DURATION OF SUCCINYLCHOLINE AND VECURONIUM BLOCKADE BUT NOT POTENCY CORRELATES WITH THE RATIO OF END-PLATE SIZE TO FIBER SIZE IN 7 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 th e morphology of fibres, endplates and acetylcholine receptors (AChR) i n muscles. Methods: In goats during thiopentone anaesthesia, the evoke d EMG response to indirect train-of-four stimulation was moni tored an d the cumulative dose-response curves and duration of action of SUX an d VEC in the diaphragm, cricoarytenoideus dorsalis, thyroarytenoideus, transversus abdominis, rectus abdominis, soleus and gastrocnemius mus cles were determined and related to their fibre composition, fibre siz e, endplate size, endplate to fibre size ratio, AChR number or AChR de nsity by regression analysis. Results: There were no differences in th e 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 a t 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 sh ow that (Ij the relative responses of muscles do not differ between de polarizing and non-depolarizing relaxants; (2) the duration of blockad e is shorter in muscles composed of small fibres with large endplates relative to fibre size; and (3) there is no relation between fibre typ e composition and sensitivity to muscle relaxants.