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Biomedical subjects

J Toyama

Publications and source records attributed to J Toyama.

At least 91 records · Page 5Linked to original sources
PubMed1995

Electrophysiological effects of Ro 22-9194, a new antiarrhythmic agent, on guinea-pig ventricular cells.

1. Cardiac effects of Ro 22-9194 were examined in papillary muscles and single ventricular myocytes isolated from guinea-pigs and compared with those of moricizine. 2. In papillary muscles, both Ro 22-9194 (> or = 10 microM) and moricizine (> or = 1 microM) caused a significant dose-dependent decrease in the maximum upstroke velocity (Vmax) and a shortening of the action potential duration. 3. In the presence of either drug, trains of stimuli at rates > or = 0.2 Hz led to an exponential decline in Vmax. This use-dependent block was enhanced at higher stimulation frequencies. A time constant (tau R) for Vmax recovery from the use-dependent block was 9.3 s for Ro 22-9194 and 26.4 s for moricizine. 4. The curves relating membrane potential and Vmax in single myocytes were shifted by Ro 22-9194 (30 microM) or by moricizine (3 microM) in a hyperpolarizing direction by 8.4 mV and 8.0 mV respectively. 5. In myocytes treated with Ro 22-9194 (30 microM), a 10 ms conditioning clamp to 0 mV caused a significant decrease in Vmax of the subsequent test action potential; further prolongation of the clamp pulse duration resulted in a modest enhancement of the Vmax inhibition. In the presence of moricizine (3 microM), a similar conditioning clamp > 200 ms caused a significant Vmax reduction; the longer the clamp pulse duration, the greater the Vmax reduction. 6. Ro 22-9194 > or = 30 microM caused a slight decrease of calcium inward current (ICa) of myocytes without affecting the delayed rectifier potassium current (IK). 7. These findings suggest that the primary electrophysiological effect of Ro 22-9194 as an antiarrhythmicagent is, like moricizine, a use- and voltage-dependent inhibition of sodium channels. From the onset and offset kinetics of the use-dependent block, Ro 22-9194 belongs to the intermediate kinetic Class I drugs, while moricizine is a slow kinetic drug. From the state-dependence of sodium channel block, Ro 22-9194 may belong to activated channel blockers, while moricizine belongs to inactivated channel blockers.

Action Potentials↗
PubMed1995

Modulation of pacemaker activity of sinoatrial node cells by electrical load imposed by an atrial cell model.

To investigate the electrotonic modulation of sinoatrial (SA) node pacemaker activity by atrial muscle, single or multiple (2-7) SA node cells isolated from rabbit hearts were connected to a membrane model [resistance-capacitance (R-C) circuit] of an atrial cell through an external circuit that mimics the gap junctional conductance (Gc) between cells. When Gc was 0 nS (uncoupled conditions), all the preparations generated regular and stable spontaneous action potentials with a mean cycle length (SCL) of 263 +/- 45 ms (+/- SD, n = 35). Step increases of Gc were associated with a progressive prolongation of SCL. At sufficiently high values of Gc, the spontaneous activity became irregular and finally stopped. We defined the threshold Gc causing an appreciable SCL irregularity as the minimum Gc at which the ratio of SD to mean of SCL was > 0.3. The threshold Gc for a single SA node cell was calculated to be 0.58 nS. In the presence of acetylcholine (ACh; 0.05-0.2 microM), the coupling-induced inhibition of spontaneous activity was greatly increased, and the threshold Gc for a single SA node cell was decreased in a concentration-dependent manner. These findings show that the pacemaker activity of SA node cells is easily inhibited when the cells are coupled to a passive atrial cell model and the inhibition is amplified by ACh. Computer simulation using a modified Oxsoft HEART model indicates that the passive atrial cell model acts as a current sink, imposing a substantial outward current on the SA node cell, and ACh amplifies the effect by activating an additional outward current.

Acetylcholine↗
PubMed1995

Effect of chronic hypoxia on ion channel development in cultured cardiac cells.

In order to investigate mechanisms underlying the postnatal development of the transient outward current (Ito) in the rat heart, we established a primary culture of neonatal rat ventricular myocytes and studied the modulated expressions of Ito under normoxic (21% O2) and hypoxic (7.5% O2) growth conditions. During development from day 5 to day 15 in the normoxic cultures, Ito density doubled without any changes to the kinetics of current inactivation. Chronic hypoxia from day 6 to day 15 prevented this increase in Ito density. The reduced expression of Ito under hypoxic conditions was due to the loss of the fast inactivating component of Ito (Ito,f) These observations suggest that ion channels in neonatal rat cardiomyocytes in primary culture are not constitutively expressed but can be regulated by extrinsic factors. The postnatal increase in Ito density among newborn rats is closely associated with an alteration in oxygen tension.

Action Potentials↗
PubMed1995

Effects of stress on Kv1.5 K+ channel gene expression in the left ventricle of rat hearts.

Voltage-dependent K+ channels contribute to the repolarization phase of the cardiac action potential. An increase in Kv1.5 K+ channel gene expression by a pharmacological dose of glucocorticoid was reported recently in the heart of an adrenalectomized rat. In this study, we examined whether physiological elevations of glucocorticoid induced by tail suspension affects the K+ channel gene expression. Five-week-old male Wistar rats were subjected to tail suspension for three days. Total RNA was extracted from the ventricle, and the level of Kv1.5 mRNA was determined by Northern blot analysis. On the first day, the Kv1.5 mRNA had increased over the non-suspended rat. By the third day, mRNA had increased significantly by more than 3-fold. knowing that urinary excretions of glucocorticoid increase in tail-suspended rats, these findings suggest that physiologically elevated glucocorticoid may upregulate Kv1.5 K+ gene expression.

Action Potentials↗
PubMed1995

Ionic basis of the chronotropic effect of acetylcholine on the rabbit sinoatrial node.

OBJECTIVE: The aim was to study the ionic basis of the chronotropic effects of bath applied acetylcholine and vagal stimulation on the rabbit sinoatrial node. METHODS: The chronotropic effect of bath applied acetylcholine was measured in single cells and small multicellular preparations from the rabbit sinoatrial node and the chronotropic effect of postganglionic vagal stimulation was measured in the intact sinoatrial node. The roles of the hyperpolarisation activated current, i(f), the acetylcholine activated potassium current, iK,ACh, and the L-type calcium current, iCa, were investigated by blocking the currents with 1-2 mM Cs+ or 10(-6) M UL-FS49, 0.2-1.0 mM Ba2+, and 6 x 10(-6) M nifedipine, respectively. RESULTS: Under control conditions, small multicellular preparations were approximately two orders of magnitude less sensitive to bath applied acetylcholine than single cells. However, after block of acetylcholinesterase by eserine in small multicellular preparations the sensitivities of the two types of preparation were approximately the same. Block of i(f) either had no discernible effect or increased the chronotropic effect of bath applied acetylcholine on single cells or small multicellular preparations, whereas partial block of iK,ACh reduced it substantially. Similarly, block of i(f) did not suppress the initial slowing of spontaneous action potentials by vagal stimulation, whereas partial block of iK,ACh reduced it. The hyperpolarisation of the arrested sinoatrial node in response to vagal stimulation was also substantially reduced by block of iK,ACh. Partial block of iCa caused large decreases in the action potential amplitude and maximum diastolic potential, but little decrease in the rate of spontaneous action potentials, and therefore did not mimic the effect of acetylcholine. CONCLUSIONS: The chronotropic effects of bath applied acetylcholine and vagal stimulation are not principally the result of a suppression of i(f) or iCa, whereas the activation of iK,ACh may play an important role.

Acetylcholine↗
PubMed1994

The utility of Nitroderm TTS in angina pectoris: long-term treatment after switching from long-acting oral isosorbide dinitrate.

Long-acting oral isosorbide dinitrate (ISDN) was replaced by Nitroderm TTS, and the utility of this drug in long-term treatment was assessed in 69 patients with angina pectoris. The frequency of attacks (p < 0.001) and the consumption of sublingual nitrate tablets (p < 0.01) were found to be significantly lower at 2 weeks to 6 months than in the observation period in patients who experienced attacks or received sublingual tablets during the observation period. On the other hand, no significant time-lapse changes from the observation period were noted in patients who experienced no attacks or received no sublingual tablets during the observation period. The symptoms of adverse effects were mild. The improvement rates (improved) of subjective symptoms and electrocardiogram (ECG) in Group A were about 50% after 6 months. On the other hand, the improvement rates (not aggravated) of subjective symptoms and ECG in Group B were more than 90%. Nitroderm TTS is considered a useful plaster preparation which can be used for sufficiently extended periods of time because its efficacy was as high as, or higher than, that of oral ISDN when this drug was administered over a long period of time after a switch from long-acting oral ISDN in angina pectoris patients.

Administration, Cutaneous↗
PubMed1994

Ca(2+)-calmodulin mediated modulation of the electrical coupling of ventricular myocytes isolated from guinea pig heart.

The mechanism for the regulation of junctional conductance (gj) by Ca2+ was examined in paired ventricular myocytes isolated from guinea pig heart. One cell of the pair was voltage-clamped by a single-patch pipette, and gj was measured after perforation of the non-junctional membrane of the partner cell. The average value of gj under control condition at pCa 9.0 was 181 +/- 13 nS (n = 30). An elevation of calcium concentration ([Ca2+]i) in the intracellular perfusate from pCa 9.0 to pCa 5.7 resulted in a decrease of 32 +/- 5% in gj (n = 18). In myocytes pretreated with W7 (10(-4) M), a similar elevation of [Ca2+]i caused a decrease of only 10 +/- 3% in gj (n = 6), indicating a protective action of W7 against Ca(2+)-mediated electrical uncoupling. W5 (10(-4) M), a non-chlorinated derivative of W7, did not show such a protective action. Calmodulin (10(-5) M) had no effects on gj at pCa 9.0. However, at moderately elevated [Ca2+]i condition at pCa 7.0, calmodulin (10(-5) M) decreased gj by 29 +/- 6% (n = 4). These results suggest that calmodulin may play an important role in the Ca(2+)-mediated regulation of gap junctional channel function in the cardiac ventricular cells.

Action Potentials↗
PubMed1994

Aftereffects of high-intensity DC stimulation on the electromechanical performance of ventricular muscle.

To clarify the mechanisms underlying cardiac dysfunction after electrical defibrillation, we investigated the effects of direct current field stimulation (10 ms, 1-80 V/cm) on isolated guinea pig papillary muscles. Shocks (S2) > 15 V/cm lowered the plateau height of the S2-induced action potential and inhibited its terminal repolarization. Subsequent responses to basic stimuli (S1, 1.0 Hz) for 1-3 min were characterized by a decrease in the maximum diastolic potential, a shortening of action potential duration, and an increase of the developed tension. With S2 > 30 V/cm, a marked delay in repolarization of the S2-induced action potential was followed by oscillation of membrane potential, resulting in repetitive spontaneous activity and often refractoriness to S1 stimulation. The aftereffects were independent of the phase of S2 application. Most of the aftereffects were preserved in the presence of nifedipine (1 microM) or ryanodine (1 microM). Only sodium channel blockade by tetrodotoxin (10 microM) modified the aftereffects by depressing the generation of spontaneous activity. These findings suggest that strong shocks (> 15 V/cm) will produce abnormal arrhythmogenic responses probably through a transient rupture of sarcolemmal membrane (electroporation) leading to a disturbance of the ionic equilibrium of the myocyte.

Animals↗
PubMed1994

Inhomogeneity of cellular activation time and Vmax in normal myocardial tissue under electrical field stimulation.

To clarify the mechanism underlying the ectopic excitation after countershock, the cellular activation processes of cardiac tissue with a low-potential-gradient electric field (LPEF) were investigated in experiments using guinea pig papillary muscles and in computer simulation. Action potential upstrokes in papillary muscles during longitudinal propagation (LP) or transverse propagation (TP) were different from those of nonpropagating ones in single ventricular cells in terms of lower maximum upstroke velocity (Vmax) (LP, 231 V/s; TP, 309 V/s) and the presence of a linear ascending segment in the phase-plane plot. High Vmax (409 V/s) close to the single cell (512 V/s) was obtained in the muscle at the collision of LP (LC). Field stimulation of the muscles with LPEF < 5 V/cm caused inhomogeneous excitation suggesting multiple wave fronts, which collide with each other, and a wide spatial dispersion of Vmax (132-388 V/s). Phase-plane plots of action potential with lower Vmax were similar to LP or TP, whereas those with higher Vmax were similar to LC. In the two-dimensional discrete sheet composed of 51 x 51 elements of modified Beeler-Reuter model, the inhomogeneous excitation induced by LPEF is mimicked by setting a random variation of stimulus onset in each element. LPEF may induce inhomogeneous excitation with multiple wave fronts through a complex electrotonic interaction. This would provide a basis for the genesis of ectopic focal excitation.

Action Potentials↗
PubMed1993

Deferoxamine reduces the reperfusion injury in isolated neonatal rabbit hearts after hypothermic preservation.

The protective action of deferoxamine, an iron chelator, against reperfusion injury following hypothermic preservation was investigated in the Langendorff-perfused hearts of neonatal rabbits. Left ventricular function and radical generation were used as parameters to evaluate functional and metabolic changes. The free radicals in coronary effluents were measured with an electron spin resonance spectroscope using a spin trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The hearts were preserved in cold St. Thomas solution at 4 degrees C for 6 h following cardiac arrest, then reperfused with oxygenated Krebs-Henseleit solution in a control. Deferoxamine (100 microM) was added to the perfusate just prior to reperfusion in group I, and 3 min after the start of reperfusion in group II. In the control group, the left ventricular developed pressure (LVDP) after 30-min reperfusion recovered up to 43.5 +/- 3.1% (mean +/- SD, n = 5) the preischemic value. Group I showed a significant improvement in LVDP recovery after 30-min reperfusion at 57.1 +/- 3.1% (P < 0.01 vs control), but group II did not (47.5 +/- 2.3%). When a burst of DMPO-OH signals was detected during the initial 5 min of reperfusion, the signal intensity in group I was significantly reduced to about 35-40% of the control value. Group II showed a similar intensity to the control group. Thus, we conclude that deferoxamine may exert a protective action against the dysfunction of neonatal mammalian hearts induced by preservation-reperfusion through an inhibition of iron-catalyzed radical formation.

Animals↗
PubMed1993

Contribution of potassium accumulation in narrow extracellular spaces to the genesis of nicorandil-induced large inward tail current in guinea-pig ventricular cells.

The mechanism of nicorandil-induced large inward tail current (Itail) in single guinea-pig ventricular cells was investigated using the whole-cell patch-clamp technique. In the presence of 0.5-1.0 mM nicorandil, an activator of adenosine 5'-triphosphate (ATP)-sensitive K+ current (IKATP), a depolarization pulse causing a large outward current was followed by a large inward Itail on the repolarization step to the holding potential at -85 mV. The larger the outward current, the greater the Itail. The amplitude of Itail increased as a single exponential function (tau = 74.9 ms) as the duration of preceding depolarization was prolonged. Both the outward current and Itail were inhibited nearly completely after application of glibenclamide (1 microM), a specific blocker of IKATP. Substitution of K+ with Cs+ in both the external and internal solutions resulted in a virtual elimination of Itail. Itail was well preserved under the condition where Ca2+ entry during the preceding depolarization was largely inhibited or where external Na+ was replaced by Li+. A transient positive shift of reversal potential for the net current was observed at the peak of Itail). At 30 mM external K+ concentration, Itail was almost eliminated. From these findings, its is concluded that the Itail is a K+ current associated with an alteration of the K+ equilibrium potential (EK) following a substantial K+ efflux. This EK change is most likely explained by an accumulation of K+ in transverse tubules (T-tubules) since Itail was not induced in atrial cells in which T-tubules are poorly developed.

Adenosine Triphosphate↗
PubMed1993

Role of platelet activating factor in ischaemia-reperfusion injury of isolated rabbit hearts: protective effect of a specific platelet activating factor antagonist, TCV-309.

OBJECTIVES: The aims were to confirm that platelet activating factor is released during reperfusion after global ischaemia in isolated blood perfused rabbit hearts, and to examine the protective action of TCV-309, a platelet activating factor antagonist, against reperfusion injury of cardiac muscle. METHODS: The hearts were mounted on a Langendorff apparatus and perfused with diluted blood perfusate. After cardiac arrest with St Thomas's cardioplegic solution, the hearts were subjected to global ischaemia for 120 minutes at 25 degrees C, and then reperfused for 60 minutes at 37 degrees C. Release of platelet activating factor into the coronary effluent was quantified by radioimmunoassay. The effect of TCV-309 on left ventricular function and release of creatine kinase was measured. RESULTS: A pronounced release of platelet activating factor occurred after the commencement of reperfusion, although it was not detectable before induction of ischaemia. Release of platelet activating factor occurred over 60 minutes of the reperfusion period. In the control, left ventricular developed pressure after 60 minutes of reperfusion recovered to 54.3(SEM 1.7)% (n = 5) of the preischaemic value. In the hearts treated with TCV-309 at concentrations above 0.3 microM, recovery of left ventricular developed pressure was significantly improved (77.6(2.0)% at 1 microM, p < 0.01 v control). Leakage of creatine kinase during the initial five minutes of reperfusion was significantly less in the hearts treated with 1 microM TCV-309 than in the controls (5.2(0.4) v 12.2(1.4) IU.g-1 wet weight, p < 0.01). CONCLUSIONS: Release of platelet activating factor occurred during the reperfusion period in Langendorff perfused hearts. Treatment with the platelet activating factor antagonist TCV-309 significantly improved postischaemic left ventricular function and decreased creatine kinase release. These results suggest that platelet activating factor is involved in myocardial injury during ischaemia-reperfusion.

Animals↗
PubMed1993

Binding properties of (+/-)[3H]benidipine hydrochloride to rat heart membranes.

Benidipine is a newly developed slow-onset and long-lasting dihydropyridine calcium antagonist. The kinetics of specific binding of (+/-)[3H]benidipine to the dihydropyridine receptor sites in rat heart membranes were assessed. The dissociation constant (Kd) and the maximal number of binding sites were 0.078 +/- 0.029 nM and 286 +/- 6 fmol/mg of protein, respectively. Association and dissociation rate constants of (+/-)[3H]benidipine binding were 4 and 50 times smaller, respectively, than those of (+/-)[3H]nitrendipine binding. In displacement studies, the decreasing order of potency was benidipine, nisoldipine, nicardipine, nitrendipine, nifedipine, and Bay K 8644. A high stereoselectivity was observed, with Ki values of 0.028 nM in the S-S-(+) isomer and 4.4 nM in the R-R-(-) isomer of this drug. Benidipine had a high affinity for specific binding, with an inhibition constant of 0.084 nM, which was in good agreement with the Kd value stated above. Among the stimulants and blockers of alpha- and beta-adrenergic, cholinergic, histaminergic, dopaminergic, serotonergic, or GABAergic receptors, no drug inhibited or enhanced specific (+/-)[3H]benidipine binding. These binding properties of (+/-)[3H]benidipine may explain its unique slow onset and long-lasting antihypertensive activities.

Animals↗
PubMed1993

[Urease test].

In order to elucidate bacterial infection, as a rule, isolated and cultured bacteria of a specimen are identified by their morphologies or biochemical characteristics. However, the culture of Helicobacter pylori (H. pylori) is difficult and takes, much time. Therefore, favorable test results are currently obtained by combining the culture method, histopathologic method, urease test and sero-immunologic test. The merits of the urease test are that judgement is available immediately after endoscopy and that the test is inexpensive. The demerits are that the existence of H. pylori is diagnosed only by estimation that endoscopy is indispensable for the test. When compared with the culture method, the urease test has 91.8% sensitivity and 94.7% specificity and when compared with the histopathologic method, it has 94.4% sensitivity 88.4% specificity. In the future, the clinical requirement will increase for quick investigation on the evidence of H. pylori-induced infection. These results show that urease test is useful because it is simple and allows quick judgement.

Ammonia↗
PubMed1993

Protective effects of diltiazem and ryanodine against ischemia-reperfusion injury in neonatal rabbit hearts.

The effects of diltiazem, a sarcolemmal Ca2+ channel blocker, and ryanodine, an inhibitor of sarcoplasmic reticulum function, were investigated in isolated newborn rabbit hearts (2 to 5 days old) subjected to ischemia and reperfusion. After cardioplegic arrest with St. Thomas' Hospital solution, global ischemia was induced at 37 degrees C (normothermia) for 45 minutes or at 20 degrees C (hypothermia) for 180 minutes. The hearts were then reperfused at 37 degrees C for 30 minutes. Diltiazem or ryanodine, at concentrations that have minimal to moderately negative inotropic effects under nonischemic conditions, was added to the cardioplegic solution. After normothermic ischemia, reperfusion of untreated hearts resulted in recovery of left ventricular developed pressure to 52.9% +/- 2.5% of the preischemic level. In hearts treated with diltiazem, recovery of left ventricular developed pressure was significantly improved (84.2% +/- 2.9% at 3 x 10(-8) mol/L; p < 0.01). Comparable improvement was achieved with ryanodine (90.5% +/- 4.1% at 10(-9) mol/L; p < 0.01). Creatine kinase leakage and structural derangement of mitochondria were also reduced by both agents. With hypothermic ischemia, left ventricular developed pressure recovered in untreated hearts to 72.7% +/- 3.3% of preischemic values. Treatment with diltiazem improved the recovery of left ventricular developed pressure to 96.9% +/- 3.5% at 3 x 10(-8) mol/L and reduced creatine kinase leakage and mitochondrial damage. Ryanodine also improved the recovery of left ventricular developed pressure and attenuated ultrastructural damage. These findings suggest that Ca2+ handling by the sarcoplasmic reticulum, like transsarcolemmal Ca2+ influx, plays an important role in the pathogenesis of myocardial ischemia-reperfusion injury in the neonatal heart despite the morphologic and functional immaturity of the sarcoplasmic reticulum in the neonate.

Animals↗