A differentiation between red and white muscle in the cat based on responses to neuromuscular blocking agents

The antagonism of muscle relaxants by ambenonium and methoxyambenonium in the cat

The ability of ambenonium and a methoxy analogue to antagonize paralysis produced either by tubocurarine or by decamethonium has been studied in the tibialis anterior muscle of the cat under chloralose anaesthesia. In small doses, both oxamides facilitated neuromuscular transmission, but in larger doses they depressed the sensitivity of the motor end plates to depolarizing substances and it is considered that this latter action is sufficient to account for their anti-decamethonium action. Although both compounds possess anticholinesterase activity, there was found to be no correlation between their relative abilities to antagonize tubocurarine paralysis and their abilities to inhibit muscle cholinesterase in vitro.

Effects of neuromuscular-blocking drugs in rats in vivo: direct measurements in the diaphragm and tibialis anterior muscle

BACKGROUND

Effects of neuromuscular-blocking drugs are often investigated in rodents using both the diaphragm in vitro and the tibialis anterior muscle in vivo. Differences in the properties of these muscles cannot be neglected. We evaluated the neuromuscular effects of drugs on the rat diaphragm in vivo in comparison with the tibialis anterior muscle, directly measuring twitch tension with a force transducer.

METHODS

Respectively using sternotomy and an approach via the gluteal space, transducers were attached to the diaphragm and the tibialis anterior muscle. Phrenic and sciatic nerves were stimulated supramaximally by single square pulses to record isometric contraction of muscles. Pancuronium, vecuronium, or rocuronium was infused continuously at a rate adjusted repeatedly until stable tension was obtained. Effective doses for 50% (ED 50) and 90% (ED 90) depression of tension were calculated, using a cumulative infusion dosing method.

RESULTS

Isometric contraction of the diaphragm could be recorded despite movements of the heart and lungs. For all three drugs, both mean ED 50 and ED 90 were more than twice as large for the diaphragm as for the tibialis anterior (P<0.05). In addition, slopes of dose-response curves for the diaphragm were significantly less steep than those of the tibialis anterior for the three drugs.

CONCLUSION

Neuromuscular-blocking effects on the diaphragm can be recorded in vivo. According to direct contractile force measurement, the diaphragm is more resistant to non-depolarizing neuromuscular-blocking drugs than the tibialis anterior muscle.

Acute partial paralysis alters perceptions of air hunger, work and effort at constant P(CO(2)) and V(E)

Breathing sensations of AIR HUNGER, WORK and EFFORT may depend on projections of central motor discharge (corollary discharge) to the forebrain. Source of motor drive (brainstem or cortex) may determine what is perceived. To test the effect of changing motor discharge at constant ventilation, we induced partial neuromuscular blockade during hypercapnic hyperpnea (31 + or - 9 L min(-1); PET(CO(2))=49 + or - 2 Torr) and during matched volitional hyperpnea (34 + or - 5 L min(-1); PET(CO(2))=41 + or - 1 Torr). Decline of vital capacity was similar between conditions (39%). Ventilation was unchanged with paralysis, indicating increased respiratory motor drive to maintain hyperpnea. Sensations were rated on a seven point ordinal scale. Median EFFORT and WORK increased 3-3.5 points with paralysis during both forms of hyperpnea (P<0.02, Wilcoxon signed rank). Median AIR HUNGER increased 2.5 points with paralysis during hypercapnic (P<0.02) but not during volitional hyperpnea. Data suggests that EFFORT and WORK arise from motor cortex activity (subjects reported engaging volitional control when paralyzed even during hypercapnia) and suggests that AIR HUNGER arises from medullary motor activity.

The neuromuscular blocking action of benzoquinonium chloride in the cat and in the hen

Benzoquinonium (Mytolon) has been shown to produce a curare-like rather than a decamethonium-like paralysis of neuromuscular transmission in the tibialis anterior and soleus muscle of cats and the gastrocnemius muscle of hens. In the cat, but not in the hen, benzoquinonium had an additional action in preventing both the twitch potentiating action and the anti-curare action of edrophonium, neostigmine, eserine, and tetraethyl pyrophosphate. Paralysis produced by benzoquinonium was antagonized by injected acetylcholine and by acetylcholine liberated by tetanic stimulation of the motor nerve but, in the cat, anticholinesterases were without effect even when they were first administered in circumstances in which acetylcholine accumulation might be expected to occur. These findings suggested that inhibition of cholinesterase played little part in the skeletal muscle effects of the anticholinesterases studied. Although anticholinesterases did not antagonize benzoquinonium paralysis in the cat, they nevertheless potentiated the antagonistic action of injected acetylcholine.

A comparison of the responses of the tenuissimus muscle to neuromuscular blocking drugs in vivo and in vitro

In view of the differing responses to decamethonium which have been reported for the isolated tenuissimus muscle on the one hand and the tibialis muscle of the cat on the other hand, the responses of the tenuissimus muscle to neuromuscular blocking drugs were studied both in vivo and in vitro and compared with those of the tibialis anterior muscle. In both muscles in vivo, the block produced by decamethonium had all the well-known characteristics of blockade due to long-lasting depolarization, independent of the number of doses given. However, when the tenuissimus muscle was studied in vitro its responses were considerably altered. When decamethonium was left in contact with the isolated muscle, maximum paralysis developed quickly, but then the muscle recovered despite the continued presence of the drug in the bath. This contrasts with the effect obtained in vivo, where a steady application of decamethonium produced steady blockade. This spontaneous recovery in vitro occurred to a smaller extent with each successive dose. In addition, the usual antagonism between depolarizing and competitive drugs was only seen during the earlier part of an experiment and rarely occurred after several doses. It is suggested that the differences between in vivo and in vitro responses of the tenuissimus muscle to decamethonium could be due to detrimental changes in ionic concentration gradients resulting from immersion in an artificial fluid.

SOME EFFECTS OF LONG CHAIN POLYMETHYLENE BISONIUM SALTS ON JUNCTIONAL TRANSMISSION IN THE PERIPHERAL NERVOUS SYSTEM

A survey has been made of the effects on junctional transmission of the complete series of polymethylene bis-trimethylammonium (BTM) and bis-triethylammonium (BTE) salts from the decamethylene compounds (BTM 10 and BTE 10) to those with twenty-one methylene groups in the chain. These were tested for their ability to cause contracture of the isolated chick biventer cervicis preparation, and for their ability to block the twitch responses of this preparation, those of the rat isolated diaphragm preparation, and those of the cat tibialis anterior preparation. They were also tested for their ability to block transmission in the cat superior cervical ganglion, to block the actions of acetylcholine on the guinea-pig isolated ileum, and for ability to inhibit the hydrolysis of acetylcholine by acetylcholinesterase. Their electrical conductivity has been measured in aqueous solution. Ability to cause contracture of the chick biventer cervicis is confined to the compounds BTM 10 to 15; BTE 10, 11 and 12 have some weak activity but the other BTE compounds, and the BTM compounds with more than fifteen methylene groups, have virtually no activity. In the BTE series both neuromuscular blocking and ganglion-blocking activities increase with chain length up to a maximum in the region of BTE 15 to 17 and then decline. In the BTM series ganglion-blocking activity increases with chain length in much the same way as in the BTE series, though the maximum activity is at a slightly longer chain length. At the neuromuscular junction an increase in chain length beyond BTM 10 leads to a decline in activity but this returns to some extent at longer chain lengths, reaching a second maximum at BTM 18, above which it declines further. At the ganglion BTE 16 is only slightly more active than BTM 16 and about five-times as active as hexamethonium; at the neuromuscular junction in the cat BTE 16 is about five-times as active as BTM 16 and about eight-times as active as (+)-tubocurarine. The affinity of the BTE compounds for the postganglionic acetylcholine receptors of the guinea-pig ileum reaches a maximum at BTE 14 but does not decline significantly with further increase in chain length. Anticholinesterase activity, likewise, does not alter significantly between BTM 12 and BTM 21 and the activity of the compounds in the BTE series appears to be similar. This property could conceivably be modifying the actions of some of the intermediate compounds but is not likely to be affecting those of the more active ones. The conductivity experiments indicate that micelle formation could be limiting the actions of the compounds with 20 or 21 methylene groups, but is not likely to be affecting those of the other compounds. The results suggest that there is a regular increase with chain length of the affinity of these compounds for the receptors in the ganglia and at the neuromuscular junction but that efficacy in causing contracture is limited to compounds with three methyl groups in the cationic head and a chain of about ten methylene groups. The connexion between this ability to depolarize and the ability to block transmission by desensitization is discussed.

Recovery of post-tetanic and train-of-four responses at the first dorsal interosseous and adductor pollicis muscles in patients receiving vecuronium

PURPOSE

To compare recovery of accelographical responses to post-tetanic twitch (PTT) and train-of-four (TOF) stimuli obtained at the first dorsal interosseous muscle (DI) with those at the adductor pollicis muscle (AP) after administration of vecuronium 70 micrograms.kg-1.

METHODS

Sixty adult patients were randomly assigned to one of four groups: PTT-DI (n = 15), PTT-AP (n = 15), TOF-DI (n = 15), or TOF-AP (n = 15) group. In PTT-DI and PTT-AP groups, responses to PTT were measured accelographically at the DI and at the AP, respectively. In TOF-DI and TOF-AP groups, responses to TOF were measured at the DI and at the AP, respectively.

RESULTS

The T1/T0 (T0 = control) was greater in the TOF-DI than in TOF-AP group throughout recovery (P < 0.05), and the T4/T1 was greater in the TOF-DI than in TOF-AP group during the 30-40 min after vecuronium injection (P < 0.05). Time to the return of the first response to PTT (post-tetanic count(1), PTC1) was less in the PTT-DI than in the PTT-AP group (17.7 +/- 4.2 vs 21.7 +/- 5.6 min, mean +/- SD, P = 0.0341). The post-tetanic count PTC (number of single twitch stimuli in response to PTT) was greater in the PTT-DI than in the PTT-AP group during the 10-30 min after vecuronium (P < 0.05). Time to the return of T1 was less in the TOF-DI than in the TOF-AP group (23.1 +/- 6.0 vs 27.6 +/- 4.9 min, P = 0.0334).

CONCLUSION

Recovery of responses to PTT and TOF stimuli occurred earlier at the DI than at the AP.

Differential effect of pancuronium at the adductor pollicis, the first dorsal interosseous and the hypothenar muscles. An electromyographic and mechanomyographic dose-response study

Cumulative dose-response curves were constructed from evoked compound electromyographic (EMG) recordings in man to compare the sensitivity to pancuronium of the adductor pollicis, the hypothenar and the first dorsal interosseous muscles. Also, the EMG and mechanomyography-based sensitivity of the adductor pollicis muscle were compared. The EMG and the mechanomyogram were evaluated in random sequence in each of 21 adult thiopental, fentanyl and diazepam anesthetized patients. The EMG-based ED50 were 36-38 micrograms.kg-1 with no differences between muscles. The EMG-based ED90 of the adductor pollicis and the hypothenar muscles were 62-65 micrograms.kg-1 compared to the 60 micrograms.kg-1 of the first dorsal interosseous muscle (P < 0.05). ED50 (34 micrograms.kg-1), and ED90 (56 micrograms.kg-1) obtained from the adductor pollicis mechanomyogram were significantly lower than those based on the EMG (P < 0.05). It is concluded that differences in sensitivity to pancuronium exist between the three muscles when evaluated from the EMG, and that the apparent sensitivity of a given muscle to a muscle relaxant may depend upon whether the response is evaluated using EMG or mechanomyography.

The effects of fructose 1,6-diphosphate, caffeine and dantrolene sodium on suxamethonium-induced contractures in denervated rat skeletal muscle

Previously unidentified forms of suxamethonium-induced contractures have been investigated in chronically denervated rat extensor digitorum longus (EDL) muscle at 20 degrees C. Contractures were assigned to groups 1-6 on the basis of the peak tension (Tp1) during 0-10 min exposure to the drug (3.0 x 10(-5) M), (7.0 x 10(-6) M), and (3.5 x 10(-6) M) and the subsequent retention, increase, or decrease in tension (Tp2), during the further 10 min. It is proposed that four stages exist in the development of contractile changes at 1-7, 8-35, 36-70 and 70-130 days after denervation (DPD) and that contractility is lost at 147 days after denervation. Initial changes, although present in EDL muscles in group 1 at 2.0 DPD s.d. +/- 1 (n = 7) in response to the drug (3.0 x 10(-5) M), were more apparent in EDL muscles in group 2 which were identified at 5.5 DPD s.d. +/- 1.6 (n = 7) by an excessive contracture response (Tp2) to the drug (3.0 x 10(-5) M), 18.3 mN s.d. +/- 10.6. At 5.0 DPD s.d. +/- 2.7 (n = 5) contracture tension (Tp2) was commensurate with membrane depolarization, 13.1 mN/33.1 mV, but residual tension increased to 23.3 mN during the Krebs wash (80 min) whilst membrane depolarization decreased to 9.2 mV. Also, at 4.3 DPD s.d. +/- 2.3 (n = 5) tension (Tp2) increased significantly (P less than or equal to 0.05) in the presence of caffeine (4.1 x 10(-3) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular, ventilatory and catecholamine responses to maximal dynamic exercise in partially curarized man

1. In ten young men the ventilatory, cardiovascular, catecholamine and metabolic responses to maximal dynamic leg exercise on a stationary bicycle were followed during partial neuromuscular blockade with tubocurarine. Maximal exercise was performed when the drug effect was at its maximum as well as during the subsequent reduction in the effect allowing a gradually increasing work intensity. The results were compared with those obtained during submaximal and maximal exercise performed without tubocurarine. Partial neuromuscular blockade decreased hand-grip strength to 41 +/- 1.1% (S.E. of mean) and the maximal work load to 27 +/- 2.4% of control values. Voluntary effort was maximal and the rate of perceived exertion was high at all levels of exercise with tubocurarine indicating a maintained intense central nervous motor command. 2. During maximal action of the drug oxygen uptake was 1.67 +/- 0.11 l/min while only 0.91 +/- 0.13 l/min (P less than 0.01) at the same work intensity without neuromuscular blockade. This difference may reflect a dominant reliance on fast-twitch muscle fibres when work was performed under the influence of tubocurarine. 3. Compared at a given oxygen uptake ventilation was higher during work with tubocurarine than during control exercise (e.g. 55 +/- 4.2 and 40 +/- 2.2 l/min, respectively (P less than 0.01), at a mean oxygen uptake of 1.9 l/min), while heart rate did not differ significantly (146 +/- 4.4 and 139 +/- 3.0 beats/min). With decreasing drug effect both variables increased towards the maximum values of 138 +/- 4.5 l/min and 183 +/- 3.9 beats/min, respectively, achieved in control experiments at an oxygen uptake of 3.8 +/- 0.2 l/min. Like heart rate the mean arterial blood pressure increased with increasing work load and was similar at a given oxygen uptake with and without tubocurarine. 4. During maximal exercise at peak tubocurarine effect plasma adrenaline and noradrenaline concentrations were smaller than during control maximum, 1.6 +/- 0.27 versus 3.4 +/- 0.55 nmol/l (P less than 0.01) and 7.5 +/- 1.3 versus 12.6 +/- 1.8 nmol/l (P less than 0.05), respectively. However, comparisons at identical oxygen uptake rates revealed that catecholamine responses were markedly enhanced during tubocurarine treatment. Also, blood lactate concentrations were smaller at peak tubocurarine action than during control maximum, 1.9 +/- 0.42 mmol/l and 6.1 +/- 0.49 mmol/l (P less than 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)

Monitoring technique affects measurement of recovery from succinylcholine

To compare recovery time from neuromuscular blockade after the administration of a single intravenous bolus of succinylcholine (1 mg/kg), we measured, both mechanically and electromyographically, the evoked twitch responses in the hand muscles of 10 patients. Electromyographic data were obtained, electronically integrated, and recorded by a newly available clinical monitoring device. Using both the mechanical and the electromyographic devices, we recorded the times for first return of twitch, as well as for 25% and 75% recovery of twitch height compared with prerelaxant twitch baseline values and compared the values by calculating least-squares regression lines. Times given by the electromyographic device for these measures of returning neuromuscular function were notably longer than those given by the mechanical evoked force monitor. No such findings have been reported in previous studies that compared these two techniques for monitoring of muscle recovery from nondepolarizing relaxants. The specific reasons for the differences found in this study are unknown.

Heart rate at the onset of static exercise in man with partial neuromuscular blockade

In human subjects the length of the first complete cardiac cycle after the onset of static finger or hand muscle contractions was recorded. Graded contractions as well as maximal contractions in which muscle fatigue was provoked by ischaemia were studied. Maximal finger muscle contractions were also weakened by partial neuromuscular blockade with either decamethonium or tubocurarine. The change in R-R interval at the onset of exercise was variable and in part dependent on the length of the last R-R interval before the onset of exercise. The largest decreases were seen when the preceding R-R interval was long. During graded submaximal and maximal contractions a correlation was demonstrated between the change of the R-R interval at the onset of exercise and the contraction intensity. At the onset of the weakest contractions a 3% prolongation of the R-R interval was seen while the strongest contractions were associated with a shortening of 8%. At the onset of all maximal contractions, whether fatigued due to ischaemia or weakened by decamethonium or tubocurarine, a decrease in R-R interval was seen independent of the force developed. When compared at the same resting R-R interval, contractions performed with hand muscles elicited a greater shortening of R-R interval than contractions with finger muscles. The results suggest that the initial change in heart rate at the onset of static muscle contractions is related to the voluntary effort rather than to the force developed, the state of the working muscles or to the type of muscle fibres involved. It may thus be elicited by 'cortical irradiation' rather than by a 'muscle-heart reflex'. Its size depends on the length of the preceding R-R interval, the intended contraction intensity and the muscle mass involved.

Partial neuromuscular blockade and cardiovascular responses to static exercise in man

In human subjects sustained static contractions of the quadriceps femoris in one leg were performed with the same absolute and the same relative intensity before and after partial neuromuscular blockade with either decamethonium or tubocurarine which reduced strength to about 50% of the control value. During the contractions performed with the same absolute force, the magnitude of the cardiovascular responses (heart rate and blood pressure) was greater during neuromuscular blockade than during control contractions. During the contractions involving the same relative force the magnitude of the cardiovascular responses was almost the same with and without neuromuscular blockade. These findings were independent of the drug used. The metabolic part of the exercise pressor reflex was assessed by the application of an arterial cuff 1/2 min before cessation of exercise and for the following 3 min of rest. Although heart rate and blood pressure decreased after cessation of exercise, application of the tourniquet resulted in higher post-exercise values and this effect was seen both with and without neuromuscular blockade. Muscle biopsies from the subjects' m. vastus lateralis were analysed for fast- and slow-twitch fibre composition showing 27-66% slow-twitch fibres. No correlation was found between cardiovascular responses to static exercise, with or without neuromuscular blockade, and fibre type predominance. The results suggest that the involvement of fast- or slow-twitch muscle fibres does not play a dominant role in the cardiovascular responses to static exercise in man. Both central command and reflex neural mechanisms are of importance, and it appears that these two control mechanisms are redundant and that neural occlusion may be operative. However, when partial neuromuscular blockade induces a disproportion between an increase in central command and a constant or decreasing muscle tension and metabolism, the larger signal arising from central command determines the magnitude of the cardiovascular responses.

The effects of d-tubocurarine, pancuronium and atracurium on the responses of gastrocnemius and soleus muscles in the cat

In vivo, the effects of d-tubocurarine (0.20 mg kg-1), pancuronium (0.015 mg kg-1) and atracurium (0.15 mg kg-1) on the responses of the indirectly stimulated cat gastrocnemius (fast) and soleus (slow) muscles to a twitch, train-of-four and tetanic stimuli were studied. The soleus muscle demonstrated a greater degree of fade than the gastrocnemius in response to tetanic stimuli (50 Hz). There was no difference between the responses of the two muscles to twitch or train-of-four stimuli with any of the drugs. Recovery of train-of-four ratio occurred more rapidly than did the tetanic fade ratio. At a time when train-of-four ratio exceeded 0.7, tetanic fade was still evident, especially in the soleus muscle.

Effects of succinylcholine chloride on the response of fast and slow muscle in the cat

Fast and slow muscle fibres differ histochemically, electrophysiologically and pharmacologically. In vivo, the effect of 0.05, 0.075 and 0.1 mg X kg-1 succinylcholine on the response of the indirectly stimulated gastrocnemius (fast) and soleus muscle (slow) to single, train-of-four and tetanic stimuli was studied. There was a significant difference between the response of the two muscles. Maximum twitch suppression occurred 2 min after injection and was more significant in the gastrocnemius than the soleus. The duration of the neuromuscular blockade was 9 min or more in both groups. The train-of-four ratio dropped below 0.7 in the gastrocnemius preparation at all doses and in the soleus when 0.1 mg X kg-1 succinylcholine was used. Sustained tetanus was achieved in both muscles at 5 and 10 min, respectively. There was evidence of fatigue in the soleus muscle at the 5-min observation suggestive of the presence of a phase II block, a phenomenon not observed in the gastrocnemius.

Effect of tubocurarine on static and dynamic muscle contractions in man

The effect of a bolus injection of tubocurarine (0.1 mg X kg-1 i.v.) was followed in six young subjects by registration of static, and slow (30 degrees X s-1) and fast (150 degrees X s-1) dynamic (isokinetic) maximal voluntary leg extensions. Mechanograms from both unblocked and curarized muscle contractions showed a "notch" after about 440 ms separating two relative maxima. The mechanograms were divided by an arbitrary straight line connecting the starting point of the contraction curve and the notch. The line separated an area (Nm X s) above and to the left (alpha-component) from an area below and to the right (beta-component) of the line. Tubocurarine affected the beta-component selectively until about 70% reduced in the static contractions. With further curarization the alpha-component was also reduced in size. The alpha-component was equally affected during the three types of contractions, while the faster the contraction the more the beta-component was reduced. The results suggest that static as well as dynamic human muscle contractions can be divided into two parts with a different sensitivity for tubocurarine, one of which seems to have a sensitivity which depends on the contraction velocity.

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 tubocurarine on human soleus and gastrocnemius muscles

The effect of a bolus injection of tubocurarine (0.1-0.13 mg X kg-1 i.v.) was followed in six young male subjects by registration of the rectified smoothed electromyogram (rsEMG) from the m. soleus (71 +/- 5.1 (s.e. mean) % slow twitch muscle fibers) and from the m. gastrocnemius (54 +/- 3.1% slow twitch muscle fibers). Volitional muscle strength was recorded in isometric plantar flexions with the knee fully (0 degrees) extended where m. soleus and m. gastrocnemius both are active, and with the knee bent 90 degrees where m. soleus is dominating force development. During maximal action of the drug, the rsEMG from the soleus muscle was reduced to 30 +/- 4.0% of the control value, while the rsEMG from the gastrocnemius muscle was reduced to 53 +/- 5.2% (P less than 0.01). Muscle strength with the knee extended showed 53 +/- 7.5% force left, while 44 +/- 6.4% of the muscle strength remained when the knee was bent (P less than 0.01). The results suggest that tubocurarine affects human muscles in proportion to their slow twitch muscle fiber content.

Morphological and biochemical correlates of skeletal muscle contractility in the cat. I. Histochemical and electron microscopic studies

Three cat hind limb muscles have been examined, histochemically and ultrastructurally, in a multiparameter correlative study of structure and function in skeletal muscle contractility. The soleus, a histochemically pure, slow-twitch muscle possesses ultrastructural features which are, in many cases, significantly different from those of almost pure fast twitch caudofemoralis muscle. Although stereological analysis of fiber types indicates a correlation between speed of relaxation and volume of sarcoplasmic reticulum, morphological features such as fenestrated collars and triad morphology are identical in all fiber types. The fast twitch-oxidative-glycolytic fiber possesses features common to both slow twitch fibers (high mitochondrial content) as well as fast twitch fibers (high sarcoplasmic reticulum content) in addition to Z band width which falls in between these two fiber types. Sarcoplasmic microtubules have been described in all three fiber types in all muscles examined. They occur in predictable orientation and their possible function(s) is described.

Labelled decamethonium in cat muscle

1 Tritium-labelled decamethonium was infused intravenously in 12 cats at final rates of 1.3-4.2 nmol kg-1 min-1 to produce a steady plasma concentration which ranged between 0.21-1.3 mumol/l in different experiments. Muscle contractions were elicited by nerve stimulation and the potential at the end-plate regions of superficial fibres was recorded by extracellular electrodes. 2 Under these conditions, it was not possible to obtain a steady degree of neuromuscular block. The initial decrease in muscle contractions was followed by recovery towards the original value although the concentration of decamethonium in the plasma remained constant, or in some cases rose. The initial depolarization of the end-plate region also waned during the constant infusion of the drug. 3 Once the twitch tension had returned to control values during infusion of the drug, prolongation of the infusion for a total of four hours did not produce a secondary neuromuscular block. 4 Scintillation counting showed that during infusion of labelled decamethonium the radioactivity of the muscles increased progressively with time. The uptake was less in the soleus muscle than in the fast-contracting flexor longus digitorum and extensor longus digitorum muscles. Muscles which had been denervated 12-13 days previously showed a greater uptake of labelled drug than control muscles from the contralateral limb. 5 The labelled drug was localized by autoradiography of frozen sections of leg muscles following intra-arterial injection of decamethonium. Grain counts in individual fibres showed that small amounts of decamethonium had entered the muscle fibres along their entire length, and there was increased uptake of the drug into the cell in the region of the end-plate. 6 The mechanisms underlying the waning of the pharmacological response during constant application of depolarizing drugs are discussed.

Characteristics and mechanism of neuromuscular block in myasthenia gravis

The neuromuscular block of myasthenia gravis has the characteristics of an antidepolarizing (competitive block), similar to that produced by d-tubocurarine in normal subjects: progressive decrease in muscle action potentials evoked by two or more nerve stimuli, posttetanic facilitation, posttetanic fatigue, inhibition of the depolarizing action of acetylcholine (ACh) or anticholinesterase compounds, and reversal of the block by ACh or anticholinesterase compounds. In myasthenic patients spontaneously recurring negative discharges were more difficult to locate in the end-plate zone than in normal subjects, suggesting that the number or density of functioning end plates may be reduced. The threshold dose of intra-arterial ACh that increased electrical activity was higher than in normal subjects; the duration of the increased electrical activity was briefer, and was followed by more depression of negative discharges than in normal subjects and by a greater increase in the threshold dose of ACh. These results indicate that the end-plate zone of myasthenic patients is less responsive than that of normal subjects to the excitatory action of ACh, and may be more readily desensitized by ACh. In both myasthenic patients and normal subjects the intra-arterial injection of ACh produced a prompt transient decrease in evoked potentials, attributable to depolarization of the end plates, followed by recovery (and in myasthenic patients by repair), and then by a more prolonged late decrease in evoked potentials, attributable to desensitization of the end plates to transmitter. This prompt depressant effect of ACh on evoked potentials was less in myasthenic patients than in normal subjects, and the late depressant effect of ACh was greater. In myasthenic patients the late block produced by ACh had the characteristics of an antidepolarizing (competitive) type of block, including inhibition of the depolarizing action of ACh and reversibility by ACh or neostigmine, while in normal subjects the characteristics were those of a depolarizing (noncompetitive) type of block, including little or no inhibition of the depolarizing action of ACh and lack of reversal by ACh or neostigmine. The differences between the late depressant action of ACh in myasthenic patients and normal subjects resembled differences in the effect of other depolarizing compounds, such as choline, succinylcholine, and decamethonium, and are best explained by differences in behavior of the postsynaptic receptor. The disease appears to be due to the presence of abnormal forms of receptor or to abnormal responses of receptor to the transmitter. The predominance of one or other form of receptor may determine the clinical state of the myasthenic patient and his response to anticholinesterase medication.

Action of some bisquaternary derivatives of phthalic acids and related substances on neuromuscular transmission

1. All the bisquaternary derivatives of terephthalic acid with three methyl groups on each nitrogen atom (PK-107, PK-95, PK-97 and PK-126) were depolarizing neuromuscular blocking agents. The most active was the compound PK-97, in which the two quaternary groups are separated by sixteen atoms and are about 20 A (2 nm) apart. Activity was reduced many fold either by decreasing the separation to twelve atoms or by increasing it to eighteen atoms. It was also reduced several hundred fold when one trimethylammonium group in PK-97 was replaced by a hydrogen atom (as in PK-119).2. The presence and position of the ester groups in these compounds is important; depolarizing activity is in most cases greatest when the ester groups are the same distance from the quaternary nitrogen atoms as in acetylcholine, that is, in carbolonium, sebacoyldicholine and PK-154. The monoquaternary analogues of carbolonium and sebacoyldicholine are appreciably active, having between about one-tenth to one-fifth of the activity of their bisquaternary analogues.3. The relationships between the structure and activity of these compounds are discussed, particular consideration being given to the structure of the chain separating the quaternary groups and the arrangement of acetylcholine receptors on cells and of esterbinding groups within these receptors.

The relationship between the level of cholinesterase in plasma and the action of suxamethonium in animals

1. The neuromuscular blocking action of suxamethonium, given by intravenous injection, and the effect upon it of iso-OMPA (tetraisopropyl pyrophosphoramide) in doses which produced marked selective inhibition of cholinesterase in blood were studied in anaesthetized rats and cats, and in mice.2. In cats experiments were also carried out in which suxamethonium was given by intravenous infusion until an effect which remained constant with time was achieved. From the degree of neuromuscular block (under equilibrium conditions) obtained with different infusion rates the infusion rate for 50% reduction in twitch tension of the indirectly stimulated soleus and gastrocnemius muscles (IR50) was calculated. The effect on it of raising the suxamethonium hydrolysing capacity of blood and of selectively reducing the level of cholinesterase in blood by various doses of iso-OMPA was then investigated.3. At relevant stages of each experiment cholinesterase activity in blood was determined with butyrylcholine or benzoylcholine and where appropriate with suxamethonium as substrate.4. The results obtained show that in rats and cats the effectiveness of suxamethonium is unrelated to the level of cholinesterase activity in blood and that raising the suxamethonium hydrolysing capacity in the blood up to 22-fold (in cats) only reduces the IR50 by a factor of 1.6.5. The enhancement of the effectiveness of suxamethonium in the three species (2- to 3-fold in rats, 2- to 4-fold in mice and 7- to 8-fold in cats under the conditions used for comparison) which follows the administration of iso-OMPA is attributable to inhibition of cholinesterase in the tissues.6. It is concluded that the results obtained clearly indicate that the species studied do not give information as regards suxamethonium and its metabolism which is applicable to man.

Development of the differential response to succinylcholine in the fast and slow-twitch skeletal muscle of the kitten

1. The response to SCh has been studied in anterior tibialis and soleus muscles of new-born and neonate cat while, in a separate group of animals of similar ages, the end-plate development in these muscles has been examined.2. At birth, when the contraction times of the anterior tibialis and soleus are not statistically different, the anterior tibialis is 1.4 times more sensitive to SCh blocking action than the soleus, but both muscles were 20-40 times less sensitive than their adult counterparts.3. With maturation, the ratio of the soleus/anterior tibialis contraction time increased to 2.7 while the ratio of the 50% blocking dose of SCh in the soleus/anterior tibialis increased to 2.8, the increases being parallel. The time to 50% recovery at 50% blocking doses of SCh was similar for both muscles at all ages although it was 3 times longer in new-born and 1-week-old animals than in animals 3 weeks of age and older.4. The areas of individual end-plates from these muscles were very similar at birth but by three weeks of age the areas of end-plates from the anterior tibialis were substantially greater than those obtained from soleus.5. We have speculated that the relationship between SCh sensitivity and end-plate area in fast and slow muscle may find its origins in the frequency of the MEPP.

The effect of some foreign anions on suxamethonium blockade of the isolated rat diaphragm preparation

1. The pharmacology of suxamethonium blockade of the rat phrenic nerve-diaphragm preparation has been studied in the presence of some foreign anions.2. Blockade proceeded in two distinct phases in both normal and modified Krebs solution.3. In normal Krebs solution the characteristics bore no close resemblance to either depolarizing or competitive type blockade.4. In the presence of foreign anions the characteristics of the blockade more closely resembled those of depolarization.5. There was an increase in the sensitivity of the motor end-plate region of the muscle to the depolarizing action of acetylcholine, carbachol and suxamethonium in the presence of the anions.6. Although the anions enhanced the depolarizing activity of suxamethonium, the blocking potency of the drug was unaltered.7. It is suggested that end-plate depolarization plays little part in suxamethonium blockade of the isolated rat diaphragm and that desensitization is the primary cause of the blockade.

An electron microscope study of fibre types in normal and dystrophic muscles of the mouse

Images

Repouso da junção neuromuscular no tratamento de crises miastênicas e colinérgicas

Os autores trataram 18 crises miastênicas e colinérgicas desenvolvidas em 12 pacientes com forma generalizada e severa de miastenia grave, mediante o "repouso" da junção neuromuscular. Êste foi conseguido, em um grupo de 6 enfermos, pela suspensão das drogas anticolinesterásicas, emprego da respiração artificial e alimentação por sonda nasogástrica — "repouso relativo". Outro grupo de 6 pacientes foi submetido ao "repouso absoluto" da junção neuromuscular, mediante o uso da respiração artificial, alimentação por sonda nasogástrica e curarização prolongada pela galamina. Em mais de 50% das crises observaram-se melhoras imediatas e acentuadas com o método de tratamento pelo "repouso" da junção neuromuscular, ao lado de redução significativa da taxa de mortalidade nas crises. A evolução mostrou que os pacientes que responderam melhor durante e logo após o tratamento da crise, tiveram, também, melhor evolução ulterior. Dos 12 enfermos somente um era portador de timoma e, mesmo nesse paciente, a evolução foi satisfatória. A sensibilidade inicial ao curare foi muito grande em todos os doentes submetidos à curarização prolongada, mas, em prazo relativamente curto (alguns dias), esta hipersensibilidade diminuiu sensivelmente. Apesar de todos os cuidados, as infecções respiratórias foram a regra, exigindo tratamento enérgico e bem orientado.