RESUMO
Endocytosis in dystrophic muscles was studied by a combination of biochemical, radiochemical, and light and electron microscopic techniques. It was observed that the uptake of horseradish peroxidase (HRP) and 3H-Inulin in vitro was increased in leg skeletal muscles from dystrophic mice compared with littermate controls. Endocytosis of HRP in vivo was also increased in dystrophic muscles. When HRP was administered intravenously, light microscopic examination of the muscles showed that the macromolecular tracer was present not only in the extracellular space but also as intracellular deposits in several dystropic muscle fibers. Ultrastructural examination of these fibers showed HRP to be present in membrane limited bodies of variable size, some of which likely represented secondary lysosomes, located preferentially close to the A-I junction. HRP was also found inside vacuoles which were sometimes in close vicinity to autophagic vacuoles. Primary uptake vesicles containing HRP appeared to originate from the sarcolemma and the transverse tubules. Biochemical determination of lysosomal enzyme activities revealed elevated levels of both cathepsin D and N-acetylglucosaminidase in dystrophic muscles as compared with controls. The results suggest an increased endocytic activity in dystrophic muscles with distribution of exogenous marcromolecular tracers into endocytic vesicles and lysosomal structures. The hypothesis is put forward that endocytic activity constitutes an important mechanism of lysosomal activation in dystrophic muscles.
Assuntos
Endocitose , Lisossomos/enzimologia , Músculos/patologia , Distrofia Muscular Animal/patologia , Acetilglucosaminidase/metabolismo , Animais , Catepsinas/metabolismo , Peroxidase do Rábano Silvestre , Inulina/metabolismo , Camundongos , Músculos/enzimologia , Músculos/ultraestrutura , Distrofia Muscular Animal/enzimologiaRESUMO
The effect of guanidine on the neuromuscular transmission in human intercostal and mouse diaphragm muscle in vitro during repetitive nerve stimulation was studied. The drug greatly increased the release of acetylcholine (ACh) quanta by nerve impulses at low frequencies of nerve stimulation and at the beginning of tetani at high frequencies of stimulation. The effect was shown to be produced by an increase in fractional release from an unchanged store of ACh quanta available fro immediate release. This seems to explain why guanidine has a poor therapeutic effect in myasthenia gravis but a good effect in the myasthenic syndrome.
Assuntos
Acetilcolina/metabolismo , Guanidinas/farmacologia , Junção Neuromuscular/efeitos dos fármacos , Neurossecreção/efeitos dos fármacos , Transmissão Sináptica/efeitos dos fármacos , Animais , Estimulação Elétrica , Potenciais Evocados/efeitos dos fármacos , Glicerol/farmacologia , Humanos , Técnicas In Vitro , Magnésio/farmacologia , Potenciais da Membrana/efeitos dos fármacos , Camundongos , Placa Motora/efeitos dos fármacos , Neurônios Motores/fisiologia , Fatores de Tempo , Tubocurarina/farmacologiaRESUMO
The effect of guanidine on neuromuscular transmission was studied in human intercostal muscle and mouse diaphragm preparations in vitro. Guanidine greatly increased the number of acetylcholine (ACh) quanta released by a single motor nerve action potential. This effect of guanidine was greater at junctions with a low quantum content. The spontaneous release of ACh quanta was not substantially changed by guanidine. No change was found in the postsynaptic sensitivity to ACh released from the motor nerve or iontophoretically applied to the muscle fiber. Effects of the drug had slow onset and were very long-lasting and resistant to wash.
Assuntos
Acetilcolina/metabolismo , Guanidinas/farmacologia , Junção Neuromuscular/efeitos dos fármacos , Neurossecreção/efeitos dos fármacos , Transmissão Sináptica/efeitos dos fármacos , Potenciais de Ação/efeitos dos fármacos , Adolescente , Adulto , Idoso , Animais , Relação Dose-Resposta a Droga , Estimulação Elétrica , Potenciais Evocados/efeitos dos fármacos , Glicerol/farmacologia , Humanos , Técnicas In Vitro , Magnésio/fisiologia , Potenciais da Membrana/efeitos dos fármacos , Camundongos , Pessoa de Meia-Idade , Potássio/fisiologia , Receptores ColinérgicosRESUMO
Botulinum toxin type A, when applied to the extensor digitorum longus muscle of rats, reduces spontaneous and evoked transmitter release to a few per cent of the normal level. The toxin, however, fails to affect the morphological redifferentiation of the postsynaptic part of the neuromuscular junction following muscle degeneration-regeneration induced by a selective myotoxic agent, bupivacaine. This indicates that neither transmitter release nor resulting muscle activity are necessary for the differentiation of the sole plate.
Assuntos
Toxinas Botulínicas/farmacologia , Diferenciação Celular/efeitos dos fármacos , Músculo Esquelético/efeitos dos fármacos , Músculo Esquelético/inervação , Regeneração Nervosa/efeitos dos fármacos , Regeneração Nervosa/fisiologia , Junção Neuromuscular/efeitos dos fármacos , Junção Neuromuscular/ultraestrutura , Anestésicos Locais/farmacologia , Animais , Bupivacaína/farmacologia , Diferenciação Celular/fisiologia , Masculino , Microscopia Eletrônica , Músculo Esquelético/ultraestrutura , Junção Neuromuscular/metabolismo , Neurotransmissores/metabolismo , Ratos , Ratos Sprague-DawleyRESUMO
Examination of spontaneous miniature endplate potentials (MEPPs) in murine skeletal muscle has revealed that in conditions such as botulinum poisoning, during nerve terminal regeneration or in the presence of the drug 4-aminoquinoline, two types of acetylcholine release are responsible for the MEPPs. In addition to the MEPPs which correspond to the quantal component of a nerve impulse-evoked endplate potential a second type of acetylcholine release occurs. The latter type of transmitter release gives rise to MEPPs with a more prolonged time-to-peak and frequently a larger than normal amplitude. It is unaffected by nerve terminal depolarization and transmembrane Ca2+ fluxes. The relationship between MEPP frequency and temperature has a Q10 of about 12 compared to 2-3 for normal MEPPs. In botulinum-poisoned muscles this secretory type of transmitter release dominates, being exclusively present in muscles where nerve stimulation fails to release transmitter. In normal muscle such a release is induced by 4-aminoquinoline which may cause up to 45% of all the spontaneous MEPPs to be of that kind. It is suggested that the described spontaneous secretion of acetylcholine serves in inductory and neurotrophic function.
Assuntos
Acetilcolina/metabolismo , Junção Neuromuscular/metabolismo , Transmissão Sináptica , Aminoquinolinas/farmacologia , Animais , Toxinas Botulínicas/farmacologia , Cálcio/farmacologia , Potenciais Evocados/efeitos dos fármacos , Placa Motora/metabolismo , Regeneração Nervosa/efeitos dos fármacos , Nervo Fibular/metabolismo , Potássio/farmacologia , Ratos , Transmissão Sináptica/efeitos dos fármacos , Vesículas Sinápticas/metabolismoRESUMO
The isolated neurotoxins taipoxin and notexin from the venoms of the Elapidae, Oxyuranus scutellatus and Notechis scutatus scutatus respectively cause a neuromuscular block when administered to the mouse in vivo or to the phrenic nerve-hemidiaphragm preparation in vitro. The block is preceded by a latency period during which the toxins bind irreversibly to the nerve. The period is shortened by nerve activity. The frequency of the miniature end-plate potentials is gradually reduced, almost to zero, and their amplitude distribution is altered; small and very large miniature endplate potentials appearing. Ultrastructurally the endplates are altered in the presynaptic portion but not in the postsynaptic part. In an early stage of poisoning the axolemma has an increased number of omega-shaped indentations similar in size to synaptic vesicles. At a later stage, when the animals die of respiratory paralysis, the axolemmal indentations are more numerous and the synaptic vesicles greatly reduced in number, the remaining vesicles having a variable and frequently larger than normal size. When impulse activity in the phrenic nerve is stopped by cutting the nerve before the administration of toxin there is no reduction in the number of synaptic vesicles, only the appearance of an increased number of axolemmal indentations. It is suggested that taipoxin and notexin irreversibly interfere with the formation of synaptic vesicles by arresting vesicle membrane recycling at the level of the axolemma. When the pre-existing store of vesicles is depleted, by nerve activity, a neuromuscular block results.
Assuntos
Venenos Elapídicos/toxicidade , Bloqueadores Neuromusculares/farmacologia , Junção Neuromuscular/efeitos dos fármacos , Junção Neuromuscular/ultraestrutura , Acetilcolina/metabolismo , Animais , Estimulação Elétrica , Potenciais Pós-Sinápticos Excitadores/efeitos dos fármacos , Potenciais Pós-Sinápticos Excitadores/fisiologia , Camundongos , Microscopia Eletrônica , Junção Neuromuscular/metabolismo , Membranas Sinápticas/efeitos dos fármacos , Membranas Sinápticas/patologia , Membranas Sinápticas/ultraestrutura , Vesículas Sinápticas/efeitos dos fármacos , Vesículas Sinápticas/metabolismo , Vesículas Sinápticas/ultraestruturaRESUMO
1 In the presence of tetrodotoxin, electrotonic depolarization of frog motor nerve terminals causes the appearance of stimulus-graded endplate potentials. When 4-aminopyridine is added, the graded endplate potential is converted into a triggered all-or-none response resulting in giant endplate potentials of about 70 mV amplitude and 50 ms duration. The triggered endplate potentials are abolished in Ca(2+)-free saline and are blocked by Mn(2+) ions. Sr(2+) but not Ba(2+) can replace Ca(2+) in supporting transmitter release. Mg(2+) fails, even in concentrations as high as 32 mM, to affect the amplitude and the shape of the endplate potential but abolishes it when the Ca(2+) concentration is reduced to 0.2 mM.2 Despite the large amplitude of the triggered endplate potential in the presence of 4-aminopyridine and tetrodotoxin, repetitive stimulation up to 10 Hz causes only a small decline in amplitude of successive endplate potentials. However, in the presence of (+)-tubocurarine or gallamine, repetitive nerve stimulation produces a marked decline in successive endplate potential amplitude. The fall is counteracted when evoked transmitter release is reduced in the presence of 0.2 mM Ca(2+). The results suggest that in the presence of 4-aminopyridine such large amounts of transmitter are released that even during repetitive stimulation (5 to 10 Hz) endplate potentials are of maximal amplitude.3 4-Aminopyridine causes a prallel shift to the right of the dose-response curve to Mg(2+) for blockade of nerve impulse-evoked transmitter release (in the absence of tetrodotoxin). A similar parallel shift occurs in the presence of tetraethylammonium and guanidine.4 It is concluded that 4-aminopyridine increases transmitter release by enhancing the transport efficacy for Ca(2+) across the nerve terminal membrane during nerve terminal depolarization.
Assuntos
Aminopiridinas/farmacologia , Neurônios Motores/metabolismo , Terminações Nervosas/metabolismo , Neurotransmissores/metabolismo , Animais , Anuros , Guanidinas/farmacologia , Técnicas In Vitro , Neurônios Motores/efeitos dos fármacos , Terminações Nervosas/efeitos dos fármacos , Rana temporaria , Transmissão Sináptica/efeitos dos fármacos , Tetrodotoxina/farmacologia , Tubocurarina/farmacologiaRESUMO
1. Tacrine (20 microM) induced, like 4-aminoquinoline (4-AQ, 200 microM), the appearance of a population of miniature endplate potentials (m.e.p.ps) with more than twice the normal amplitude or time-to-peak. The times-to-peak of nerve impulse-evoked endplate potentials were not similarly affected. 2. Cholinesterase inhibition by edrophonium (25 microM) did not prevent tacrine or 4-AQ from inducing this population of m.e.p.ps. 3. Nerve-muscle preparations in which the normal calcium-sensitive quantal release of acetylcholine had been blocked by botulinum neurotoxin type A also responded to tacrine by an increase in the frequency of giant or slow m.e.p.ps. 4. Reduction of the temperature from 30 degrees to 14 degrees C reduced the frequency of giant or slow m.e.p.ps induced either by tacrine or by 4-AQ. A similar effect was obtained by colchicine (5 mM). This supports the idea that proximo-distal axonal transport is required for the secretory activity. 5. The neurosecretion evoked by tacrine could explain the therapeutic effects of the drug claimed in the treatment of Alzheimer's type of dementia.
Assuntos
Aminoacridinas/farmacologia , Placa Motora/metabolismo , Junção Neuromuscular/metabolismo , Tacrina/farmacologia , Acetilcolina/metabolismo , Aminoquinolinas/farmacologia , Animais , Toxinas Botulínicas/farmacologia , Edrofônio/farmacologia , Eletrofisiologia , Exocitose/efeitos dos fármacos , Técnicas In Vitro , Masculino , Potenciais da Membrana/efeitos dos fármacos , Camundongos , Placa Motora/efeitos dos fármacos , Nervo Frênico/efeitos dos fármacos , Músculos Respiratórios/efeitos dos fármacos , Vesículas Sinápticas/efeitos dos fármacos , Vesículas Sinápticas/metabolismo , TemperaturaRESUMO
1. The acute effects of a purified toxin from Australian Tiger snake (Notechis scutatus scutatus) venom have been investigated at the mammalian neuromuscular junction.2. The toxin was injected into the tail vein of mice. Death was due to respiratory paralysis.3. The resting membrane potential, and action potential of muscle fibres in muscles from in vivo intoxicated animals were normal.4. The frequency of miniature end plate potentials (m.e.p.p.s) from intoxicated nerve-muscle preparations was reduced, although m.e.p.p. amplitude was unaltered.5. Nerve stimulation resulted in end plate potentials (e.p.p.s) of quantal amplitude; only rarely was the e.p.p. large enough to give rise to an action potential.6. High (20 mM) K(+) did not increase m.e.p.p. frequency in intoxicated preparations.7. The toxin was largely ineffective in vitro.8. The similarities and differences between this toxin, beta-bungarotoxin and botulinum toxin are discussed.
Assuntos
Junção Neuromuscular/efeitos dos fármacos , Serpentes , Toxinas Biológicas/farmacologia , Peçonhas/farmacologia , Potenciais de Ação/efeitos dos fármacos , Animais , Estimulação Elétrica , Concentração de Íons de Hidrogênio , Masculino , Potenciais da Membrana/efeitos dos fármacos , Camundongos , Camundongos Endogâmicos , Contração Muscular/efeitos dos fármacos , Peptídeos/farmacologia , Potássio/farmacologia , Transmissão Sináptica/efeitos dos fármacos , Fatores de Tempo , Toxinas Biológicas/isolamento & purificaçãoRESUMO
In frogs poisoned with botulinum toxin type A the quantal content of endplate potentials is greatly reduced. Lowering the temperature of the preparation increases quantum content; between 14 and 4 degrees C the mean Q10 for this effect is 6.3. Facilitation of synaptic transmission is marked with pairs of stimuli and cooling further enhances facilitation. The time constant of decay of facilitation is 34 ms at 20 degrees C and 116 ms at 4 degrees C. The increase in facilitation and in its time constant of decay at low temperature are presumably not a result of a prolongation of the duration of the nerve terminal action potential since such changes are not seen in the presence of K+-channel blockade by 3,4-diaminopyridine. Electrotonic depolarization of nerve terminals in the presence of tetrodotoxin and 3,4-diaminopyridine induces all-or-none endplate currents. Such endplate currents, at a holding potential of -50 mV, show that the amount of charge entry is about 1/3 of that in unpoisoned junctions but still corresponds to 5-10 X 10(3) transmitter quanta. Transmitter release at this level is maintained during repetitive stimulation even in the presence of 82 mM Ca2+ in the extracellular solution. We speculate that the blockade of transmitter release in BoTx -poisoned muscles results from a stimulatory effect of the toxin on metabolic systems of Ca2+ disposal in the nerve terminal.
Assuntos
Toxinas Botulínicas/farmacologia , Junção Neuromuscular/efeitos dos fármacos , Transmissão Sináptica/efeitos dos fármacos , Animais , Cálcio/metabolismo , Estimulação Elétrica , Potenciais Evocados/efeitos dos fármacos , Canais Iônicos/efeitos dos fármacos , Placa Motora/efeitos dos fármacos , Músculos/inervação , Potássio/metabolismo , Rana esculenta , Rana temporaria , Sódio/metabolismo , Sinapses/efeitos dos fármacos , TemperaturaRESUMO
Intracellular recordings from botulinum toxin type A (BoTx)-poisoned extensor digitorum longus muscles from adult rats have shown that the toxin initially reduced the frequency of miniature endplate potentials (m.e.p.ps) to about 1/200 of normal. After a few days the m.e.p.p. frequency rose and was subsequently maintained at a level of about 1/3 of that at normal endplates. Depolarization of the nerve terminals with 20-30 mM KCl-Ringer initially failed to affect the frequency of m.e.p.ps and later caused only a 2-3--fold increase in their frequency. The temperature dependence of m.e.p.p. frequency at BoTx-poisoned endplates had a Q10 of about 12 compared to 2-3 for normal junctions. The time to peak of a population of m.e.p.ps at Botx-poisoned junctions was prolonged as compared to normal and fast- and slow-rising m.e.p.ps originated within the same post-synaptic membrane field area. M.e.p.ps in BoTx-poisoned muscles resembled the m.e.p.ps which 4-aminoquinoline (4-AQ) has been shown to induce in normal muscle, and we therefore examined and compared these two release processes for acetylcholine. Procedures known to markedly affect m.e.p.p. frequency at normal junctions, such as nerve terminal depolarization or changes in extra- and intracellular Ca2+ concentrations, failed to affect m.e.p.p. frequency in BoTx-poisoned muscles and similarly the frequency of m.e.p.ps induced by 4-AQ in normal muscle. Tonicity changes in the extracellular medium altered m.e.p.p. frequency in both the experimental conditions, but in a direction opposite to that at normal junctions. The temperature dependence of the frequency of 4-AQ-induced m.e.p.ps was similar to that of m.e.p.ps at BoTx-poisoned junctions. It is concluded that BoTx poisoning induces an abnormal type of spontaneous quantal transmitter release, characterized by being insensitive to nerve terminal depolarization and to transmembrane Ca2+ fluxes. This transmitter release has characteristics similar to that previously described for the release induced, at normal junctions, by 4-AQ.
Assuntos
Aminoquinolinas/farmacologia , Toxinas Botulínicas/farmacologia , Placa Motora/metabolismo , Junção Neuromuscular/metabolismo , Neurotransmissores/metabolismo , Animais , Potenciais Evocados/efeitos dos fármacos , Cinética , Masculino , Microeletrodos , Placa Motora/efeitos dos fármacos , Placa Motora/fisiologia , Potássio/farmacologia , Ratos , Ratos EndogâmicosRESUMO
Ouabain (100 microM) applied for 60 min to botulinum A (BoTx) poisoned motor junctions increases, in a time-dependent manner, the mean number of acetylcholine quanta released by nerve stimulation and enhances the delayed transmitter release. The drug does not affect spontaneous quantal release. The observed effects on evoked transmitter release cannot be explained by changes in the configuration of presynaptic currents recorded from motor terminals. They suggest that in BoTx-poisoned motor endings the level of intraterminal Ca2+, lower than that required for the activation of quantal transmitter release, can be effectively increased through the reversed operation of an Na+-Ca2+ exchange system that normally uses the Na+ gradient to extrude Ca2+.
Assuntos
Toxinas Botulínicas/farmacologia , Cálcio/metabolismo , Junção Neuromuscular/efeitos dos fármacos , Ouabaína/farmacologia , Sódio/metabolismo , Transmissão Sináptica/efeitos dos fármacos , Acetilcolina/metabolismo , Animais , Proteínas de Transporte/metabolismo , Técnicas In Vitro , Masculino , Potenciais da Membrana/efeitos dos fármacos , Junção Neuromuscular/metabolismo , Junção Neuromuscular/fisiologia , Rana temporaria , Trocador de Sódio e CálcioRESUMO
The hypothesis according to which botulinum A toxin blocks acetylcholine release from motor endings by stimulating intracellular Ca2+ disposal systems was tested by recording presynaptic membrane currents from poisoned muscles. Calcium and calcium-activated potassium currents displayed amplitudes, time courses and stimulation frequency-dependent inactivation similar to those observed in unpoisoned preparations. This indicates that poisoned endings are no more efficient than normal ones in dealing with Ca2+ overloads.
Assuntos
Toxinas Botulínicas/farmacologia , Canais de Cálcio/fisiologia , Cálcio/metabolismo , Neurônios Motores/metabolismo , Junção Neuromuscular/metabolismo , Animais , Cálcio/fisiologia , Canais de Cálcio/efeitos dos fármacos , Masculino , Potenciais da Membrana/efeitos dos fármacos , Neurônios Motores/efeitos dos fármacos , Junção Neuromuscular/efeitos dos fármacos , Potássio/fisiologia , Rana esculentaRESUMO
The effects of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on spontaneous quantal transmitter release and nerve terminal ultrastructure were studied on isolated cutaneous pectoris nerve-muscle preparations from frogs that were completely paralysed by a single sublethal dose of Clostridium botulinum type A toxin (BoTx). CCCP enhanced miniature endplate potential frequency at poisoned junctions and caused a reduction in the density of clear synaptic vesicles and of large dense core vesicles in motor nerve terminals. However, the intensity of these effects was much less important than that previously reported at unpoisoned junctions. The moderate depletion of synaptic vesicles can be related to the low levels of transmitter release detected with CCCP at BoTx-poisoned terminals.
Assuntos
Toxinas Botulínicas/toxicidade , Carbonil Cianeto m-Clorofenil Hidrazona/farmacologia , Neurônios Motores/ultraestrutura , Terminações Nervosas/ultraestrutura , Neurotransmissores/metabolismo , Animais , Técnicas In Vitro , Placa Motora/efeitos dos fármacos , Neurônios Motores/efeitos dos fármacos , Neurônios Motores/metabolismo , Músculos/inervação , Terminações Nervosas/efeitos dos fármacos , Terminações Nervosas/metabolismo , Rana temporariaRESUMO
The segmental uptake of horseradish peroxidase (HRP) in the endplate region of denervated skeletal muscle fibres has been studied ultrastructurally using a method for selecting single muscle fibres with high segmental peroxidase staining from denervated mouse tibialis anterior muscle. Segments containing large peroxidase positive phagosomes could already be seen 10-15 min after i.v. injection of HRP. Such segments were still present 24 h after HRP injection. The localization of phagosomes, deep in the fibres rather than immediately under the sarcolemma, suggests that the uptake occurs from t-tubuli. Vivid proliferation of t-tubuli, consisting of vesiculation, enlargement and encircling of cytoplasmic components, was also observed. The HRP accumulates in phagosomes of varying size and shape. Similar membrane-limited bodies without or with very weak peroxidase staining were also observed. The peroxidase-positive phagosomes participate in autophagic processes as suggested by their content of undegraded cellular material. Golgi profiles, which occurred deep in the muscle fibres, and enlarged components of the sarcoplasmic reticulum were frequently encountered in the segments. Myofibrillar degeneration occurs in the segments and progresses with time after denervation. The described segments may be related to the increased membrane turnover in denervated muscle fibres and/or they may be related to processes aimed at establishing new synaptic contacts.
Assuntos
Denervação Muscular , Músculos/ultraestrutura , Animais , Complexo de Golgi/ultraestrutura , Peroxidase do Rábano Silvestre , Masculino , Camundongos , Microscopia Eletrônica , Microtúbulos/ultraestrutura , Miofibrilas/ultraestrutura , Fagossomos/ultraestrutura , Retículo Sarcoplasmático/ultraestrutura , Coloração e RotulagemRESUMO
The injection of botulinum toxin type A into the hind-leg of adult rats causes complete paralysis of the leg lasting for several weeks. In the extensor digitorum longus (EDL) muscle transmitter release is reduced to a level of less than 1% of normal. Tetraethylammonium (TEA) and guanidine in concentrations of about 3 mM restore, in EDL muslces in vitro, neuromuscular transmission to about the normal level, provided that the external calcium concentration is 4 mM or higher. 4-Aminopyridine (4-AP) has similar restorative effect but is about 20-30 times more potent. Unlike TEA and guanidine, 4-AP is effective when the ambient calcium concentration is 2 mM; this drug is therefore also active in vivo. The intravenous injection of 4-AP (5 mg/kg body weight) restores neuromuscular transmission from complete paralysis by botulinum toxin to a normal level as shown by the recording of almost normal twitch and tetanic tensions in the EDL muscle. In rats paralysed by a lethal dose of botulinum toxin, the intraperitoneal administration of 4-AP restores general motor activity, the effect lasting 1-2 hours. A study of the effects of these drugs on spontaneous and evoked transmitter release suggests that all three compounds increase the level of free calcium inside the nerve terminals. In botulinum poisoning the transmitter release mechanism appears to be intact, but a reduced sensitivity to calcium has been shown (Cull-Candy et al. 1976), and this could explain why the drugs restore evoked transmitter release in botulinum poisoning.
Assuntos
Antitoxina Botulínica , Guanidinas/farmacologia , Paralisia/induzido quimicamente , Piridinas/farmacologia , Compostos de Tetraetilamônio/farmacologia , Potenciais de Ação/efeitos dos fármacos , Animais , Cálcio/farmacologia , Relação Dose-Resposta a Droga , Técnicas In Vitro , Masculino , Contração Muscular/efeitos dos fármacos , Junção Neuromuscular/efeitos dos fármacos , Ouabaína/farmacologia , Potássio/farmacologia , Ratos , Transmissão Sináptica/efeitos dos fármacos , Fatores de TempoRESUMO
The neuromuscular toxicity of stonefish (Synanceia trachynis) venom was characterized by electrophysiological and electron microscopic examination of isolated murine and frog nerve-skeletal muscle preparations exposed to various concentrations of venom. Low concentrations of venom (2.5-10 micrograms/ml) acted presynaptically by causing release and depletion of neurotransmitter from the nerve terminal. The response was Na+ channel-independent (resistant to tetrodotoxin), required the presence of either Ca2+ or Mg2+, and was observed with botulinum neurotoxin-paralyzed nerve-muscle preparations. Higher concentrations of venom (100-300 micrograms/ml) acted postsynaptically and presynaptically. They caused irreversible depolarization of muscle cells and microscopically observable muscle and nerve damage. We conclude that the previously observed neuromuscular toxicity of stonefish venom is a consequence of the venom's dose-dependent, presynaptic and postsynaptic actions at the myoneural junction.
Assuntos
Venenos de Peixe/farmacologia , Peixes/metabolismo , Junção Neuromuscular/efeitos dos fármacos , Acetilcolina/metabolismo , Animais , Toxinas Botulínicas/farmacologia , Membrana Celular/efeitos dos fármacos , Membrana Celular/ultraestrutura , Eletrofisiologia , Técnicas In Vitro , Masculino , Camundongos , Microscopia Eletrônica , Placa Motora/efeitos dos fármacos , Placa Motora/ultraestrutura , Músculos/patologia , Terminações Nervosas/efeitos dos fármacos , Terminações Nervosas/metabolismo , Terminações Nervosas/ultraestrutura , Fármacos Neuromusculares Despolarizantes/farmacologia , Junção Neuromuscular/ultraestrutura , Rana esculenta , Ratos , Ratos Sprague-Dawley , Sinapses/efeitos dos fármacos , Sinapses/metabolismo , Sinapses/ultraestruturaRESUMO
The classical studies of Katz and co-workers have shown that nerve impulses release quanta of acetylcholine at the neuromuscular junction. This release is regulated by presynaptic calcium and accounts for the trans-synaptic transmission of nerve impulses. In resting conditions it gives rise to small spontaneous potentials, i.e. miniature endplate potentials. In addition these investigators described a spontaneous molecular leakage of acetylcholine from the motor nerve. I have studied a third type of acetylcholine release. It is a spontaneous intermittent secretion of acetylcholine which postsynaptically causes large, generally slow rising potentials. This release is unaffected by presynaptic calcium and is therefore not influenced by nerve activity. The acetylcholine responsible for these potentials comes from the same pool of transmitter as that liberated by nerve impulses. The observation that the release is blocked by drugs that prevent the accumulation of acetylcholine into synaptic vesicles indicates that the secretion originates from clusters of vesicles or large vesicle-like structures in the nerve terminal. This type of release is present at a low frequency at normal neuromuscular junctions. It is markedly accelerated whenever the calcium-dependent quantal release of acetylcholine is blocked or impaired. The drug 4-aminoquinoline selectively stimulates this release. I speculate that this type of transmitter secretion is important for the development of synaptic connexions.