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N Tohse

Publications and source records attributed to N Tohse.

At least 37 records · Page 2Linked to original sources
PubMed1995

Effects of BQ-485, a selective ETA antagonist, on endothelin-mediated vasomotion in rat coronary vascular beds.

The effects of BQ-485, a selective endothelin (ET)-A receptor antagonist, on the vasomotion induced by a low dose of ET were investigated. In the isolated rat heart perfused with Krebs-Henseleit solution at a constant flow, intracoronary bolus injection of ET-1 or ET-3 (10 pmol) elicited a rapid transient decrease, followed by a slight sustained increase, in the coronary perfusion pressure (CPP). The decrease in CPP induced by ET-1 was similar in magnitude to (approximately 30%) but shorter in duration than that induced by ET-3. Pretreatment of the heart with saponin (30 micrograms/ml) to denude the coronary endothelium abolished the decrease and markedly enhanced the increase in CPP induced by ETs, indicating that the vasorelaxing action of ETs is endothelium-dependent. The selective ETA receptor antagonist BQ-485 (1 microM) significantly prolonged the duration of the ET-1-induced decrease in CPP, made the vasodilatation by ET-1 indistinguishable from that by ET-3, and eliminated the subsequent increase in CPP. In the saponin-treated heart, BQ-485 also eliminated the ET-1-mediated increase in CPP. These findings suggest that, in rat coronary vascular beds, a low dose of ET-1 elicits vasoconstriction and endothelium-dependent vasodilatation through the ETA receptor on the vascular smooth muscle and presumably the ETB receptor on the endothelium, respectively. Furthermore, it is expected that selective ETA receptor antagonists, including BQ-485, may be able to protect the heart against ET-1-induced coronary spasm in situations, such as hyperlipidemia or artherosclerosis, in which the release and/or function of endothelium-derived vasorelaxing substances is impaired.

Animals↗
PubMed1994

Depressed responsiveness to angiotensin II in ventricular myocytes of hypertrophic cardiomyopathic Syrian hamster.

Electromechanical responsiveness to angiotensin II (Ang II) receptor stimulation in ventricular myocardium and myocytes of hypertrophic cardiomyopathic Syrian hamsters (BIO 14.6) was examined and compared with that in preparations of normal hamsters (F1B) using conventional microelectrode and patch clamp techniques. Action potential duration (APD) and developed tension (DT) corrected for the cross-sectional area of the papillary muscles of 14-20 week-old BIO 14.6 hamsters were significantly smaller than those in preparations of age-matched normal hamsters. An Ang II (1 microM)-induced increase in DT in BIO 14.6 papillary muscles (24.7 +/- 11.0%) was significantly smaller than that in F1B papillary muscles (53.8 +/- 8.5%), which was associated with a smaller increase in APD in BIO 14.6 papillary muscles. In ventricular myocytes of both BIO 14.6 and F1B hamsters. Ang II increased the calcium current (ICa) following a transient decrease in ICa. However, the magnitude of the Ang II-induced increase in ICa in BIO 14.6 myocytes (35.5 +/- 7.5%) was significantly smaller than that in F1B myocytes (86.0 +/- 19.7%), suggesting a causal relationship between ICa and mechanical response to Ang II in these hamsters. The depressed responsiveness to Ang II receptor stimulation in hypertrophic cardiomyopathic hamster is in a marked contrast with the enhanced responsiveness to alpha 1-adrenergic stimulation, which was demonstrated by previous studies, and may be one of adaptational changes to the activated renin-angiotensin system in the cardiomyopathy.

Action Potentials↗
PubMed1993

Voltage-dependent modulation of L-type Ca2+ current by manidipine in guinea-pig heart cells.

Effects of manidipine, a dihydropyridine derivative, on L-type Ca2+ currents were examined in guinea-pig ventricular cells, using the whole-cell patch-clamp method. At a holding potential of -37 mV, manidipine decreased the Ca2+ current at concentrations above 0.1 nM, and abolished it at 100 nM (IC50 = 2.6 nM). At a holding potential of -78 mV, manidipine did not suppress the Ca2+ current at concentrations less than 100 nM, but increased the Ca2+ current slightly at a concentration of 10 nM. The antagonistic effect of manidipine was significant at concentrations above 100 nM (IC50 = 400 nM). The voltage-dependent effect of manidipine on the Ca2+ current may explain the weak negative inotropism of manidipine in ventricular muscles, and provide an electrophysiological basis for its vascular selectivity.

Animals↗
PubMed1993

Tetrodotoxin-sensitive sodium current in rat fetal ventricular myocytes--contribution to the plateau phase of action potential.

In cardiomyocytes of some animals, the Na+ current shows slow gating properties, and seems to contribute to the plateau phase of the action potential. In the present study, we characterized the fast Na+ current in freshly-isolated ventricular cardiomyocytes from 18-day-old fetal rats, using whole-cell patch clamp, and examined a possible contribution of the Na+ current to action potential duration (APD). Currents were recorded (at 25 degrees C) in Ca(2+)-K(+)-free external and internal (pipette) solutions with 143 mM [Na]o and 20 mM [Na]i. The fast Na+ current was elicited by depolarizing steps above -47 mV from a holding potential of -97 mV. The maximal amplitude of the current was 735 +/- 94 pA (mean +/- S.E., n = 11) at -7 mV. The current density was 39.0 +/- 6.7 pA/pF (n = 9). The reversal potential was 51 +/- 3 mV (n = 11), agreeing with the equilibrium potential for Na+ (51 mV). The inward current was completely blocked in absence of [Na]o and by tetrodotoxin (TTX, 10 microns). The inactivation curve (h infinity) was sigmoidal between -127 and -27 mV, the half-inactivation potential and slope factor (k) being -69 and 9.5 mV, respectively. The activation curve (normalized gNa) was also sigmoidal, with half-activation voltage and k of -27 mV and -8.4 mV, respectively. The two curves overlapped from -60 to -30 mV. The current decay was best fitted by a two-exponential equation: (a) the time constant of the fast component (tau if) decreased from 3.9 +/- 0.6 ms at -37 mV to 0.8 +/- 0.1 ms at 23 mV; (b) the slow component (tau is) declined from 31 +/- 8 ms to 16 +/- ms at the same potentials. The recovery from inactivation had also two components: fast (tau rf = 10 +/- 2 ms) and slow (tau rs = 307 +/- 44 ms). TTX (1 micron) decreased the slow inactivating component and ADP50 by 69.5 +/- 4.2% and 25 +/- 6%, respectively. Therefore, the Na+ current in rat fetal cardiomyocytes may significantly contribute to the plateau phase of the action potential. The kinetics properties of the fast Na+ current of the 18-day fetal cardiomyocytes were compared with those from 1-day old neonatal rats. The current density was lower and the current decay was slower in the fetal cells in comparison with neonatal cells.

Action Potentials↗
PubMed1993

Membrane currents of porcine granulosa cells in primary culture: characterization and effects of luteinizing hormone.

In ovarian granulosa cells, LH depolarizes the membrane potential and induces steroid production. The membrane currents of porcine granulosa cells in primary culture were studied by means of the whole cell configuration of the patch-clamp technique to investigate whether the ionic channels are involved in the effects of LH. We identified and characterized two types of K+ currents--a transient outward current (Ito) and a delayed rectifier K+ current (IK)--and one Ca2+ current (ICa). Ito and IK were voltage- and calcium-dependent. Both currents were blocked by 4-aminopyridine, a K+ channel blocker, but only IK was sensitive to tetraethylammonium, another K+ channel blocker. ICa was inactivated within 50 ms of the test pulse. Nifedipine and verapamil, L-type CA2+ channel blockers, did not suppress ICa even at a concentration of 10 microM. Tetramethrin (1 microM), a T-type Ca2+ channel blocker, decreased ICa by 38.4 +/- 5.6% (n = 3). These findings suggest that the current may be a T-type Ca2+ current. LH and dibutyryl cAMP, potent stimulants of steroid production, attenuated Ito only by 13.9 +/- 1.8% (n = 7) and 21.0 +/- 1.5% (n = 4), respectively. However, they did not affect IK and ICa. These results indicated that LH does not modulate CA2+ current directly, but it slightly decreased Ito through cAMP elevation. The LH-induced inhibition of Ito may be involved in the depolarizing effect of LH and may play some role in steroid secretion or other functions in granulosa cells.

4-Aminopyridine↗
PubMed1993

Possible contribution of potassium channels to the endothelin-induced dilatation of rat coronary vascular beds.

The mechanism for the endothelin (ET)-induced vasodilatation of the endothelium-intact rat coronary vascular bed was investigated. Continuous infusion (0.1-1 nM) or bolus injection (1-100 pmol) of ET-1 or ET-3 elicited a dose-related transient decrease, followed by a slight sustained increase, in the coronary perfusion pressure (CPP). The decrease in CPP induced by an injection (10 pmol) of ET-1 or ET-3 was not modified by indomethacin (5 microM). However, oxyhemoglobin (5 mM) shortened the duration of the ET-induced decrease in CPP, although it did not affect the magnitude. The ET-induced decrease in CPP was abolished by raising K+ in the perfusing solution from 5.9 to 16.9 mM. These findings suggest that the ET-induced dilatation of the rat coronary vascular beds may not be mediated by cyclooxygenase products. The ET-induced vasorelaxation may be mediated, at least in part, by endothelium-derived relaxing factor and may be related to opening of K+ channels.

Animals↗
PubMed1993

Effects of MS-551, a new class III antiarrhythmic drug, on action potential and membrane currents in rabbit ventricular myocytes.

1. Electrophysiological effects of MS-551, a new class III antiarrhythmic drug, were examined and compared with those of (+)-sotalol in rabbit ventricular cells. 2. In rabbit ventricular muscles stimulated at 1.0 Hz, MS-551 (0.1-10 microM) and (+)-sotalol (3-100 microM) prolonged action potential duration (APD) and effective refractory period without affecting the maximum upstroke velocity of phase 0 depolarization (Vmax). The class III effect of MS-551 was approximately 30 times more potent than that of (+)-sotalol. 3. Class III effects of MS-551 and (+)-sotalol showed reverse use-dependence, i.e., a greater prolongation of APD at a longer cycle length. 4. In rabbit isolated ventricular cells, 3 microM MS-551 and 100 microM sotalol inhibited the delayed rectifier potassium current (IK) which was activated at more positive potentials than -50 mV and saturated around +20 mV. 5. MS-551 at a higher concentration of 10 microM decreased the transient outward current (Ito) and the inward rectifier potassium current (IK1) although 100 microM sotalol failed to inhibit these currents. 6. MS-551 is a non-specific class III drug which can inhibit three voltage-gated K+ channels in rabbit ventricular cells.

Action Potentials↗
PubMed1992

Mechanism of the membrane depolarization induced by oxidative stress in guinea-pig ventricular cells.

Mechanism of the membrane depolarization induced by oxidative stress was examined using ion-selective microelectrode and patch clamp techniques. In guinea-pig papillary muscles stimulated at 0.5 Hz, cumene hydroperoxide (CH) at a concentration of 300 microM decreased the resting membrane potential and shortened the action potential, concomitantly with muscle contracture. The membrane depolarization was not associated with a significant decrease in intracellular potassium ion activity, indicating that the depolarization is not due to a decrease in potassium equilibrium potential resulting from leak of intracellular K+. In isolated guinea-pig ventricular cells. CH (10-30 microM) consistently decreased the inward rectifier potassium current and slightly decreased the calcium current. In cell-attached patches CH inhibited the opening of the inward rectifier K+ channel without affecting the unit amplitude of the single channel current. Thus, the depolarization of the resting membrane induced by oxidative stress is, at least in part, due to the inhibition of the inward rectifier K+ channel activity, and may play an important role in the genesis of reperfusion-induced arrhythmias.

Animals↗
PubMed1992

Novel isoform of Ca2+ channel in rat fetal cardiomyocytes.

1. Single cardiomyocytes of 18-day-old rat fetuses were isolated to characterize the cardiac Ca2+ channels in the fetal period, using whole-cell voltage clamp (Na+, K(+)-free external solution and K(+)-free internal solution), and depolarizing test pulses from a holding potential (HP) of -87 mV were applied. 2. The Ca2+ current was completely blocked by 2 mM-CO2+, but not completely blocked by the dihydropyridine (DHP) Ca2+ antagonist nifedipine. Nifedipine (3 microM) decreased the amplitude of the current (at -7 mV) by 65.9 +/- 3.4% (n = 20). At a HP of -47 mV, nifedipine decreased the Ca2+ current to about the same degree. Diltiazem (1 microM) did not block the nifedipine-resistant current which remained. 3. Nitrendipine, another DHP Ca2+ antagonist, had effects on the Ca2+ current similar to those of nifedipine. 4. The DHP-resistant current was not blocked by T-type channel blockers (Ni2+, tetramethrine) or an N-type blocker (omega-conotoxin). 5. In conclusion, rat fetal cardiomyocytes may have a unique type of Ca2+ channel (ICa(fe)), which decreases in amplitude and becomes less prominent during subsequent development.

Animals↗
PubMed1992

Developmental changes in long-opening behavior of L-type Ca2+ channels in embryonic chick heart cells.

In the early (3-day) stage of development, long-lasting openings of the L-type Ca2+ channels (mode 2) occur in embryonic chick heart cells. Since mode-2 behavior is infrequently observed in adult heart cells of other species, in the present study, developmental change in behavior of the Ca2+ channel was examined in young (3-day) and old (17-day) embryonic chick heart cells. In the whole-cell voltage clamp, the L-type Ca2+ current carried by Ca2+ ions was smaller in amplitude and had a faster inactivation in 17-day cells than in 3-day cells. The peak current density was 8.1 +/- 0.2 microA/cm2 (mean +/- SEM, n = 5) and 5.1 +/- 0.3 microA/cm2 (n = 5) in 3-day and 17-day cells, respectively. When the charge carrier was Ba2+, the L-type Ca2+ channel current density was also smaller in 17-day cells (22.7 +/- 1.8 microA/cm2) than in 3-day cells (28.3 +/- 2.1 microA/cm2). In single-channel recordings, the mode-2 behavior was infrequent in 17-day cells compared with 3-day cells. High-open probability sweeps (with an open probability of greater than 0.25), reflecting mode-2 behavior, accounted for 20.2% and 3.7% in 3-day and 17-day cells, respectively. The ensemble-averaged currents in 17-day cells was 37% of that current in 3-day cells. In addition, decay of the averaged current appeared to be faster in 17-day cells than in 3-day cells. All data from the single-channel analysis agreed with the data from the whole-cell voltage clamp.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Alpha 1-adrenoceptor stimulation enhances the delayed rectifier K+ current of guinea pig ventricular cells through the activation of protein kinase C.

The effect of alpha 1-adrenoceptor stimulation on the delayed rectifier K+ current (IK) was examined in isolated guinea pig ventricular cells by use of the patch-clamp method. IK was evoked by a 3-second depolarizing pulse from a holding potential of -30 mV in a Na(+)- and K(+)-free solution containing 3 microM nifedipine. Phenylephrine (30 microM) in the presence of propranolol (1 microM) produced an increase in IK. In five cells, phenylephrine increased the tail current of IK by 23 +/- 5%. This effect of phenylephrine was blocked by prazosin (0.3 microM), a selective alpha 1-blocker. Phenylephrine produced only a small effect on the voltage and time dependence of IK. Pretreatment with 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7, 10 microM) abolished the phenylephrine-induced increase in IK. In addition, pretreatment with a maximally effective concentration of 12-O-tetradecanoylphorbol 13-acetate (100 nM) abolished the phenylephrine-induced increase in IK. In conclusion, alpha 1-adrenoceptor stimulation increases IK in guinea pig cardiomyocytes. This alpha 1-adrenoceptor-mediated response may be related to an activation of protein kinase C. The increase in IK may explain a shortening of action potential duration observed after alpha 1-adrenoceptor stimulation in guinea pig cells.

Action Potentials↗
PubMed1991

Bay K 8644 enhances Ca2+ channel activities in embryonic chick heart cells without prolongation of open times.

The effects of the Ca2+ channel agonist, Bay K 8644, on the slow (L-type) Ca2+ channels was examined in young (3-day-old) embryonic chick heart cells, which naturally exhibit long-lasting openings. Bay K 8644 (5 microM) increased (a) the peak amplitude of the ensemble-averaged current by 3.9 +/- 0.9-fold (mean +/- S.E.) and (b) the maximal number of simultaneous opening from 2.6 +/- 0.4 to 4.4 +/- 0.9. Bay K 8644 had no effect on the unitary conductance (27 pS in control), and relatively little effect in the open-close kinetic analysis. The mean open times were 4.2 ms and 5.2 ms, in control and Bay K 8644, respectively. These results suggest that the agonistic effect of Bay K 8644 involves a mechanism other than open-time prolongation, such as activation of silent channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Effects of ATP-sensitive K+ channel blockers on the action potential shortening in hypoxic and ischaemic myocardium.

1. In order to determine whether activation of adenosine triphosphate (ATP)-sensitive K+ channels exclusively explains the hypoxia- and ischaemia-induced action potential shortening, effects of tolbutamide and glibenclamide on changes in action potential duration (APD) during hypoxia, metabolic blockade or experimental ischaemia were examined in guinea-pig and canine isolated myocardium by standard microelectrode techniques. 2. With use of patch clamp techniques, activity of ATP-sensitive K+ channels was recorded from open cell-attached patches of guinea-pig isolated ventricular myocytes. The probability of opening of the K+ channels was decreased by 2 mM tolbutamide and 20 microM glibenclamide to almost the same extent, whereas it was increased by 100 microM pinacidil. 3. In guinea-pig papillary muscles a marked shortening of the action potential produced by 100 microM pinacidil was completely antagonized by 2 mM tolbutamide or 20 microM glibenclamide. 4. In guinea-pig papillary muscles exposed to hypoxic, glucose-free solution or dinitrophenol (10 microM)-containing, glucose-free solution, APD declined gradually and twitch tension decreased. Pretreatment with glibenclamide partially but significantly inhibited the action potential shortening, whereas tolbutamide failed to improve it during hypoxia or metabolic blockade. 5. When in canine isolated myocardium, experimental ischaemia was produced by the cessation of coronary perfusion, APD was gradually shortened. The action potential shortening was partially but not completely inhibited by pretreatment with 20 microM glibenclamide. 6. These results suggest that changes in membrane current(s) other than the outward current through ATP-sensitive K+ channels also contribute to the action potential shortening in hypoxic or ischaemic myocardium.

Action Potentials↗
PubMed1991

Protein kinase C is not involved in alpha 1-adrenoceptor-mediated positive inotropic effect.

This study was performed to determine whether activation of protein kinase C is responsible for the positive inotropic effect of alpha 1-adrenoceptor stimulation in rat papillary muscle. In the presence of 1 microM propranolol, phenylephrine (10 microM) produced triphasic inotropic response that was accompanied by prolongation of action potential duration (APD) and hyperpolarization of membrane potential. Phorbol 12,13-dibutyrate (PDBu, 0.1 microM) abolished the negative inotropic effect of phenylephrine and apparently resulted in enhancement of the positive inotropic effect. PDBu also attenuated the phenylephrine-induced hyperpolarization without affecting the APD prolongation. However, such changes were not observed with 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 microM). Neither PDBu nor TPA increased the force of contraction or prolonged APD similar to phenylephrine. The protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H 7, 10 microM) did not suppress the changes induced by PDBu, and more importantly H 7 did not affect the inotropic and electrophysiological effects of phenylephrine. Both TPA and PDBu significantly inhibited the phenylephrine-induced phosphoinositide hydrolysis as measured by [3H]inositol monophosphate, and these inhibitory effects were eliminated in the presence of H 7. Our results provide an argument against a role of protein kinase C activation in the alpha 1-adrenoceptor-mediated inotropic and electrophysiological effects.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1991

cGMP inhibits the activity of single calcium channels in embryonic chick heart cells.

Effects of cGMP on the slow (L-type) Ca2+ channels of cultured chick embryonic cardiomyocytes were investigated by a cell-attached patch-clamp method. Superfusion of the single cells with 8-bromo-cGMP, a membrane-permeable derivative of cGMP, inhibited the single-channel activity. The cyclic nucleotide decreased, in a concentration-dependent manner, the ensemble averaged currents obtained from multichannel patches. 8-Bromo-cGMP (1 mM) completely abolished the currents (n = 8), whereas 0.1 mM only slightly decreased the currents (n = 4). The influence of cGMP on the characteristics of the single Ca2+ channels was examined using 0.3 mM 8-bromo-cGMP. Unit amplitude and slope conductance of the Ca2+ channel was not changed (25 pS in control versus 24 pS in the presence of cGMP). Analysis of single-channel kinetics showed that cGMP prolonged the slow time constant for the closed-time histogram (from 6.7 to 15.4 msec); the other time constants (for the open-time and closed-time histograms) were not affected. cGMP-induced inhibition of the Ca2+ channels may be mediated by cGMP-dependent protein kinase, because 8-bromo-cGMP is a potent activator of this protein kinase and does not stimulate cAMP hydrolysis. The present results suggest that cGMP opposes the effects of cAMP on the L-type Ca2+ channels in myocardial cells.

Animals↗