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T Tokimasa

Publications and source records attributed to T Tokimasa.

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

Effects of 1, 9-dideoxy forskolin on delayed rectifier potassium current of bullfrog dorsal root ganglion cells.

Whole-cell voltage-clamp recordings were made from cultured bullfrog dorsal root ganglion cells to examine the channel blocking activity of 20 microM of 1, 9-dideoxy forskolin (d-FSK) on the delayed rectifier potassium current. The drug reduced the maximum amplitude of the current (10-20 nA at +65 mV) by 40-80%. However, the voltages for 50%-activation (approximately -12 mV) and for 50%-inactivation (approximately -37 mV) remained unaffected by d-FSK. These results indicate that the channel block by d-FSK occurs in a voltage-independent manner.

Animals↗
PubMed1994

Cyclic ADP-ribose modulates Ca2+ release channels for activation by physiological Ca2+ entry in bullfrog sympathetic neurons.

Although Ca(2+)-induced Ca2+ release (CICR) via ryanodine receptors has been found to occur in intact neurons, little is known about the physiological processes that regulate it. We studied the effects of cyclic ADP-ribose (cADPR) on CICR in cultured bullfrog sympathetic neurons by fura-2 fluorescence recording and patch-clamp techniques. cADPR applied through a patch pipette augmented action potential- or depolarizing pulse-induced rises in intracellular Ca2+ without a change in Ca2+ entry initiating the responses, but not in the presence of ryanodine. Likewise, cADPR enhanced a single or oscillatory rise(s) in intracellular Ca2+ induced by caffeine. These results strongly suggest that cADPR can be an endogenous modulator of ryanodine receptors in neurons.

Adenosine Diphosphate Ribose↗
PubMed1993

Intracellular ATP changes the voltage-dependence of delayed rectifier potassium current in bullfrog primary afferent neurons.

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration to study the steady-state activation and inactivation curves for a delayed rectifier potassium current. The 50%-activation of the current occurred at +15 mV when measured with ATP (5 mM) in the pipette solution as opposed to -11 mV with 5'-adenylylimidodiphosphate (AMP-PNP, 5 mM) and -15 mV with adenosine 5'-O-(3-thiotriphosphate) (5 mM). The 50%-inactivation of the current occurred at -6 mV with ATP but at -31 mM with AMP-PNP. The results suggest that intracellular ATP modulates voltage-dependence of the delayed rectifier in amphibian afferent neurons.

Adenosine Triphosphate↗
PubMed1993

Chemosensitivity of C-cells in bullfrog dorsal root ganglia to substance P and adenosine 5'-triphosphate.

Dissociated bullfrog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. In small C-cells having 20 microns as averaged diameter, substance-P (0.1-1 microM) inhibited an M-type potassium current while ATP (1-10 microM) activated a sodium-potassium current. In large A-cells (approximately 65 microns in diameter) in which ATP has been shown to inhibit M-current, substance P (0.1-1 microM) also inhibited this potassium current without activating the sodium-potassium current. Results provided evidence for the distinction between A- and C-cells in terms of their chemosensitivity.

Adenosine Triphosphate↗
PubMed1993

Myosin light chain kinase occurs in bullfrog sympathetic neurons and may modulate voltage-dependent potassium currents.

A polyclonal antibody against myosin light chain kinase (MLCK) of chicken gizzard recognized a 130 kd peptide of bullfrog sympathetic ganglia as MLCK. MLCK immunoreactivity was confined to the neuronal cell body. A synthetic peptide corresponding to an inhibitory domain of MLCK (Ala783-Gly804) was applied intracellularly to isolated sympathetic neurons during whole-cell recordings of ionic currents. The peptide inhibitor reversibly decreased M-type potassium current (IM) while not affecting A-type of delayed rectifier-type potassium currents. Intracellular application of an active fragment of MLCK enhanced IM, whereas application of an inactive MLCK fragment did not. The results suggest that IM can be modulated by MLCK-catalyzed phosphorylation.

Amino Acid Sequence↗
PubMed1992

Cellular metabolism regulating H and M currents in bullfrog sympathetic ganglia.

Much evidence has accumulated suggesting that neurons in autonomic and dorsal root ganglia possess voltage-dependent currents that link with transmitter receptors through intracellular signal transduction systems. The M current (IM), a voltage-dependent potassium current, was activated at potentials more positive than -65 mV, while the H current (IH), a voltage-dependent nonselective cationic current, was activated at potentials more negative than -50 mV. The hydrolyzable form of ATP was required to activate IM and IH. Intracellular application of calmodulin enhanced the amplitude of IM in a calcium-dependent manner. IM was reduced by W-7, a calmodulin antagonist, and by ML-9, an inhibitor of calmodulin-dependent protein kinase. IH was enhanced by intracellular loading with cyclic adenosine monophosphate (AMP) or bath application of forskolin and membrane-permeable cyclic AMP analogues. Isobutylmethylxanthine also increased the maximal conductance of IH. IH was depressed by H-8 but not by phorbol ester. It is concluded that the resting membrane conductance of these ganglion cells can be regulated by basal activities of calmodulin-dependent protein kinase and A kinase.

Animals↗
PubMed1991

Slowly inactivating potassium current in cultured bull-frog primary afferent and sympathetic neurones.

1. Cultured bull-frog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. The cells were superfused with a nominally calcium-free Ringer solution containing tetrodotoxin (3 microM), magnesium (10 mM), cobalt (1 mM), barium (2 mM), 4-aminopyridine (3 mM) and caesium (2 mM). 2. Step depolarizations (10-40 mV, 100-300 ms) from a holding potential close to the rest (typically -70 mV) evoked an outward current (IK) followed by an outward tail current. The peak amplitude of the current was reduced to less than 10% by tetraethylammonium (30 mM). 3. IK developed to its peak in 200 ms at -30 mV. Tail currents reversed at potentials that changed according to the logarithm of the extracellular potassium concentrations. 4. Tail currents declined to the baseline according to an exponential function of time (tau congruent to 40 ms at -60 mV) and its reciprocal time constant increased e-fold with a 13 mV hyperpolarization. 5. The current inactivated during sustained (1-20 s) depolarizing pulses according to a single exponential function (tau congruent to 3 s). 6. The peak amplitude of IK at -30 mV was progressively increased as the holding potential was made more negative than -70 mV reaching the maximum with step depolarizations from -120 mV. Reversed phenomenon was observed as the holding potential was made less negative than -70 mV. 7. The removal of the steady-state inactivation occurred along with a single exponential function and the time constant was decreased from 70 ms at -70 mV to 10 ms at -120 mV. 8. It is suggested that a slowly inactivating potassium current which we called IK in amphibian sensory neurones could be a class of a 'delayed' rectifier potassium current. A potassium current with properties indistinguishable from those which have been described for the sensory IK also occurred in cultured bull-frog sympathetic neurones. 9. Forskolin (1-30 microM) and 1,9-dideoxy forskolin (10 microM) reduced the amplitude of IK by up to 85% but these actions were not mimicked by any of 8-bromo-cyclic AMP (1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (1 mM). A hydrophilic forskolin analogue, 7-O-hemisuccinyl-7-deacetyl forskolin (10 microM), was about one-tenth as potent as forskolin (10 microM).

Animals↗
PubMed1991

A slow calcium-dependent chloride current in rhythmic hyperpolarization in neurones of the rabbit vesical pelvic ganglia.

1. Voltage-clamp recordings were made from neurones of vesical pelvic ganglia isolated from the rabbit urinary bladder. A rhythmic outward current, ISH, which corresponds to the spontaneous hyperpolarization, occurred at fairly constant intervals in fifty-eight of eighty-four neurones superfused with Krebs solution. The peak amplitude of the ISH was 0.5 +/- 0.2 nA (n = 48; mean +/- S.E.M.). 2. The ISH was eliminated in a Krebs solution containing nominally zero calcium and 12 mM-magnesium. Lowering the temperature of the superfusing solution from 36 to 22 degrees C also inhibited the occurrence of the ISH. 3. Bath application of caffeine increased the frequency of ISH. In contrast, ryanodine and procaine reversibly blocked ISH. 4. In thirty-four of fifty-eight neurones, the ISH was composed of two current components, an initial fast ISH with duration of 1-10 s and a slow ISH lasting 15-60 s. In the remaining twenty-four neurones, ISH showed only the fast component. 5. The fast ISH was associated with an increased membrane conductance and the slow ISH was associated with a decreased membrane conductance. The reversal potentials of the fast and the slow ISH were -88 +/- 7 mV (n = 4) and -30 +/- 6 mV (n = 4), respectively. 6. Tetraethylammonium (5 mM) and barium (1 mM) blocked the fast ISH but not the slow ISH. Intracellular caesium injected by ionophoresis through a Cs(+)-filled microelectrode blocked the fast ISH, without affecting the slow ISH. Apamin and (+)-tubocurarine selectively suppressed the fast component of the ISH. 7. Substitution of isethionate (67 mM) for chloride increased the amplitude of the slow ISH and shifted the reversal potential of the slow ISH to +1 +/- 8 mV (n = 5). A slow ISH with amplitude of 0.1-1 nA and was still observed in a low-sodium (26.2 mM) solution. The stilbene derivative, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS), a chloride channel blocker, suppressed the slow ISH. 8. These results suggest that ISH is composed of two distinct calcium-dependent currents, a fast ISH produced by activation of potassium conductance and a slow ISH produced by inactivation of chloride conductance. 9. The after-hyperpolarization (AHP) following the action potential was also composed of apamin-sensitive and insensitive spontaneous hyperpolarizing oscillations. The apamin-insensitive component of IAHP was increased by lowering external chloride activity, while it was depressed by SITS.

Animals↗
PubMed1990

Morphological and electrophysiological properties of C-cells in bullfrog dorsal root ganglia.

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration. Small spheroidal C-cells had a mean diameter of 14-30 microns and shared about 10% of the total population of the cells. The C-cells were characterized by a prominent calcium-activated potassium current underlying a hyperpolarization following the action potential. In contrast, a hyperpolarization-activated cationic inward rectifier was missing in all C-cells tested. These properties were completely different from those which have been observed for large spheroidal A-cells.

Animals↗
PubMed1990

Extracellular calcium ions are required for muscarine-sensitive potassium current in bullfrog sympathetic neurons.

Cultured bullfrog sympathetic neurons were voltage-clamped in the whole-cell configuration. The extracellular medium contained tetrodotoxin (3 microM) and cesium (1 mM) to block and inward sodium current and a hyperpolarization-activated cation current Attempts were made to separate the M-current from four other potassium currents. Tetraethylammonium (30 mM) was used to block a classical delayed rectifier current (IK) and a fast calcium-activated current (IC). Apamin (30 nM) was used to block a slow calcium-activated current (IAHP). 4-Aminopyridine (1 mM) was used to reduce the amplitude of a transient current (IA). In these conditions, the maximum M-conductance near 0 mV was reduced by as much as 90% when divalent cations such as cobalt (1 mM) were added to the superfusate. The maximum M-conductance was also reduced by as much as 60% when calcium ions were removed from the superfusate. The half-activation voltage in the steady-state activation curve and the reversal potential of the M-current were not significantly changed in the calcium-free solution. It is suggested that the presence of calcium ions in the extracellular space is required for the M-current activation.

Adrenergic Fibers↗
PubMed1990

Volatile anaesthetics inhibit a cyclic AMP-dependent sodium-potassium current in cultured sensory neurones of bullfrog.

1. Cultured dorsal root ganglion cells of the bullfrog were voltage-clamped in the whole-cell configuration. 2. An adenosine 3':5'-cyclic monophosphate (cyclic AMP)-dependent cationic inward rectifier (IH) was inhibited by bath application of enflurane (0.2-0.8 mM) and halothane (0.2-0.5 mM), which thereby induced an outward current at the resting potential, and a membrane hyperpolarization in unclamped cells. 3. The main effect of enflurane (0.5 mM) was to displace the steady-state IH activation curve to a hyperpolarizing direction by about 10 mV, as well as to reduce the maximum H-conductance to about 20%. 4. Forskolin (1-10 microM), which enhances IH by producing a depolarizing shift in the IH activation curve and increasing the maximum H-conductance, recruited IH even when the current had already been eliminated by enflurane (1 mM).

Anesthetics↗
PubMed1990

Cyclic AMP regulates an inward rectifying sodium-potassium current in dissociated bull-frog sympathetic neurones.

1. Bull-frog sympathetic neurones in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 2. A hyperpolarization-activated sodium-potassium current (H-current: IH) was separated from other membrane currents in a nominally calcium-free solution containing cobalt (2 mM), magnesium (4 mM), barium (2 mM), tetraethylammonium (20 mM), tetrodotoxin (3 microM), apamin (30 nM) and 4-aminopyridine (1 mM). IH was selectively blocked by caesium (10-300 microM). 3. The steady-state activation of IH occurred between -60 and -130 mV. The H-conductance was 4.1-6.6 nS at the half-activation voltage of -90 mV. With the concentrations of potassium and sodium ions in the superfusate at 20 and 70 mM, respectively, the reversal potential of IH was about -20 mV. IH was activated with a time constant of 2.8 s at -90 mV and 22 degrees C. The Q10 between 16 and 26 degrees C was 4.3. 4. A non-hydrolysable ATP analogue in the pipette solution did not support IH activation. Intracellular 'loading' of GTP-gamma-S (30-500 microM) led to a progressive activation of IH. 5. Forskolin (10 microM) increased the maximum conductance of IH by 70%. This was associated with a depolarizing shift in the half-activation voltage (5-10 mV) and in the voltage dependence of the activation/deactivation time constant of IH. 6. Essentially the same results as with forskolin were obtained by intracellular 'loading' with cyclic AMP (3-10 microM) or bath application of 8-bromo cyclic AMP (0.1-1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (0.1-1 mM). 7. The protein kinase inhibitor H-8 (1-10 microM) decreased the peak amplitude of IH. Phorbol 12-myristate 13-acetate (10 microM), a protein kinase C activator, was without effect. 8. It is concluded that a voltage-dependent cation current can be regulated by the basal activity of adenylate cyclase, presumably through protein kinase A, in vertebrate sympathetic neurones.

1-Methyl-3-isobutylxanthine↗
PubMed1990

Calcium-dependent chloride current in neurones of the rabbit pelvic parasympathetic ganglia.

1. Voltage-clamp recordings were made from neurones in rabbit vesical pelvic ganglia by using single microelectrodes filled with 2 M-caesium chloride. Neurones were superfused with Krebs solution containing 300 nM-tetrodotoxin and 50 mM-tetraethylammonium. 2. Depolarizing voltage jumps activated inward currents followed by slowly decaying inward tail currents at -30 to +30 mV, which were accompanied by a large increase in membrane conductance. Both the inward current and tail current were blocked by cobalt (2 mM) or in a Krebs solution containing zero calcium and 12 mM-magnesium. 3. Substitution of barium for calcium enhanced the inward current, while it strongly reduced the tail current. Strontium substitution still exhibited both the inward current and the tail current. 4. Lowering external chloride activity increased the tail current amplitudes without affecting an initial calcium current. The reversal potentials of the tail current, measured using a twin-pulse protocol, were -18 +/- 5 mV (mean +/- S.E.M., n = 8) and +5 +/- 3 mV (n = 5) in Krebs solution and low-chloride (62 mM) solution, respectively, suggesting a calcium-dependent chloride current. 5. Stilbene derivatives, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS, 0.01-1 mM) and 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 0.01-1 mM), reversibly and concentration dependently depressed the tail current without affecting the calcium current. 6. Transient (T) and sustained (N and L) types of calcium current were likely to co-exist in neurones of the rabbit pelvic ganglia. Calcium-dependent chloride current was activated by N- and L-type calcium currents but not by T-type current. 7. Activation of the tail current at 0 to +20 mV was described by a single-exponential function. The tail current decayed exponentially at a holding membrane potential of -70 mV. Tail decay time constants were dependent on voltage and duration of the step command. 8. Substantial activation of the calcium-dependent chloride conductance could occur during a post-tetanic after-potential when pelvic ganglia neurones fired action potentials repetitively.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1990

ATP regulates muscarine-sensitive potassium current in dissociated bull-frog primary afferent neurones.

1. Bull-frog dorsal root ganglion cells in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 2. Step depolarizations (5-70 mV, 0.1-1 s) from a holding potential close to the resting potential (range, -64 to -79 mV) evoked a non-inactivating potassium current with properties indistinguishable from those which have been reported for the M-current of bull-frog sympathetic neurones. 3. An unhydrolysable ATP analogue APP(NH)P (5 mM), substitute with ATP in the pipette solution, did not support the M-current activation. 4. Bath application of ATP (30 nM-30 microM) reduced the amplitude of the M-current in a concentration-dependent manner, congruent to 50% inhibition of the current occurring with 1 microM-ATP. The main effect of ATP was to reduce the maximum M-conductance without changing the activation and deactivation kinetics of the M-current. 5. Essentially the same results were obtained with ADP (0.1-30 microM) and alpha, beta-methylene-ATP (10-30 microM). AMP (10-100 microM) and adenosine (10-30 microM) were without effect on the M-current. 6. The ATP-induced inhibition of the M-current was irreversible when an unhydrolysable GTP analogue GTP-gamma-S (10-30 microM) was present in the pipette solution. ATP (3 microM) reduced the amplitude of the M-current only by about 10% when GDP-beta-S (100 microM) was present in the pipette solution. Pre-treatment of the cells with pertussis toxin (IAP; 500 ng ml-1) for 24 h at 24 degrees C did not prevent the ATP-induced M-current inhibition. 7. Phorbol 12-myristate 13-acetate (PMA; 1-3 microM) reduced the amplitude of the M-current to about 50%. A reduction in the M-current amplitude by PMA (3 microM) and ATP (10 microM) was attenuated when staurosporine (200 nM) was present in the pipette solution. Forskolin (10 microM) was without effect on the M-current. 8. It is concluded that ATP acting at P2 receptors, associated with an IAP-insensitive GTP-binding protein, inhibits the M-current in amphibian primary afferent neurones.

Adenosine Diphosphate↗
PubMed1990

Reduction of the N-type calcium current by noradrenaline in neurones of rabbit vesical parasympathetic ganglia.

1. Intracellular and single-electrode voltage-clamp recordings were made from neurones of vesical parasympathetic ganglia (VPG) isolated from the rabbit urinary bladder. 2. Noradrenaline (NA, 0.5-5 microM) shortened the duration of the action potentials and depressed the amplitudes of both spike after-hyperpolarization and after-current. 3. Voltage-dependent calcium currents (ICa) were recorded by using microelectrodes filled with 2 M-caesium chloride in a superfusing solution containing tetraethylammonium (TEA, 50 mM) and tetrodotoxin (TTX, 500 nM). Noradrenaline (0.5-5 microM) depressed both the ICa and the tail current evoked by depolarizing voltage jumps from -100 to -50 mV to -30 to +20 mV. 4. Substitution of barium for calcium also produced an inward current (IBa) with no obvious tail current. Noradrenaline (1 microM) reduced the magnitude of the IBa without affecting the voltage dependence of the current-voltage relationship for IBa. 5. Yohimbine (1 microM), but not prazosin (1 microM) or propranolol (1 microM), antagonized the NA-induced inhibition of the IBa. UK 14304, a potent alpha 2-adrenoceptor agonist, mimicked NA in depressing the IBa. 6. The transient low-threshold (T), the transient high-threshold (N) and the slowly inactivating high-threshold (L) calcium currents co-existed in VPG neurones. 7. Noradrenaline reduced the IBa evoked at clamp potentials more positive than -20 mV from holding potentials near the resting membrane potential (-70 to -50 mV). Under these conditions, the IBa consisted primarily of N- and L-current components. In contrast, NA had no effect on the isolated T- and L-currents. It is concluded that NA selectively inhibits the N-type calcium channels by an action at alpha 2-adrenoceptors in the rabbit VPG neurones.

Action Potentials↗
PubMed1989

Modulation of the sensitivity of nicotinic receptors in autonomic ganglia.

This article reviews some of the evidence suggesting that a variety of endogenous substances either facilitates or inhibits the sensitivity of nicotinic acetylcholine (ACh) receptors at the subsynaptic membrane of cholinergic synapses. It is noteworthy that 5-hydroxytryptamine and histamine act as competitive antagonists, like curare, presumably changing the affinity of ACh for the specific binding site on the nicotinic receptor. Catecholamine, neuropeptides, prostaglandin and glucocorticoids act as non-competitive antagonists on an allosteric site on the receptor-ionic channel complex. ATP and LH-RH (in a subpopulation of sympathetic neurons) caused a facilitation of the sensitivity of nicotinic receptors. The mode of actions of endogenous substances which modulate the nicotinic receptor-sensitivity is similar to those of pharmacological agents. Therefore, these neurotransmitters and neurohormones have been termed endogenous 'antagonists' or 'sensitizers'.

Acetylcholine↗
PubMed1989

Histamine H2 receptor mediates postsynaptic excitation and presynaptic inhibition in submucous plexus neurons of the guinea-pig.

Intracellular recordings were made from submucous plexus neurons of the guinea-pig cecum maintained in vitro. Histamine (0.3-10 microM) produced a dose-dependent membrane depolarization (congruent to 13 mV with 3 microM) in about 28% of the cells tested; most of these cells showed a prominent calcium-activated potassium conductance (AH cells). The depolarization was due primarily to an inactivation of potassium conductance which is available at the resting membrane potential of -60 mV. Peak amplitude of the fast excitatory postsynaptic potential was depressed by histamine (0.1-10 microM) in a dose-dependent manner (congruent to 62% depression with 1 microM). This was observed even in those cells in which histamine did not produce any membrane depolarizations (mostly S cells). The depression of the fast excitatory postsynaptic potential resulted from the presynaptic inhibition of acetylcholine release. Histamine also reduced the amplitude of the non-cholinergic, presumably peptidergic, slow excitatory postsynaptic potential by suppressing peptide release from presynaptic nerve terminals. Peak amplitude of the adrenergic inhibitory synaptic potential was not depressed by histamine suggesting that histamine receptors are not present on presynaptic terminals of sympathetic nerve fibres. Both postsynaptic and presynaptic actions of histamine were blocked by cimetidine or ranitidine but not by pyrilamine implying that H2 receptors are involved.

Action Potentials↗