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T Shimahara

Publications and source records attributed to T Shimahara.

At least 37 records · Page 2Linked to original sources
PubMed1992

Charge movement and Ca2+ release in normal and dysgenic foetal myotubes.

Intramembrane charge movement and Ca2+ release from sarcoplasmic reticulum was studied in foetal skeletal muscle cells from normal and mutant mice with 'muscular dysgenesis' (mdg/mdg). It was shown that: 1) unlike normal myotubes, in dysgenic myotubes membrane depolarization did not evoke calcium release from the sarcoplasmic reticulum; 2) when all ionic currents are pharmacologically suppressed, membrane depolarization produced an asymmetric intramembrane charge movement in both normal and dysgenic myotubes. The relationship between the membrane potential and the amount of charge movement in these muscles could be expressed by a two-state Boltzmann equation; 3) the maximum amount of charge movement associated with depolarization (Qon max) in normal and in dysgenic myotubes was 6.3 +/- 1.4 nC/microF (n = 6) and 1.7 +/- 0.3 nC/microF (n = 6) respectively; 4) nifedipine (1-20 microM) applied to the bath reduced Qon max by about 40% in normal muscle cells. In contrast, the drug had no significant effect on the charge movement of dysgenic myotubes; and 5) the amount of nifedipine-resistant charge movement in normal and in dysgenic myotubes was 3.5 nC/microF (n = 3) and 1.7 nC/microF 1 maximum (n = 3), respectively.

Animals↗
PubMed1992

Ciguatoxin-induced changes in acetylcholine release and in cytosolic calcium levels.

In this communication we summarize our current knowledge concerning the mode of action of ciguatoxin (CTX) on acetylcholine (ACh) release either from motor nerve terminals or from pure cholinergic synaptosomes. The results obtained indicate that CTX affects Ca(2+)-dependent ACh release via distinct actions mediated by Na+ which alter presynaptic excitability and Ca2+ influx through both voltage-sensitive channels and the reversed operation of the Na+/Ca2+ exchange system. The external calcium-independent ACh release induced by CTX in motor terminals seems to be due to a Na(+)-dependent and tetrodotoxin-sensitive mechanism which mobilizes Ca2+ from intraterminal stores, as determined by fluorometrical recordings in single mouse neuroblastoma x rat glioma NG108-15 cells.

Acetylcholine↗
PubMed1991

Barium currents in developing skeletal muscle cells of normal and mutant mice foetuses with 'muscular dysgenesis'.

The ontogenesis of Ca channel activities was studied in the developing myotubes of normal mice and mutant mice foetuses with 'Muscular Dysgenesis'. The ionic current through Ca channels was measured with Ba2+ as charge carrier using the whole cell clamp technique. All dissociated myotubes from foetuses (14th to 18th day of gestation) showed two distinct inward Ba currents: a low threshold, transient current (T-type) and a high threshold sustained current. In normal myotubes, T-type current density increased from the 14th day to the 16th day of gestation. After day 16, T-type current density decreased gradually until birth. Similar changes in T-type current density were observed in developing dysgenic myotubes where the current density was about 40% of that measured in normal myotubes throughout the prenatal period studied. The high threshold sustained current (L-type current) density increased gradually with age in normal myotubes while absent in dysgenic muscle. The latter, regardless of age, showed a high threshold current (Idys) which is distinct from the L-type current. Idys density did not change during the prenatal myogenesis period studied.

Animals↗
PubMed1990

Mobilization of intracellular Ca2+ and suppression of inward currents in a neuronal hybrid cell line triggered by bradykinin.

Bradykinin triggered intracellular Ca mobilizations and ionic conductance changes were studied in the neuroblastoma x glioma hybrid cell line NG108-15 using Ca-sensitive fluorescent indicator fura-2 under patch pipette whole cell voltage clamp condition. The time course of outward current induced by bradykinin was closely related to the time-course of [Ca2+]i change. Following application of bradykinin, [Ca2+]i increased transiently and then decreased below the basal level before bradykinin application. The inward currents activated by step-depolarization were suppressed after bradykinin application, but the time-course of the suppression did not go in parallel with the [Ca2+]i changes: the suppression started before the [Ca2+]i change emerged and outlasted the phase of [Ca2+]i increase. Both transient type and long-lasting type Ca current were suppressed by bradykinin. [Ca2+]i increase induced by high potassium depolarization was suppressed by bradykinin. Pertussis toxin did not affect the Ca transient nor the suppression of Ca channel induced by bradykinin. Our results suggest that the modifications of ionic channels by bradykinin could be through the other mechanisms than the well established activation of the G-protein leading to the IP3 mechanisms and that the bradykinin receptor might couple with the pertussis toxin-insensitive G protein which regulates the calcium channels.

Animals↗
PubMed1990

Reduced intramembrane charge movement in the dysgenic skeletal muscle cell.

Intramembrane charge movement in skeletal muscle cells has been proposed to underlie the process leading to Ca release from the sarcoplasmic reticulum. A number of recent studies suggest that the dihydropyridine receptor located in the transverse-tubular membrane is responsible for the generation of intramembrane charge movement. The skeletal muscle cell of the mutant mouse with "Muscular Dysgenesis" is characterized by absence of excitation-contraction coupling. Here we investigated the charge movement in freshly dissociated skeletal muscle cells from dysgenic mice. In 9 out of 34 dysgenic mouse cells the charge movement was completely absent, in the remaining cells the charge movement was never more than 30% of control. The amount of maximum charge movement (Qmax) in mutant muscle cells was less than 30% of Qmax in normal muscle. Nifedipine, a dihydropyridine derivative, reduced the amount of charge movement in normal muscle cells but it was less effective on charge movement in mutant muscle cells. We conclude that there is an alteration of nifedipine-sensitive charge movement in the skeletal muscle cells from the mutant mice.

Animals↗
PubMed1989

Appearance of the slow Ca conductance in myotubes from mutant mice with "muscular dysgenesis".

Voltage gated Ca conductance in skeletal muscle cells from mice with muscular dysgenesis (mdg/mdg) and from normal mice was studied using the whole cell recording technique. The physiological properties of the myotubes from the mutant mice (uncoupling of excitation-contraction, deficiency in the voltage gated slow Ca conductance) were changed to normal when the mdg/mdg myotubes were cocultured with spinal cord cells from normal mice. Spinal cord cells from mutant mice failed to induce normal muscle activity in the mutant myotubes. In aged mutant myotubes cultured without spinal cord cells, the slow Ca conductance sometimes developed, although with smaller amplitude. The number of mdg/mdg myotubes with partial development of the slow Ca conductance increased with the age of the culture. E-C coupling was never established in aged mutant myotubes. The phenotypic reversion did not require functional synaptic transmission since it was also obtained when neuromuscular transmission was chronically blocked with alpha-bungarotoxin (4-40 micrograms/ml).

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

Ionic currents in neurones cultured from embryonic cockroach (Periplaneta americana) brains.

Neurones isolated from embryonic cockroach brains were maintained in culture for up to 8 weeks. A single patch electrode was used to record voltage changes in response to injected current, membrane ionic currents under whole-cell voltage-clamp conditions or single-channel currents from isolated membrane patches. The voltage changes in response to injected current that depolarized the cell indicated increases in membrane permeability to calcium and potassium. These observations were confirmed using a voltage clamp. The potassium current observed in the youngest cultures turned on with a delay and was blocked by tetraethylammonium (TEA) and 4-aminopyridine (4-AP). Two kinds of decrease in the outward potassium current were observed. One may be associated with extracellular potassium accumulation, inactivation of the potassium channel or inactivation of a calcium channel. The other appears to be a voltage-dependent inactivation. The magnitude of the calcium permeability appeared to increase as the cultures developed, being most prominent in cultures more than 2 weeks old. Single-channel conductance measured from an analysis of records from six isolated membrane patches ranged from 15 to 110 pS. Except for one channel, the probability of the channels being open did not change appreciably with membrane potential. Our results suggest that much of the increase in potassium permeability may be due an increase in intracellular calcium level.

Animals↗
PubMed1987

Activation of enkephalin receptors reduces calcium conductance in neuroblastoma cells.

Enkephalin actions on the voltage-gated Ca channel were studied under voltage clamp in the neuroblastoma X glioma hybrid cell line, NG 108-15. We found that in sodium-free external solutions containing Ba2+ (20 mM), a depolarizing step pulse from a holding potential of -50 mV evoked both a rapidly decaying inward current and a sustained inward current similar to those described in other preparations. External application of 1 microM [D-Thr2,Leu5]enkephalin-Thr (DTLET), an agonist at the delta-opioid receptors specifically inhibited the sustained inward current. Naloxone, an antagonist at this receptor, blocked the effect of DTLET. Furthermore, this effect of DTLET was not observed if the cells were dialyzed with a low Ca2+ internal buffer solution [( Ca2+]i less than 10(-9) M). We conclude that in neuroblastoma cells: (1) there is a functional coupling between delta-receptors and voltage-gated sustained Ca channels, and (2) the coupling is mediated by intracellular free Ca ions.

Animals↗
PubMed1986

Acetylcholine-activated currents in mouse neuroblastoma cells.

The nicotine and muscarinic responses of differentiated mouse neuroblastoma cells from the clonal line N1E 115 to applied cholinergic agents were recorded using single channel and whole cell patch clamp techniques. An inward macroscopic current induced by acetylcholine (ACh) at the resting potential was blocked by curare; cell-attached recordings revealed a single channel conductance of 18 pS and a lifetime of 36 ms at 30 degrees C, with 200 nM ACh. The zero current potential was close to 0 mV. The kinetics of these nicotinic currents were described by multiexponential functions for both the open and closed time distributions. An outward single channel current, present at resting and slightly depolarized potentials, was also observed and has been tentatively described as being dependent on muscarinic receptor activation, as it was usually blocked by atropine. Under our conditions of whole cell clamp, no macroscopic outward current sensitive to ACh was observed.

Acetylcholine↗
PubMed1986

Arsenazo III transients and calcium current in a normally non-spiking neuronal soma of crayfish.

Arsenazo III was used to investigate Ca2+ transients in the normally non-excitable soma of the motor giant neurones of the crayfish Procambarus clarkii. Two kinds of regenerative potentials could be obtained depending on membrane potential conditioning: a fast spike after a pre-hyperpolarization to -90 mV and a slow action potential after a pre-depolarization to -50 mV. Only the second of these was accompanied by an Arsenazo III transient. In voltage-clamped, somata injected, with tetraethylammonium chloride, an absorbance change could be obtained by pulsing the membrane potential above -44 mV. The relationship between absorbance change and potential peaked between 0 and +10 mV then fell off to zero at ca. +150 mV. Changes in light absorbance studied using double-pulse protocols suggested that the inactivation of Ca2+ entry was predominantly mediated by the intracellular free Ca2+ concentration. External application of 1 mM-CdCl2 abolished both the absorbance changes and the (Ca2+) inward current. The voltage dependence of this current was similar to that of the absorbance change. For positive membrane potential the current-voltage relationship showed a voltage-dependent conductance property, the origin of which is discussed.

Action Potentials↗
PubMed1983

Presynaptic modulation of transmitter release by the early outward potassium current in Aplysia.

The mechanism involved in presynaptic modulation of transmitter release was studied in an identified synapse of Aplysia californica. Presynaptic hyperpolarization induces a decrease in he evoked postsynaptic potential amplitude. This is shown to be due to a reduction in the presynaptic spike amplitude during the hyperpolarization. The decreased presynaptic spike amplitude with hyperpolarization is explained s resulting from the superimposition of an early outward potassium current on the transient inward current. It is suggested that the presynaptic hyperpolarizing conditioning pulse decreases inactivation of the early outward current, which shunts the transient inward current. The superimposition of these two currents (transient inward current and the early outward current) induces a decrease in presynaptic spike amplitude, which in turn reduces the synaptic output from the terminal.

Animals↗
PubMed1981

Modulation of synaptic output by the transient outward potassium current in aplysia.

The mechanisms involved in synaptic output modulation by presynaptic membrane potential was studied in identified Aplysia synapses, where a presynaptic hyperpolarization reduces the postsynaptic potential amplitude. The experiment reported here reveals that a presynaptic conditioning hyperpolarization induces a decreased presynaptic spike amplitude and that the reduction is due to a superimposition of a transient outward potassium current on the inward current. This is demonstrated by the external application of 4-aminopyridine which suppresses the early outward current and thus suppresses the modulation both of the presynaptic spike amplitude and the synaptic output.

4-Aminopyridine↗
PubMed1980

Quantal transmitter release in an identified inhibitory cholinergic synapse of Aplysia.

A sustained postsynaptic potential is observed in an identified synapse of Aplysia when the presynaptic neuron is depolarized in the presence of tetrodotoxin (TTX). This prolonged postsynaptic potential appears to be at least in part due to the summation of quantal events. It is still observed when 30 mM CoCl2, which is known to inhibit Ca2+ influx, is added to the external media.

Acetylcholine↗
PubMed1978

The role of cyclic AMP in the modulation of synaptic efficacy.

1. Heterosynaptic facilitation (modification of synaptic transmission by a neuron influencing the terminals of the presynaptic neuron) was studied in the pleural ganglion of Aplysia. Among several identified synapses, heterosynaptic facilitation was observed only in one type (EIPSP synapses) when repetitive stimulation was applied to the tentacular nerve or to a particular identified neuron. 2. Serotonin was shown to increase the amplitude of the EIPSP at this synapse; this facilitatory effect was prolonged in the presence of theophylline and mimicked by cyclic AMP. 3. When transmission was abolished by calcium-free solution, calcium injected in the region of the synapse caused partial recovery of the EIPSP; when calcium injection was preceded by serotonin injection near the same terminal, the EIPSP was much larger than with calcium injection alone. 4. It was concluded that the activation of one neuron (the heterosynaptic neuron) caused it to release serotonin, which activated an adenylate cyclase in the pre-synaptic terminals of another neuron. Consequent accumulation of cyclic AMP in these terminals is supposed to have increased their voltage-dependent calcium conductance and hence the amount of transmitter released during an action potential.

Animals↗