Tetanic fade during partial transmission failure produced by non-depolarizing neuromuscular blocking drugs in the cat

Abstract

1. A comparison has been made of the effects of three acetylcholine antagonists--hexamethonium, tubocurarine and pancuronium--on maximal tetani of limb muscles of cats under chloralose anaesthesia. In most experiments, the indirectly stimulated soleus muscle was studied, but observations were also made on the tibialis anterior and flexor digitorum longus muscles. 2. When neuromuscular block was produced by intra-arterial injections of the acetylcholine antagonists, tetanic tension, though depressed in amplitude did not wane and there was little or no post-tetanic relief of the block as judged by the amplitude of subsequent twitches. On the other hand, during similar degrees of block produced by intravenous injections, tetanic tension rapidly waned, and, after the tetanus, transmission was temporarily facilitated, as evidenced by an increase in the amplitude of post-tetanic twitches. 3. Intravenously injected hexamethonium caused complete waning of tetanic tension in doses too small to depress twitch amplitude and which caused only a small depression of peak tetanic tension. In contrast, pancuronium caused only partial tetanic fade even in doses that produced pronounced depressions of twitch and tetanic tensions. The effects of tubocurarine fell between these extremes. 4. The results suggest that depression of peak tension and tetanic fade are independent effects of acetylcholine antagonists. It is postulated that the former is a consequence of block of post-junctional cholinoceptors, whereas the latter arises from an action of pre-junctional cholinoceptors. 5. The results obtained, together with those of other workers, led to the suggestion that transmitter acetylcholine, in addition to evoking the endplate potential, acts on the nonmyelinated nerve terminals in a positive feed-back mechanism that mobilizes transmitter to keep pace with release during high frequencies of stimulation.