A comparison of the responses of the tenuissimus muscle to neuromuscular blocking drugs in vivo and in vitro.

In view of the differing responses to decamethonium which have been reported for the isolated tenuissimus muscle on the one hand and the tibialis muscle of the cat on the other hand, the responses of the tenuissimus muscle to neuromuscular blocking drugs were studied both in vivo and in vitro and compared with those of the tibialis anterior muscle. In both muscles in vivo, the block produced by decamethonium had all the well-known characteristics of blockade due to long-lasting depolarization, independent of the number of doses given. However, when the tenuissimus muscle was studied in vitro its responses were considerably altered. When decamethonium was left in contact with the isolated muscle, maximum paralysis developed quickly, but then the muscle recovered despite the continued presence of the drug in the bath. This contrasts with the effect obtained in vivo, where a steady application of decamethonium produced steady blockade. This spontaneous recovery in vitro occurred to a smaller extent with each successive dose. In addition, the usual antagonism between depolarizing and competitive drugs was only seen during the earlier part of an experiment and rarely occurred after several doses. It is suggested that the differences between in vivo and in vitro responses of the tenuissimus muscle to decamethonium could be due to detrimental changes in ionic concentration gradients resulting from immersion in an artificial fluid.