The importance of peripheral changes in determining the sensitivity of striated muscle to depolarizing drugs

Cross-innervated mammalian skeletal muscle: histochemical, physiological and biochemical observations

1. Cross-innervation of the slow soleus and fast flexor hallucis longus or flexor digitorum longus muscles has been performed in new-born kittens and rabbits and in adult cats.2. The effects on the histochemical and structural properties of the muscle have been studied and compared with the changes in the contractile properties.3. Cross-innervation has produced a dramatic change in histochemical pattern in the fast muscles, with the development of areas of muscle fibres indistinguishable from normal soleus muscle. The converse change from the histochemical pattern of slow soleus to that of fast muscle has also occurred, but has been less consistent.4. It is concluded that the neural influence determining the contractile properties of fast and slow muscle also has a profound controlling influence on the structure and metabolic activity of the muscle fibres.5. No significant changes could be demonstrated biochemically in the ATPase activities of the fast and slow muscles following cross-innervation.

Properties of fibres, endplates and acetylcholine receptors in the diaphragm, masseter, laryngeal, abdominal and limb muscles in the goat

PURPOSE

Although differences in fibre composition, fibre size or acetylcholine receptor (AChR) density between muscles have often been proposed to explain the unequal sensitivities of muscles to muscle relaxant drugs, it is not clear whether or how these parameters differ among muscles or are related to one another, In this study, several muscles were examined to determine the composition and cross-sectional area (CSA) of types I and II fibres, the surface area of their motor endplates (ESA), and their AChR density.

METHODS

Biopsies from the thyroarytenoideus, cricoarytenoideus dorsalis, masseter, diaphragm, transversus abdominis, rectus abdominis, gastrocnemius and soleus muscles of goats were processed by muscle histochemistry and morphometry and the ESA:CSA ratio was computed. The number and density of AChRs per endplate were estimated by 125I-alpha-bungarotoxin binding studies.

RESULTS

The mean type I fibre composition (range: 0-100%), fibre diameter (28-50 microns) and the ESA:CSA ratio (0.27-1.01) differed among muscles (P = 0.0001), but there were no significant differences (P > 0.05) in the mean endplate size (577-725 microns 2) AChR number (6.6-14.5 x 10(6)) or AChR density (8,900-22,300 microns-2) probably because of marked individual variations. Fibre size increased and the ESA:CSA ratio decreased in the order laryngeal, diaphragm, jaw, limb and abdominal muscles.

CONCLUSION

It is concluded that between muscles fibre size varies more than endplate size or AChR number.

Duration of succinylcholine and vecuronium blockade but not potency correlates with the ratio of endplate size to fibre size in seven muscles in the goat

PURPOSE

Muscles differ in their response to neuromuscular relaxants. This study investigated whether (1) the relative responses of muscles is inverted between succinylcholine (SUX) and vecuronium (VEC), and (2) differences in dose-response or duration of action are related to the morphology of fibres, endplates and acetylcholine receptors (AChR) in muscles.

METHODS

In goats during thiopentone anaesthesia, the evoked EMG response to indirect train-of-four stimulation was monitored and the cumulative dose-response curves and duration of action of SUX and VEC in the diaphragm, cricoary-tenoideus dorsalis, thyroarytenoideus, transversus abdominis, rectus abdominis, soleus and gastrocnemius muscles were determined and related to their fibre composition, fibre size, endplate size, endplate to fibre size ratio, AChR number or AChR density by regression analysis.

RESULTS

There were no differences in the ED50S of SUX [range, 119 +/- 11 (SE) to 159 +/- 20 micrograms.kg-1] or VEC [range, 2.8 +/- 0.2 to 3.7 +/- 0.8 microgram.kg-1] among muscles. With either drug, duration to 25% or 50% T1 recovery was shortest at the laryngedl muscles and longest at abdominal muscles (P = 0.0001), and correlated directly with fibre size (r > or = 0.40; P < 0.004) and inversely with the endplate to fibre size ratio (r > or = 0.40; P < 0.008).

CONCLUSION

The results show that (1) the relative responses of muscles do not differ between depolarizing and non-depolarizing relaxants; (2) the duration of blockade is shorter in muscles composed of small fibres with large endplates relative to fibre size; and (3) there is no relation between fibre type composition and sensitivity to muscle relaxants.

Succinylcholine and vecuronium blockade of the diaphragm, laryngeal and limb muscles in the anaesthetized goat

The purpose of the study was to compare the response of the cricoarytenoideus dorsalis muscle (CD) to neuromuscular blocking drugs with those of the thyroarytenoideus (TA), diaphragm (DI) and ulnaris lateralis (UL) muscles. Evoked electromyographic response to indirect supramaximal stimulation at 1 Hz was monitored in ten adult goats under thiopentone-halothane anaesthesia. The onset time and duration of neuromuscular blockade after intravenous administration of 500 micrograms.kg-1 of succinylcholine or 4 micrograms.kg-1 of vecuronium were determined. Times to 100% paralysis in CD, TA, DI and UL after succinylcholine were (mean +/- SD) 39 +/- 11, 39 +/- 11, 42 +/- 8 and 57 +/- 8 seconds, respectively; the corresponding times for vecuronium were 5.6 +/- 2.3, 4.6 +/- 1.7, 6.0 +/- 1.9 and 9.6 +/- 1.7 min. The order of recovery to 25% spontaneous EMG activity was TA, CD, DI and UL after succinylcholine (durations: 9.7 +/- 3.6, 11.0 +/- 3.0, 15.3 +/- 1.3 and 22.0 +/- 1.2 min, respectively) but DI, CD, TA and UL after vecuronium (durations: 31.9 +/- 18.6, 35.2 +/- 19.5, 47.1 +/- 19.9 and 71.7 +/- 16.1 minutes, respectively). Thus, as in the diaphragm and thyroarytenoideus muscles, onset time and duration of succinylcholine or vecuronium blockade were shorter in the abductor muscle of the glottis, cricoarytenoideus dorsalis, than in the limb muscle.

Histochemical and morphometric properties of muscles of the upper airway of goats

This study investigated the histochemical and morphometric properties of fibres in laryngeal, hyoid, tongue and pharyngeal muscles which contribute in maintaining patency of the upper airway. Muscle specimens from adult female goats were stained for nicotinamide adenine dinucleotide dehydrogenase-tetrazolium reductase and myosin adenosine triphosphatase activities, and the composition and size of the fibre types determined. These muscles contained types 1, 2A, 2B and 2C fibres with type 2 fibres predominating and the fibres possessed oxidative enzyme activity suggesting fast contraction speed and yet moderate resistance to fatigue. Abductor laryngeal muscles contained more type 1 fibres than the adductors. Among pharyngeal muscles fibre size and type 1 fibre composition increased progressively from the hyopharyngeus caudally. Upper airway muscles contained relatively small fibres (range of mean diameter: 25.7 to 46.1 microns) with the pharyngeal and lingualis proprius muscles containing the smallest fibres. These properties might influence the response of upper airway muscles to neuromuscular blocking drugs.

Type, diameter and distribution of fibres in some respiratory and abdominal muscles of the goat

This study was designed to determine the histochemical properties, size and composition of fibres in the diaphragm, intercostal and abdominal muscles of goats to clarify whether reported similarities in respiratory muscle physiology between goats and humans have a structural basis. Serial sections (10 microns) of muscular tissue from adult female goats were stained for myosin adenosine triphosphatase and reduced nicotinamide adenine dinucleotide dehydrogenase-tetrazolium reductase activities; the fibres were classified into type I, IIA and IIB; and their mean diameter and composition were determined. Abdominal and intercostal muscles contained types I, IIA and IIB fibres in the ratio 1:1:1, and the mean diameter of the fibres ranged from 49.2 to 62.2 microns. In contrast, the diaphragm contained 58.9% type I and 41.1% type II fibres, and the latter could not be differentiated into types IIA and IIB. Diaphragmatic fibres were also smaller (36.9-40.9 microns). These findings contrast with those in humans, where the diaphragm, intercostal and abdominal muscles contain > 50% type I fibres and have fibres of identical diameter. The differences in fibre characteristics between the diaphragm, intercostal and abdominal muscles of goats and the differences between goats and humans need to be taken into consideration in interpreting the results from studies in respiratory muscle physiology.

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.

Pharmacological properties, cholinesterase activity and anatomy of nerve-muscle junctions in vagus-innervated frog sartorius

1. The pharmacological properties of the nerve-muscle junctions of vagus-innervated frog sartorius muscles were investigated. Vagus-evoked junctional potentials were ten times more sensitive to hexamethonium than were control end-plate potentials from muscles re-innervated with sartorius nerve. Hexamethonium did not affect passive electrical muscle membrane properties or quantal content. The sensitivity of vagus junctions to D-tubocurarine (dTC) did not differ from the controls.2. The amplitude and time course of vagus-evoked junctional potentials were not affected by eserine or neostigmine. Furthermore, histologically detectable acetylcholinesterase activity was not demonstrable at vagus-muscle junctions.3. The ultrastructure of vagus-innervated muscle fibres was normal in appearance indicating that the vagus was capable of structurally maintaining the muscle fibres.4. Junctional contacts were made by varicosities of unmyelinated vagal fibres. Although such varicose vagal fibres ran along the muscle fibres for long distances, they seemed to make synaptic contacts primarily at old end-plate regions, as deduced from persisting junctional folds.5. It is concluded that at least several ;trophic' factors are probably involved in the different effects of the vagus nerve on the sartorius muscle.

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.

Chronotropic and inotropic effects of isoprenaline and L-noradrenaline in the dog heart

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