The Autonomic Margins of Safety of Metocurine and d-Tubocurarine in the Cat

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Abstract
The potencies of metocurine and d-tubocurarine for neuromuscular and autonomic blockade and histamine release were determined in cats anesthetized with chloralose and pentobarbital. The autonomic margins of safety of these drugs were determined by measuring the ratios of ED50 for sympathetic block to ED95 for neuromuscular block; ED50 for vagal block to ED95 for neuromuscular block; ED50 for histamine release to ED95 for neuromuscular block. Metocurine was 14 times more potent than d-tubocurarine as a neuromuscular blocking agent in the cat, but its autonomic blocking action is 3 times weaker than that of d-tubocurarine and its histamine-releasing action is less than 1/2 that of d-tubocurarine. The combination of higher neuromuscular blocking potency and weaker autonomic effect gives metocurine a much higher autonomic margin of safety than d-tubocurarine in the cat.