THE EFFECT OF TUBOCURARINE ON ACETYLCHOLINE RELEASE FROM MOTOR NERVE TERMINALS

Nicotinic autoreceptor function in rat brain during maturation and aging: possible differential sensitivity to organophosphorus anticholinesterases

Acetylcholine (ACh) release is modulated pre-synaptically by both muscarinic and nicotinic receptor-mediated processes. While muscarinic autoreceptors inhibit ACh release, nicotinic autoreceptors enhance ACh release and thus disruption of these processes could potentially affect cholinergic toxicity following exposure to anticholinesterases. Marked age-related differences in sensitivity to some organophosphorus (OP) anticholinesterases have been reported. We compared nicotinic autoreceptor function (NAF) during maturation and aging and evaluated its potential modulation by the common OP insecticide, chlorpyrifos (CPF). Cortical synaptosomes were pre-loaded with [3H]choline, superfused (0.6 ml/min) with physiological buffer and [3H]ACh release was evoked with potassium (KCl, 9 mM), with or without co-addition of exogenous ACh to stimulate nicotinic autoreceptors. Fractions of perfusate were subsequently collected and area under the curve (AUC) for [3H] was analyzed by scintillation counting. The difference in evoked release due to co-addition of exogenous ACh was defined as NAF. Under these conditions, atropine (ATR, 0.1 microM) appeared requisite for NAF; thus this muscarinic antagonist was subsequently added to all perfusion buffers. In synaptosomes from adult tissues, exogenous ACh (3-100 microM) significantly increased release in a concentration-dependent manner. The nicotinic antagonist mecamylamine (MEC, 100 microM) substantially reduced the potassium-evoked release elicited by co-addition of ACh (10 microM). Interestingly, the nicotinic agonists nicotine (NIC) and dimethylphenylpiperazinium (DMPP; 0.1-10 microM) had no effect on release. The active metabolite of CPF (i.e. chlorpyrifos oxon (CPO), 1-10 microM) inhibited NAF in vitro. Maturation-related expression of NAF was noted (AUC with co-addition of 10 microM ACh: 7-day rats, 7+/-6; 21-day rats, 44+/-6; 90-day rats, 196+/-37; 24-month rats, 173+/-52). NAF was substantially reduced (67-91%) 96 h after maximum tolerated dosages of CPF in adult and aged rats (279 mg/kg, sc) but not in juveniles (127 mg/kg, sc), even though AChE inhibition was similar among the age groups (>80%). Together these data suggest that NAF is differentially expressed during maturation and that this neuromodulatory process may be selectively altered by some OP insecticides, potentially contributing to age-related differences in response to AChE inhibitors. As NAF has been postulated to be activated under conditions of 'impaired' cholinergic function, selective alteration of this pre-synaptic process by OP anticholinesterases may be also important in age-related conditions associated with cholinergic hypofunction.

Side effects of nondepolarizing muscle relaxants: relationship to their antinicotinic and antimuscarinic actions

Since acetylcholine (ACh) is the 'master key' to different subtypes of nicotinic and muscarinic receptors, and muscle relaxants (MRs) available in clinical practice are structurally related to it, MRs may exert their unwanted effects through inhibition of these receptors. Since the subunit composition of nicotinic ACh receptors (nAChRs) of pre- and/or postsynaptic location and the binding potency of MRs to these and muscarinic receptors are different, a search for selective muscle nAChR antagonists without or with less side effects seems to be promising and important for clinical practice.

Prejunctional actions of muscle relaxants: synaptic vesicles and transmitter mobilization as sites of action

1. Nicotinic antagonists such as tubocurarine affect acetylcholine release from motor nerve terminals at the neuromuscular junction. 2. Electrophysiological studies comparing the prejunctional actions of tubocurarine to those of vesamicol and vecuronium have been used to provide an insight into the mechanisms involved in the prejunctional effects of tubocurarine-like compounds. 3. The observed prejunctional actions of tubocurarine can be accounted for by a model in which the compound has two separately identifiable effects on the nerve terminal. At low frequencies of nerve stimulation tubocurarine augments acetylcholine release while at high frequencies of nerve stimulation tubocurarine depresses acetylcholine release. 4. Both of the effects of tubocurarine on acetylcholine release are a consequence of a change in the number of quanta within the nerve terminal immediately available for release upon nerve stimulation. 5. On the basis of our experimental observations, we suggest that the two prejunctional effects of tubocurarine are mediated through two pharmacologically distinct prejunctional nAChRs.

The effect of acetylcholine and related drugs on currents at the frog motor nerve terminal

Acetylcholine, acetylthiocholine, carbachol, suberyldicholine, propionylcholine, succinylcholine, methylfurmethide and F 2268 were tested on motor nerve ending currents recorded with an extracellular microelectrode. The isolated and transversally cut cutaneous pectoris muscle of frog Rana ridibunda was used. Only acetylcholine and acetylthiocholine affected the spike waveforms in a concentration-dependent manner. Lower concentrations (1-6 x 10(-4) M) prolonged the inward Na+ current and increased the outward K+ current at the proximal and central parts of the nerve terminal. Most remote parts of the terminal were not affected. At 7 x 10(-4) M and higher, both drugs further prolonged the Na+ current and eliminated the K+ component of the spike. The potentiating effect of acetylcholine and acetylthiocholine on the K+ phase of nerve terminal current disappeared after treatment with tetraethylammonium and 4-aminopyridine. The effect also disappeared when synaptic cholinesterase was inhibited by the anticholinesterases or by treatment with collagenase. Reactivation of cholinesterase by dipyroxime restored the facilitating effect of acetylcholine. Choline and slight acidification to pH 6.8 did not mimic the acetylcholine action on the terminal currents. Facilitation of the K+ current by acetylcholine was not calcium-dependent. The results indicate that lower acetylcholine concentrations inhibit the delayed rectifier only, whereas 7 x 10(-4) M and higher concentrations of acetylcholine depress all outward currents of the terminal.

Nicotinic actions of oxotremorine on murine skeletal muscle. Evidence against muscarinic modulation of acetylcholine release

The effects of oxotremorine, arecoline and muscarine on neuromuscular transmission of mouse or rat phrenic nerve-diaphragm were investigated. For some studies of endplate potentials (e.p.p.s) the preparation was immobilized by cutting muscle fibers. Oxotremorine (0.3-10 microM) depolarized endplate membranes, reduced miniature e.p.p. amplitudes but increased frequency, induced spontaneous neural discharges and muscle fasciculations, and produced contracture of denervated mouse diaphragm. In mouse and young rat preparations pretreated with Mn2+, Co2+, Ni2+, Cd2+ or low Ca2+ Tyrode to depress evoked acetylcholine release, oxotremorine 0.3-1 microM increased indirect twitches as well as amplitudes and quantal contents of e.p.p.s. These increases were not observed when the synaptic transmission was not depressed, nor in adult rat preparations. The augmentation by oxotremorine of evoked acetylcholine release persisted in preparations pretreated with neostigmine (1 microM) and tetrodotoxin (20 nM), which inhibited acetylcholinesterase and oxotremorine-induced spontaneous neural discharges. These effects of oxotremorine were mimicked by arecoline but not by muscarine and were antagonized by tubocurarine (0.3 microM) but not by atropine (0.1-10 microM). Atropine alone did not affect indirect twitches, synaptic transmission, tetanic responses evoked by direct stimulation of diaphragms, nor the durations of muscle action potential. The direct twitch responses were only slightly increased by oxotremorine at 2-3 microM. Oxotremorine at high concentrations (greater than 2 microM), depressed indirect twitches and e.p.p. amplitude, and accelerated the run-down of trains of e.p.p.s. The IC50 on indirect twitches was reduced by pretreatment with diltiazem or proadifen, which are known to promote receptor desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic nicotine receptors mediating a positive feed-back on transmitter release from the rat phrenic nerve

The effects of 1,1-dimethyl-4-phenylpiperazinium (DMPP) and of nicotine receptor antagonists on [3H]acetylcholine release from the rat phrenic nerve preincubated with [3H]choline were investigated in the absence and presence of cholinesterase inhibitors (presynaptic effects). Additionally, the effects of hexamethonium and tubocurarine on the muscle contraction of the indirectly stimulated diaphragm were examined (postsynaptic effects). DMPP (1-30 microM) increased (76-92%), whereas hexamethonium (0.001-1 mM) and tubocurarine (1-10 microM) decreased (52-60%) the release of [3H]acetylcholine following a train of 100 pulses at 5 Hz. The release caused by a longer train (750 pulses at 5 Hz) was only slightly affected by DMPP and tubocurarine. In the presence of neostigmine (10 microM) neither tubocurarine nor DMPP significantly modulated the evoked [3H]acetylcholine release. High DMPP concentrations (10 and 30 microM) enhanced the evoked release only when the pretreatment interval was reduced from 15 min to 20 s. Tubocurarine and hexamethonium concentration-dependently inhibited the end-organ response. Hexamethonium was 250-fold more potent on presynaptic than on postsynaptic nicotine receptors. It is concluded that the motor nerve terminals are endowed with presynaptic nicotine receptors. These autoreceptors mediate a positive feed-back mechanism that can be triggered by previously released endogenous acetylcholine. Receptor desensitization can be produced by high agonist concentrations (endogenous or exogenous agonists) and is probably one mechanism to limit the autofacilitatory process. The presynaptic receptors appear to differ in their pharmacological properties from the postsynaptic receptors.

Frequency-dependence of neuromuscular blockade in the presence of d-tubocurarine

The frequency-dependence of neuromuscular blockade in the presence of d-tubocurarine (dTC) was studied in the vascular perfused rat phrenic nerve-hemidiaphragm preparation by intracellular recording. Endplate potential (EPP) amplitude decreased as the frequency of stimulation increased (2.5-40 Hz). Additionally, the slopes of the regression lines of EPP amplitude versus the log of stimulation frequency decreased when the dTC concentration was increased. Since the dependence of neuromuscular blockade on frequency was not enhanced by higher concentrations of dTC, the postjunctional channel-blocking hypothesis does not sufficiently explain frequency-dependent neuromuscular blockade in the presence of dTC. However, the present results are compatible with the hypothesis that dTC influences neuromuscular blockade via a prejunctional action, affecting acetylcholine release.

The effect of (+)-tubocurarine on neuromuscular transmission during repetitive stimulation in the rat, mouse, and frog

1. The effect of tubocurarine on amplitudes of end-plate currents in response to trains of repetitive stimulation (50-150/sec) was investigated in voltage-clamped muscle fibres of the rat, mouse and frog. 2. In rat and mouse muscle, the presence of tubocurarine led to a more rapid decline (rundown) in the amplitudes of successive end-plate currents during trains of impulses. In frog, tubocurarine caused an increase in apparent facilitation of end-plate current amplitudes during the first few impulses of repetitive stimulation; this increase was followed by a more rapid rundown of end-plate current amplitude. 3. These effects of tubocurarine appear not to be an artifact resulting from inadequate control of membrane potential in voltage-clamped fibres. 4. The more rapid rundown during trains of end-plate currents in the presence of tubocurarine showed little variation with membrane potential indicating that voltage-sensitive channel blockade by tubocurarine was not a major factor contributing to the rundown. 5. The effect of tubocurarine on the apparent facilitation and rundown of end-plate current amplitudes was typically decreased by reducing the frequency of stimulation. 6. These results suggest that tubocurarine affects transmitter release at neuromuscular junctions during repetitive stimulation.

Presynaptic action of curare

1. As a result of a conditioning phrenic nerve stimulus, end-plate currents (e.p.c.s) in a voltage clamped uncurarized cut diaphragm show a facilitation which reaches its maximum at 30-40 msec and subsequently decays with a time constant from 150 to 200 msec. In curarized (cut or uncut) diaphragms, however, the conditioning stimulus causes a depression which reaches its maximal value at 10 msec and then decays slowly with a time constant of about 3 sec. This indicates that curare strongly interferes with the process of transmitter release. 2. The presynaptic action of curare is also evident if short tetanic trains are given. In uncurarized preparations e.p.c.s decay in size much more slowly than in curarized preparations, and usually show a transient facilitation. 3. These results can be explained in terms of a model where curare blocks presynaptic depolarizing action of ACh. As a result of this presumed curare action a small increase in Ca permeability and subsequent entry of Ca associated with depolarization are also blocked, and the facilitation resulting from that entry of Ca is abolished.

Factors affecting the rate of incorporation of a false transmitter into mammalian motor nerve terminals

1. The incorporation of acetylmonoethylcholine (AMECh) into transmitter store at the mammalian neuromuscular junction has been studied using electrophysiological techniques. 2. Incubation of rat muscle in the presence of 0.1 mM-monoethylcholine (MECh) and 40 mM-K+ for 60-90 min produced a maximal reduction in the time constant of decay of synaptic currents and potentials, indicating that acetylcholine (ACh) had been replaced by AMECh in the released quanta. 3. Under resting conditions, muscles incubated for up to 24 hr in the presence of MECh showed no incorporation of AMECh into the released transmitter. In contrast, muscles pre-loaded with AMECh and then incubated in choline-containing medium showed a substantial reversion to ACh in the released transmitter within 4 hr. 4. It is suggested that this difference results from the rate of synthesis or packaging of transmitter being considerably slower with MECh than with choline, so that stimulation in the presence of MECh causes an over-all depletion of transmitter stores that does not occur with choline as the precursor. Measurements of m.e.p.c. amplitude following K+-evoked release in the presence of MECh or choline confirmed this interpretation. 5. In order to test whether newly formed AMECh is incorporated into a single homogeneous pool of transmitter from which the released quanta are derived, the rate of incorporation of AMECh into the released transmitter was measured as a function of the number of quanta released when transmitter output was increased by various methods. 6. The number of quanta released before 63% conversion of the released transmitter to AMECh was brought about varied from 1.3 X 10(5) (nerve stimulation at 3 Hz) to about 4 X 10(5) (release by high-K+ solution). With nerve stimulation in Mg2+-blocked muscles the value was 2.5 X 10(5). 7. Incorporation of AMECh into the quanta released by nerve stimulation appeared to take place more rapidly than its incorporation into spontaneously released quanta. 8. These results are discussed in terms of the compartmentation of transmitter stores in the nerve terminals.

Noradrenaline inhibition of acetylcholine release from guinea-pig brain

The effect of noradrenaline (NA) on acetylcholine (ACh) release from guinea-pig brain was investigated in superfused cerebral cortex slices and in unrestrained unanaesthetized animals provided with epidural cups. The amine reduced the ACh release from electrically stimulated tissue and its effect was antagonized by phentolamine and phenoxybenzamine, but not by propranolol and spiroperidol. The injection of NA (150 microgram) into the cerebral ventricles caused sedation, E.Co.G. synchronization and reduced ACh outflow from the parietal cortex. This inhibition was counteracted by alpha-blocking agents. A lower dose of NA (50 microgram) did not change the behaviour, but produced a late increase in ACh outflow, prevented by spiroperidol. These results fit well with the hypothesis that NA restrains, via alpha-receptors, the ACh secretion from the nerve endings and indirectly support the view that the amine reduces the firing rate of the corticopetal cholinergic neurones. The late increase in ACh outflow, observed in vivo, may be referred to secondary activation of the dopaminergic neurones, known to enhance the cortical ACh release in this animal species.

Tetanic fade during partial transmission failure produced by non-depolarizing neuromuscular blocking drugs in the cat

1. A comparison has been made of the effects of three acetylcholine antagonists--hexamethonium, tubocurarine and pancuronium--on maximal tetani of limb muscles of cats under chloralose anaesthesia. In most experiments, the indirectly stimulated soleus muscle was studied, but observations were also made on the tibialis anterior and flexor digitorum longus muscles. 2. When neuromuscular block was produced by intra-arterial injections of the acetylcholine antagonists, tetanic tension, though depressed in amplitude did not wane and there was little or no post-tetanic relief of the block as judged by the amplitude of subsequent twitches. On the other hand, during similar degrees of block produced by intravenous injections, tetanic tension rapidly waned, and, after the tetanus, transmission was temporarily facilitated, as evidenced by an increase in the amplitude of post-tetanic twitches. 3. Intravenously injected hexamethonium caused complete waning of tetanic tension in doses too small to depress twitch amplitude and which caused only a small depression of peak tetanic tension. In contrast, pancuronium caused only partial tetanic fade even in doses that produced pronounced depressions of twitch and tetanic tensions. The effects of tubocurarine fell between these extremes. 4. The results suggest that depression of peak tension and tetanic fade are independent effects of acetylcholine antagonists. It is postulated that the former is a consequence of block of post-junctional cholinoceptors, whereas the latter arises from an action of pre-junctional cholinoceptors. 5. The results obtained, together with those of other workers, led to the suggestion that transmitter acetylcholine, in addition to evoking the endplate potential, acts on the nonmyelinated nerve terminals in a positive feed-back mechanism that mobilizes transmitter to keep pace with release during high frequencies of stimulation.

The effect of curare on the release of acetylcholine from mammalian motor nerve terminals and an estimate of quantum content

Curarized and non-curarized rat hemidiaphragm muscles were indirectly stimulated in vitro. 2. The fluid bathing the active curarized muscles was eluted through a dextran gel (Sephadex G-10), effecting a complete separation of ACh from curare. The acetylcholine fraction was then assayed on an isometric leech muscle preparation. 3. Prostaglandin (PGE1) in a concentration fifteen times that estimated to be released from the skeletal muscle preparation did not affect the response of leech muscle to ACh. 4. The amount of ACh released by curarized muscles (4-9 X 10(-18) mole/impulse-junction) was not significantly different from that released by non-curarized muscles (4-6 X 10(-18) mole/impulse-junction). These quantities are similar to those obtained by previous workers. It is concluded that curare in a paralytic dose does not affect the output of ACh from motor nerve terminals stimulated at low frequencies. 5. Spontaneous release of ACh from non-curarized muscles was estimated at 0-45-0-65 p-mole/min. hemidiaphragm. It is calculated that only 2% of this amount could give rise to post-synaptic electrical events, the remainder having a non-synaptic source. 6. The number of molecules of 'quantal' ACh released by stimulated muscle is calculated as 2-5 X 10(6)/impulse-junction, taking account of the non-synaptic release. The number of ACh molecules in one quantum was estimated to be 6250, an amount that could be easily accommodated in one synaptic vesicle.

Cholinergic agents affect two receptors that modulate transmitter release at a central synapse in Aplsia californica

A unitary, monosynaptic and presumably cholinergic EPSP recorded in cell R15 of the abdominal ganglion of Aplysia californica undergoes depression followed by facilitation when the presynaptic axon is repetitively stimulated at a rate of 1-3 pulses/sec. During trains of stimulation which produced this sequence of phenomena, the effects of a large number of agents known to affect cholinergic transmission in other systems were studied. The agents could be divided into 4 classes: (1) agents having no effect upon transmission at this cholinergic junction; (2) agents of a class typified by curare, which depressed all EPSPs of a train to the same extent, and which are believed to be acting in this system solely as competitive postsynaptic blockers; (3) agents typified by acetylcholine and carbachol (ACh class), which selectively depressed earlier EPSPs of a train more than later EPSPs and which appear to act by reducing the fractional release of transmitter; (4) agents typified by trimethidinium (trimethidinium class), which selectively depress later EPSPs of a train more than earlier EPSPs and which appear to act by reducing the rate of transmitter supply into the readily releasable pool. Neither the ACh class nor the trimethidinium class produced these selective effects on different pulses in the train by changes in the postsynaptic membrane potential or membrane resistance. Nor did they act by stimulating or inhibiting other recorded inputs onto R15. Iontophoretic application of acetylcholine onto R15 indicated that the effect of trimethidinium could not be explained by an alteration in desensitization of a postsynaptic acetylcholine receptor. The structural specificity of the presynaptic receptors mediating the action of the ACh and trimethidinium classes was demonstrated by the use of a larger number of structurally related compounds.

Increased extrajunctional acetylcholine sensitivity produced by chronic acetylcholine sensitivity produced by chronic post-synaptic neuromuscular blockade

1. Anaesthetized rats were paralysed for periods of up to 3 days by chronic administration of D-tubocurarine (DTC), succinylcholine or alpha-bungarotoxin. 2. After 3 days of treatment with DTC, the phrenic nerve remained active. Neuromuscular transmission and spontaneous miniature end-plate potentials (m.e.p.p.s) were restored after removal of the DTC. Resting potentials and input resistances of muscle fibres that had been paralysed for 3 days were similar to those in denervated fibers. 3. Chronic neuromuscular blockade increased the binding of [125-I]-alpha-bungarotoxin by extrajunctional regions of muscle. The time course of the increase was similar to that seen after denervation. Binding to muscles from animals that were anaesthetized and respirated, but not paralysed, was not increased. 4. Three days of paralysis increased the sensitivity of the extrajunctional muscle membrane to acetylcholine (ACh) applied by iontophoresis. 5. Approximately the same proportion of muscle fibres from muscles paralysed for 3 days gave overshooting action potentials in the presence of tetrodotoxin 10-minus 6 g/ml. as did fibres form muscles denervated for 3 days. 6. Chronic paralysis did not change the accumulation of acetylcholinesterase above a ligation in the sciatic nerve. 7. These results are consistent with the idea that extrajunctional ACh sensitivity is normally controlled by muscle activity.

Neuromuscular transmission in a mammalian preparation in the absence of blocking drugs and the effect of D-tubocurarine

1. Neuromuscular transmission in the presence and absence of D-tubocurarine was examined in cut muscle preparations of the rat diaphragm using intracellular recording techniques. The method of localizing the end-plates is described and evidence is presented indicating that end-plate potential (e.p.p.) recordings were made close to the end-plate region and that reliable estimates of the quantum content of e.p.p.s could be made.2. During repetitive nerve stimulation at rates between 2 and 10 Hz, e.p.p. amplitudes declined in response to the first 6-8 stimuli to reach a plateau level which was maintained for 1500 stimuli. At higher rates of stimulation some 500-700 stimuli were needed before a plateau level of amplitude was reached. It was demonstrated that these amplitude changes reflected similar changes in the quantal release of ACh.3. Tubocurarine in a concentration of 4 x 10(-8) g/ml. had no effect on quantal release but did reduce quantal size. In the presence of tubocurarine, 8 x 10(-8) g/ml., small reductions in the quantum content of the first e.p.p. in response to a train of stimuli and the maintained quantal release were observed. In the presence of tubocurarine 4 x 10(-7) g/ml., the quantum content of the first e.p.p. and the maintained release were further reduced, to the level previously described for curarized preparations. At this concentration a significant effect of tubocurarine upon the presynaptic measurements could be detected (t test). Studies on single junctions revealed that the post- and presynaptic effects of tubocurarine developed with different time courses.4. Neuromuscular preparations in vivo have been reported to be much more resistant to maintatined stimulation than curarized neuromuscular preparations in vitro. It is suggested that this disparity is explained by the presynaptic action of tubocurarine.5. From the quantal release rates of the present investigation and reports of ACh release/stimulus/end-plate in previous investigations it can be calculated that a quantum of ACh contains between 12,000 and 21,000 molecules.

The prejunctional actions of some non-depolarizing blocking drugs

1. Trains of end-plate potentials (e.p.p.s) have been recorded from the isolated tenuissimus of the cat. The muscle was paralyzed either by transversely cutting the muscle fibres or by non-depolarizing blocking drugs.2. The following parameters of transmitter synthesis, storage and release have been calculated: the quantal content of the first e.p.p. in the train, the size of the available store, fractional release, quantum size, and the rate of refilling of the available store.3. Tubocurarine and benzoquinonium depressed the rate of refilling of the available store causing its depletion at high rates of stimulation. This was offset by an increase in fractional release, which in the case of tubocurarine was sufficient for the quantal content of the first e.p.p. to be unchanged.4. Dimethyltubocurarine and pancuronium had a similar effect to tubocurarine on the rate of refilling of the store and depletion of the store at high rates of stimulation but did not increase fractional release. There was, therefore, a decrease in the quantal content of the first e.p.p.5. Lignocaine depressed the rate of refilling of the store and depleted the store at high rates of stimulation. Fractional release was also depressed.6. It is suggested that the non-depolarizing drugs have a weak local anaesthetic action retarding the influx of sodium into the nerve terminal which slows the rate of refilling of the store. This effect is due to the quaternary ammonium head. The presence of a phenolic group increases fractional release due either to an increased influx of calcium into the nerve terminal or to a potentiation of the actions of calcium.

Curare and pancuronium compared: effects on previously undepressed mammalian myoneural junctions

Curare and pancuronium have multiple effects on previously undepressed rat diaphragm; these include depression of transmitter output and prolongation of the refractory period of prejunctional structures. The effect of curare on motor nerve terminals is greater than that of pancuronium. Both drugs depress postjunctional receptors; but curare, in addition, raises the threshold for the generation of muscle action potentials. In addition, these results raise questions about the validity of statistical methods used to calculate transmitter output.

Does curare affect transmitter release?

1. The effect of curare on the amount of transmitter released by a nerve stimulus was studied in frog and rat nerve-muscle preparations using electrophysiological techniques.2. When the frog sartorius nerve-muscle preparation was exposed to low doses of curare, the amplitudes of spontaneous miniature end-plate potentials and end-plate currents (e.p.c.s, measured under ;voltage clamp' conditions) were reduced to the same extent, suggesting that the drug did not alter the number of transmitter quanta released by nerve stimulation.3. With higher doses of curare in frog muscle treated with glycerol to abolish twitching, quantum content was estimated from the coefficient of variation (CV) of e.p.c.s. The measured CV increased slightly in curare; this increase probably resulted from a relatively greater contribution of random noise to the observed fluctuations when the e.p.c. was reduced by curare.4. In the rat diaphragm, muscle fibres were cut to block twitching. This procedure produced, among other changes, a reduction in the muscle fibre space constant, so that junctional signals were distorted by the cable properties of the muscle fibre when, as often occurred, micro-electrodes were more than 100-200 mu from the end-plate focus. This produced errors in estimates of quantum content; when these errors were accounted for, it appeared that curare did not significantly alter quantum content.5. It is concluded that if curare affects transmitter release at all, its effect must be much smaller than its well known post-synaptic blocking action.

The effect of catecholamines and sympathetic stimulation on the release of acetylcholine from the guinea-pig colon

1. In isolated guinea-pig terminal colon, the effect of sympathetic stimulation on contraction and acetylcholine release elicited by pelvic and transmural stimulation was investigated.2. Sympathetic stimulation reduced the nerve-mediated contractile responses more than those produced by added acetylcholine.3. Sympathetic stimulation also reduced the acetylcholine released during pelvic and transmural stimulation at low frequency. The inhibitory effect on acetylcholine released from resting colons is concealed by the simultaneous release of acetylcholine in considerable amounts from stimulated periarterial nerves which probably contain parasympathetic fibres.4. The inhibitory effect of endogenous and exogenous catecholamines prevails when cholinergic neurones fire at low rates. It was confirmed that adrenaline is more active than noradrenaline.5. The release of acetylcholine from unstimulated colons was for the most part maintained by nerve-conducted activity, because tetrodotoxin was able to reduce it to about one-tenth.6. It is suggested that the sympathetic control of gastrointestinal tone and motility is exerted through two different routes: inhibition of intramural cholinergic plexuses and direct relaxation of smooth muscle cells.7. The possible site and mechanism of action of catecholamines on intramural cholinergic structures is briefly discussed.

The action of tubocurarine and atropine on the normal and denervated rat diaphragm

1. The potency with which tubocurarine chloride (TC) and atropine sulphate (AS) influence the amplitude of the end-plate potentials was measured in the rat diaphragm. This effect was compared with the action of these drugs on brief depolarizations evoked by iontophoretic application of ACh to end-plate-free spots of the chronically denervated fibre.2. TC and AS act similarly on e.p.p.s, but the concentrations necessary to cause the same effect are 2000-times higher for AS.3. The dose-response curves for both inhibitors are unchanged by prostigmine.4. Ten to twenty-five days after denervation the ACh-potentials of the sensitized end-plate-free parts of the membrane are less responsive to curare than the normal e.p.p.s are. AS is as effective in blocking ACh potentials of denervated muscles as it is in blocking normal e.p.p.s. The curare/atropine coefficient (dose ratio for equal effect) is 0.0005 for e.p.p.s and 0.003 for ACh potentials of the denervated membrane.5. Both blocking drugs reduce the amplitude of ACh-potentials evoked in the end-plate region of normally innervated rat diaphragm fibres as effectively as they reduce the amplitude of e.p.p.s.6. Neither TC nor AS have a presynaptic action in concentrations markedly reducing the e.p.p. amplitude.

Effects of some polymethylene bis(hydroxyethyl)dimethylammonium salts on neuromuscular transmission

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Effect of choline on the release of acetylcholine and on its tissue stores in the phrenic nerve-diaphragm preparation treated with dyflos

The release of acetylcholine from the guinea-pig phrenic nerve-diaphragm preparation treated with different concentrations of dyflos during four successive stimulation periods, was detected. At 38°, after 50 and 200 μg/ml of dyflos, the amount of acetylcholine released fell abruptly after the first stimulation period; a less evident decrease in the output was observed from the preparations preincubated with 500 and 1,000 μg/ml of dyflos. A reasonably constant release was detected at 28° after 1,000 μg/ml of dyflos; at this temperature the initial output was about 40% of that at 38°. Choline chloride (3 μg/ml) added to the perfusion fluid prevented the decreasing output only in the preparations incubated with the anticholinesterase at concentrations of 500 and 1,000 μg/ml. Dyflos did not affect the time course of acetylcholine release at 28°, the initial output of acetylcholine and the release at rest under any experimental conditions or the tissue stores of acetylcholine at the end of the fourth period of stimulation. A probable impairment by dyflos of the choline transfer system is discussed.