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T Tokimasa

Publications and source records attributed to T Tokimasa.

At least 55 records · Page 3Linked to original sources
PubMed1989

Muscarinic agonists and potassium currents in guinea-pig myenteric neurones.

1. Intracellular electrophysiological recordings were obtained from single neurones of the guinea-pig myenteric plexus in vitro. Using single electrode voltage clamp techniques, four distinct potassium currents were described and the effects of muscarinic agonists on these currents were studied. 2. A calcium-dependent potassium current (gKCa) was present in AH neurones at rest, and was much increased following a brief depolarization (50 ms, to 0 mV). Muscarinic agonists reduced both the resting current and the current evoked by depolarization. Pirenzepine competitively antagonized the suppression by muscarine of the calcium-dependent potassium current (or after-hyperpolarization) following an action potential. The dissociation equilibrium constant for pirenzepine was about 10 nM. 3. The conductance of AH neurones increased two to three fold when they were hyperpolarized negative to -90 mV. This inward rectification was blocked by extracellular caesium (2 mM) or rubidium (2 mM), but not by tetraethylammonium (TEA, 40 mM), 4-aminopyridine (100 microM) or cobalt (2 mM). The inward rectification was unaffected by muscarinic agonists. 4. When AH neurones were depolarized from very negative holding potentials (less than -80 mV) a brief outward current was recorded with a duration of about 200 ms. This transient or A current was completely blocked by 4-aminopyridine (100 microM) but was not affected by tetrodotoxin (300 nM), TEA (40 mM) or cobalt (2 mM). Muscarinic agonists did not affect the A current. 5. In S neurones, and in AH neurones in calcium-free solutions, the potassium conductance (in TEA and caesium) behaved according to constant field assumptions. This background conductance was suppressed by muscarinic agonists. 6. It is concluded that the depolarization by muscarinic agonists of myenteric AH neurones is due to a suppression of both a calcium-dependent potassium conductance and a background potassium conductance. Muscarinic depolarization of S neurones results only from suppression of the background potassium conductance. Effects on both conductances result from M1-receptor activation. Inward rectifying and transient outward (A) potassium currents are unaffected.

Animals↗
PubMed1989

Potassium currents in submucous neurones of guinea-pig caecum and their synaptic modification.

1. Intracellular recordings were made from submucous neurones of the guinea-pig caecum. In most experiments, membrane currents were measured using a single-electrode voltage clamp. 2. A potassium current dependent on calcium influx occurred at rest (approximately equal to 200 pA at -60 mV). The amplitude of the current was increased up to 1 nA at -35 mV and decreased to zero at -100 mV; when fully activated the current did not show any inactivation. An inward calcium current, of 15-25 pA in amplitude near -60 mV and insensitive to omega-conotoxin (0.5 microM), probably activated the potassium current. 3. Step depolarizations from potentials negative to -80 mV evoked a transient (less than or equal to 200 ms at -40 mV) potassium current which was blocked by 4-aminopyridine (1-3 mM). Hyperpolarizing commands to potentials negative to -87 mV evoked an inwardly rectifying potassium current which was selectively blocked by caesium (1-2 mM). The residual cell current between -100 and -40 mV in calcium-free solution containing tetraethylammonium (20 mM), caesium (2 mM) and 4-amino-pyridine (3 mM) conformed to constant field assumptions. This current was called a background potassium current. 4. Decrease in membrane conductance during the slow excitatory postsynaptic current (EPSC) was due predominantly (greater than or equal to 90%) to a reduction in the calcium-activated potassium current at -35 mV, but due almost exclusively to a reduction in the background potassium current at potentials more negative than -100 mV. The relative contribution of the two currents to the slow EPSC was entirely dependent on the relative contribution of the currents to the membrane conductance at given potentials. 5. The transient potassium current was unaffected or slightly enhanced during the slow EPSC. The inwardly rectifying potassium current was unaffected during the slow EPSC. 6. Three tachykinins (substance P, substance K and neurokinin B; 3-800 nM), forskolin (1-30 microM), 8-bromoadenosine 3':5'-cyclic monophosphate (8-bromo cyclic AMP; 1-3 mM), 3-isobutyl-1-methylxanthine (0.3-1 mM) mimicked the conductance changes during the slow EPSC in a concentration-dependent manner. 7. It is concluded that the slow excitatory synaptic potential in the submucous plexus, presumably mediated by peptidergic transmitters, results from an inactivation of two distinct potassium currents, at least one of which is controlled by intracellular calcium ions.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Electrophysiological properties of cultured dog myocytes obtained by endomyocardial biopsy.

Right ventricular cardiac tissue (10-20 mg wet weight) was obtained from anesthetized adult dogs by endomyocardial biopsy. The biopsy could be repeated in one dog every 2 weeks for up to 3 months. Fifty to 200 cardiomyocytes, dispersed with collagenase and trypsin, were collected by centrifugation of the cells with 50% polysucrose-sodium diatrizoate solution (Ficoll-Paque). Single cardiomyocytes were suspended in a minimum essential medium containing 20% fetal bovine serum and 8-bromoadenosine 3': 5'-cyclic monophosphate (0.1 mM) for up to 3 weeks. Approximately 70-80% of the cultured cardiomyocytes were rod shaped after 24 hours (10-20% after 7 days). Cytoplasmic organelles of the cultured cells, examined with a transmission electron microscope, were within the normal range of canine heart morphology in vivo. Resting membrane potential of the cells was about -80 mV when superfused with a Krebs' solution containing 4.7 mM potassium ions. The action potential lasted for 300 msec and had a peak amplitude of about 120 mV. Voltage-clamp experiments demonstrated the presence of an inward calcium current (congruent to 0.9 nA at +9 mV), which was facilitated by isoproterenol (0.1-1 microMs). The background potassium current showed typical inward rectification at potentials more negative than -80 mV. The results indicate that morphological, electrophysiological, and pharmacological properties of the cultured cardiomyocytes were intact. We propose that the culture techniques we have developed can be useful for repeated investigation on functional aspects of cardiac muscles in myocardial disease.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1989

Delayed rectifier potassium current in dissociated bullfrog primary afferent neurons.

Cultured bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration. The classical delayed rectifier potassium current (IK) was separated from other ionic currents. Tetraethylammonium (1-50 mM) depressed the amplitude of IK in a concentration-dependent manner, a complete block occurring with 30 mM. With the concentration of potassium ions in the superfusate at 20 mM, the reversal potential of IK amounted to about -30mV. IK was activated between -30 and +70 mV. The half activation of IK occurred at +15 mV. The amplitude of IK was increased e-fold with 13.6 mV depolarization. The time constant of IK de-activation was shortened with membrane hyperpolarization (tau congruent to 4 ms at -100 mV). Finally, reciprocal time constant (tau -1) of the de-activating IK was increased e-fold with congruent to 13 mV hyperpolarization. It appears that the properties of IK in amphibian afferent neurons are comparable to those which have been observed with respect to the IK of the squid giant axons (Hodgkin and Huxley, 1952).

Action Potentials↗
PubMed1988

5-hydroxytryptamine inhibits cholinergic transmission through 5-HT1A receptor subtypes in rabbit vesical parasympathetic ganglia.

Intracellular recordings were made from parasympathetic neurons of the rabbit vesical pelvic ganglia (VPG) maintained in vitro. 5-Hydroxytryptamine (5-HT) inhibited cholinergic transmission in the VPG by reducing the fast excitatory postsynaptic potential (EPSP) evoked by stimulations of pelvic nerves. 8-Hydroxy-2-(di-n-propyl-amino) tetralin hydrochloride mimicked the inhibitory effect of 5-HT on the ganglionic transmission. 5-HT-induced inhibition of the fast EPSP was antagonized by spiperone. The results suggest that 5-HT1A receptor subtypes mediate the inhibition of cholinergic transmission in the rabbit VPG.

Animals↗
PubMed1988

Calcium-activated chloride conductance in parasympathetic neurons of the rabbit urinary bladder.

Intracellular recordings were made from vesical pelvic ganglion cells of the rabbit in a Krebs solution containing tetrodotoxin (1 microM). Experiments were carried out during complete suppression of the calcium-dependent potassium conductance by tetraethylammonium (greater than or equal to 20 mM) and/or intracellular injection of cesium ions. The action potential was followed by a depolarizing afterpotential which lasted for 0.3-10 s and had a peak amplitude of 5-20 mV at about -50 mV. The afterdepolarization (ADP) could not be observed when the preceding calcium-dependent action potential was blocked in a nominally calcium-free solution. Intracellular injection of ethyleneglycol-bis(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA) or total substitution of extracellular calcium ions with barium ions selectively blocked the ADP. The ADP, associated with an increased membrane conductance, reversed its polarity at -17 mV, when ganglion cells were impaled with microelectrodes filled with potassium chloride or cesium chloride. This reversal level was similar to that of the depolarization induced by gamma-aminobutyric acid. The reversal potential shifted to about -50 mV when acetate or sulphate were injected as counter anions. The peak amplitude and the total duration of the ADP was increased by substitution of external sodium chloride with sucrose or sodium isethionate. These results suggest that the ADP results from calcium entry during the spike and subsequent opening of chloride channels in parasympathetic neurons of the rabbit.

Action Potentials↗
PubMed1988

Calcium-dependent potassium conductance in neurons of rabbit vesical pelvic ganglia.

Intracellular recordings were made from neurons of vesical pelvic (parasympathetic) ganglia (VPG) isolated from the rabbit urinary bladder. Spontaneous hyperpolarizations (SH), occurring at intervals of 30 s to 5 min, could be recorded from 53% of VPG neurons in Krebs solution. The action potential was associated with inward sodium and calcium currents and was followed by fast and slow afterhyperpolarizations (AHPs). The action potential also evoked an additional hyperpolarization which was identical to the SH. The SH and the AHPs were associated with a decrease in the input resistance and reversed their polarity close to the potassium equilibrium potential. Intracellular cesium ions blocked the AHPs and the SH. Superfusing the preparation with a calcium-free solution produced a depolarization associated with an increased input resistance. The outward rectification activated at the resting membrane potential was depressed in the calcium-free solution. The removal of extracellular calcium ions also depressed both the SH and the spike AHPs. Bath-application of caffeine (1-3 mM) increased the frequency of the appearance of the SH. Injection of EGTA into VPG neurons caused a depolarization due to a blockade of the outward rectification. EGTA also depressed the slow AHP and the SH. These results suggest that the neuronal membrane of the rabbit VPG is endowed with a calcium-dependent potassium conductance (gKCa). Apamin (0.3-5 nM) and (+)-tubocurarine (30-300 microM) blocked the slow AHP and the SH without affecting the fast AHP and the resting membrane potential. Tetraethylammonium (TEA, 0.3-5 mM) suppressed the fast AHP and the SH without affecting the outward rectification. TEA augmented the slow AHP. Barium ions (0.1-1 mM) depressed the AHPs, the SH and the outward rectification. These pharmacological properties imply that at least 3 kinds of gKCa systems underlie the generation of the outward rectification, the spike AHPs and the SH.

Action Potentials↗
PubMed1988

Norepinephrine inhibits calcium action potential through alpha 2-adrenoceptors in rabbit vesical parasympathetic neurons.

Intracellular and voltage-clamp recordings were made from neurons in rabbit vesical parasympathetic ganglia (VPG) maintained in vitro. Norepinephrine (NE, 10 nM-10 microM) reduced the Ca2+ component of the action potential and the afterhyperpolarization. Clonidine and UK14304, the selective alpha 2-adrenoceptor agonists, mimicked the inhibitory effects of NE on the action potential. NE and UK14304 blocked the Ca2+ spike elicited in the presence of tetrodotoxin and tetraethylammonium. UK14304 suppressed the inward Ca2+ current induced by depolarizing step command under the voltage-clamp condition. These inhibitory actions were antagonized by yohimbine and idazoxan but not by prazosin and propranolol. It is suggested that alpha 2-adrenoceptors mediate the inhibition of voltage-dependent Ca2+ entry during the action potential.

Action Potentials↗
PubMed1988

Action of naloxone on myenteric neurons removed from morphine-treated guinea pigs.

Guinea pigs were treated chronically with morphine by implantation of pellets and the ileum was later removed for experiments in vitro. Intracellular recording from myenteric neurons showed that chronic morphine treatment led to the development of an increased calcium-dependent afterhyperpolarization, whether this followed an action potential or the depolarization resulting from activation of nicotinic acetylcholine receptors. The properties of the neurons were otherwise normal. Naloxone depolarized about one-third of neurons and led to the appearance of spontaneous fast excitatory postsynaptic potentials in many cells. Extracellular recordings from neurons induced to fire with suction electrodes showed that the excitation caused by naloxone was prevented in solutions that contained sufficient cobalt to block synaptic transmission. Measurements of longitudinal muscle tension of intact pieces of ileum showed that the contracture caused by naloxone was much reduced by prior exposure of the ileum in vitro for 30 min to beta-funaltrexamine. The beta-funaltrexamine appeared to have irreversibly blocked a significant fraction of the mu receptors because the effectiveness of Tyr-D-Ala-Gly-MePhe-Gly-ol to inhibit nerve-evoked contractile responses (twitches) was much reduced in tissues treated with beta-funaltrexamine. These results indicate that at least a portion of the contracture results from an action of naloxone at kappa rather than mu receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1988

Inward rectification of resting and opiate-activated potassium currents in rat locus coeruleus neurons.

Intracellular recordings were made from rat locus coeruleus neurons in vitro, and membrane currents were measured at potentials from -50 to -130 mV. In the absence of any applied agonists, the slope conductance of the cells increased 3-fold when the cell was hyperpolarized from -60 to -120 mV. This conductance increase was complete within 5 msec of the onset of a hyperpolarizing command and was subsequently independent of time for several seconds. The conductance increase was blocked by cesium chloride (1-2 mM), rubidium chloride (1-2 mM), or barium chloride (1-100 microM). The membrane potential range over which the conductance increased was centered at the potassium equilibrium potential (EK; extracellular potassium concentration, 2.5-10.5 mM): the current/voltage (I/V) relation of the cell could be well described by supposing that there were 2 potassium conductances, one voltage independent (G1) and the other (inward rectifier, Gir) activated according to the expression Gir = Gir,max/(1 + exp[(V - EK)/k]), where k ranged from 15 mV in 2.5 mM potassium to 6 mV in 10.5 mM potassium. The additional membrane potassium conductance that developed when agonists at mu-opioid and alpha 2-adrenoceptors were applied also became larger with membrane hyperpolarization, and this voltage dependence was also reduced or blocked by rubidium, cesium, and barium; in the presence of these agonists the current also reached its final value within 5 msec. However, the conductance increased by the agonists (Gag) was not well expressed by simply increasing the values of G1 and Gir,max. It was best described by a potassium conductance that increased according to Gag,max/(1 + exp[(V - Vm)/k]), where Vm (the potential at which the conductance was half-maximum) was close to the resting potential of the cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenergic alpha-Agonists↗
PubMed1988

Postsynaptic modulation of cholinergic transmission by endogenous substances.

1. Recent concept of postsynaptic modulation is reviewed on the basis of literature data and the results of our investigation using conventional intracellular and voltage-clamp recording methods, in vitro. 2. Experimental evidence provided that the sensitivity of nicotinic ACh receptors endowed on the postsynaptic membrane of the bullfrog sympathetic ganglia and of the frog skeletal muscle end-plate is either facilitated or inhibited by other neurotransmitters or neurohormones. 3. We propose that one neurotransmitter not only initiates its own postsynaptic potential but also regulates the efficacy of synaptic transmission mediated by a distinct neurotransmitter, as an endogenous "antagonist" or "sensitizer".

Animals↗
PubMed1987

Effects of three mammalian tachykinins on single enteric neurons.

The effects of substance P (SP), substance K (SK) and neuromedin K (NMK) were compared on single neurons of the guinea pig myenteric plexus. The tachykinins depolarized all myenteric neurons with a rank order of potency SP greater than SK greater than NMK. In approximately 80% of neurons studies the depolarization was associated with an increase in membrane resistance: in 20% membrane resistance decreased. The depolarizations were not affected by tetrodotoxin (TTX) (300 nM), hyoscine (1 microM) or by [D-Arg1, D-Pro2, D-Trp7,9, Leu11]SP (1 microM). These results provide no evidence for more than one type of tachykinin receptor on myenteric neurons.

Animals↗
PubMed1987

Mianserin blocks alpha 2 adrenoceptors in submucous neurones of the guinea-pig caecum.

Intracellular recordings were made from submucous plexus neurones of the guinea-pig caecum in vitro. The peak amplitude of the adrenergic inhibitory postsynaptic potential (IPSP) was depressed by mianserin in a dose-dependent manner (300 nM-100 microM). This was due to a direct blockade of postsynaptic alpha 2 adrenoceptors. The nicotinic excitatory postsynaptic potential (EPSP) and the non-cholinergic EPSP were not affected by mianserin (100 microM). The presynaptic inhibition of the release of acetylcholine, mediated by presynaptic alpha 2 receptors, was also blocked by mianserin (30 microM). The results suggest that mianserin antagonizes both pre- and post-synaptic alpha 2 adrenoceptors in enteric plexus neurones.

Adrenergic alpha-Antagonists↗
PubMed1987

Postsynaptic inhibition of the frog neuromuscular transmission by prostaglandin E1.

Intracellular recordings were made from the frog sartorius muscle end plate. Prostaglandin (PG) E1 (100 nM-10 microM) decreased the amplitude of the end plate potential (EPP). PGE1 decreased the quantal size of EPPs, while it rather increased the quantum content. The frequency of miniature (m) EPPs was not affected by PGE1. PGE1 depressed the acetylcholine (ACh)-induced depolarization, as well as the amplitude of mEPPs. These results suggest that PGE1 decreases the sensitivity of nicotinic ACh receptors at the end plate membrane, resulting in postsynaptic depressions of neuromuscular transmission.

Animals↗
PubMed1987

Activation of 5-HT3 receptor subtypes causes rapid excitation of rabbit parasympathetic neurones.

Intracellular recordings were made from parasympathetic neurones of the rabbit vesical pelvic ganglia maintained in vitro. 5-Hydroxytryptamine (5-HT) caused a membrane depolarization which was antagonized by ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester) but not by methysergide. ICS 205-930 caused a parallel shift to the right of the dose-response curve for 5-HT. These results suggest that the 5-HT3 receptor is involved in the membrane depolarization.

Animals↗
PubMed1987

Persistent calcium-sensitive potassium current and the resting properties of guinea-pig myenteric neurones.

1. Intracellular recordings were made from neurones in the guinea-pig myenteric plexus which show a long-lasting after-hyperpolarization following the action potential (AH neurones). In most experiments membrane currents were measured using a single-electrode voltage clamp. Tetrodotoxin was present. 2. A step depolarization (10-15 mV, 5-10 s) from a holding potential close to the resting level (typically -60 mV) caused a slowly developing outward current. The current increased exponentially with a time constant of about 1.3 s at -50 mV. At the termination of the step the current declined over a similar time period (time constant congruent to 2.5 s). Hyperpolarizing step commands resulted in a slowly declining outward current (tau congruent to 3.5 s at -70 mV) which developed again at the termination of the hyperpolarization (tau congruent to 2.1 s at -60 mV); these tail currents reversed at the potassium equilibrium potential. 3. The current was not observed in solutions containing no calcium, high magnesium concentrations and cobalt. It did not inactivate during changes in holding potential of up to 5 min. The current is therefore called the persistent calcium-sensitive potassium current. 4. A brief depolarizing command to less-negative potentials (typically to -10 mV for 10-30 ms) was followed by a potassium current which increased and decreased according to the sum of two exponentials having time constants tau on congruent to 0.4 s and tau off congruent to 2.5 s. This after-current disappeared in calcium-free, high-magnesium and cobalt solution. 5. Both the after-current and the persistent calcium-sensitive current were similarly sensitive to tetraethylammonium ions, being unaffected by 5 mM but substantially reduced by 20 mM solutions. The time constant of decline of the persistent calcium-sensitive current at the end of a depolarizing step was not different from tau off for the after-current; these time constants had a similar sensitivity to voltage and temperature. 6. The conductance underlying the after-current became progressively smaller as the persistent calcium-sensitive current was increased by membrane depolarization. In a given neurone, the sum of the two conductances was constant. This finding implies that the persistent calcium-sensitive potassium conductance is the same conductance as that which is increased during the after-hyperpolarization.

Action Potentials↗
PubMed1985

Spontaneous muscarinic suppression of the Ca-activated K-current in bullfrog sympathetic neurons.

Neurons in bullfrog sympathetic ganglia were voltage-clamped using a single microelectrode. A prolonged outward current which was identified as the Ca-activated K-current secondary to a transient Ca entry through voltage-operated channels was shortened by oxotremorine. An inward Ca-current was not significantly depressed by oxotremorine. It was suggested that muscarinic agonists accelerate the re-closure of K-channels either directly or secondarily via their effects on an intracellular sequestration process of Ca ions. It was also suggested that a small amount of acetylcholine only sufficient to cause a miniature synaptic current via nicotinic receptors could shorten the Ca-activated K-current via muscarinic receptors.

Animals↗