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 (I) the relative responses of muscles is inverted between succinylcholine (SUX) and vecuronium (VEC), and (2) differences in dose-response or duration of action ave 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 moni tored and the cumulative dose-response curves and duration of action of SUX and VEC in the diaphragm, cricoarytenoideus 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 ED(50)s of SUX [range, 119 +/- 11 (SE) to 159 +/- 20 mu g . kg(-1)] or VEC [range, 2.8 +/- 0.2 to 3.7 +/- 0.8 mu g . kg(-1)] among muscles. With either drug, duration to 25% or 50% T-1 recovery was shortest at the laryngeal muscles and longest at abdominal muscles (P = 0.0001), and correlated directly with fibre size (r greater than or equal to 0.40; P < 0.004) and inversely with the endplate to fibre size ratio (r greater than or equal to 0.40; P < 0.008). Conclusion: The results show that (Ij 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.