Biomedical subjects

R Niwa

Publications and source records attributed to R Niwa.

17 recordsLinked to original sources

PubMed2005

The ecdysteroidogenic P450 Cyp302a1/disembodied from the silkworm, Bombyx mori, is transcriptionally regulated by prothoracicotropic hormone.

During larval and pupal development of insects, ecdysone is synthesized in the prothoracic gland (PG). Although several Drosophila genes, including Halloween P450 genes, are known to be important for ecdysteroidogenesis in PG, little is known of the ecdysteroidogenic genes in other insects. Here we report on Cyp302a1/disembodied (dib-Bm), one of the Halloween P450s in the silkworm Bombyx mori that is a carbon-22 hydroxylase. dib-Bm is predominantly expressed in PG and its developmental expression profile is correlated with a change in the ecdysteroid titre in the haemolymph. Furthermore, dib-Bm expression in cultured PGs is significantly induced by treatment with prothoracicotropic hormone. This is the first report on the transcriptional induction of a steroidogenic gene by the tropic hormone in insects.

Amino Acid Sequence↗

PubMed2003

Sarcoplasmic reticulum Ca2+ release is not a dominating factor in sinoatrial node pacemaker activity.

Recent work on isolated sinoatrial node cells from rabbit has suggested that sarcoplasmic reticulum Ca2+ release plays a dominant role in the pacemaker potential, and ryanodine at a high concentration (30 micromol/L blocks sarcoplasmic reticulum Ca2+ release) abolishes pacemaking and at a lower concentration abolishes the chronotropic effect of beta-adrenergic stimulation. The aim of the present study was to test this hypothesis in the intact sinoatrial node of the rabbit. Spontaneous activity and the pattern of activation were recorded using a grid of 120 pairs of extracellular electrodes. Ryanodine 30 micromol/L did not abolish spontaneous activity or shift the position of the leading pacemaker site, although it slowed the spontaneous rate by 18.9+/-2.5% (n=6). After ryanodine treatment, beta-adrenergic stimulation still resulted in a substantial chronotropic effect (0.3 micromol/L isoproterenol increased spontaneous rate by 52.6+/-10.5%, n=5). In isolated sinoatrial node cells from rabbit, 30 micromol/L ryanodine slowed spontaneous rate by 21.5+/-2.6% (n=13). It is concluded that sarcoplasmic reticulum Ca2+ release does not play a dominating role in pacemaking in the sinoatrial node. The full text of this article is available at http://www.circresaha.org.

Action Potentials↗

PubMed2001

Pacemaker shift in the rabbit sinoatrial node in response to vagal nerve stimulation.

Effects of brief postganglionic vagal nerve stimulation on the activation sequence of the rabbit sinoatrial (SA) node were investigated. Activation sequences in a small area (7 mm x 7 mm) on the epicardial surface were measured in a beat-to-beat manner using an extracellular potential mapping system composed of 64 modified bipolar electrodes with high-gain and low-frequency band-pass filtering. The leading pacemaker site was recognised clearly from both the activation sequence and the characteristic morphology of the potentials. Vagal stimulation resulted in a short-lasting initial slowing of spontaneous rate followed by a long-lasting secondary slowing; a brief period of relative or absolute acceleration was interposed between the two slowing phases. During these changes of spontaneous rate, the leading pacemaker site shifted in a complex beat-to-beat manner by 1-6 mm alongside the crista terminalis in the superior or inferior direction. For the first spontaneous excitation following stimulation, the greater the slowing, the larger the distance of the pacemaker shift. There was no such linear relationship between the extent of slowing and the distance of pacemaker shift for the subsequent beats. These changes in the leading pacemaker site in response to vagal stimulation may be the result of the functional and morphological heterogeneity of the mammalian SA node in terms of innervation, receptor distribution and ion channel densities. Experimental Physiology (2001) 86.2, 177-184.

Animals↗

PubMed2000

Regional differences in arrhythmogenic aftereffects of high intensity DC stimulation in the ventricles.

Regional differences of the aftereffects of high intensity DC stimulation were investigated in isolated rabbit hearts stained with a voltage-sensitive dye (di-4-ANEPPS). Optical action potential signals were recorded from the epicardial surface of the right and left ventricular free wall (RVep, LVep) and from the right endocardial surface of the interventricular septum (IVS). Ten-millisecond monophasic DC stimulation (S2, 20-120 V) was applied to the signal recording spots during the early plateau phase of the action potential induced by basic stimuli (S1, 2.5 Hz). There was a linear relationship between S2 voltage and the S2 field intensity (FI). S2 caused postshock additional depolarization, giving rise to a prolongation of the shocked action potential. With S2 > or = 40 V (FI > or = approximately 20 V/cm), terminal repolarization of action potential was inhibited, and subsequent postshock S1 action potentials for 1-5 minutes were characterized by a decrease in the maximum diastolic potential and a decrease in the amplitude and a slowing of their upstroke phase. The higher the S2 voltage, the larger the aftereffects. The changes in postshock action potential configuration in RVep were significantly greater than those observed in LVep and IVS when compared at the same levels of S2 intensity. In RVep, 12 of 20 shocks of 120 V resulted in a prolonged refractoriness to S1 (> 1 s), and the arrest was often followed by oscillation of membrane potential. Ventricular tachycardia or fibrillation ensued from the oscillation in five cases. No such long arrest or serious arrhythmias were elicited in LVep and IVS. These results suggest that RVep is more susceptible than LVep and IVS for arrhythmogenic aftereffects of high intensity DC stimulation.

Animals↗

PubMed1999

Carvedilol blocks the repolarizing K+ currents and the L-type Ca2+ current in rabbit ventricular myocytes.

Carvedilol ((+/-)-1-(carbazol-4-yloxy)-3-[[2-(o-methoxyphenoxy)ethyl]am ino]-2-propanol), a beta-adrenoceptor-blocking agent with vasodilator properties, has been reported to produce dose-related improvements in left ventricular function and reduction in mortality in patients with chronic heart failure. However, its electrophysiological effects have not been elucidated. We studied ion channel and action potential modulation by carvedilol in rabbit ventricular preparations using whole-cell voltage-clamp and standard microelectrode techniques. In ventricular myocytes, carvedilol blocked the rapidly activating component of the delayed rectifier K+ current (I(Kr)) in a concentration-dependent manner (IC50 = 0.35 microM). This block was voltage- and time-independent; a prolongation of the depolarizing pulses from a holding potential of -50 mV to +10 mV within the range of 100-3000 ms did not affect the extent of I(Kr) block. Carvedilol also inhibited the L-type Ca2+ current (I(Ca)), the transient outward K+ current (I(to)) and the slowly activating component of the delayed rectifier K+ current (I(Ks)) with IC50 of 3.59, 3.34, and 12.54 microM, respectively. Carvedilol (0.3-30 microM) had no significant effects on the inward rectifier K+ current. In papillary muscles from rabbits pretreated with reserpine, action potential duration was prolonged by 7-12% with 1 microM and by 12-24% with 3 microM carvedilol at stimulation frequencies of 0.1-3.0 Hz. No further action potential duration prolongation was observed at concentrations higher than 3 microM. These results suggest that concomitant block of K+ and Ca2+ currents by carvedilol resulted in a moderate prolongation of action potential duration with minimal reverse frequency-dependence. Such electrophysiological effects of carvedilol would be beneficial in the treatment of ventricular tachyarrhythmias.

Action Potentials↗

PubMed1999

Downward gradient in action potential duration along conduction path in and around the sinoatrial node.

Regional differences in electrical activity in rabbit sinoatrial node have been investigated by recording action potentials throughout the intact node or from small balls of tissue from different regions. In the intact node, action potential duration was greatest at or close to the leading pacemaker and declined markedly in all directions from it, e.g., by 74 +/- 4% (mean +/- SE, n = 4) to the crista terminalis. Similar data were obtained from the small balls. The gradient is down the conduction pathway and will help prevent reentry. In the intact node, a zone of inexcitable tissue with small depolarizations of <25 mV or stable resting potentials was discovered in the inferior part of the node, and this will again help prevent reentry. The intrinsic pacemaker activity of the small balls was slower in tissue from more inferior (as well as more central) parts of the node [e.g., cycle length increased from 339 +/- 13 ms (n = 6) to 483 +/- 13 ms (n = 6) in transitional tissue from more superior and inferior sites], and this may help explain pacemaker shift.

Action Potentials↗

PubMed1999

Regional differences in effects of E-4031 within the sinoatrial node.

Effects of block of the rapid delayed rectifier K+ current (IK,r) by E-4031 on the electrical activity of small ball-like tissue preparations from different regions of the rabbit sinoatrial node were measured. The effects of partial block of IK,r by 0.1 microM E-4031 varied in different regions of the node. In tissue from the center of the node spontaneous activity was generally abolished, whereas in tissue from the periphery spontaneous activity persisted, although the action potential was prolonged, the maximum diastolic potential was decreased, and the spontaneous activity slowed. After partial block of IK,r, the electrical activity of peripheral tissue was more like that of central tissue under normal conditions. One possible explanation of these findings is that the density of IK,r is greater in the periphery of the node; this would explain the greater resistance of peripheral tissue to IK,r block and help explain why, under normal conditions, the maximum diastolic potential is more negative, the action potential is shorter, and pacemaking is faster in the periphery.

Animals↗

PubMed1999

Preparation and application of anti-idiotypic antibody against anti-gibberellin A4 antibody.

A monoclonal anti-idiotypic antibody was raised against anti-gibberellin A4 (GA4) antibody, which recognizes biologically active gibberellins such as GA1 and GA4 specifically. Amino acid sequences of variable regions of both anti-GA4 and anti-idiotypic antibodies were analyzed. By using the property of the anti-idiotypic antibody to compete with GA1/4 in binding to the anti-GA4 antibody, we successfully applied the anti-idiotypic antibody to ELISA as a tracer for measuring GA1/4. The single-chain Fv (scFv) gene of the anti-idiotypic antibody was constructed, and scFv expressed in E. coli showed binding activity to anti-GA4 antibody. These results suggest the possible application of anti-idiotypic antibody as a handy and stable source of an enzymatic tracer for ELISA by production of fusion protein of the scFv and an appropriate enzyme.

Amino Acid Sequence↗

PubMed1998

Na+ channel blocking effects of cibenzoline on guinea-pig ventricular cells.

The effects of cibenzoline on transmembrane action potentials were examined in right ventricular papillary muscles and in single ventricular myocytes isolated from guinea-pig hearts. In papillary muscles, cibenzoline > or = 3 microM caused a significant decrease in the maximum upstroke velocity (Vmax) of the action potential without affecting the action potential duration. The inhibition of Vmax was enhanced at higher stimulation frequencies. In the presence of cibenzoline, trains of stimuli at rates > or = 0.2 Hz led to a use-dependent inhibition of Vmax. The time constant for Vmax recovery (tauR) from the use-dependent block was 26.2 s. The use-dependent block of Vmax with cibenzoline was enhanced and tauR was shortened when the resting potential was depolarized by high (8, 10 mM) [K+]o. The curve relating membrane potential and Vmax in single myocytes was shifted by cibenzoline (10 microM) in a hyperpolarizing direction by 7.1 mV. In myocytes treated with cibenzoline (10 microM), a 10-ms conditioning clamp to 0 mV caused a significant decrease in Vmax of the subsequent test action potential; the Vmax inhibition was enhanced modestly in association with a prolongation of the 0 mV clamp pulse duration. In the presence of cibenzoline (3 microM), application of a train of depolarizing pulses (10 ms, 200 ms) to myocytes from the resting level (-80 mV) to 0 mV resulted in a progressive Vmax reduction in a pulse number-dependent manner. Unlike glibenclamide (30 microM), cibenzoline (10 microM) did not prevent the hypoxia-induced shortening of action potential duration in papillary muscles. These findings indicate that the onset and offset kinetics of use-dependent Na+ channel block by cibenzoline are slow. Given its state dependence, cibenzoline may be a blocker of activated Na+ channels. The inhibitory action of this compound on the ATP-sensitive K+ current (I(K), ATP) would be minimal or negligible at concentrations causing sufficient Na+ channel block.

Animals↗

PubMed1998

Low-frequency extracellular potentials recorded from the sinoatrial node.

OBJECTIVE: To study the morphology of small extracellular potentials localized to the sinoatrial (SA) node and to elucidate its potential usefulness in evaluating SA node dysfunction. METHODS: Extracellular potentials were recorded from the endocardial surface of the SA node in isolated right atrial preparations of rabbits through custom-made modified bipolar electrodes with high-gain amplification and a low-frequency (0.5-32 Hz) filter setting. RESULTS: The potentials in and around the SA node under control conditions showed a variety of morphologies. In a small area near the leading pacemaker site, slow primary negative deflections were preceded by a gradual increase of the negativity (73.5 +/- 5.6 microV in amplitude, n = 12). In the periphery of the SA node cranial and caudal to the leading pacemaker site, slow positive/negative deflections were recorded. In the septal side of the SA node showing very slow conduction, the electrograms showed slow primary positive deflections. Transient pacemaker shifts induced by atrial stimulation or vagal nerve stimulation were reflected well in morphologies of the extracellular potentials. In the presence of 20 microM TTX, wide and slow negative deflections were observed in the center and periphery of the SA node in association with extremely slow conduction restricted to a corridor-like area along the crista terminalis, whereas the atrial muscle surrounding the area was made inexcitable. In the presence of 1 microM nifedipine, the leading pacemaker site was shifted to the periphery of the SA node close to the crista terminalis. The negative deflection in the center and septal side of the SA node disappeared reflecting no excitation of the area. CONCLUSION: The endocardial extracellular electrograms recorded in and around the SA node under appropriate conditions reflect two dimensional activation sequences. They would provide useful information in recognizing the leading pacemaker site and alterations of the conductivity and excitability.

Animals↗

PubMed1998

In vivo analysis of hydrogen peroxide and lipid radicals in the striatum of rats under long-term administration of a neuroleptic.

It has been hypothesized that free radicals play a causative role in tardive dyskinesia, which is an inveterate movement disorder caused by chronic administration of neuroleptics. To verify this hypothesis, rats were reared while being regularly treated with water containing a neuroleptic, haloperidol (HPD), for 1 year (HPD group). The changes in the striatal hydrogen peroxide content of the rats in the HPD and control groups were measured by using a Pt-disk microelectrode while the animals were in a freely moving state following intraperitoneal administration of HPD (HPD challenge). We also performed electron spin resonance (ESR) detection of lipid radicals in the striatum before the HPD challenge. HPD challenge led to significant elevation of the intrastriatal hydrogen peroxide in all animals, but the elevation in the HPD group was smaller than that in the control group. However, in the HPD group, marked ESR signals of intrastriatal lipid radicals were observed. We think that these results support the hypothesis on the role of free radicals in tardive dyskinesia.

Animals↗

PubMed1998

Regional differences in effects of 4-aminopyridine within the sinoatrial node.

4-Aminopyridine (4-AP)-sensitive transient outward current (Ito) has been observed in the sinoatrial node, but its role is unknown. The effect of block of Ito by 5 mM 4-AP on small ball-like tissue preparations (diameter approximately 0.3-0.4 mm) from different regions of the rabbit sinoatrial node has been investigated. 4-AP elevated the plateau, prolonged the action potential, and decreased the maximum diastolic potential. Effects were greater in tissue from the periphery of the node than from the center. In peripheral tissue, 4-AP abolished the action potential notch, if present. 4-AP slowed pacemaker activity of peripheral tissue but accelerated that of central tissue. Differences in the response to 4-AP were also observed between tissue from more superior and inferior regions of the node. In the intact sinoatrial node, 4-AP resulted in a shift of the leading pacemaker site consistent with the regional differences in the response to 4-AP. It is concluded that 4-AP-sensitive outward current plays a major role in action potential repolarization and pacemaker activity in the sinoatrial node and that its role varies regionally.

4-Aminopyridine↗

PubMed1997

Generation of lipid radicals in the hippocampal extracellular space during kainic acid-induced seizures in rats.

We report direct electron spin resonance (ESR) evidence of extracellular free radical formation during kainic acid-induced seizures obtained using in vivo brain microdialysis in freely moving rats. Saline solution containing the spin trap agent alpha-(4-pyridyl-N-oxide)-N-tert-butylnitrone was perfused through the hippocampus. ESR analysis of the dialysate samples revealed a six-line spectra, for which the hyperfine coupling constants corresponded to those of the ESR signal from the lipoxygenase/linoleic acid system, a lipid radical generating system. This result is direct evidence that lipid peroxidation of the neuronal membrane progresses during seizure activity. Increased formation of lipid radicals may participate in the cascade of reactions leading to neuronal damage in the hippocampus following kainic acid-induced seizure activity.

Animals↗

PubMed1997

Synthesis of spin labels for ESR imaging of living rat head.

Spin labels (7, 10, 13, 16, 22, 27) were synthesized from piperidinyloxyl (1), pyrrolidinyloxyl (2), and oxazolidinyloxyl (3). These compounds were injected into the carotid artery of anesthetized rats, and the ESR spectra of the rat brain were immediately recorded by the use of an L-band ESR spectrometer. Based on the spectra obtained, we considered whether or not these spin labels can pass the blood brain barrier and bind to brain tissue components.

Animals↗

PubMed1997

Role of QT interval prolongation in the creation of spiral wave type reentry.

The inducibility of reentry was compared for four QT patterns in a heart conduction simulation model. Local (L) and gradual (G) QT prolongation models are more susceptible to reentry induction than the no (N) QT prolongation model (reentry induced episodes for N, L, and G numbered 90, 120, and 122, respectively). This increased vulnerability was diminished when the QT interval was prolonged at all simulation sites (reentry induced episodes for the diffuse QT prolongation model, D model, numbered 82). Decreased QT dispersion might be important for the prevention of reentry induction regardless of whether the QT interval is increased.

Action Potentials↗

PubMed1996

Synthesis of isomelamines and isocyanurates and their biological evaluation.

The reaction of cyanogen bromide (1) with primary amines (2a-p), including arylmethylamines (2l-p), gave the corresponding cyanamides (3a-p). Trimerization of 3a-p gave 1,3,5-trisubstituted 2,4,6-triiminohexahydro-1,3,5-triazines (isomelamines) (4a-p), which were treated with hydrochloric acid to give the corresponding 1,3,5-trisubstituted 2,4,6-trioxohexahydro-1,3,5-triazines (isocyanurates) (5a-c, f) and 1,3,5-trisubstituted 2-imino-4,6-dioxohexahydro-1,3,5-triazines (5b'-e'). Biological evaluation of 4a-p, 5a-c,f, and 5b'-e' was carried out, and some of these compounds showed bronchodilator and positive inotropic activities.

Animals↗

PubMed1995

Synthesis and evaluation of DMPO-type spin traps.

The spin traps substituted with some groups at the 4-position of dimethyl-1-pyrroline N-oxide(DMPO) were compared with DMPO itself regarding their abilities as spin traps and their physical properties. 4,5,5-Trimethyl-1-pyrolline N-oxide (4MDMPO) and 5,5-dimethyl-4-phenyl-1-pyrolline N-oxide (4PDMPO) were synthesized by the Bonnett method, and 5,5-dimethyl-4-hydroxymethyl-1-pyrolline N-oxide (4HMDMPO) was made by a unique method from 2(5H)-furanone. The melting points of 4MDMPO, 4PDMPO and 4HMDMPO were higher than that of DMPO. The magnitude of hydrophilicity was in the order of 4HMDMPO, DMPO, 4MDMPO, and 4PDMPO based on the partition coefficient experiments in a 1-octanol--water system. Several radicals, O2-., HO., .CH3, .CH2OH, .CH(CH3)OH, (CH3)3CO. and H. radicals, were trapped with these DMPO derivatives for comparison with the trapping by DMPO itself. Spin adducts of O2-. with the three DMPO derivatives showed ESR spectra similar to that of DMPO. In spite of the formation of diastereomers arising from spin trapping, the line-width enlargement was very small. The intensities and the decay rates of the spectra of 4MDMPO-O2-, 4PDMPO-O2-, 4HMDMPO-O2- and DMPO-O2- were almost equal. In the trapping of the .OH radical by 4MDMPO, 4PDMPO and 4HMDMPO, the eight-line ESR spectra observed were different from the well-known four-line spectrum of DMPO-OH.

Cyclic N-Oxides↗