Junichiro Morikawa

Ethanol inhibition of Ca2+ and Na+ currents in the guinea-pig heart

The effects of ethanol on L-type Ca2+ and fast Na+ currents (ICa and INa, respectively) were examined using the whole-cell patch-clamp experiments on guinea-pig ventricular cells. At a clinically relevant concentration of 24 mM, ethanol slightly but significantly shortened the action potential duration, and reduced the ICa by 7 +/- 4% (mean +/- S.D.). This concentration of ethanol did not affect INa, but a lethal concentration of ethanol (80 mM) significantly inhibited INa by 13 +/- 5%. The voltage dependence of INa activation was not affected by ethanol, whereas the inhibitions of ICa by 80 mM ethanol and INa by 240 mM were both accompanied by a several mV shift in the channel availability curve toward more negative potentials, suggesting that the channels in the inactivated state are more susceptible to ethanol. The ICa inhibition by ethanol at clinically relevant concentrations could contribute to a negative inotropic effect, action potential shortening and development of arrhythmias, while the pathophysiological significance of ethanol inhibition of INa seems less important.

Tyrosine kinase-dependent modulation by interferon-α of the ATP-sensitive K+ current in rabbit ventricular myocytes

We examined the effects of interferon‐α on the ATP‐sensitive K+ current (I K,ATP) in rabbit ventricular cells using the patch‐clamp technique. I K,ATP was induced by NaCN. Whole‐cell experiments indicated that interferon‐α (5×102–2.4×104 U/ml) inhibited I K,ATP in a concentration‐dependent manner (60.7±7.5% with 2.4×104 U/ml). In cell‐attached configuration, interferon‐α (2.4×104 U/ml) applied to the external solution also inhibited the activity of the single ATP‐sensitive K+ (KATP) channel by 56.0±5.8% without affecting the single channel conductance. The inhibitory effect of I K,ATP by interferon‐α was blocked by genistein and herbimycin A, tyrosine kinase inhibitors, but was not affected by N‐(2‐metylpiperazyl)‐5‐isoquinolinesulfoamide (H‐7), an inhibitor of protein kinase C and cAMP‐dependent protein kinase. These findings suggest that interferon‐α inhibits the cardiac KATP channel through the activation of tyrosine kinase. The tyrosine kinase‐mediated inhibition of I K,ATP by cytokines may aggravate cell damage during myocardial ischemia.

Dopamine stimulation of cardiac β‐adrenoceptors: the involvement of sympathetic amine transporters and the effect of SKF38393

1 Mechanisms underlying β‐adrenoceptor stimulation by dopamine were examined on guinea‐pig Langendorff‐perfused hearts and isolated cells from the right atrium, by using the chronotropic effects and the enhancement of L‐type Ca2+ current (ICa,L) in the presence of prazosin as indicators of β‐adrenoceptor stimulation. Dopamine‐induced overflow of noradrenaline (NA) concentrations was measured by high‐performance liquid chromatography. 2 Dopamine caused positive chronotropic effects with an EC50 of 2.5 μm and induced NA overflow with a similar EC50 (1.3 μm). The chronotropic effect of dopamine was abolished by bisoprolol (1 μm). 3 The effects of dopamine were maintained during prolonged application, whereas the effects of tyramine faded with time. Dopamine (3 μm) restored the chronotropic effects and the NA release suppressed by pretreatment with tyramine, suggesting a de novo synthesis of NA during the exposure to dopamine. 4 Dopamine (3 μm)‐induced NA release was not affected by tetrodotoxin, ω‐conotoxin, rauwolscine, ICI118551 or sulpiride, but was inhibited by desipramine, a NA uptake inhibitor (IC50 ∼1 μm). It was also not affected by GBR12909 and bupropion, dopamine uptake inhibitors in the central nervous system. 5 SKF38393, a D1 receptor partial agonist, potently inhibited the 3 μm dopamine‐induced release of NA (IC50 ∼0.1 μm). D1 receptors are not involved in the DA‐induced release of NA, since SCH23390 (3 μm), a potent D1 antagonist, inhibited the NA release only slightly, and dihydrexidine (1 μm) and chloro‐APB (1 μm), full D1 agonists, caused no significant NA release. 6 SKF38393 inhibited tyramine‐induced overflow of NA, and potentiated the field stimulation‐induced NA release. SKF38393 and desipramine retarded the decay of the stimulation‐induced tachycardia in a similar manner. These results indicate that SKF38393 is a potent monoamine transport inhibitor and a useful tool for the functional evaluation of indirectly‐acting sympathomimetic agonists in the heart. In the presence of SKF38393 (10 μm), dopamine at 1 μm showed no chronotropic effect. 7 Voltage clamp experiments with isolated atrial cells revealed that dopamine is a weak partial agonist. The EC50 for ICa,L stimulation by dopamine was high (13 μm). As a result, dopamine at 1 μm did not affect ICa,L. Bisoprolol abolished the stimulation of ICa,L by dopamine (30 μm), and dihydrexidine (1 μm) did not affect ICa,L. 8 It was concluded that the cardiac effects of dopamine at clinically relevant concentrations (<1 μm) result almost exclusively from the indirect effect of β adrenoceptor stimulation, involving the release of NA from sympathetic nerve terminals. The roles of the direct stimulation of β adrenoceptors by dopamine at these concentrations and the stimulation of postjunctional D1 receptors seem negligible. The desipramine‐ and SKF38393‐sensitive monoamine transporter mediates the release of NA.

Negative chronotropic actions of endothelin-1 on rabbit sinoatrial node pacemaker cells

1 The effects of endothelin‐1 (ET‐1) on sinoatrial (SA) node preparations of the rabbit heart were studied by means of whole‐cell clamp techniques. 2 ET‐1 at 1 nM slowed the spontaneous beating activity and rendered half of the cells quiescent. At a higher concentration of 10 nM, the slowing and cessation of spontaneous activity were accompanied by hyperpolarization. 3 In voltage‐clamp experiments, ET‐1 decreased the basal L‐type Ca2+ current (ICa(L)) dose‐dependently with a half‐maximal inhibitory concentration (EC50) of 0.42 nM and maximal inhibitory response (Emax) of 49.5%. The delayed rectifying K+ current (IK) was also reduced by 33.2±11.1% at 1 nM. In addition, an inwardly rectifying K+ current was activated by ET‐1 at higher concentrations (EC50=4.8 nM). These ET‐1‐induced changes in membrane currents were abolished by BQ485 (0.3 μM), a highly selective ETA receptor antagonist. 4 When ICa(L) was inhibited by ET‐1 (1 nM), subsequent application of 10 μM ACh showed no additional decrease in ICa(L), suggesting the involvement of cyclic AMP in the effects of ET‐1 on ICa(L). In contrast, 1 nM ET‐1 further decreased ICa(L) in the presence of 10 μM ACh, suggesting that ET‐1 activates some additional mechanism(s) which inhibit ICa(L). The ET‐1‐induced ICa(L) inhibition was abolished by protein kinase A inhibitory peptide (PKI, 20 μM) or H‐89 (5 μM). However, the ICa(L) inhibition was not affected by methylene blue (10 μM), suggesting a minor role for cyclic GMP in the effect of ET‐1 under basal conditions. 5 ET‐1 failed to inhibit ICa(L) when the pipette contained GDPβS (200 μM). However, incubation of the cells with pertussis toxin (PTX, 5 μg ml−1, >6 h) only reduced the ET‐1‐induced inhibition to 21.5±9.5%, whereas it abolished the inhibitory effect of ACh on ICa(L). 6 Intracellular perfusion of 8‐bromo cyclicAMP (8‐Br cyclicAMP, 500 μM) attenuated, but did not abolish the inhibitory effect of ET‐1 on ICa(L). This 8‐Br cyclicAMP‐resistant component (17.5±14.4%, n=20) was not affected by combined application of 8‐Br cyclicAMP with 8‐bromo cyclicGMP (500 μM), ryanodine (1 μM) or phorbol‐12‐myristate‐13‐acetate (TPA; 50 nM). 7 In summary, ET‐1 exerts negative chronotropic effects on the SA node via ETA‐receptors. ET‐1 inhibits both ICa(L) and IK, and increases background K+ current. The inhibition of ICa(L) by ET‐1 is mainly due to reduction of the cyclicAMP levels via PTX‐sensitive G protein, but some other mechanism(s) also seems to be operative.

Effects Of Hydrogen Peroxide On The Transient Outward Current In Rabbit Atrial Myocytes

1. In the present study, we investigated the effects of hydrogen peroxide (H2O2) on the 4‐aminopyridine‐sensitive transient outward current (ITO) in rabbit atrial myocytes using the amphotericin B‐perforated patch voltage‐clamp method.

Delineation of premature P waves on four-dimensional electrocardiography, a new display of electrical forces by computer techniques.

This study investigated the feasibility of four-dimensional electrocardiography (4-D ECG), a new display in which the vector loop was rotated and scanned along a timed axis to overcome the shortcomings of vectorcardiography (VCG). The subjects consisted of 38 patients with premature atrial complexes and 30 controls. The orthogonal Frank elec trocardiograms were rotated three-dimensionally according to the right-hand rectangular coordinate system and scanned along a timed axis. The P wave delineation score, signi fying good agreement with the intraobserver and interobserver variability, was signifi cantly higher in 4-D ECG than those in the orthogonal leads or those on the transverse and frontal projections (P < 0.001). The authors measured the premature P loop areas as viewed from 361 directions. P loop areas were best delineated when viewed from cranial directions of 42.6 ±34.0 degrees and from rightward directions of 11.3 ±30.7 degrees. Adequate cranial rotation followed by scanning along a timed axis will maximally delineate premature atrial signals and provide comprehensive visualization of electrical forces.

Decreased sensitivity to beta-adrenergic stimulation of the ventricular cells isolated from the spontaneously hypertensive rat heart.

1. The stirnulatory effects of isoproterenol on the L‐type Ca2+ current (Ic.) were compared between the control (WKY) and hypertensive (SHR) rat heart cells, using the patch‐clamp method.

Modulation of L-type Ca current by denopamine, a nonparenteral partial β1 stimulant, in rabbit ventricular cells

The effects of denopamine, a nonparenteral partial β agonist which is used clinically in Japan, on the L-type Ca2+ current (ICa) were examined in rabbit ventricular cells. Denopamine stimulated basal ICa with a maximum response of +33.2% and a concentration for half-maximal response (EC50) of 0.039 μM. The maximun response of ICa was only a quarter of that induced by isoprenaline (ISO), while 10 μM denopamine elicited 70–75% of the maximum inotropic response in the papillary muscle preparations. The denopamine stimulation of ICa was abolished by selective β1 antagonists (atenolol or bisoprolol). Pretreatment with forskolin or dialysis with cAMP also abolished the stimulation. Denopamine, in turn, inhibited ISO-stimulated ICa. This inhibition was not affected by pretreatment with pertussis toxin or prazosin. The presence of denopamine at various concentrations caused a rightward shift in the concentration/response curve for ISO stimulation of ICa. The Schild plot for this effect had a slope of 0.99 and Kp of 0.20 μM. In the presence of guanosine-5′-O-(3-thiotriphosphate) (GTPγS) (0.5 mM) in the pipette, denopamine (10 μM) stimulated the ICa to 86±5% of the maximum response induced by ISO. These findings indicate that denopamine modulates ICa exclusively through the β1 adrenoceptor-adenylate cyclase pathway, that the stimulatory GTP-binding protein regulates the agonistic potency of denopamine, and that the signal from the β1 adrenoceptors is amplified between ICa and the tension development, which would contribute to the spare capacity of β adrenoceptors.

Sensitivity and accuracy in recording his-purkinje activity by surface-averaged electrocardiography

A clinical evaluation of the surface-averaged ECG (SAE) to record His-Purkinje activity (HPA) was made on 70 patients who underwent His bundle electrograms (HBE). The recorded signals first judged as HPA in 43 patients by the noninvasive method alone were later verified in 37 patients by HBE; the accuracy of the HPA recordings (predictive value) was 86.0%. The HPA-V interval measured noninvasively had a high correlation with the HV interval by HBE (r = 0.89, p less than 0.01). The verified detection rate in all 70 patients was 52.9%: HPA was detected in 12 of 18 patients (66.7%) with sclerotic and hypertensive heart disease (Group I), five of 19 patients (26.3%) with rheumatic heart disease (Group II), 11 of 17 patients (64.7%) with congenital heart disease (Group III), and nine of 16 patients (56.2%) with miscellaneous conditions (Group IV). The detection rate was markedly lower in Group II than in other groups (Group II vs Group I or III, p less than 0.025). The PR segment was significantly longer in the patients in whom HPA was detected than in those in whom it was not detected (71.5 +/- 22.3 msec vs 43.9 +/- 19.5 msec, p less than 0.001). His-Purkinje activity (HPA) was detected in 32 of 52 recordings (61.5%) with sinus rhythm and seven of 20 recordings (35.0%) with atrial fibrillation, including two recordings in each of two cardioverted patients (p less than 0.05). We conclude that the surface-averaged ECG (SAE) has clinically acceptable sensitivity and accuracy except in patients with rheumatic heart disease, short PR segments or atrial fibrillations.

Double ventricular parasystole associated with complete atrioventricular block

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