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

The effect of high concentration of magnesium with ropivacaine, gentamicin, rocuronium, and their combination on neuromuscular blockade

Background

Magnesium, ropivacaine, gentamicin, and rocuronium block neuromuscular (NM) transmission by different mechanisms. Therefore, concurrent administration of these agents may induce prolonged muscle paralysis via synergistic interaction. This study investigated the efficacy and safety of NM block caused by the administration of high concentrations of magnesium in combination with ropivacaine, gentamicin, and rocuronium.

Methods

Eighty-three left phrenic nerve-hemidiaphragms from male SD rats (150-250 g) were hung in Krebs solution. Three consecutive single twitch tension (ST, 0.1 Hz) and one tetanic tension (TT, 50 Hz for 1.9 s) were obtained before drug application and at each new drug concentration. The concentration of MgCl₂ and MgSO₄ in Krebs solution was increased until an 80 to 90% reduction in ST was reached. To test the effects of combinations of NM agents, a Krebs solution was premixed with MgCl₂ alone, MgCl₂ and ropivacaine, or MgCl₂, ropivacaine, and gentamicin. The concentration of ropivacaine, gentamicin, or rocuronium was then progressively increased until an 80 to 90% reduction in ST was reached. The effective concentrations were estimated with a probit model.

Results

The potency of MgCl₂ was greater than that of MgSO₄, and pretreatment with MgCl₂ increased the potency of gentamicin and rocuronium. Unexpectedly, MgCl₂ did not potentiate ropivacaine, and the potency of gentamicin and rocuronium failed to show an increase when premixed with 0.5 µM ropivacaine.

Conclusions

The concomitant administration of high concentrations of magnesium and ropivacaine together with clinically relevant concentrations of gentamicin or rocuronium potentiated NM blockade but not with clinically relevant concentrations of ropivacaine.

Do bupivacaine, clindamycin, and gentamicin at their clinical concentrations enhance rocuronium-induced neuromuscular block?

Background

Bupivacaine, clindamycin, and gentamicin inhibit neuromuscular (NM) conduction. When they are combined, they may synergistically reduce the effective concentration of each to the therapeutic concentration in augmenting rocuronium-induced NM block. Thus, the aim of this study was to investigate whether combinations of the three drugs, at around their therapeutic concentrations, potentiate rocuronium-induced NM block.

Methods

Fifty-seven left-phrenic nerve hemidiaphragms (Male S-D rats, 150-250 g) were hung in a 20-ml organ bath filled with Krebs solution. Three consecutive single-twitch tensions (0.1 Hz) and one tetanic tension (50 Hz for 1.9 s) were obtained. A Krebs solution was premixed with concentration sets of bupivacaine and clindamycin, bupivacaine and gentamicin, or bupivacaine, clindamycin and gentamicin. Then, the concentration of rocuronium was cumulatively increased in the Krebs solution (1, 3, 5, 7, 9, 12, 14, 16, 18, and 20 µM) until an 80% to 90% reduction in single twitch was attained. The effective concentrations for each experiment were determined with the probit model.

Results

The combinations of bupivacaine, clindamycin, and gentamicin enhanced rocuronium-induced NM block. When the three drugs were applied simultaneously, their concentrations were reduced to near-therapeutic levels in potentiating the action of rocuronium.

Conclusions

Bupivacaine, clindamycin, and gentamicin blocked NM conduction, and when all three drugs were applied together, they augmented rocuronium-induced NM block at their near-therapeutic concentrations. Clinicians should be aware of the cooperability in NM block between drugs that interrupt NM conduction.

Priming with rocuronium to accelerate the onset time of cisatracurium during intubation

BACKGROUND

The priming technique, in which a small dose of nondepolarizing muscle relaxant is administered 3-6 minutes before giving the intubation dose, can speed up the onset of muscle relaxation in patients with paralysis during intubation. We investigated the priming technique and compared 2 different priming agents (rocuronium and cisatracurium) at a priming time of 3 minutes and its effect on decreasing the onset time of cisatracurium.

METHODS

A total of 60 patients with ASA physical status I-II scheduled for elective surgery were enrolled. After induction with propofol and fentanyl, the patients were randomized into 1 of 3 groups. Group 1 received rocuronium 0.06 mg/kg as a priming dose. Group 2 received cisatracurium 0.01 mg/kg as a priming dose. Group 3 received normal saline and constituted the control group. After a 3-minute priming time, intubation doses of cisatracurium were given (Groups 1 and 2, 0.14 mg/kg; Group 3, 0.15 mg/kg). First twitch height percentage (T1/T0%; % of control) and train-of-four percentage (T4/T1%) were recorded every 10 seconds from baseline until T1/T0% reached 0.

RESULTS

Rocuronium (Group 1) and cisatracurium (Group 2) significantly accelerated the onset of cisatracurium (Group 1, 117.0 +/- 29.0 seconds; Group 2, 151.0 +/- 37.5 seconds; Group 3, 221.5 +/- 36.6 seconds; all p < 0.001).

CONCLUSION

Priming with rocuronium or cisatracurium for 3 minutes significantly accelerated the onset of cisatracurium. Priming with rocuronium for 3 minutes improved the onset time of cisatracurium even more than priming with cisatracurium itself.

Sevoflurane increases fade of neuromuscular response to TOF stimulation following rocuronium administration in children. A PK/PD analysis

BACKGROUND

Sevoflurane enhances neuromuscular block produced by rocuronium, affecting not only single twitch response but also the response to high-frequency stimulation, increasing tetanic [or train-of-four (TOF)] fade.

METHODS

We compared the degree of fade during spontaneous recovery from rocuronium-induced neuromuscular block in 24 children (3-11 years old, ASA groups I and II), anesthetized with nitrous oxide-sevoflurane (one MAC, endtidal concentration) or nitrous oxide-fentanyl. Neuromuscular transmission was monitored electromyographically (EMG), stimulating the ulnar nerve at the wrist with TOF, 2 Hz for 2 s, repeated at 20-s intervals and recording EMG potential from adductor pollicis brevis. Depression of the fourth twitch, T4, was used as a measure of fade. Following an intubating dose of rocuronium, 0.6 mgxkg(-1), continuous infusion of rocuronium was given to maintain stable 90-99% T1 depression. Plasma concentration of rocuronium was determined with high performance liquid chromatography with electrochemical detection (HPLC-EC) method at the moment of discontinuation of rocuronium infusion and 10, 20, 30, 40, 50, 60, and 75 min afterwards. A two compartment model was used for pharmacokinetic (PK) calculations. PK parameters were fixed and pharmacodynamic data were fitted to effect compartment model proposed by Sheiner.

RESULTS

Sevoflurane reduced rocuronium concentration in effect compartment producing 50% inhibition of both T1 and T4 response and significantly delayed not only T1, but also T4 recovery.

CONCLUSIONS

Potentiating effect of sevoflurane on rocuronium-induced neuromuscular block influences not only postsynaptic, but also the presynaptic part of the neuromuscular junction, enhancing fade of neuromuscular response to high-frequency stimulation. The intensity of this latter effect is clinically relevant.

The guinea-pig oesophagus is a versatile in vitro preparation for pharmacological studies

1. The isolated oesophagus of the guinea-pig is a useful preparation that can be used as an alternative to the phrenic nerve-diaphragm preparations that have been used traditionally in the determination of the actions of drugs and toxins at the neuromuscular junction. The guinea-pig isolated oesophagus can also provide information about effects at ganglionic nicotinic receptors, which are not present in the diaphragm preparations. 2. The muscularis externa of the body of the oesophagus consists exclusively of striated muscle fibres. The myenteric plexus is found between the outer longitudinally arranged layer and the inner circular layer. The muscularis mucosae contains smooth muscle fibres arranged longitudinally. 3. The vagal nerves are comprised of special vagal efferents that innervate the striated muscle fibres directly and 'parasympathetic' vagal fibres that synapse in the myenteric ganglia and, subsequently, affect the smooth muscle of the muscularis mucosae. Thus, both striated and smooth muscle responses to vagal nerve stimulation can be studied. In addition, afferent neurons run in the vagus. 4. Studies using various isolated oesophageal preparations have been reviewed. These consist of the whole oesophagus (including striated muscle, myenteric plexus and smooth muscle), the isolated mucosal layer (striated muscle and the myenteric plexus both absent) and the whole oesophagus with the vagus nerve attached. Comparative studies of the effects of drugs acting directly on the muscularis mucosae and/or indirectly via the intramural plexuses can be performed using the whole oesophagus and the isolated mucosal layer. 5. The use of the guinea-pig isolated vagus nerve-oesophagus preparation allows potency determinations for both neuromuscular and ganglion blockade of various non-depolarizing muscle relaxants to be performed simultaneously under identical conditions. Furthermore, the phenomenon of fade, a waning of tetanic tension during the stimulation period, can be studied. 6. A potential application of this preparation is the simultaneous screening of the constituents of snake venoms for activity at the neuromuscular junction and/or the ganglion. It is also suggested that new calcium channel blockers could be screened in this preparation because different voltage-sensitive calcium channels are involved in neurotransmitter release from nerve terminals at the neuromuscular junction and autonomic cholinergic neuroeffector sites.

Isobolographic analysis of non-depolarising muscle relaxant interactions at their receptor site

Administration of certain combinations of non-depolarising muscle relaxants produces greater than expected neuromuscular blockade. Synergistic effects may be explained by drug interactions with the postsynaptic muscle nicotinic acetylcholine receptor. To investigate this hypothesis, the adult mouse muscle nicotinic acetylcholine receptor (alpha(2)beta delta epsilon) was heterologously expressed in Xenopus laevis oocytes and activated by the application of acetylcholine (10 microM). The effects of five individually applied muscle relaxants and six combinations of structurally similar and dissimilar compounds were studied. Drug combinations containing equipotent concentrations of two agents were tested and dose-response curves were determined. All compounds tested alone and in combination produced rapid and readily reversible, concentration-dependent inhibition. Isobolographic and fractional analyses indicated additive interactions for all six tested combinations. These findings suggest that synergistic neuromuscular blocking effects, observed for the administration of certain combinations of muscle relaxants, do not result from purely postsynaptic binding events at the muscle nicotinic acetylcholine receptor, but rather from differential actions on pre- and postsynaptic sites.

Train-of-four fade and neuromuscular block in rats: a comparison between pancuronium, vecuronium, and rocuronium

PURPOSE

To clarify the relationship between neuromuscular block and train-of-four fade and to investigate the causes of these drug-dependent differences, we compared the neuromuscular block and TOF fade after pancuronium, vecuronium and rocuronium.

METHODS

In 24 anesthetized rats, the sciatic nerve was stimulated, and the twitch of left tibialis anterior muscle was recorded. After T1 (first twitch response) was kept constant at 95% block by administration of pancuronium, vecuronium, or rocuronium (n=8, in each), the TOF fade was measured when T1 block was decreased to 40% and 20%. In addition, using 24 phrenic nerve-diaphragm preparations, the fade was measured when the T1 block increased to 20% and 40% by titrating of either one of the three drugs (n=8, in each).

RESULTS

In in vivo experiments, the fade produced by pancuronium was greater than that by vecuronium or rocuronium when T1 block was at 40% (81 +/- 9 vs 63 +/- 15 and 63 +/- 6%, respectively) and at 20% (66 +/- 13 vs 34 +/- 17 and 40 +/- 6%, respectively). In contrast, in in vitro experiments, the differences did not reach significant levels among the three drugs either at 20% (32 +/- 19 vs 33 +/- 10 and 32 +/- 17%) or 40% of block (62 +/- 29 vs 65 +/- 14 and 55 +/- 14%).

CONCLUSIONS

For vecuronium and rocuronium, the results were similar in vivo and in vitro. For pancuronium, fade was greater in vivo. These results suggest that different neuromuscular blocking agent have different relationships between the fade and the block. In vitro results might not be the same as in vivo, possibly due to pharmacokinetic differences.

The neuromuscular transmission fade (Wedensky inhibition) induced by L-arginine in neuromuscular preparations from rats

L-Arginine (4.7-18.8 mM) and 3-(4-morpholinyl)-sydonone imine hydrochloride (SIN-1; 1.15 mM) induced an increase in tetanic fade caused by indirect stimulation (180-200 Hz) of muscle. However, Wedensky inhibition, different from control, was not observed when the preparations treated with d-tubocurarine were directly stimulated by the same frequency. D-Arginine (9.4 mM) was ineffective in changing R values caused by indirect stimulation (180-200 Hz) of muscle. N(omega)-Nitro-L-arginine (73 mM) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM) did not produce any effect on Wedensky inhibition but did antagonize the tetanic fade induced by L-arginine (9.4 mM). The SIN-1 effect was antagonized by previous administration of ODQ (108 microM), which alone did not produce any effects on R values. These results indicate that NO acting at the presynaptic level increases the Wedensky inhibition induced by high frequency of stimulation applied on motor nerves, and its effect may be produced through the cGMP-GC pathway.

Neuromuscular blocking effects and train-of-four fade with cisatracurium: comparison with other nondepolarising relaxants

Neuromuscular blocking drugs exhibit different degrees of fade in response to train-of-four stimulation believed to represent their relative prejunctional effects. The present study was designed to compare the train-of-four fade after cisatracurium and compare this with other commonly used muscle relaxants. Train-of-four fade during onset and recovery of block were recorded after administration of cisatracurium 0.05 or 0.1 mg.kg-1, atracurium 0.5 mg.kg-1, vecuronium 0.08 mg.kg-1, mivacurium 0.15 mg.kg-1 or rocuronium 0.6 mg.kg-1 to patients anaesthetised with fentanyl, nitrous oxide and a propofol infusion. Neuromuscular monitoring was by stimulation of the ulnar nerve and recording the force of contraction of the adductor pollicis muscle. The onset and recovery of block were also measured. Train-of-four fade during onset of block was greater with the lower dose of cisatracurium compared with the higher dose of cisatracurium and all other relaxants. Train-of-four fade during recovery was similar. The median times (and ranges) for the onset of maximum block were 3.4 (2.1-5.6), 1.5 (1.2-2.3), 2.1 (1.2-2.6), 2.0 (1.5-2.7) and 1.0 (0.7-1.3) min for cisatracurium 0.1 mg.kg-1 and atracurium, mivacurium, vecuronium and rocuronium, respectively. The median times (and ranges) for the recovery of T1 to 25% of control and to a train-of-four ratio of 0.8 were 41 (21-50) and 65 (40-78); 43 (37-54) and 69 (58-79); 15 (11-20) and 25 (19-30); 31 (23-46) and 60 (45-117); and 33 (18-57) and 50 (28-76) min following cisatracurium, 0.1 mg.kg-1, atracurium, mivacurium, vecuronium and recuronium, respectively.

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.

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.

Nondepolarizing neuromuscular blocking drugs and train-of-four fade

The aim of this study was to examine differences in prejunctional effects of different relaxants by measuring the train-of-four (TOF) fade during the onset and recovery of neuromuscular block. The relaxants studied were atracurium (225 micrograms.kg-1), mivacurium (65 micrograms.kg-1) rocuronium (300 micrograms.kg-1) and vecuronium (40 micrograms.kg-1). The TOF ratios were measured at approximate heights of T1 (first response in the TOF) of 90, 75, 50, and 25%. The TOF fade (as shown by lower TOF ratios) increased with a decrease in the T1 during onset of neuromuscular block. Although there was a slightly greater fade with atracurium and rocuronium during the onset of block, the differences among the relaxants were insignificant. It is concluded that the relative prejunctional effects of these relaxants are similar.

Monitoring of neuromuscular transmission by electromyography (II). Evoked compound EMG area, amplitude and duration compared to mechanical twitch recording during onset and recovery of pancuronium-induced blockade in the cat

The feasibility of the compound electromyogram (EMG) was evaluated during onset and recovery from pancuronium block in the tibialis anterior muscle of ten cats. The evoked EMG area, amplitude and duration of the total response and of the major negative deflection were evaluated and compared to the mechanomyogram during 0.1 Hz and train-of-four (TOF) stimulation. EMG areas and amplitudes were found to be linearly and similarly related to the mechanomyogram during onset and recovery. Slopes of the regression lines ranged between 1.00-1.02 and between 1.10-1.22 during onset and recovery, respectively, with high individual correlation coefficients (> 0.95). The TOF ratio of the mechanomyogram was linearly related to the EMG TOF ratio during onset and to the square root of the EMG TOF ratio during recovery, with no differences between EMG areas and amplitudes, suggesting a higher initial recovery of the TOF ratio of the mechanomyogram during recovery. EMG duration increased as the level of block increased but was unsuitable for neuromuscular monitoring. Evaluation of the agreement between the two methods showed that the EMG may be from 15% below to 10% above the mechanomyogram during onset and from 40% below to 45% above the mechanomyogram during recovery, in spite of high correlation coefficients. In contrast, agreement between EMG parameters was found to be high. In conclusion, EMG is more reliable than the mechanomyogram for evaluation of neuromuscular transmission in the cat. EMG amplitudes and areas both reflect the degree of neuromuscular blockade equally well.

A comparison between vecuronium and atracurium in myasthenia gravis

We evaluated the effect of vecuronium bromide and atracurium besylate on the train-of-four response in the management of muscle relaxation in 20 patients with myasthenia gravis (MG) who were undergoing thymectomy. We confirmed the safe use of these two non-depolarizing muscle relaxants in MG patients. Vecuronium (0.04 mg.kg-1) demonstrated a lesser clinical duration than did atracurium (0.2 mg.kg-1) (38 +/- 19 vs 50 +/- 21 min, mean +/- s.e.mean). The recovery time for vecuronium patients was shorter than that for atracurium patients (22 +/- 18 vs 38 +/- 18 min), but the time until onset of neuromuscular blockade was longer with vecuronium (246 +/- 105 vs 107 +/- 103 s). During spontaneous recovery from neuromuscular relaxation, at T1/C of 25% and 100%, the train-of-four fade with vecuronium was significantly greater than that with atracurium (0.04 +/- 0.02, 0.16 +/- 0.03 vs 0.17 +/- 0.01, 0.83 +/- 0.03), suggesting that vecuronium had a greater prejunctional effect than did atracurium.

Pharmacological studies on cutaneous inflammation induced by ultraviolet irradiation (1): quantification of erythema by reflectance colorimetry and correlation with cutaneous blood flow

This study was conducted to quantify the intensity of ultraviolet (UV) erythema in guinea pigs, a method for evaluating anti-inflammatory drugs, and to clarify any correlation of erythema with cutaneous blood flow. Skin color and cutaneous blood flow in non-administered and indomethacin-administered animals were measured by a colorimeter and a laser Doppler flowmeter over time after UV-irradiation treatment. Skin color was indicated by a XYZ colorimetric system and L*a*b* color space. In either colorimetric system, the values of two indices, x and y or a* and b*, increased along with the intensification of erythema. The increase in the chroma (C*) value calculated from a* and b* was UV-dose-dependent. This value was significantly suppressed by indomethacin 0.5-4 hr after irradiation, and it was found to be a clear and sensitive index for evaluating the suppressive effect of drugs. Cutaneous blood flow also increased with UV irradiation. Indomethacin significantly suppressed this increase 2-3 hr after UV irradiation. The changes of cutaneous blood flow correlated with those of C*. These results suggested C* was a suitable parameter to quantify UV erythema, and the change of skin color in UV erythema reflected the change of cutaneous blood flow.

Fading responses in the evoked EMG after rocuronium in cats

This study was performed to evaluate the inhibitory effect on motor nerve terminals by rocuronium using recovery curves of muscle compound action potentials (CAPs) and train-of-four ratios (TOFRs) in anaesthetized cats, and to compare the results with other relaxants reported previously. Recovery curves were derived from the amplitude of the CAP induced in the gastrocnemius muscle by the second of a paired stimulus (test response) to the sciatic nerve and compared with results evoked by the first component (conditioning response). The interval between the paired stimuli was increased stepwise from 7 to 1,000 msec, and the differences in amplitude of the test and conditioning responses were plotted on a graph by relating the changes in paired intervals. The recovery curve after rocuronium was less inhibited than after pancuronium, (100.4 +/- 5.9%, 82.3 +/- 6.7% and 68.5 +/- 6.7% at 60, 100 and 500 msec intervals, compared with 70.3 +/- 3.3%, 59.0 +/- 4.7% and 46.7 +/- 4.3% after pancuronium (P < 0.05). The recovery curves with d-tubocurarine were more depressed than with pancuronium; however, the RC with vecuronium was similar to that of rocuronium. The degree of fade in TOF by rocuronium was also less than those seen with d-tubocurarine and pancuronium. The results obtained suggest that rocuronium has less inhibitory effect on motor nerve terminals than do d-tubocurarine and pancuronium, and has a similar effect to that of vecuronium.

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.

Relationships between block-of-twitch and train-of-four fade in the mouse phrenic nerve-diaphragm preparation

The relationships between the block-of-twitch and train-of-four fade in the presence of nondepolarizing neuromuscular blocking drugs (d-tubocurarine, vecuronium and pancuronium) were examined in vitro by measuring the contractile tension from mouse phrenic nerve-diaphragm preparations. The slope of the block/fade relationship differed between onset of and recovery from neuromuscular block following single doses of d-tubocurarine, vecuronium or pancuronium. Decreasing the dose of d-tubocurarine or using a divided dose technique to accelerate onset (i.e., priming) increased the amount of fade for a given amount of block. In addition, the block/fade relationships for cumulative dosing and sequential dilution were the same when measurements were made at steady-state for several doses. It is concluded that the block/fade relationship in the mouse phrenic nerve-diaphragm preparation is variable, and is related to the time course of the neuromuscular block. In addition, the block/fade relationships for d-tubocurarine, vecuronium and pancuronium did not differ when determined at steady-state.

Atropine-resistant transmission in partially depolarized rat urinary bladder

1. Phasic contractile responses of the intact rat urinary bladder to the muscarinic agonists carbachol and pilocarpine became nearly blocked as the concentrations were progressively increased to 200-500 microM. In contrast, tonic contractile responses remained elevated throughout progressive increases in agonist concentration. 2. Nerve-induced phasic contractions to 1 Hz stimuli were potentiated throughout progressive increases in the concentration of muscarinic agonists. However, these responses were more atropine sensitive than untreated controls and responses to 1 Hz stimuli were nearly abolished. 3. After inhibition of cholinesterase, the action of cholinergic transmitter released during prolonged nerve stimulation may extend to the tonic contractile state of the bladder and potentiate responses to 1H stimuli. Nerve-induced responses were more atropine sensitive than untreated controls. 4. Bladder tone was increased and nerve-induced contractions to 1-Hz stimuli were also potentiated in an elevated K+ environment. However, atropine sensitivity of nerve-induced responses w s reduced. 5. Nerve-induced bladder contractions were linked to the tonic contractile state of the bladder muscle, controlled physiologically by muscarinic receptors. Since phasic contractile responses to muscarinic agonists were abolished at high concentrations by receptor desensitization, nerve-induced responses must be elicited under these conditions by a non-cholinergic transmitter.

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.

Attenuation of soman-induced lesions of skeletal muscle by acetylcholinesterase reactivating and non-reactivating antidotes

It has been reported recently that some oximes reactivating acetylcholinesterase (AChE) exhibit concomitant ganglion-blocking effects which presumably could contribute independently to their powerful antidotal action in organophosphate inhibitor (OPI) poisoning, thus mimicking some unrelated substances which are effective antidotes without reactivating AChE. This raises the question whether OPI-induced muscle lesions, like some other symptoms could also be attenuated by oximes and other antidotes in the absence of AChE reactivation. To test this possibility, the oxime HI-6 was applied at increasing time intervals after the injection of soman until and beyond the point when soman-AChE complex becomes completely "aged" and not capable of reactivation. As the examples of OPI antidotes which do not reactivate AChE, the muscarinic antagonist atropine and the ganglion-blocking agent hexamethonium were also tested on possible attenuation of muscle lesions. The proportions of fibers with lesions, AChE inhibition and muscle fasciculations in experimental groups relative to the controls treated with soman only were evaluated. The results show that HI-6 can attenuate lesions only if AChE is partially reactivated and muscle fasciculations are permanently eliminated. However, atropine does not affect either AChE inhibition or muscle fasciculations and is also ineffective in counteracting the lesions in spite of its potency as an effective general antidote. Hexamethonium also does not affect AChE inhibition, but abolishes fasciculations and effectively attenuates muscle lesions. The latter findings reveal the existence of lesion-protecting mechanisms unrelated to AChE reactivation, which if further elucidated might become potentially relevant for additional treatment in OPI poisoning.

Prejunctional modulation of acetylcholine release from the skeletal neuromuscular junction: link between positive (nicotinic)- and negative (muscarinic)-feedback modulation

1. Presynaptic receptor-mediated modulation of stimulation-evoked [3H]-acetylcholine[( 3H]-ACh) release from the neuromuscular junction was studied in the region of the mouse hemidiaphragm which contains the motor endplates, and which can easily be loaded with [3H]-choline. This method made it possible to detect exclusively the [Ca2+]0-dependent, quantal release of [3H]-ACh in response to axonal stimulation. 2. Atropine enhanced, and non-depolarizing muscle relaxants [+)-tubocurarine, pancuronium and pipecuronium) reduced, the release of [3H]-ACh evoked by high frequency trains of stimulation (50 Hz, 40 shocks) of the phrenic nerve. The effect of (+)-tubocurarine was frequency-dependent as at 5 Hz (40 shocks) it was less effective than at 50 Hz. The resting release of [3H]-ACh was not affected by these compounds. These findings indicate that ACh released into the synaptic gap by axonal firing reaches a concentration sufficient to influence its own release by a prejunctional effect. 3. The anticholinesterase, physostigmine sulphate, enhanced the release of [3H]-ACh in a concentration-dependent manner. This effect was mediated via prejunctional nicotinic receptor stimulation: (+)-tubocurarine, pancuronium and pipecuronium completely prevented the effect of physostigmine. 4. When the prejunctional nicotinic and muscarinic receptors were stimulated by a high concentration of extracellular ACh which had accumulated in the junctional gap in the presence of physostigmine, atropine did not influence the evoked release of [3H]-ACh. However, when the effect of endogenous ACh on nicotinic receptors was prevented by (+)-tubocurarine, atropine enhanced the release. 5. It is concluded that quantally-released ACh from motor endplates is subject to prejunctional automodulation: (a) ACh facilitates its own release via an effect on prejunctional nicotinic receptors (positive feedback), (b) ACh release is reduced by an action on muscarinic receptors. When the nicotinic receptor-mediated facilitation is fully operative, the muscarinic receptor-mediated negative feedback is much less effective. It is supposed that there is a link between the two feedback mechanisms possibly at the level of the second messenger system(s).

Train-of-four fade during onset of neuromuscular block with nondepolarising neuromuscular blocking agents

Fade in the train-of-four (TOF) responses during onset of neuromuscular block was studied following administration of atracurium (225 or 450 micrograms/kg), vecuronium (40 or 80 micrograms/kg), pancuronium (60 or 120 micrograms/kg) and tubocurarine (450 micrograms/kg). TOF ratios were measured at approximate heights of T1 (first response in the TOF) of 75, 50 and 25%. Fade in TOF increased as the height of T1 decreased, with maximum fade being observed at T1 of 25%. The greatest difference between relaxants was observed at T1 of 25%, vecuronium showing the least fade and pancuronium, atracurium and tubocurarine showing increasing fade, in that order. The difference between atracurium and tubocurarine or between vecuronium and pancuronium was not significant, but the degree of TOF fade was significantly greater with atracurium and tubocurarine in comparison to vecuronium or pancuronium.

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.

Train-of-four fade of non depolarizing muscle relaxants: an insight into the mechanism of precurarization

This study was carried out to assess the prejunctional effect of non depolarizing muscle relaxants during the onset of neuromuscular blockade using the train-of-four ratio (TR). The prejunctional effect was compared with previous results concerning the ability of the relaxants to prevent suxamethonium-induced fasciculations. Fifty-three adult patients were relaxed with small incremental doses of either alcuronium (0.03 mg.kg-1), atracurium (0.04 mg.kg-1), pancuronium (0.01 mg.kg-1), d-tubocurarine (0.05 mg.kg-1) or vecuronium (0.01 mg.kg-1) during anaesthesia with thiopentone, fentanyl and nitrous oxide in oxygen. The muscle relaxant was given after recovery from an initial suxamethonium blockade needed for tracheal intubation. The evoked integrated EMG response to supramaximal train-of-four (2 Hz) stimulation was recorded every 20 s. TR % was calculated at different first twitch (T1) levels during the onset of neuromuscular blockade. Significant changes occurred at the 100% and 90% T1 levels, alcuronium having the lowest mean TR values. Atracurium, pancuronium and vecuronium gave similar TR values. Results with d-tubocurarine placed it between alcuronium and the others. These train-of-four ratio results were compared with the ability of non depolarizing muscle relaxants to prevent fasciculations. In conclusion, the stronger the train-of-four fade, the greater was the ability of the relaxant to prevent suxamethonium-induced fasciculations. This supports the theory that the blockade of prejunctional cholinergic receptors is the mechanism of action of precurarization.

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.

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.

Effect of ethanol on neuromuscular function in rats. Its interaction with alcuronium

The effect of chronic ethanol intake on neuromuscular function has been analyzed by using a rat tibial muscle preparation. The time-course of single twitches, trains-of-four, tetanus and post-tetanic facilitation with and without blockade with alcuronium was evaluated. A decrease in these parameters was observed, being more pronounced in ethanol fed rats during 10 than 30 days. The twitch was the most affected parameter. After recovery of alcuronium blockade, the depressant effects of ethanol were completely reversed. These data suggest that low but sustained ethanol blood levels causes evident alterations of neuromuscular function due, probably, to a postjunctional action.

Experimental and clinical evaluation of neuromuscular blocking agents

Neuromuscular blocking agents can be evaluated in anesthetized cats, dogs, or rhesus monkeys using dose-response relationships to assess the degree of separation between neuromuscular blocking activity and effects on cardiovascular and autonomic systems. Histamine release can be studied in anesthetized dogs by establishing whether any induced hypotension is reduced by pretreatment with H1 and H2 receptor antagonists. In anesthetized cats, the role of the liver in metabolism is investigated after administration of the drug by the hepatic portal vein and determining whether this "first pass" effect through the liver reduces potency. The involvement of the kidneys in the elimination of the drug can be assessed by ascertaining whether or not the potency and duration of action are enhanced after bilateral renal ligation. In anesthetized man, the simultaneous measurement of the tetanic (50 Hz) and single twitch contractions of the adductor pollicis muscle of the hand has proved a valuable technique for the quantitative assessment of skeletal muscle relaxants. Furthermore, the tetanic response overcomes the relative insensitivity of the single twitch, and tetanic fade provides a measure of the effects of these drugs at presynaptic sites. Cumulative effects may be evident as a prolonged recovery after repeated dosage or after an infusion. The effectiveness of neostigmine as an antagonist should be investigated both from partial and complete block, and recovery of respiration should be compared with that of peripheral neuromuscular blockade.

Pre-and post-junctional effects of tubocurarine and other nicotinic antagonists during repetitive stimulation in the rat

The effects of tubocurarine and trimetaphan have been examined at voltage-clamped rat diaphragm neuromuscular junctions during (a) single and repetitive stimulation of the phrenic nerve in cut muscles and (b) repetitive ionophoretic application of acetylcholine (ACh). Tubocurarine (2.5 X 10(-7)-10(-6)M) produced a concentration-dependent reduction in the amplitude of neurally evoked end-plate currents (e.p.c.s). It also reduced their time constant of decay (tau e.p.c.) in a manner that was independent of membrane potential, and not markedly dependent on the tubocurarine concentration. Likewise the snake alpha-neurotoxin, erabutoxin b, reduced the e.p.c. amplitude and produced a voltage-independent shortening of tau e.p.c. Estimates of mean channel lifetime (tau noise) from ACh-induced e.p.c. fluctuations revealed that (a) tau noise was 46.4 +/- 3.7% shorter than tau e.p.c. measured at the same end-plate. At these same end-plates in the presence of tubocurarine (2.5 X 10(-7)M) tau e.p.c. was 32.6 +/- 1.0% shorter than the control tau e.p.c. but tubocurarine did not change tau noise, (b) trimetaphan (2.5 X 10(-5)-2 X 10(-4)M) produced a concentration-dependent and voltage-dependent reduction of tau e.p.c., and a concentration-dependent reduction of peak e.p.c. amplitude. Trimetaphan (2.5 X 10(-5)M) produced a 50% reduction of tau noise. (a) Both tubocurarine and trimetaphan produced concentration-dependent increases in the run-down of trains of neurally evoked e.p.c.s (50 Hz, 0.4 s). This effect did not vary with membrane potential in tubocurarine, but was voltage dependent when induced by trimetaphan. (b) Erabutoxin b reduced the e.p.c. amplitude but did not produce any increase in the run-down of trains of neurally evoked e.p.c.s. During 50 Hz repetitive ionophoretic application of ACh, tubocurarine (2.5 X 10(-7)M) reduced the amplitude of each current in the train without inducing any run-down of the current amplitudes. This effect was not dependent on the membrane potential. In contrast trimetaphan (2.5 X 10(-5)M) induced a voltage-dependent run-down of trains of ionophoretically evoked e.p.c.s. We conclude that tubocurarine and erabutoxin b reduce the e.p.c. amplitude by blocking the post-junctional ACh receptor. Tubocurarine produces tetanic rundown of e.p.c.s. by a prejunctional mechanism, whereas the effects of trimetaphan during single and repetitive stimulation are at least partly due to block of the open ion channel associated with the ACh receptor.

Effect of tubocurarine and decamethonium on voluntary muscle contractions in man

Six healthy, young male subjects performed repeated brisk maximal voluntary muscle contractions (MVC) with the knee and hip extensors. Three MVCs were performed every minute. On separate days decamethonium 0.03 mg X kg-1 and tubocurarine 0.01 mg X kg-1 were administered intravenously during repeated MVCs. While ordinary MVCs showed a slow rate of rise of tension over approximately 1 s, brisk MVCs showed a steep rate of rise of tension and a biphasic configuration appeared, as a notch was seen 370-480 ms after the initiation of the contraction curve. An arbitrary straight line was drawn connecting the starting point of the contraction curve and the notch. The tension time integral to the left and above this line (alpha component), respectively to the right and below the line (beta component) was measured during the first 600 ms of the contraction. Tubocurarine affected the beta component until 70% reduced. With further curarization, the remainder of the beta component was reduced together with the alpha component. Decamethonium, in contrast, affected the alpha component together with 30% of the beta component. Thereafter, the rest of the beta component was increasingly affected. The results suggest that the isometric mechanogram is composed of a phasically active component with a high innervation threshold primarily sensitive to decamethonium, and a tonically active component with a lower innervation threshold, and primarily sensitive to tubocurarine.

Effect of dithiothreitol on agonist and antagonist actions in frog muscle

The effects of the disulphide bond reducing agent dithiothreitol (DTT) in the frog rectus abdominis preparation have been investigated. DTT, 1 mM, reduced the potency of the monoquaternary agonists acetylcholine, carbachol and tetramethylammonium and the response to electrical field stimulation; the same applied to nicotine, but the action of edrophonium was unaffected and that of the bisquaternary agonist, decamethonium, was increased. The potency of tubocurarine and gallamine as antagonists was unaltered or slightly reduced by DTT when monoquaternary agonists were used and increased when decamethonium was used as agonist. All these effects of DTT were reversed by the oxidizing agent 5-5'dithiobis(2-nitrobenzoic acid) and can be explained by a reduction of a disulphide bond in the vicinity of the anionic site of the nicotinic cholinoceptor. Comparison between these results and published data indicate that there are species differences between nicotinic cholinoceptors and motor endplates. In the guinea-pig ileum preparation DTT reduced the potency of nicotine acting at ganglionic nicotinic cholinoceptors, but had no effect on the agonist response mediated via muscarinic cholinoceptors.

Anomalous action of tubocurarine in the tibialis anterior muscle of the cat

Low dose infusion of tubocurarine (20 micrograms/kg per min) causes an increase in muscle contraction in tibialis anterior of the cat. Tibialis anterior muscle of 13 adult cats was indirectly stimulated via the sciatic nerve using a square wave pulse of 0.2 ms duration and supramaximal voltage at 0.06 Hz. The increase is from 5-133% of control with a mean of 36%. This is sustained for a mean of 97 min, if the infusion is ceased when maximum potentiation is achieved. Duration of potentiation is decreased by increasing the frequency of stimulation. The potentiation occurs in the presence of either rising or falling concentrations of tubocurarine, and continuing the infusion results in neuromuscular blockade. One explanation of these observations is that low dose tubocurarine may interfere with presynaptic negative feedback control of acetylcholine release.

The influence of stimulation parameters on the potency and reversibility of neuromuscular blocking agents

Voluntary muscle movements in mammalian muscles are initiated by short trains of 16 to 60 Hz impulses (Zierler, 1974). Despite this in most neurophysiological and neuropharmacological studies either single stimuli of 0.1 to 2 Hz or 5 to 10 sec 50 to 500 Hz tetani have been employed. Neither of these two types of stimuli are ideal for the testing of the functional integrity of the motor unit. Stimulation with single impulses, at slow rates, does not reveal incipient pathological or drug induced defects. Recovery of neuromuscular (NM) activity after 5 to 10 sec tetanic stimulation is prolonged and after repeated stimulation of this type the preparations decay rapidly. Stimulation of 0.1 sec trains of 50 Hz impulses applied every 10 to 20 sec eliminate the above disadvantages. This type of stimulation represents adequate challenge for revealing more moderate degrees of functional defects of the myoneural apparatus without causing rapid decay of the in vitro or in vivo preparations. In agreement with this the ED50 of NM blocking agents were found to be significantly lower in both the in vitro phrenic nerve-hemidiaphragm preparation and the in vivo sciatic nervetibialis anterior muscle preparation of rats during stimulation with 0.1 sec trains of 50 Hz impulses, than when single stimuli of 0.1 Hz were used. Recovery of the in vitro preparations after washout or in vivo after discontinuation of the infusion was also slower during stimulation with short trains of tetani. The antagonist potency of anticholinesterases or 4-aminopyridine and maximal recovery after the use of the optimal concentrations of these antagonists was less in the preparations stimulated with short trains of tetani than in those stimulated with single impulses.

Diphenylhydantoin-induced block of the rat phrenic nerve-diaphragm preparation pretreated with p-hydroxymercuribenzoate

Pretreatment of the rat phrenic nerve-diaphragm preparation with the sulfhydryl-(SH) blocking agent p-hydroxymercuribenzoate (pOHMB) increased the blocking efficiency of the antiepileptic drug diphenylhydantoin (DPH) during indirect stimulation. Another SH-blocking agent, N-ethyl-maleimide (NEM) did not potentiate the block, but SH-group protection with dithiothreitol (DTT) abolished the effect of pOHMB. SH-binding was thus necessary but not sufficient for enhancement of DPH-block. High Ca2+-concentration potentiated the block. Well-maintained response of the isolated phrenic nerve, and of the diaphragm during direct stimulation, located the block at the neuromuscular junction. Microelectrode records in preparations which were curarized, cut or Mg2+ paralyzed to abolish action potential activity, disclosed an abrupt cessation of end-plate potentials (EPPs) by DPH, and pOHMB pretreatment reduced the time period to abrupt EPP fallout in the curarized preparation, suggesting depressed nerve terminal excitability as the cause of the block and its potentiation. Observation of miniature EPPs beyond the time of EPP cessation excluded a postsynaptic block. The pOHMB-treated preparation is suggested as a model for testing antiepileptic drugs.

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.