pharmacological sciences. 02/2011; 115(2):176-81.

  The pulmonary vein is known as an important source of ectopic beats, initiating frequent paroxysms of atrial fibrillation. We analyzed electrophysiological and pharmacological characteristics ofThe pulmonary vein is known as an important source of ectopic beats, initiating frequent paroxysms of atrial fibrillation. We analyzed electrophysiological and pharmacological characteristics of triggered activity elicited in the isolated pulmonary vein from the guinea pig. Immediately after the termination of train stimulation (pacing cycle length of 100 ms), spontaneous activities accompanied with phase-4 depolarization were detected in 43 out of 45 pulmonary vein preparations. Such triggered activities were not observed in the isolated left atrium. The incidence of triggered activity was higher at a shorter pacing cycle length (100 - 200 ms), and the coupling interval was shorter at a shorter pacing cycle length. Verapamil (1 µM), ryanodine (0.1 µM), and pilsicainide (10 µM) suppressed the occurrence of triggered activities. The resting membrane potential of the pulmonary vein myocardium was more positive than that of the left atrium. Carbachol (0.3 µM) hyperpolarized the resting membrane potential and completely inhibited the occurrence of triggered activities. These results suggest that the pulmonary veins have more arrhythmogenic features than the left atrium, possibly through lower resting membrane potential. The electrophysiological and pharmacological characteristics of triggered activity elicited in the pulmonary vein myocardium were similar to those previously reported using ventricular tissues.

• Developmental changes in action potential prolongation by K+-channel blockers in chick myocardium.

  Authors: Hideaki Nouchi, Naoaki Kiryu, Mikio Kimata, Yayoi Tsuneoka, Shogo Hamaguchi, Iyuki Namekata, Akira Takahara, Koki Shigenobu, Hikaru Tanaka

  Journal of pharmacological sciences. 02/2011; 115(2):235-8.

  The effects of K(+)-channel blockers on the action potential duration of the myocardium were examined in isolated right ventricles from the 7 - 10-day-old, 11 - 13-day-old, and 14 - 20-day-old embryoThe effects of K(+)-channel blockers on the action potential duration of the myocardium were examined in isolated right ventricles from the 7 - 10-day-old, 11 - 13-day-old, and 14 - 20-day-old embryo and 1 - 7-day-old hatched chicks. E-4031 significantly prolonged action potential duration at all developmental stages examined; the prolongation was largest in the 11 - 13-day-old embryo and was accompanied by early after-depolarizations. Chromanol 293B showed smaller prolongation at all stages examined. Terfenadine prolonged action potential duration in the 11 - 13-day-old embryo, but not in other stages. Thus, the chick ventricular myocardium changes its repolarization properties during development.

• Blocking effect of NIP-142 on the KCNQ1/KCNE1 channel current expressed in HEK293 cells.

  Authors: Iyuki Namekata, Noriko Tsuruoka, Yayoi Tsuneoka, Tomoyuki Matsuda, Akira Takahara, Yoshio Tanaka, Takeshi Suzuki, Tetsuo Takahashi, Naoko Iida-Tanaka, Hikaru Tanaka

  Biological & pharmaceutical bulletin. 01/2011; 34(1):153-5.

  We examined the effect of NIP-142, a benzopyran compound with terminating effect on experimental atrial arrhythmia, on the KCNQ1/KCNE1 channel, which underlies the slow component of the cardiacWe examined the effect of NIP-142, a benzopyran compound with terminating effect on experimental atrial arrhythmia, on the KCNQ1/KCNE1 channel, which underlies the slow component of the cardiac delayed rectifier potassium channel (I(Ks)). NIP-142, as well as chromanol 293B, showed concentration-dependent blockade of the current expressed in HEK293 cells; the EC(50) value of NIP-142 and chromanol 293B for the inhibition of tail current was 13.2 µM and 4.9 µM, respectively. These results indicate that NIP-142 has blocking effect on the KCNQ1/KCNE1 channel current.

• Effects of S(+)-efonidipine on the rabbit sinus node action potential and calcium channel subunits Ca(V)1.2, Ca(V)1.3 and Ca(V)3.1.

  Authors: Hikaru Tanaka, Iyuki Namekata, Toru Ogawa, Yayoi Tsuneoka, Chisa Komikado, Akira Takahara, Naoko Iida-Tanaka, Hiroko Izumi-Nakaseko, Hiromichi Tsuru, Satomi Adachi-Akahane

  European journal of pharmacology. 12/2010; 649(1-3):263-7.

  The effect of S(+)-efonidipine on sinus node action potential and calcium channel α-subunits was examined. The slope of the phase 4 depolarization of isolated rabbit sinus node tissue wasThe effect of S(+)-efonidipine on sinus node action potential and calcium channel α-subunits was examined. The slope of the phase 4 depolarization of isolated rabbit sinus node tissue was significantly reduced by S(+)-efonidipine (1 μM), slightly reduced by nifedipine (1 μM), but was not affected by R(-)-efonidipine. S(+)-efonidipine (1 μM), inhibited the expressed Ca(V)1.2, Ca(V)1.3 and Ca(V)3.1 channel currents by 75.7%, 75.3% and 94.0%, nifedipine 84.0%, 43.2% and 14.9%, and R(-)-efonidipine 30.0%, 19.6% and 92.8%, respectively. Thus, the prolongation of the phase 4 depolarization of the rabbit sinus node by S(+)-efonidipine may be explained by blockade of the Ca(V)1.3 channel current.

• Role of transient receptor potential vanilloid 2 in LPS-induced cytokine production in macrophages.

  Authors: Kenji Yamashiro, Tetsuo Sasano, Katsuyoshi Tojo, Iyuki Namekata, Junko Kurokawa, Naoki Sawada, Takayoshi Suganami, Yasutomi Kamei, Hikaru Tanaka, Naoko Tajima, Kazunori Utsunomiya, Yoshihiro Ogawa, Tetsushi Furukawa

  Biochemical and biophysical research communications. 07/2010; 398(2):284-9.

  There is considerable evidence indicating that intracellular Ca2+ participates as a second messenger in TLR4-dependent signaling. However, how intracellular free Ca2+ concentrations ([Ca2+]i) isThere is considerable evidence indicating that intracellular Ca2+ participates as a second messenger in TLR4-dependent signaling. However, how intracellular free Ca2+ concentrations ([Ca2+]i) is increased in response to LPS and how they affect cytokine production are poorly understood. Here we examined the role of transient receptor potential (TRP), a major Ca2+ permeation pathway in non-excitable cells, in the LPS-induced cytokine production in macrophages. Pharmacologic experiments suggested that TRPV family members, but neither TRPC nor TRPM family members, are involved in the LPS-induced TNFalpha and IL-6 production in RAW264 macrophages. RT-PCR and immunoblot analyses showed that TRPV2 is the sole member of TRPV family expressed in macrophages. ShRNA against TRPV2 inhibited the LPS-induced TNFalpha and IL-6 production as well as IkappaBalpha degradation. Experiments using BAPTA/AM and EGTA, and Ca2+ imaging suggested that the LPS-induced increase in [Ca2+]i involves both the TRPV2-mediated intracellular and extracellular Ca2+ mobilizations. BAPTA/AM abolished LPS-induced TNFalpha and IL-6 production, while EGTA only partially suppressed LPS-induced IL-6 production, but not TNFalpha production. These data indicate that TRPV2 is involved in the LPS-induced Ca2+ mobilization from intracellular Ca2+ store and extracellular Ca2+. In addition to Ca2+ mobilization through the IP3-receptor, TRPV2-mediated intracellular Ca2+ mobilization is involved in NFkappaB-dependent TNFalpha and IL-6 expression, while extracellular Ca2+ entry is involved in NFkappaB-independent IL-6 production.

• Effect of NIP-142 on potassium channel alpha-subunits Kv1.5, Kv4.2 and Kv4.3, and mouse atrial repolarization.

  Authors: Hikaru Tanaka, Iyuki Namekata, Shogo Hamaguchi, Taro Kawamura, Hiroyuki Masuda, Yoshio Tanaka, Naoko Iida-Tanaka, Akira Takahara

  Biological & pharmaceutical bulletin. 01/2010; 33(1):138-41.

  Effects of NIP-142, a benzopyran compound which terminates experimental atrial arrhythmia, on potassium channel alpha-subunits and mouse atrial repolarization were examined. NIP-142Effects of NIP-142, a benzopyran compound which terminates experimental atrial arrhythmia, on potassium channel alpha-subunits and mouse atrial repolarization were examined. NIP-142 concentration-dependently blocked the outward current through potassium channel alpha subunits Kv1.5, Kv4.2 and Kv4.3 expressed in Xenopus oocytes. In isolated mouse atrial myocardia, NIP-142 prolonged the action potential duration and effective refractory period, and increased the contractile force. These results suggest that NIP-142 blocks the potassium channels underlying the transient and sustained outward currents, which may contribute to its antiarrhythmic activity.

• H₂receptor-mediated positive inotropic effect of histamine in neonatal guinea-pig left atria.

  Authors: Naoki Agata, Yoshimitsu Kato, Iyuki Namekata, Akira Takahara, Hikaru Tanaka, Daisuke Chino, Katsuo Koike, Yoshio Tanaka

  Biological & pharmaceutical bulletin. 01/2010; 33(12):2033-5.

  The receptor type mediating the positive inotropic effect of histamine was examined in left atria from neonatal guinea pigs. The positive inotropic effect of histamine, as well as its actionThe receptor type mediating the positive inotropic effect of histamine was examined in left atria from neonatal guinea pigs. The positive inotropic effect of histamine, as well as its action potential prolonging effect, was antagonized by ranitidine, but not by chlorpheniramine or thiperamide. The positive inotropic effect was enhanced by isobutylmethylxantine. Receptor binding studies revealed the presence of both H₁ and H₂ receptor types. These results suggest that the positive inotropic effect of histamine in the neonatal guinea-pig atrium is mediated by H₂ receptors.

• Clobutinol delays ventricular repolarization in the guinea-pig heart: comparison with cardiac effects of hERG K+ channel inhibitor E-4031.

  Authors: Akira Takahara, Rieko Sasaki, Mariko Nakamura, Akiko Sendo, Yukiko Sakurai, Iyuki Namekata, Hikaru Tanaka

  Journal of cardiovascular pharmacology. 09/2009;

  Clobutinol has been clinically reported to induce long QT syndrome. To clarify its cardiac electrophysiological properties, we compared effects of clobutinol on the isolated myocardium andClobutinol has been clinically reported to induce long QT syndrome. To clarify its cardiac electrophysiological properties, we compared effects of clobutinol on the isolated myocardium and anesthetized guinea-pig heart with those of a hERG K channel blocker E-4031. In isolated guinea-pig ventricular tissues, clobutinol (3 muM) as well as E-4031 (10-100 nM) prolonged the action potential duration without affecting maximum upstroke velocity, but no further prolongation was observed after application of 30 muM clobutinol. In anesthetized closed-chest guinea pigs, clobutinol (1 and 10 mg/kg, i.v.) and E-4031 (0.01 and 1 mg/kg, i.v.) prolonged the QT interval and duration of the monophasic action potential (MAP) in a dose-dependent manner, and at the same time increased the beat-to-beat variability of the MAP duration, reverse use-dependent prolongation of the MAP duration and triangulation of the MAP configuration. These results suggest that clobutinol delayed the ventricular repolarization and increased the proarrhythmic parameters linked to the hERG K channel inhibitor-induced torsade de pointes arrhythmias.

• Involvement of the Na(+)/Ca(2+) Exchanger in the Automaticity of Guinea-Pig Pulmonary Vein Myocardium as Revealed by SEA0400.

  Authors: Iyuki Namekata, Yayoi Tsuneoka, Akira Takahara, Hideaki Shimada, Takahiko Sugimoto, Kiyoshi Takeda, Midori Nagaharu, Koki Shigenobu, Toru Kawanishi, Hikaru Tanaka

  Journal of pharmacological sciences. 06/2009; 110(1):111-6.

  We examined the involvement of the Na(+)/Ca(2+) exchanger in the automaticity of the pulmonary vein myocardium with a specific inhibitor, SEA0400. Action potentials were recorded from the myocardialWe examined the involvement of the Na(+)/Ca(2+) exchanger in the automaticity of the pulmonary vein myocardium with a specific inhibitor, SEA0400. Action potentials were recorded from the myocardial layer of isolated guinea-pig pulmonary vein preparations, and Ca(2+) transients were recorded from the cardiomyocytes. Spontaneous electrical activity was observed in 17.7% of the preparations, which was inhibited by either SEA0400 or ryanodine. In quiescent preparations, ouabain induced electrical activity and spontaneous Ca(2+) transients, which were inhibited by SEA0400, as well as ryanodine. These results provide pharmacological evidence that the Na(+)/Ca(2+) exchanger underlies the automaticity of the pulmonary vein myocardium.

• New aspects for the treatment of cardiac diseases based on the diversity of functional controls on cardiac muscles: diversity in the excitation-contraction mechanisms of the heart.

  Authors: Hikaru Tanaka, Iyuki Namekata, Hideaki Nouchi, Koki Shigenobu, Toru Kawanishi, Akira Takahara

  Journal of pharmacological sciences. 04/2009; 109(3):327-33.

  The waveform of the myocardial action potential (AP) triggering contraction differs among the species, developmental stage, and pathological state. The species difference in heart rate, whichThe waveform of the myocardial action potential (AP) triggering contraction differs among the species, developmental stage, and pathological state. The species difference in heart rate, which inversely correlates with body size, originates in the ion-channel mechanisms responsible for diastolic depolarization of the sinoatrial node. In some cases, such as the chronically AV-blocked dog and 11- to 13-day chick embryo, the repolarization reserve is decreased making the heart useful for drug evaluation. The degree of dependence of contraction on sarcoplasmic reticulum (SR) function increases during development. The large SR dependence and short AP of the adult mouse and rat support their rapid contraction under high heart rate. The function of the Na(+)/Ca(2+) exchanger is affected by AP waveform and ion concentrations; its major role is Ca(2+) extrusion, but under pathological conditions such as ischemia-reperfusion, it allows Ca(2+) influx and leads to myocardial injury, including loss of mitochondrial function. The role of mitochondria in ATP supply is less in the fetus where glycolysis plays a greater role. The pharmacological properties of the myocardium are affected by all of these factors and also by autonomic innervation and the hormonal status. Such comprehensive understanding is indispensable for the development of novel therapeutic strategies.

• Chronic left atrial volume overload abbreviates the action potential duration of the canine pulmonary vein myocardium via activation of IK channel.

  Authors: Hideaki Nouchi, Akira Takahara, Hideki Nakamura, Iyuki Namekata, Takahiko Sugimoto, Yayoi Tsuneoka, Kiyoshi Takeda, Toshikazu Tanaka, Koki Shigenobu, Atsushi Sugiyama, Hikaru Tanaka

  European journal of pharmacology. 10/2008;

  Electrophysiological properties of the pulmonary vein myocardium were assessed in a canine chronic atrioventricular block model resulting in left atrial volume overload. Five chronic atrioventricularElectrophysiological properties of the pulmonary vein myocardium were assessed in a canine chronic atrioventricular block model resulting in left atrial volume overload. Five chronic atrioventricular block dogs and five sham-operated dogs were used. The heart was removed two months after a surgical procedure causing atrioventricular block, when atrial structural remodeling was established. Standard microelectrode penetrations were made with glass microelectrodes to obtain action potential signals of left atrium and pulmonary vein myocardia. The resting membrane potential in the pulmonary vein was more positive than that in the left atrium (-69 mV vs -74 mV) in both animal groups. The action potential duration at 50% repolarization of the pulmonary vein was shorter in the chronic atrioventricular block dogs than in the sham-operated dogs (38 ms vs 63 ms), whereas no significant difference was detected in the action potential duration of the left atrium between the two animal groups (67 ms vs 61 ms). The action potential duration of the pulmonary vein in the chronic atrioventricular block dogs was prolonged by charybdotoxin but not by iberiotoxin. Such prolongation was not observed in the normal pulmonary vein. These results suggest that long-term left atrial dilatation shortened the action potential duration of pulmonary vein myocardium, which may be associated with activation of the intermediate conductance Ca(2+)-activated K(+) channel (IK channel).

• Species difference in the contribution of T-type calcium current to cardiac pacemaking as revealed by r(-)-efonidipine.

  Authors: Hikaru Tanaka, Chisa Komikado, Iyuki Namekata, Hideki Nakamura, Mariko Suzuki, Yayoi Tsuneoka, Koki Shigenobu, Akira Takahara

  Journal of pharmacological sciences. 06/2008; 107(1):99-102.

  The contribution of the T-type Ca2+ current to cardiac pacemaking was examined in isolated right atrial tissue from the mouse, guinea pig, and rabbit using a specific blocker, R(-)-efonidipine. AtThe contribution of the T-type Ca2+ current to cardiac pacemaking was examined in isolated right atrial tissue from the mouse, guinea pig, and rabbit using a specific blocker, R(-)-efonidipine. At 10(-6) M, R(-)-efonidipine produced negative chronotropy, which was prominent in the mouse and small but significant in the guinea pig. No effect was observed in the rabbit. Microelectrode recordings revealed that R(-)-efonidipine significantly prolongs the pacemaker (phase 4) depolarization of the sinoatrial-node action potential in the mouse and guinea pig. These results provide the first pharmacological evidence that the contribution of T-type Ca2+ current to cardiac pacemaking differs among experimental animal species.

• Membrane-labeled MDCK cells and confocal microscopy for the analyses of cellular volume and morphology.

  Authors: Naoko Iida-Tanaka, Iyuki Namekata, Miku Tamura, Yuko Kawamata, Toru Kawanishi, Hikaru Tanaka

  Biological & pharmaceutical bulletin. 05/2008; 31(4):731-4.

  A clone of Madin-Darby canine kidney (MDCK) cells whose cell membrane was stably labeled with expressed cyan fluorescent protein (CFP) was established, and changes in their volume and shape inducedA clone of Madin-Darby canine kidney (MDCK) cells whose cell membrane was stably labeled with expressed cyan fluorescent protein (CFP) was established, and changes in their volume and shape induced by hyposmotic stress were analyzed with confocal microscopy. The membrane-targeted CFP was present not only on the cell membrane but also in the endoplasmic reticulum and Golgi apparatus, but was excluded from the mitochondria and cell nucleus. During hyposmosis, the initial swelling and the following regulatory volume decrease could be accurately measured by summation of the cellular volume in every confocal slice crossing the cell at different heights. Changes in the cellular height roughly paralleled the changes in cellular volume when the mean value was compared, but dissociation as much as 30% was observed for individual cells due to changes in cell shape. The present experimental system, which enables direct measurement of cell volume and simultaneous observation of various intracellular phenomena, would be useful for further investigation of cellular volume regulation.

• Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium.

  Authors: Iyuki Namekata, Shinpei Fujiki, Yuko Kawakami, Rina Moriwaki, Kentaro Takeda, Toru Kawanishi, Akira Takahara, Koki Shigenobu, Hikaru Tanaka

  Naunyn-Schmiedeberg's archives of pharmacology. 03/2008; 376(6):385-95.

  We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue andWe examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

• (R)-ACX, a pancreatic beta-cell selective sulfonylurea, does not aggravate myocardial ischemia-reperfusion damage.

  Authors: Iyuki Namekata, Yusuke Yamaguchi, Shuhei Moriguchi, Satoshi Yamazaki, Akihiko Terasawa, Reiko Yamagishi, Tokiko Aikawa, Tomoaki Saito, Katsumi Kurashima, Kenji Seri, Yorishige Imamura, Hiroyuki Akita, Koki Shigenobu, Hikaru Tanaka

  European journal of pharmacology. 01/2008; 577(1-3):211-8.

  The organ selectivity and the effect on myocardial ischemia-reperfusion injury of (R)-acetoxyhexamide ((R)-ACX), a novel sulfonylurea, were examined. (R)-ACX, as well as glibenclamide,The organ selectivity and the effect on myocardial ischemia-reperfusion injury of (R)-acetoxyhexamide ((R)-ACX), a novel sulfonylurea, were examined. (R)-ACX, as well as glibenclamide, concentration-dependently stimulated insulin release from INS-1 cell, a cell line derived from pancreatic beta-cells. The potency of (R)-ACX was about 1/10 of that of glibenclamide. In isolated guinea pig ventricular myocardial tissue, glibenclamide concentration-dependently inhibited the action potential shortening by NIP-121, an ATP-sensitive potassium channel opener, but (R)-ACX showed only slight inhibition. In isolated rat aortic rings contracted with norepinephrine, glibenclamide concentration-dependently inhibited the relaxation by NIP-121, while (R)-ACX showed only slight inhibition. In coronary-perfused guinea pig ventricular preparations, glibenclamide reduced the recovery of contractile force after ischemia-reperfusion, while (R)-ACX did not. In conclusion, (R)-ACX is a beta-cell selective sulfonylurea which, unlike glibenclamide, does not aggravate cardiac ischemia-reperfusion damage.

• Involvement of intracellular Ca(2+) in the regulatory volume decrease after hyposmotic swelling in MDCK cells.

  Authors: Naoko Iida-Tanaka, Iyuki Namekata, Megumi Kaneko, Miku Tamura, Toru Kawanishi, Ryu Nakamura, Koki Shigenobu, Hikaru Tanaka

  Journal of pharmacological sciences. 09/2007; 104(4):397-401.

  We examined the source of Ca(2+) involved in the volume regulation of Madin-Darby canine kidney (MDCK) cells with confocal microscopy and fluoroprobes. Hyposmosis induced a transient increase in cellWe examined the source of Ca(2+) involved in the volume regulation of Madin-Darby canine kidney (MDCK) cells with confocal microscopy and fluoroprobes. Hyposmosis induced a transient increase in cell volume, as well as cytoplasmic Ca(2+), which peaked at 3 to 5 min and gradually decreased to reach the initial value within about 30 min. This late decrease in cell volume, as well as the transient rise in cytoplasmic Ca(2+), was reduced in Ca(2+)-free solution and was abolished by pretreatment with thapsigargin. In conclusion, Ca(2+) released from the intracellular store contributes to the regulatory volume decrease following hyposmotic swelling in MDCK cells.

• Involvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis in guinea-pig myocardium as revealed by SEA0400.

  Authors: Hikaru Tanaka, Hideaki Shimada, Iyuki Namekata, Toru Kawanishi, Naoko Iida-Tanaka, Koki Shigenobu

  Journal of pharmacological sciences. 03/2007; 103(2):241-6.

  Involvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis was examined with a specific inhibitor, SEA0400. In right ventricular papillary muscle isolated from guinea-pigInvolvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis was examined with a specific inhibitor, SEA0400. In right ventricular papillary muscle isolated from guinea-pig ventricle, 1 microM SEA0400, which specifically inhibits the Na+/Ca2+ exchanger by 80%, reduced the ouabain (1 microM)-induced positive inotropy by 40%, but had no effect on the inotropy induced by 100 microM isobutyl methylxantine. SEA0400 significantly inhibited the contracture induced by low Na+ solution. In HEK293 cells expressing the Na+/Ca2+ exchanger, 1 microM ouabain induced an increase in intracellular Ca2+, which was inhibited by SEA0400. The arrhythmic contractions induced by 3 microM ouabain were significantly reduced by SEA0400. These results provide pharmacological evidence that the Na+/Ca2+ exchanger is involved in ouabain-induced inotropy and arrhythmogenesis.

• Analysis of arrhythmogenic profile in a canine model of chronic atrioventricular block by comparing in vitro effects of the class III antiarrhythmic drug nifekalant on the ventricular action potential indices between normal heart and atrioventricular block heart.

  Authors: Akira Takahara, Hideki Nakamura, Hideaki Nouchi, Takeshi Tamura, Toshikazu Tanaka, Hideaki Shimada, Miku Tamura, Noriko Tsuruoka, Kentaro Takeda, Hikaru Tanaka, Koki Shigenobu, Keitaro Hashimoto, Atsushi Sugiyama

  Journal of pharmacological sciences. 03/2007; 103(2):181-8.

  The chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of thisThe chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of this model, we recorded the action potentials of the isolated ventricular tissues in the presence and absence of the class III antiarrhythmic drug nifekalant. The action potential durations of the Purkinje fiber and free wall of the right ventricle were longer in the chronic atrioventricular block dogs than in the dogs with normal sinus rhythm. Nifekalant in concentrations of 1 and 10 microM prolonged the action potential durations of Purkinje fiber and the free wall in a concentration-dependent manner. The extent of prolongation was greater in the chronic atrioventricular block dogs than in the normal dogs. However, increase of temporal dispersion of ventricular repolarization including early afterdepolarization was not detected by nifekalant in either group of dogs, indicating lack of potential to trigger arrhythmias in vitro. These results suggest that the ventricular repolarization delay in the chronic atrioventricular block model by nifekalant may largely depend on the decreased myocardial repolarization reserve, whereas the trigger for lethal arrhythmia was not generated in the in vitro condition in contrast to the in vivo experiment.

• Muscarinic receptor subtypes mediating positive and negative inotropy in the developing chick ventricle.

  Authors: Hideaki Nouchi, Seri Kaeriyama, Akiko Muramatsu, Masumi Sato, Kunihiko Hirose, Nagako Shimizu, Hikaru Tanaka, Koki Shigenobu

  Journal of pharmacological sciences. 02/2007; 103(1):75-82.

  The inotropic response to muscarinic receptor stimulation of isolated chick ventricular myocardium was examined at various developmental stages, and the receptor subtype involved wasThe inotropic response to muscarinic receptor stimulation of isolated chick ventricular myocardium was examined at various developmental stages, and the receptor subtype involved was pharmacologically characterized. In embryonic chick ventricles, carbachol (CCh) produced positive inotropy at micromolar concentrations. In hatched chick ventricles, CCh produced negative inotropy at nanomolar concentrations. Neither positive nor negative inotropy was observed in the 19 - 21-day-old embryos. Both positive and negative inotropy were also observed with acetylcholine and oxotremoline-M. The CCh-induced positive inotropy in 7 - 9-day-old embryonic ventricles and the negative inotropy in 1 - 3-day-old hatched chick ventricles were antagonized by muscarinic receptor antagonists; pA(2) values for the positive and negative responses of pirenzepine were 7.5 and 7.2, those of AF-DX116 (11-[(2-[(diethylamino)methyl]-1-piperidinyl)acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one) were 6.8 and 6.9, those of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) were 9.0 and 8.5, and those of himbacine were 7.0 and 8.0, respectively. CCh had no effect on action potential configuration. In conclusion, the positive inotropy is most likely mediated by muscarinic M(1) receptors and the negative inotropy is mostly likely mediated by muscarinic M(4) receptors.