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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.

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.

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.

In order to investigate the relationship between muscle fibre composition or size and muscle sensitivity to neuromuscular blocking drugs, data on the fibre composition and size of the ulnaris lateralis, thyroarytenoideus, cricoarytenoideus dorsalis and diaphragm muscles of the goat were related to the rate of recovery of these muscles from suxamethonium or vecuronium block. Muscle sensitivity (time to spontaneous 25% recovery of evoked EMG response after 100% block) correlated well with fibre size, the association being significant for the diameter of type I (r > or = 0.94; P < or = 0.016) and types I and II fibres taken together (r > or = 0.95; P < or = 0.013), but not for type II fibre diameter (r < or = 0.83; P > or = 0.084), for both suxamethonium and vecuronium. There was no association between the composition of types I or II fibres in these muscles and sensitivity (r < or = 0.37; P > or = 0.546). This suggests that muscle sensitivity to both depolarizing and non-depolarizing drugs increases with fibre size, and may account partly for the unequal sensitivities of different muscles.

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