Muscle fibre diameter and sensitivity to neuromuscular blocking drugs

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.