Nicotinic antagonists produce differing amounts of tetanic fade in the isolated diaphragm of the rat

The relationship between twitch depression and twitch fade during neuromuscular block produced by vecuronium: correlation with the release of acetylcholine

Background

Train-of-four stimulation pattern following the administration of non-depolarizing neuromuscular blocking drugs reveals fade on successive contractions. Fade is caused by the release of fewer acetylcholine molecules by the fourth (A₄) than by the first stimulus (A₁). The current study was conducted to define the relationship between the clinically observed fade and the simulated decline in acetylcholine release (A₄/A₁) that would be necessary to produce it.

Methods

The T₄/T₁ ratios produced by different doses of vecuronium (15–80 μg·kg^-1) were plotted as a function of the concomitant T₁. Separately in a model of neuromuscular transmission, T₁, T₄, and T₄/T₁ were estimated using simulations in the presence and in the absence of a neuromuscular blocking drug and a stepwise decrease in A₄, but constant A₁.

Results

Vecuronium induced neuromuscular block was associated with larger T₄/T₁ ratios (less fade) during the onset than during the offset of the block. All doses caused similar fade during offset. Simulations revealed that the smallest T₄/T₁ was associated with the nadir of A₄/A₁ and occurred at the beginning of T₁ recovery. The nadir of A₄/A₁ was only marginally affected by the dose of vecuronium: 15 μg·kg^-1 producing the minimum A₄/A₁ of 0.8 and 80 μg·kg^-1 the minimum A₄/A₁ of 0.7.

Conclusion

The hysteresis in the fade between onset and offset appears to be caused by a delayed decrease of A₄/A₁ as compared with the decrease in T₁. Tentative estimates of the decrease in A₄/A₁ during fade produced by vecuronium are offered. However, the validity of these estimates is dependent on the validity of the assumptions made in simulations.

Cisatracurium: myographical and electrophysiological studies in the isolated rat muscle

Myographical and electrophysiological studies of cisatracurium were performed, in vitro, in the isolated sciatic nerve-extensor digitorum longus muscle preparation of the rat. Indirect twitches were generated at 0.1 Hz and tetanic contractions at 50 Hz. endplate potentials (epps) were generated in trains of 50 Hz. The electrophysiological variables used in the analysis of the epps were: amplitude of the first epp in the train, average amplitude of the 30 degrees to the 59 degrees epp in the train (epps-plateau), tetanic rundown (percent loss in amplitude of epps-plateau relative to the first epp in the train), quantal size and quantal content. The myographical results showed that the inhibitory concentration 50% (IC(50)) of cisatracurium for the blockade of twitches (0.48 microm) is 12 times its IC(50) for the induction of tetanic fade (0.04 microm). The electrophysiological results showed a concentration dependent decrease in the amplitudes of first epps in the trains and of epps-plateau in the two used concentrations (0.13 microm and 0.38 microm). The tetanic rundown was intensified only in the presence of the higher (0.38 microm) concentration of cisatracurium. In cisatracurium 0.13 microm (a concentration which affects only tetanic contractions, inducing their fade, while leaving the twitch unaffected) there was a decrease in the quantal content of the first epp and of epps-plateau in the train. In cisatracurium (0.38 microm), a concentration, which affects the twitch, there was a decrease of the quantal size and of quantal content of epps-plateau, but not of the quantal content of the first epp in the train. The results indicate that the fade of the tetanic contraction induced by cisatracurium at the concentration of 0.13 microm is entirely because of a pre-synaptic blocking effect while the decrease in the twitch induced by cisatracurium at the concentration of 0.38 microm is due to a post-synaptic blocking effect.

Cellular mechanisms of atracurium-induced tetanic fade in the isolated rat muscle

Although atracurium is a widely used neuromuscular blocker, we still lack knowledge regarding some of its cellular mechanisms of action. Thus, similar to other clinically used blockers atracurium induces, both in vivo and in vitro, fade of the tetanic contraction. However, the cellular mechanisms underlying this tetanic fade have never been systematically studied. In the present work these mechanisms were investigated in vitro. A sciatic nerve extensor digitorum longus muscle preparation of the rat was used. A combination of myographical and electrophysiological techniques was employed. Indirect twitches were evoked at 0.1 Hz and tetanic contractions at 50 Hz. Trains of end-plate potentials were evoked at a frequency of 50 Hz. The electrophysiological variables used in the analysis of the trains of end-plate potentials were: peak amplitude of the first end-plate potential in the train, peak amplitude of plateau end-plate potentials in the train, tetanic run-down of the end-plate potentials' train, quantal content of first and plateau end-plate potentials in the train, quantal size. In the myographical study atracurium, at a concentration of 2.4 microM, induced a complete fade of the tetanic contraction while only slightly affected the twitch. In the electrophysiological study atracurium, at the same 2.4 microM concentration, significantly decreased the amplitude of both first end-plate potentials in the train (control: 14.4 mV; atracurium: 3.2 mV) and plateau end-plate potentials (control: 10.8 mV; atracurium: 2.4 mV) and reinforced the tetanic run-down of the train of end-plate potentials, evaluated as the percent loss in amplitude of plateau end-plate potentials compared to first end-plate potentials in the trains (control: 25.2%; atracurium: 33.2%). Atracurium also significantly decreased the quantal content of first end-plate potentials in the train (control: 231; atracurium: 68), the quantal content of plateau end-plate potentials (control: 159; atracurium: 42) and the quantal size (control: 0.119 mV; atracurium: 0.075 mV). In relative terms the decrease in quantal content was about twice as large as the decrease in quantal size. This indicates that the fade of the tetanic contraction induced by atracurium (2.4 microM) is due to both pre- and postsynaptic blocking effects, the presynaptic one being stronger.

Blockade of neuronal facilitatory nicotinic receptors containing alpha 3 beta 2 subunits contribute to tetanic fade in the rat isolated diaphragm

Nicotinic receptor (nAChR) subtypes involved in pre- and postjunctional actions underlying tetanic fade were studied in rat phrenic-nerve hemidiaphragms. We investigated the ability of subtype-specific nAChR antagonists to depress nerve-evoked contractions and [(3)H]-acetylcholine ([(3)H]-ACh) release. Muscle tension was transiently increased during brief high frequency trains (50 Hz for 5 sec). The rank potency order of nAChR antagonists to reduce tetanic peak tension was alpha-bungarotoxin > d-tubocurarine >> mecamylamine > hexamethonium. Reduction of maximal tetanic tension produced by dihydro-beta-erythroidine (0.03-10 microM), methyllycaconitine (0.003-3 microM), and alpha-conotoxin MII (0.001-0.3 microM) did not exceed 30%. Besides reduction of peak tension d-tubocurarine (0.1-0.7 microM), mecamylamine (0.1-300 microM), and hexamethonium (30-3,000 microM) also caused tetanic fading. With alpha-conotoxin MII (0.001-0.3 microM) and dihydro-beta-erythroidine (0.03-10 microM), tetanic fade was evident only after decreasing the safety factor of neuromuscular transmission (with high magnesium ions, 6-7 mM). The antagonist rank potency order to reduce evoked (50 Hz for 5 sec) [(3)H]-ACh release from motor nerve terminals was alpha-conotoxin MII (0.1 microM) > dihydro-beta-erythroidine (1 microM) approximately d-tubocurarine (1 microM) > mecamylamine (100 microM) > hexamethonium (1,000 microM). When applied in a concentration (0.3 microM) above that producing tetanic paralysis, alpha-bungarotoxin failed to affect [(3)H]-ACh release. Data obtained suggest that postjunctional neuromuscular relaxants interact with alpha-bungarotoxin-sensitive nicotinic receptors containing alpha1-subunits, whereas blockade of neuronal alpha3beta2-containing receptors produce tetanic fade by breaking nicotinic autofacilitation of acetylcholine release.

Neuromuscular effects of candoxin, a novel toxin from the venom of the Malayan krait (Bungarus candidus)

1 Candoxin (MW 7334.6), a novel toxin isolated from the venom of the Malayan krait Bungarus candidus, belongs to the poorly characterized subfamily of nonconventional three-finger toxins present in Elapid venoms. The current study details the pharmacological effects of candoxin at the neuromuscular junction. 2 Candoxin produces a novel pattern of neuromuscular blockade in isolated nerve-muscle preparations and the tibialis anterior muscle of anaesthetized rats. In contrast to the virtually irreversible postsynaptic neuromuscular blockade produced by curaremimetic alpha-neurotoxins, the neuromuscular blockade produced by candoxin was rapidly and completely reversed by washing or by the addition of the anticholinesterase neostigmine. 3 Candoxin also produced significant train-of-four fade during the onset of and recovery from neuromuscular blockade, both, in vitro and in vivo. The fade phenomenon has been attributed to a blockade of putative presynaptic nicotinic acetylcholine receptors (nAChRs) that mediate a positive feedback mechanism and maintain adequate transmitter release during rapid repetitive stimulation. In this respect, candoxin closely resembles the neuromuscular blocking effects of d-tubocurarine, and differs markedly from curaremimetic alpha-neurotoxins that produce little or no fade. 4 Electrophysiological experiments confirmed that candoxin produced a readily reversible blockade (IC(50) approximately 10 nM) of oocyte-expressed muscle (alphabetagammadelta) nAChRs. Like alpha-conotoxin MI, well known for its preferential binding to the alpha/delta interface of the muscle (alphabetagammadelta) nAChR, candoxin also demonstrated a biphasic concentration-response inhibition curve with a high- (IC(50) approximately 2.2 nM) and a low- (IC(50) approximately 98 nM) affinity component, suggesting that it may exhibit differential affinities for the two binding sites on the muscle (alphabetagammadelta) receptor. In contrast, curaremimetic alpha-neurotoxins have been reported to antagonize both binding sites with equal affinity.

Endogenous adenosine prevents post-tetanic release facilitation mediated by alpha3beta2 nicotinic autoreceptors

We investigated the modulatory role of endogenous adenosine on tetanic-induced (50 Hz for 5 s) nicotinic facilitation of [3H]acetylcholine release (5 Hz for 50 s) from rat motoneurons. Adenosine deaminase (0.5 U/ml) and the adenosine A(2A) receptor antagonist, 3,7-dimethyl-1-propargyl xanthine (DMPX, 30 microM), facilitated post-tetanic [3H]acetylcholine release. Release inhibition caused by tubocurarine (1 microM), dihydro-beta-erythroidine (1 microM) and alpha-conotoxin MII (0.1 microM) was attenuated after tetanic preconditioning. Nicotinic inhibitory action was fully restored after adenosine A(2A) receptor block by DMPX or adenosine deaminase. DMPX (10 microM) caused a leftward shift of the inhibitory dose-response curves for d-tubocurarine (0.1-1 microM), dihydro-beta-erythroidine (0.03-10 microM) and alpha-conotoxin MII (1-300 nM) on post-tetanic twitch amplitude. In contrast, the post-tetanic twitch depression caused by alpha-bungarotoxin (3-100 nM, which had no effect on transmitter release) was attenuated by DMPX (10 microM). It is concluded that activation of adenosine A(2A) receptors by endogenously generated adenosine prevents the post-tetanic release facilitation mediated by nicotinic alpha3beta2 autoreceptors.

A new short-acting non-depolarizing muscle relaxant (SZ1677) without cardiovascular side-effects

BACKGROUND

In order to facilitate rapid tracheal intubation, the development of a rapid onset, short duration, non-depolarizing muscle relaxant without cardiovascular side-effects would be a significant accomplishment in the field of anesthesiology. The aim of the present study was to test the action of a new non-depolarizing muscle relaxant (SZ1677) on neuromuscular transmission, muscarinic (M2, M3) receptors and cardiovascular reactions and to compare it with clinically used muscle relaxants.

METHODS

Neuromuscular transmission was studied by recording muscle contractions elicited by indirect electrical stimulation, using (i). in vitro isolated phrenic nerve-hemidiaphragm preparation of mice, rats and guinea pigs and (ii). in vivo sciatic nerve-anterior tibial muscle preparation of anesthetized rats, guinea pigs and cats. Cardiovascular effects of muscle relaxants were evaluated on the grounds of their effects on changes of blood pressure and heart rate induced by electrical stimulation of the right vagal nerve in anesthetized cats. To study postsynaptic antimuscarinic affinity of muscle relaxants on M3 receptors, oxotremorine-induced contractions of longitudinal muscle strip of guinea pig ileum were registered in their presence and absence.

RESULTS

One of more than 120 newly synthesized non-depolarizing muscle relaxants compounds, 1-3[alpha-hydroxy-17beta-acetyloxy-2beta-(1,4-dioxa-8-azaspiro[4,5]dec-8-yl)-5alpha-androstane-16beta-il] -1-(2-propenyl)pyrrolidinium bromide (SZ1677), excelled with its advantageous pharmacological properties: relatively short duration of action, no accumulation and lack of unwanted side-effects. Pharmacodynamic studies show that SZ1677 is a non-depolarizing neuromuscular blocking agent with a relatively short duration and rapid onset of action in a variety of laboratory animal species. It is without cumulative effect, does not reduce blood pressure, and fails to produce tachycardia. Significant cardiac vagal blocking effects were not observed even at concentrations or dosages of 8 times the ED90. This compound, unlike many other muscle relaxants, does not have atropine-like effects on human atrial tissue; it does not increase the release of NA from sympathetic innervation in the heart. In all practical ways, at least from the vantage point of the preclinical study, SZ1677 compares favorably with all presently available short-acting muscle relaxants, including rapacuronium.

CONCLUSION

In experiments, SZ1677 proved to be a short-acting neuromuscular blocking compound having a large safety margin between the doses required to produce neuromuscular block and those likely to lead to cardiovascular side-effects.

Pharmacological characterization of mikatoxin, an alpha-neurotoxin isolated from the venom of the New-Guinean small-eyed snake Micropechis ikaheka

Symptoms of envenomation by the New-Guinean small-eyed snake Micropechis ikaheka (Elapidae) include peripheral neurotoxicity and myotoxicity. We have now purified to homogeneity a long-chain neurotoxin, mikatoxin, from M. ikaheka venom by successive gel filtration and reverse-phase chromatography. Electrospray ionization mass spectrometry showed mikatoxin to be a homogenous peptide of MW 7775.6. Mikatoxin was devoid of any phospholipase A(2) activity associated with the crude venom and did not exhibit any intrinsic anticholinesterase activity. In the chick biventer cervicis muscle, it produced an irreversible, concentration-dependent block of responses to exogenously applied acetylcholine and carbachol as well as twitches evoked by nerve, but not by direct muscle stimulation. Moreover, mikatoxin, like alpha-bungarotoxin and erabutoxin-b, did not show significant fade response to train-of-four stimulation of the mouse phrenic nerve-hemi diaphragm muscle. It also failed to block ganglionic transmission in the guinea pig ileum and muscarinic responses in the rat anococcygeus muscle. Our study provides strong evidence for the presence of a neurotoxin (mikatoxin) in M. ikaheka venom that produces neuromuscular blockade in skeletal muscle attributable to selective and irreversible antagonism of postsynaptic nicotinic acetylcholine receptors of the neuromuscular junction and likely contributes to the peripheral neurotoxicity observed in M. ikaheka envenomation.

Modulation by presynaptic adenosine A1 receptors of nicotinic receptor antagonist-induced neuromuscular block in the mouse

We have investigated how altering the activation of adenosine A1 receptors modifies nicotinic receptor antagonist-induced fade of tetanic contractions in the mouse isolated hemi-diaphragm. Vecuronium-induced tetanic fade was attenuated by an adenosine A1 receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX, 10(-7) M) and by an inhibitor of the synthesis of extracellular adenosine from ATP (alpha,beta-methylene ADP, MeADP, 5 x 10(-5) M). Conversely, vecuronium-induced tetanic fade was potentiated by an adenosine A1 receptor agonist (N6-cyclohexyladenosine, CHA, 10(-7) M) and an inhibitor of the extracellular destruction of adenosine (erythro-9-[2-hydroxy-3-nonyl]adenine, EHNA, 10(-4) M). The ability of an adenosine A1 receptor antagonist to attenuate vecuronium-induced tetanic fade indicates that a component of this fade is due to endogenous adenosine. Further, the ability of the inhibitor of adenosine synthesis to attenuate vecuronium-induced tetanic fade indicates that this endogenous adenosine is derived from ATP. Hexamethonium-induced tetanic fade was also potentiated by increasing adenosine A1 receptor activation, albeit with a higher concentration of CHA (10(-4) M). However, unlike for vecuronium, hexamethonium-induced tetanic fade was not attenuated by reducing adenosine A receptor activation. This latter observation suggests that the tetanic fade produced by hexamethonium and vecuronium does not share a common mechanism of action.

The effect of rate of stimulation on force of contraction in a partially paralyzed rat phrenic nerve hemidiaphragm preparation

This study was performed to determine whether presynaptic receptor blockade could be differentiated from postsynaptic blockade by examining the effect of increasing rates of indirect stimulation on twitch height depression (THD) on partially paralyzed in vitro rat diaphragm preparations. We calculated the T200/T1 ratio (force of the 200th stimuli divided by the force of the first stimuli) at rates of 0.2 Hz, 0.5 Hz, 1 Hz, and 2 Hz using a drug concentration which provided approximately 20% THD during stimulation at 0.1 Hz. Markedly different T200/T1 ratios were demonstrated when hexamethonium, a drug with predominantly presynaptic effects, was compared with alpha bungarotoxin, a drug with predominantly postsynaptic effects. These results were then compared with those from vecuronium, rocuronium, mivacurium, and tubocurarine. Both hexamethonium and rocuronium caused a marked decrease in T200/T1 ratio at higher rates of stimulation; alpha bungarotoxin caused a slight increase in T200/T1 ratio at higher rates of stimulation. The T200/T1 ratios produced by vecuronium, mivacurium, and tubocurarine lay intermediate between hexamethonium and alpha bungarotoxin. Significant differences in T200/T1 ratios were found when alpha bungarotoxin was compared with all other drugs at 2 Hz. Hexamethonium and rocuronium produced significant differences in T200/T1 ratio from those of all the other drugs at 1 Hz and 2 Hz. There were significant differences in the T200/T1 ratio found after hexamethonium and rocuronium compared to alpha bungarotoxin at 0.5 Hz. No significant differences at any rate of stimulation were found between hexamethonium and rocuronium. No difference was observed in the effect of vecuronium, mivacurium, and tubocurarine. We conclude that, if the observed effect is the result of hexamethonium acting predominantly at presynaptic sites and alpha bungarotoxin acting predominantly at postsynaptic sites, the relative contribution of small doses of nondepolarizing drugs at each site can be differentiated by determining the T200/T1 ratio at rates of 1 Hz or 2 Hz. Our results are consistent with the suggestion that small doses of rocuronium have marked presynaptic activity, but that vecuronium, mivacurium, and tubocurarine have both pre- and postsynaptic effects.

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.

Neuromuscular blocking profile of the vecuronium analogue, Org-9487, in the rat isolated hemidiaphragm preparation

1. The neuromuscular effects of the short-acting aminosteroid muscle relaxant Org-9487 have been studied in the in vitro rat phrenic nerve/hemidiaphragm preparation by use of twitch tension and electrophysiological recording techniques. 2. Org-9487 (5-100 microM) produced a concentration-dependent decrease in the amplitude of twitches (0.1 Hz) and tetanic contractions (50 Hz) evoked by motor nerve stimulation. The compound produced fade of force during both 50 Hz stimulation and train-of-four stimulation at 2 Hz, indicating a prejunctional component of action. 3. Anticholinesterases only partially reversed the effect of Org-9487 on twitch responses. This was possibly because, at the concentrations required to block twitches in the rat, Org-9487 itself was found to possess significant anticholinesterase activity. 4. Org-9487 (3 microM) increased the rundown of endplate current amplitudes during a 2 s train of 50 Hz nerve stimulation. This was because Org-9487 increased the quantal content of the first endplate current in the train without affecting acetylcholine release towards the latter part of the train. 5. Org-9487 (10 microM) produced a voltage-dependent decrease in the time constant of decay of endplate currents at 32 degrees C and 0.5 Hz, indicative of a block of endplate ion channels. The blocking rate constant increased with membrane hyperpolarization.

Simultaneous comparison of nicotinic receptor antagonists on three nicotinic acetylcholine receptors

The relative potencies of several nicotinic cholinoceptor antagonists in producing tetanic fade and reduction of striated muscle contraction were investigated in the isolated guinea-pig oesophagus as well as the guinea-pig and rat phrenic nerve-diaphragm preparations. Contractile smooth muscle responses to vagal stimulation, which involves ganglionic activation, were also measured simultaneously with striated muscle responses in the oesophagus. The relative potency for inhibiting the response of oesophageal smooth muscle to vagal stimulation (20 Hz) was trimetaphan > mecamylamine > hexamethonium > tubocurarine > pancuronium. For oesophageal striated muscle, production of tetanic fade at 100 Hz and reduction in peak tetanic tension at 20 or 100 Hz showed a similar relative potency; pancuronium > tubocurarine > mecamylamine > trimetaphan > hexamethonium and similar results were obtained in the guinea-pig diaphragm for the antagonists investigated (pancuronium, tubocurarine and mecamylamine). In the rat phrenic nerve-diaphragm preparation, production of tetanic fade at 50 Hz and reduction in twitch or tetanic tension all showed the relative potency; tubocurarine > pancuronium > mecamylamine > trimetaphan > hexamethonium. These findings indicate differences in the nicotinic cholinoceptor subtypes involved in vagal ganglionic responses and those in tetanic fade.

Interaction of decamethonium with hexamethonium or vecuronium in the rat: an isobolographic analysis

We used isobolographic analysis to investigate the interaction of decamethonium with either hexamethonium or vecuronium in the rat phrenic nerve hemidiaphragm preparation. EC50 values of decamethonium, hexamethonium, and vecuronium were (mean +/- SEM) 47.36 +/- 9.58 microM, 4.27 +/- 0.53 mM, and 5.19 +/- 1.17 microM, respectively. Combinations of drugs in concentrations corresponding to the 1:2, 1:1, and 2:1 ratios of their EC50 values were used to determine three points of each isobole. Decamethonium and hexamethonium showed antagonism: significant deviations from the line of additivity were found at EC50 ratios of 2:1 and 1:1 (P < 0.01 and P < 0.05, respectively) indicating that hexamethonium is a potent antagonist of decamethonium. For decamethonium and vecuronium none of the three points on the isobole was significantly different from the corresponding point on the line of additivity. Hexamethonium is known to be a weak antagonist at the postsynaptic nicotinic acetylcholine receptor but a potent antagonist at the presynaptic nicotinic receptor. Vecuronium is a more potent antagonist at the postsynaptic nicotinic receptor but a much weaker antagonist at the presynaptic site. It was postulated that in the rat the primary site of action of decamethonium is at the presynaptic nerve terminal. Our findings suggest that presynaptic rather than postsynaptic potency of a nondepolarizing drug determines ability to antagonize the effect of a depolarizing drug in the rat.

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.

Inhibitory action of nicotinic antagonists on transmitter release at the neuromuscular junction of the rat

The effects of two nicotinic antagonists, d-tubocurarine (TC) and hexamethonium (HEX) were tested on the rat diaphragm neuromuscular junction during train-of-six stimuli to determine if a second action of these antagonists on evoked release could be demonstrated, in addition to its known impact of blocking the autoreceptor pathway. To minimize the autoreceptor pathway, the preparations were examined under low transmitter release conditions. It was observed that both compounds significantly depressed the end-plate potential amplitudes more than the miniature end-plate potential amplitudes, while also significantly depressing quantal release output. This inhibitory action is contrary to what is observed when transmitter release is high, where feedback regulation via the autoreceptors serves a prominent role. It is concluded that this depressive action on transmitter output contributes to onset of tetanic fade and that when higher concentrations of these antagonists are used this inhibitory action of TC and HEX may override autoreceptor feedback regulation.

In vitro potency and mode of action of ANQ9040: a novel fast acting muscle relaxant

1. The in vitro potency and mode of action of the novel, rapid-onset steroidal relaxant ANQ9040 were characterized in the rat isolated phrenic nerve hemidiaphragm. 2. At 32 degrees C, ANQ9040 antagonized neurally evoked contractures with EC50s of 21.5 microM for unitary twitches; 14.4 microM for 2 Hz 'trains of four'; and 7.5 microM for 50 Hz (2 s) tetanic stimulus trains. 3. (+)-Tubocurarine was 22-24 times more potent than ANQ9040 in comparative organ bath experiments. 4. Intracellular recording from endplates revealed that ANQ9040 (0.53-10.0 microM) dose-dependently and reversibly decreased the amplitude of miniature-endplate potentials (IC50 of circa 0.95 microM) without changing transmembrane potential. 5. Surmountable antagonism of subthreshold responses to exogenous (ionophoretic) acetylcholine provided evidence for a non-depolarizing and competitive blockade of post-junctional nicotinic receptors. 6. Sucrose gap recordings of phrenic nerve action potentials revealed that, at concentrations up to 32 microM, ANQ9040 produced no tonic or frequency-dependent antagonism of axonic Na+ channels. 7. We conclude that ANQ9040 is a relatively low-affinity, non-depolarizing, nicotinic antagonist. The in vitro results are discussed in relation to factors impinging on relaxant kinetics and current models for frequency-dependent fade.

Effects of hexamethonium on transmitter release from the rat phrenic nerve

Hexamethonium (HEX) was applied to isolated transected diaphragm-phrenic nerve preparations of the rat in order to further elucidate the functional role of the presynaptic nicotinic autoreceptors. End-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) were recorded from the neuromuscular junctions in the presence and absence of HEX to determine the relative effect of this nicotinic antagonist on end-plate sensitivity and evoked release. In this study we show that HEX enhances transmitter release for the first few stimuli, but this action is not maintained during a train-of-six stimulation. While these results support the hypothesis that transmitter released from the nerve terminal normally has a negative feedback effect by depressing transmitter release it is proposed that HEX also has secondary actions on the neuromuscular junction that are unrelated to autoreceptor blockage. The results with HEX suggests that the presynaptic receptors may differ pharmacologically from the postsynaptic receptors.

Effects of velnacrine, tacrine and physostigmine on tetanic twitch responses at the rat neuromuscular junction

The effects of velnacrine (1-hydroxytacrine), tacrine and physostigmine on indirectly elicited twitch at low and high stimulation frequencies were analyzed in the rat phrenic hemidiaphragm preparation. At 0.2 Hz, velnacrine and physostigmine behaved in a similar manner, the latter showing a higher potentiating effect. This potentiation was observed at 3-100 microM velnacrine, whereas a slight depression appeared at higher concentrations. When tetanic responses were studied, the drug concentrations needed to depress tetanic tension and tetanic fade were quite different in the case of velnacrine (depression of tetanic tension from 1 microM and tetanic fade from 170 microM), whereas physostigmine and tacrine were able to affect these parameters at very similar concentrations. The results suggest that some effects of velnacrine could differ from those of tacrine in spite of the chemical similarity.

alpha-Conotoxin GI produces tetanic fade at the rat neuromuscular junction

The ability of the marine snail toxin, alpha-conotoxin GI, to produce blockade of singly evoked twitches and to produce tetanic and train-of-four fade has been determined in the isolated rat hemidiaphragm preparation. Results were compared to those obtained with a reversible (vecuronium) and an irreversible (alpha-bungarotoxin) nicotinic acetylcholine antagonist and have been interpreted in terms of relative effects on post- and prejunctional nicotinic acetylcholine receptors at the neuromuscular junction. alpha-Conotoxin GI (0.5-2 microM) produced a concentration-dependent, readily reversible, decrease in the peak amplitude of single twitches and 50 Hz tetani, and an increase in tetanic and train-of-four fade. alpha-Conotoxin GI was consistently 2-3-fold more potent than vecuronium with respect to all of the measured tension parameters. Both alpha-conotoxin GI and vecuronium were approximately 2-fold more potent in producing tetanic fade and in blocking tetanic contractions than in blocking single twitches. In contrast to both alpha-conotoxin GI and vecuronium, alpha-bungarotoxin (0.13 microM) reduced the peak amplitude of both single twitches and 50 Hz tetani to the same extent without the appearance of a large degree of tetanic or train-of-four fade. Based on a comparison of the in vitro time course of neuromuscular block and of the relative effects of vecuronium, alpha-conotoxin GI and alpha-bungarotoxin on twitches, tetani and trains-of-four, we conclude that alpha-conotoxin GI has both pre- and postjunctional activity at the neuromuscular junction. In this respect, alpha-conotoxin GI resembles the clinically used competitive neuromuscular blocking drugs rather than the irreversible snake alpha-neurotoxins.

Nicotinic receptors on the rat phrenic nerve: evidence for negative feedback

The effects of low concentrations (nanomolar) of d-tubocurarine (TC) on end-plate potential (EPP) and miniature end-plate potential (MEPP) amplitude, and quantal transmitter release were examined at the rat neuromuscular junction in an attempt to identify the functional role of nicotinic receptors on the nerve terminal. TC (50 and 75 nM) significantly depressed the MEPP amplitude but not the amplitude of the initial EPPs during a train-of-six stimulation at 50 Hz. The lack of depression in EPP amplitude by TC was due to an increase in quantal release. The nearly equipotent response of the pre- and post-synaptic effects of TC suggests that the autoreceptors on the nerve terminal are very similar to the nicotinic receptors on the end-plate. These results suggest that nicotinic autoreceptors are functional even with a single action potential. The results support the hypothesis that ACh released from the nerve terminal normally has a negative feedback effect by depressing transmitter release.

Run-down of neuromuscular transmission during repetitive nerve activity by nicotinic antagonists is not due to desensitization of the postsynaptic receptor

1. Whether the function of the postsynaptic acetylcholine receptor is use-dependently affected by repetitive nerve stimulation in the presence of competitive antagonists was studied in the mouse phrenic nerve-hemidiaphragm preparation. 2. For electrophysiological experiments, the preparation was immobilized by synthetic mu-conotoxin, which preferentially blocks muscular Na-channels causing neither depolarization of the membrane potential, inhibition of quantal transmitter release, nor depression of nicotinic receptor function. 3. High concentrations of cobratoxin depressed indirect twitches and endplate potentials (e.p.ps) without inducing waning of contractilities or run-down of trains of e.p.ps evoked at 10-100 Hz. However, waning and run-down were accelerated after washout of the toxin despite diminished postsynaptic receptor blockade. Once the run-down of e.p.ps was produced by washout or low concentrations of cobratoxin, further depression of e.p.p. amplitude with high concentrations of cobratoxin did not attenuate the e.p.p. run-down. 4. The degrees of waning of tetanus and trains of e.p.ps produced by a very high concentration of tubocurarine (20 microM) were also less than that caused at a 100 fold lower concentration, albeit the amplitudes of twitches and the first e.p.p. were depressed more rapidly and markedly. 5. Tubocurarine, like cobratoxin, depressed the amplitude of miniature endplate potentials (m.e.p.ps) more than e.p.ps. 6. In contrast to the steepened run-down of successive e.p.ps in the presence of low concentrations of either nicotinic antagonists, the amplitude of m.e.p.ps observed during repetitive stimulation was uniform and was not different from that before stimulation. 7. The results suggest that the e.p.p. run-down and tetanic fade induced by nicotinic antagonists are due to a slow kinetic blockade of presynaptic receptors and confirm that the e.p.p. run-down is not produced by a use-dependent failure of postsynaptic nicotinic receptors. The roles of the presynaptic nicotinic receptor in positive or negative feedback modulations of transmitter release are discussed.

The alpha-neurotoxin erabutoxin b causes fade at the rat end-plate

d-Tubocurarine and the alpha-neurotoxins from snake venom are antagonists at the nicotinic acetylcholine receptor. It is well established that d-tubocurarine causes fade in neuromuscular transmission during repetitive nerve stimulation but paradoxically there are many reports which indicate that the alpha-neurotoxins do not cause such fade. We found that high concentrations of erabutoxin b (100-150 nM) from the venom of Laticauda semifasciata did not cause much fade in the rat diaphragm preparation. However, low concentrations of toxin (5 nM) caused severe fade which was similar to the effects of d-tubocurarine. The data suggest that fade may be caused by toxin binding to a high-affinity site on the postsynaptic acetylcholine receptor.

Influence of extracellular calcium in the effects of dibekacin and diltiazem on indirectly elicited tetanic responses in the rat phrenic-hemidiaphragm preparation

1. Dibekacin (70 microM-3 mM) produced a decrease of peak tetanic tension in a concentration-dependent manner and this effect was dependent on extracellular calcium (0.3-2.5 mM Ca2+). Only minimal fade was observed and it was not related with extracellular calcium concentrations. 2. Diltiazem (30-300 microM) decreased peak tetanic tension and produced tetanic fade. Both effects were independent of extracellular calcium, although a significant potentiation was observed at 0.3 mM calcium. 3. It is concluded that tetanic parameters are related differently to extracellular calcium.

In vitro time course studies on train-of-four fade induced by hexamethonium, pancuronium and decamethonium in the rat hemidiaphragm

1. In vitro time course studies on the effects of hexamethonium (7 mmol/L), pancuronium (5 mumol/L) and decamethonium (220 mumol/L) on nerve-evoked (2 Hz for 2 s every 20 s) maximal twitches (T1, T2, T3, T4) of the rat hemidiaphragm were conducted. All three drugs progressively depressed all four twitches in a given train but at different rates (T4 greater than T3 greater than T2 much greater than T1). 2. The response-time profiles for T1 and T4 varied widely for the three drugs such that, for the same degree of T1-block, each drug produced a different magnitude of T4-block during the onset of and recovery from neuromuscular blockade. 3. Analysis of the T1 versus T4/T1 plot showed that, at 50% T1-block, the corresponding T4/T1 (i.e. train-of-four ratios) during the onset (and recovery) phase were 0.16 (0.29), 0.46 (0.40) and 0.66 (0.53) for hexamethonium, pancuronium and decamethonium, respectively. Thus, for the same degree (i.e. 50%) of twitch (T1) tension depression, the three drugs differed widely in their ability (hexamethonium much greater than pancuronium greater than decamethonium) to produce fade as reflected in the respective train-of-four ratio. 4. Our results therefore show that the train-of-four ratio (T4/T1) at 50% T1-block obtained from such in vitro time course studies is a useful quantitative index of the potential of various drugs to cause train-of-four fade. Based on this index a classification of various compounds already studied is proposed as follows: hexamethonium much greater than pancuronium approximately (+)-tubocurarine greater than decamethonium approximately succinylcholine much greater than alpha-bungarotoxin.

Blocking effect of diltiazem in the isolated rat phrenic hemidiaphragm

The effects of diltiazem on indirectly and directly elicited twitch were studied in the isolated rat phrenic hemidiaphragm preparation. Diltiazem (30-500 microM) blocked the indirectly elicited twitch response and this effect was not affected by reducing the extracellular calcium from 2.5 to 1.25 mM. An effect on the directly elicited twitch was also observed (100-300 microM). Diltiazem (30-300 microM) blocked the peak tetanic tension and tetanic fade was present. The results were consistent with an action of diltiazem on the nicotinic receptor-ion channel complex.

The effects of tetraethylammonium during twitch and tetanic stimulation of the phrenic nerve diaphragm preparation in the rat

Tetraethylammonium (TEA) (2.6 x 10(-3) M) potentiated the twitches of the indirectly- or directly-stimulated phrenic nerve diaphragm of the rat at 37 degrees C by prolonging the action potential of the sarcolemma, due to an inhibition of the repolarizing K+ current. With indirect stimulation, TEA caused a use-dependent inhibition of tetanic contractions, induced every 10 min by 10 sec of 50 Hz stimulation, and a post-tetanic depression of the twitches was observed after about 40 min. Recording of the electromyogram (EMG) and compound action potentials of the phrenic nerve, localized the two inhibitory effects to the neuromuscular junction. They were caused by different mechanisms of action. Choline (3.6 x 10(-4) M) antagonized the depression of the twitch but not the use-dependent inhibition. Lowering the temperature to 20 degrees C reduced the depression of the twitch, whereas the use-dependent inhibition was enhanced. The release of transmitter was probably normal during tetanic stimulation; a post-synaptic desensitization of acetylcholine (ACh) receptors caused the inhibition. Microelectrode recordings of endplate potentials supported this conclusion. The depression of the twitch was due to a presynaptic depletion of transmitter. This was confirmed by inducing an additional depletion and depression of the twitch with N-ethyl-maleimide (2.5 x 10(-5) M). Since the depression of the twitch was antagonized by choline, the depletion was probably due to an inhibited uptake of choline into the nerve terminals.

Assessment of neuromuscular blockade produced by atracurium in the rat diaphragm preparation. Measurements of tetanic fade, depression and recovery profile

The effect of atracurium, a relatively new muscle relaxant, on neuromuscular transmission, in the rat diaphragm preparation, was studied, by analysing the characteristic features of tetanic fade and recovery pattern following a blocking concentration of atracurium (10 microns). Tetanic fade (TF) and peak tetanic tension (Tp) and its depression by atracurium, were analysed and the results were interpreted in terms of atracurium action at the neuromuscular junction. Atracurium reduced the sustained tetanic tension, elicited at 50 Hz for 0.5 s duration, and produced a marked tetanic fade in 38 s. Atracurium also reduced the peak tetanic tension by 40%, of the control value, in 38 s. Maximum tetanic tension was 5.7 g tension, and the time taken to completely block the tetanus was 4.75 +/- 0.15 min (means +/- SE, n = 8). Recovery from atracurium-induced blockade occurred in 30s (tetanic fade) and in 3-4 min (peak tetanic tension). It was concluded that atracurium produces a profound tetanic fade, at a time when the peak tetanic tension is reduced by only 40%. The data presented indicate that atracurium has a rapid onset of blockade, intermediate duration and a quick recovery profile at the rat neuromuscular junction.

Effects of tubocurarine on end-plate current rundown and quantal content during rapid nerve stimulation in the snake

1. The effects of tubocurarine, pancuronium, vecuronium and trimethyl-tubocurarine were examined on the amplitudes of end-plate currents (EPC) produced by repetitive nerve stimulation in cut voltage-clamped fibres of the costocutaneous muscle of the garter snake. The effects of tubocurarine were further studied on the amplitudes of miniature end-plate currents (MEPC) recorded immediately before the trains of nerve stimulation, thus allowing estimates to be made of EPC quantal content. 2. All four compounds increased the decline of EPC amplitude with successive impulses (rundown), compared with that observed in control end-plates. There was little effect on the decay time constants of EPC and the effects on rundown were not dependent on membrane potential, indicating that end-plate ion channel blockade was not a major effect in producing rundown. 3. In the presence of tubocurarine the quantal content of the EPC in the plateau phase of the trains was reduced when compared with control, indicating a reduction of transmitter mobilization. However tubocurarine had no significant effect on the quantal content of the first EPC of the trains, indicating that the decreased mobilization is not due to an initial effect on transmitter release. 4. It is suggested that the prejunctional effect of tubocurarine which results in increased EPC train rundown is primarily on transmitter mobilization. It is also suggested that the actions of tubocurarine and the other neuromuscular blockers tested are mediated through a prejunctional nicotinic receptor which may act as a positive feedback mechanism modulating transmitter mobilization.

Multiple effects of alpha-toxins on the nicotinic acetylcholine receptor

Very low concentrations (5 nM) of alpha-toxin from the venom of Naja naja atra produced a characteristic fade in muscle compound action potential and tetanus induced by repetitive nerve stimulation which was identical to the effects of curare. High concentrations of alpha-toxin and all concentrations of alpha-bungarotoxin reduced the response but produced very little fade in comparison to curare. These results suggest that alpha-toxins have more than one effect at the neuromuscular junction.

Tetanic fade during neuromuscular blockade produced by atracurium in the rat diaphragm preparation

In the present investigation, we studied and measured the phenomenon of tetanic fade and peak tetanic tension depression in the rat diaphragm preparation in the presence of a blocking concentration of atracurium (e.g., 10 mumol.l-1). Atracurium (10 mumol.l-1) produced a pronounced tetanic fade (i.e., 47-69% reduction of total sustained tetanic tension) at a time (15 s) when it reduced the peak tetanic tension by only 25%. The time course for total tetanic fade was 30-35 s, whereas the time taken for complete peak tetanic tension depression was 3-3.5 min, suggesting that the two effects were produced via different mechanisms, involving presynaptic and postsynaptic mechanism. It was concluded that atracurium produces a profound tetanic fade, with respect to its effect on twitch or tetanic tension, suggesting that the drug is a potent neuromuscular blocker, with rapid onset of blockade.