Glycopyrrolate intensifies neuromuscular blockade produced by atracurium in the rat diaphragm preparation

The effect of glycopyrrolate (Glycopyrronium, a muscarinic antagonist) (10 mumol.litre-1) and neostigmine (1 mumol.litre-1) on atracurium (0.1-100 mumol.litre-1) - induced neuromuscular blockade was studied in the rat isolated phrenic nerve-diaphragm preparation, to see if glycopyrrolate intensified the neuromuscular blockade produced by atracurium in this preparation. Atracurium had a rapid onset of blockade, reaching a complete block in 30-40 s. Glycopyrrolate had no significant effect on indirectly-elicited twitch (0.2 Hz) tension, whereas it significantly increased atracurium-induced depression of twitch tension and shortened the time needed to a complete block by 10 s. Combinations of glycopyrrolate+ neostigmine, only slightly reversed atracurium-induced blockade, if compared to the reversal by neostigmine alone. The mean concentrations to produce 50% depression of twitch tension were: 1.6 +/- 0.1 (atracurium), 0.3 +/- 0.1 (atracurium +glycopyrrolate), 4.8 +/- 0.2 (atracurium +neostigmine) and 2.7 +/- 0.1 mumol.litre-1 (atracurium +glycopyrrolate +neostigmine) (means +/- SEM, n = 6, P less than 0.001, with respect to control value of atracurium alone). It was concluded that glycopyrrolate enhanced atracurium-induced neuromuscular blockade in the rat diaphragm preparation, and that this effect should be noted when dosing glycopyrrolate in man.     

Assessment of neuromuscular blockade produced by atracurium in frog sartorius muscle

The neuromuscular effects of atracurium in frog sartorius muscle were studied and the results were compared with those obtained by tubocurarine in the same preparation. Single twitch (at 1 per 5 s) and tetanic stimulation (at 1-100 per s) of the motor nerve, stimulated with 5-10 V (maximum) and 0.2 ms pulse duration, were used to assess the neuromuscular blockade produced by atracurium and tubocurarine in the frog sartorius muscle. The results showed that atracurium was twice as potent as tubocurarine in reducing the amplitude of the indirectly-elicited twitch contractions in the frog sartorius muscle. The mean IC50 values (concentration to produce 50% inhibition of twitch tension) of atracurium and tubocurarine-induced depression of the indirectly-elicited twitch tension were 0.64 +/- 0.1 microM and 1.2 +/- 0.2 microM respectively (means +/- s.e., n = 6, P less than 0.001). Furthermore, atracurium had a shorter time of onset, time to 50% block and time to 95% recovery than tubocurarine.     

Presynaptic snake β-neurotoxins produce tetanic fade and endplate potential run-down during neuromuscular blockade in mouse diaphragm

The present study investigated the ability of a number of presynaptic snake neurotoxins (snake β-neurotoxins) to produce nerve-evoked train-of-four fade, tetanic fade and endplate potential run-down during the development of neuromuscular blockade in the isolated mouse phrenic-hemidiaphragm nerve-muscle preparation. All the snake β-neurotoxins tested, with the exception of notexin, produced train-of-four and tetanic fade of nerve-evoked isometric muscle contractions. Train-of-four fade was not present during the initial depressant or facilitatory phases of muscle tension produced by the snake β-neurotoxins but developed progressively during the final depressant phase that precedes complete neuromuscular blockade. The ‘non-neurotoxic’ bovine pancreatic phospholipase A₂ and the ‘low-toxicity’ phospholipase A₂ from Naja naja atra venom failed to elicit train-of-four fade, indicating that the phospholipase activity of the snake β-neurotoxins is not responsible for the development of fade. Intracellular recording of endplate potentials (EPPs) elicited by nerve-evoked trains of stimuli showed a progressive run-down in EPP amplitude during the train following incubation with all snake β-neurotoxins except notexin. Again this run-down in EPP amplitude was confined to the final depressant phase of snake β-neurotoxin action. However when EPP amplitude fell to near uniquantal levels (<3mV) the extent of toxin induced-fade was reduced. Unlike postjunctional snake α-neurotoxins, prejunctional snake β-neurotoxins interfere with acetylcholine release at the neuromuscular junction during the development of neuromuscular blockade. This study provides further support for the hypothesis that fade in twitch and tetanic muscle tension is due to an underlying rundown in EPP amplitude resulting from a prejunctional alteration of transmitter release rather than a use-dependent block of postjunctional nicotinic receptors. Received: 5 May / Accepted: 16 July 1997     

Examination of the mechanisms involved in tetanic fade produced by vecuronium and related analogues in the rat diaphragm.

The effects of vecuronium (Org NC45), Org 7678 and Org 7684 were examined on twitches and tetani recorded from rat isolated diaphragms. Org 7678 and Org 7684 exhibited approximately one tenth of the neuromuscular blocking potency of vecuronium. At concentrations producing equivalent amounts of twitch block, Org 7684 produced significantly less tetanic fade than did vecuronium or Org 7678. In cut muscles both vecuronium and Org 7684 reduced the endplate current (e.p.c.) amplitude (Ie.p.c.), reduced e.p.c. decay time constant (tau e.p.c.), and increased the e.p.c. train rundown. The effects of vecuronium were not voltage-dependent and vecuronium did not change tau noise. The effect of Org 7684 on Ie.p.c. and tau e.p.c. became greater with hyperpolarization, but the effect on e.p.c. train rundown was not voltage-dependent. It is concluded that both vecuronium and Org 7684 produce e.p.c. train rundown and tetanic fade by a prejunctional mechanism. However, whereas postjunctionally vecuronium blocks only the acetylcholine receptor, Org 7684 blocks both the receptor and its associated ion channel.     

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

The effects of nicotine antagonists on single twitches, trains of four twitches and tetanic contractions of the isolated diaphragm of the rat were examined. Different drugs were found to produce different amounts of tetanic fade relative to depression of twitch tension. The order of activity from most able, to least able to produce fade was: hexamethonium greater than trimetaphan=atracurium=tubocurarine greater than pancuronium greater than erabutoxin b. The effect of erabutoxin b was distinctive for its almost complete lack of tetanic fade. 3,4-Diaminopyridine increased tetanic fade produced by tubocurarine, atracurium and hexamethonium, but not that produced by erabutoxin b. It is concluded that nicotinic antagonists act at more than one site at the neuromuscular junction. Assuming block of the postjunctional acetylcholine receptor produces tension depression, a second or third site must be involved in producing tetanic fade. The possibility that tetanic fade results from block of the ion channel associated with the postjunctional acetylcholine receptor or from the block of a prejunctional nicotinic receptor is discussed.     

Suramin reverses non-depolarizing neuromuscular blockade in rat diaphragm.

Unexpectedly, it was observed that the P2-purinoceptor antagonist, suramin (10 microM to 1 mM), reversed the muscle paralysis caused by structurally unrelated non-depolarizing relaxants. Suramin competitively reversed the blocking action of pancuronium. Both the pre- and postsynaptic blockade of nicotinic receptors by pancuronium was counteracted, as shown by the action of suramin, using train-of-four stimulation. Suramin did not affect the paralysis caused by the depolarizing relaxant, succinylcholine. The reversal action of suramin was not due to an increase in the acetylcholine concentration in the synaptic cleft, since neither the contraction of preparations partially paralysed by diminished acetylcholine release in the presence of low Ca2+ or high Mg2+ nor acetylcholinesterase activity were affected. Suramin did not affect the reduction in twitch tension caused by adenosine and potentiated the ATP-induced reduction in twitch, indicating that ATP-sensitive receptors are not involved in the reversal action of suramin. Consequently, these results suggest that the action of suramin is due to binding with a site on the acetylcholine receptor also occupied by non-depolarizing relaxants, but different from the site occupied by succinylcholine.     

Selective potentiation of subtetanic and tetanic contractions by the calcium-channel antagonist nifedipine in the rat diaphragm preparation

1. Nifedipine (1.5-3.0 x 10(-5) M) potentiated the (sub)tetanic tension during 10-50 Hz indirect or direct stimulation of the rat diaphragm preparation; the twitch contractions were not potentiated. 2. The effect was antagonized in high Ca2+ (5-10 x normal) solutions. 3. A comparison with the twitch potentiators caffeine (1.0 x 10(-3) M), quinine (1.4 x 10(-5) M) and phenytoin (2.0 x 10(-5) M), showed that only phenytoin, a putative Ca-antagonist, caused a nifedipine-like frequency-dependent potentiation, indicating a Ca-antagonistic rather than an unspecific effect. 4. A similar (sub)tetanic potentiation was found in a K(+)-free solution. 5. The slow development of the potentiation during repetitive stimulation is in accordance with an effect on the slow Ca channels known to be present in mammalian skeletal muscle. 6. A delay of the fatigue-inducing accumulation of K+ in the T tubules, which may occur during a nifedipine-induced reduction of a Ca2(+)-stimulated K+ efflux, as well as in a K(+)-free solution, may explain the effect.     

Effects of (+)-tubocurarine on neuromuscular facilitation and depression in rat diaphragm.

Effects of (+)-tubocurarine on neuromusclular facilitation and depression were studied in the rat phrenic nerve-diaphragm. The paired end-plate potential (EPP) method of analysis was used. Tubocurarine caused a significant decrease in the EPP₂/EPP₁ ratio of EPP pairs elicited at a stimulus interval of 20 msec in the cut muscle preparation. Studies of tubocurarine with respect to the equilibrium potential of EPP₁ and EPP₂ and iontophoretic acetylcholine potential pairs, indicated that the effect of tubocurarine on the EPP ratio was not associated with a postsynaptic action of tubocurarine. Furthermore, tubocurarine did not alter the amplitude ratio of the nerve terminal action potentials.The effects of tubocurarine on the EPP₂/EPP₁ ratio at various stimulus intervals were studied in the cut muscle preparation. In the control condition, facilitation (EPP₂/EPP₁ ratio > 1.01 occurred at 5 and 10 msec stimulus intervals; depression (EPP₂/EPP₁ ratio < 1.0) occurred at longer stimulus intervals. Tubocurarine significantly decreased the EPP ratio at stimulus intervals of 10 100 msec. In the low Ca²⁺/high Mg²⁺ Krebs solution, only facilitation occurred and under these conditions tubocurarine did not decrease the EPP₂/EPP₁ ratio. These results indicate that tubocurarine preferentially enhances depression and that facilitation is relatively unaffected by tubocurarine. Altogether, the results are consistent with the notion that, in addition to its postsynaptic actions, tubocurarine affects acetylcholine storage and release in the nerve terminal.     

Electromyographic assessment of the neuromuscular blockade produced in vivo by anatoxin-a in the rat

The indirectly evoked compound action potentials (ECAP) of the plantar muscles of the rat were used to investigate the pharmacodynamics in vivo of the neuromuscular blockade produced by anatoxin-a. Onset time to maximum depression and the magnitude of maximum depression in amplitude of the ECAP were dose-dependent. The mean maximum percent depression (+/- S.D.) of the ECAP induced by single, supramaximal stimulations of the posterior tibial nerve after i.v. doses of (+)anatoxin-a hydrochloride at 0, 50, 100, 200 and 800 micrograms/kg were 3 (4), 53 (15), 82 (7), 95 (2), and 100 (1), respectively. The ED50 (95% confidence limits) for depression of the ECAP was 47 mg/kg (39-57 micrograms/kg). Rats administered 200 micrograms/kg or less of (+)anatoxin-a hydrochloride had 75% return of the pretoxin amplitude of the ECAP within 93 min. Animals dosed at 800 micrograms/kg did not have return of neuromuscular function and died despite mechanical ventilation, suggesting a lethal mechanism(s) of action in addition to respiratory paralysis. Percent decrements (+/- S.D.) in the amplitude of the fourth ECAP following repetitive stimulation at 10 Hz were 6 (5), 13 (22), 46 (18) and 59 (8) from (+)anatoxin-a hydrochloride given i.v. at 0, 50, 100 and 200 micrograms/kg, respectively. The decrement observed following repetitive stimulation was attributed to a presynaptic site of action. No change in maximal motor nerve conduction velocity or latency of the ECAP was observed after i.v. administration of (+)anatoxin-a hydrochloride at 100 micrograms/kg. LD50 values (95% confidence limits) for anatoxin-a administered i.v. to mice were 386 micrograms/kg (365-408 micrograms/kg, for (+)anatoxin-a hydrochloride and 913 micrograms/kg (846-985 micrograms/kg) for racemic anatoxin-a hydrochloride. No deaths were observed in mice after i.p. administration of (-)anatoxin-a hydrochloride at doses up to 73 mg/kg.     

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.     

Effectiveness of pyridostigmine in reversing neuromuscular blockade produced by soman.

The effects of pyridostigmine pretreatment on the neuromuscular blockade produced by soman in anaesthetized, atropinized animals have been studied on the soleus and anterior tibialis muscle (rhesus monkeys, cats and rabbits) and the gastrocnemius muscle (guinea-pigs and rats). Pyridostigmine pretreatment produced a complete recovery of neuromuscular function following blockade by soman; the rate of recovery was similar in all the species, suggesting a common mechanism of action. In the absence of pyridostigmine or if pyridostigmine was delayed until after blockade by soman, there was no recovery of neuromuscular function. Detailed studies in the guinea-pig showed that the recovery of neuromuscular function was related to the dose of soman and to the degree of carbamoylation of blood cholinesterase at the time of nerve agent challenge, i.e. to the dose of pyridostigmine and the time interval between the administration of pyridostigmine and soman. It is suggested that the effectiveness of pyridostigmine pretreatment is due to the carbamoylation of a portion of the tissue acetylcholinesterase, which protects it against irreversible inhibition by soman: after poisoning spontaneous decarbamoylation produces sufficient free acetylcholinesterase to restore normal function.     

Effects of presynaptic muscarinic cholinoreceptor blockade on neuromuscular transmission as assessed by the train‐of‐four and the tetanic fade response to rocuronium

This study investigated the effect of muscarinic M₁ and M₂ receptor antagonists on the rocuronium‐induced train of four (TOF ) fade and tetanic fade, respectively. Ex‐vivo phrenic nerves and diaphragms were obtained from adult Sprague‐Dawley rats and stabilized in Krebs buffer; the nerve‐stimulated muscle TOF fade was observed at 20 s intervals. For the TOF study, phrenic nerves and diaphragms were incubated with pirenzepine (an M₁ blocker) at concentrations of 0 nmol L^?1 (control), 10 nmol L^?1 (PZP 10), or 100 nmol L^?1 (PZP 100). Rocuronium was then administered incrementally until the first twitch tension had depressed by >95% during TOF stimulation. The mean TOF ratios were compared when the first twitch tensions were depressed by 40%‐50%. For the tetanic fade study, 50 Hz/5 s tetani was applied initially, 30 min after the administration of a loading dose of rocuronium and methoctramine (an M₂ receptor blocker, loaded at 0 μmol L^?1 [control], 1 μmol L^?1 [MET 1], or 10 μmol L^?1 [MET 10]). The EC ₉₅ of rocuronium was significantly lower in the PZP 10 group than in the control group. In the PZP 10 group, the TOF ratios at 50% and first twitch tension depression were significantly lower than those in the control group (P =.02). During tetanic stimulation, the tetanic fade was significantly enhanced in the MET 10 group compared to the other groups. This study shows that antagonists of muscarinic M₁ and M₂ receptors affect the rocuronium‐induced neuromuscular block as demonstrated by the reduced EC ₉₅ and TOF ratios (M₁ antagonist, pirenzepine) or the enhanced 50‐Hz tetanic fade (M₂ antagonist, methoctramine).     

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.     

Verapamil intensifies neuromuscular blockade produced by gallamine and pancuronium at the chick neuromuscular junction

Experiments were performed to study the effect of verapamil on neuromuscular transmission and muscle contraction at a chick skeletal muscle-nerve preparation. In addition, the effects and interactions of verapamil with some muscle relaxants were studied in the same preparation. These effects were explored by studying the effects of verapamil on: directly-and indirectly-elicited twitch contractions, and neuromuscular blockade produced by gallamine and pancuronium. The results showed that verapamil (2-200 microM) had a differential effect on the twitch responses; more reductions occurred in the indirectly-elicited twitch tension, whereas the directly-elicited twitch response was reduced only by 20-30% of maximum indirectly-elicited twitch tension. Furthermore, in low concentrations (1-20 microM), verapamil significantly increased the neuromuscular blockade produced by gallamine (28-1280 nM) and pancuronium (18-573 nM). In high concentrations (greater than 200 microM), verapamil completely blocked the indirectly-elicited twitch response and produced a marked contracture in the chick skeletal muscle (1.0 +/- 0.1 g, n = 6). It was concluded that by reducing twitch tension and inhibiting neuromuscular transmission, verapamil increases (intensifies) neuromuscular blockade produced by muscle relaxants, e.g. gallamine and pancuronium.     

Effectiveness of pyridostigmine in reversing neuromuscular blockade produced by soman

The effects of pyridostigmine pretreatment on the neuromuscular blockade produced by soman in anaesthetized, atropinized animals have been studied on the soleus and anterior tibialis muscle (rhesus monkeys, cats and rabbits) and the gastrocnemius muscle (guinea-pigs and rats). Pyridostigmine pretreatment produced a complete recovery of neuromuscular function following blockade by soman; the rate of recovery was similar in all the species, suggesting a common mechanism of action. In the absence of pyridostigmine or if pyridostigmine was delayed until after blockade by soman, there was no recovery of neuromuscular function. Detailed studies in the guinea-pig showed that the recovery of neuromuscular function was related to the dose of soman and to the degree of carbamoylation of blood cholinesterase at the time of nerve agent challenge, i.e. to the dose of pyridostigmine and the time interval between the administration of pyridostigmine and soman. It is suggested that the effectiveness of pyridostigmine pretreatment is due to the carbamoylation of a portion of the tissue acetylcholinesterase, which protects it against irreversible inhibition by soman: after poisoning spontaneous decarbamoylation produces sufficient free acetylcholinesterase to restore normal function.     

罗库溴铵、维库溴铵和阿曲库铵用于全麻气管插管肌松效应的对比研究

目的观察比较罗库溴铵、维库溴铵及阿曲库铵在全麻诱导气管插管时的肌松效应及不良反应。方法ASAⅠ~Ⅱ级择期全麻下行腹腔镜胆囊切除手术患者120例,随机分为:罗库溴铵组(Ⅰ组)、维库溴铵组(Ⅱ组)、阿曲库铵组(Ⅲ组),每组给2倍ED95剂量肌松剂。Ⅰ组按用量再分为:Ⅰa组(2倍ED95,0.6 mg/kg)、Ⅰb组(3倍ED95,0.9 mg/kg)2个亚组。每组各30例。麻醉诱导依次静注咪达唑仑0.06 mg/kg、芬太尼4μg/kg,异丙酚2 mg/kg后,Ⅰa、Ⅰb组分别静注罗库溴铵0.6 mg/kg、罗库溴铵0.9 mg/kg,Ⅱ组静注维库溴铵0.15 mg/kg,Ⅲ组静注阿曲库铵0.5 mg/kg。观察各组气管插管时的肌松效果及不良反应。结果Ⅰ组的起效时间及插管状态优良率均明显高于Ⅱ、Ⅲ组,Ⅰb组的插管条件好于Ⅰa组。Ⅰ组的支气管痉挛、皮疹等不良反应的发生率明显低于其他组,对循环系统的影响也较小。结论罗库溴铵比维库溴铵、阿曲库铵起效快,恢复迅速,不良反应少,是较好的全麻气管插管肌松药。     

Inhibition by propranolol of the contractile response of the rat diaphragm to tetanic field stimulation in vitro.

Contraction of the rat isolated diaphragm in response to maximal tetanic stimulation was examined before and after isoprenaline or propranolol. Isoprenaline (10(-4)M) did not affect maximum isometric force, whereas propranolol depressed maximum force in a concentration-dependent manner (10(-6)-10(-4)M). Inhibition due to propranolol (10(-4)M) could not be overcome by increasing the intensity or duration of electrical stimulation, and was only partially reversed (mean 73% +/- 10 s.e. mean) after washing. Pretreatment with isoprenaline did not alter the response of the muscle to propranolol, nor did neuromuscular blockade with (+)-tubocurarine. The response to either stereoisomer of propranolol was similar to that obtained with the racemate. Atenolol, a beta-adrenoceptor blocking agent without membrane stabilizing activity, had minimal (less than 10%) depressant effects on diaphragmatic force development. Lignocaine (8.5 X 10(-6)-8.5 X 10(-5)M) produced a concentration-related decrease in isometric force, similar to that with propranolol. It is concluded that propranolol decreases the contractile force of the rat isolated diaphragm by a mechanism related to stabilization of excitable membranes.     

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.