THE CONTRACTILE RESPONSE OF DIRECTLY STIMULATED MUSCLE AFTER ADMINISTRATION OF NEUROMUSCULAR BLOCKING COMPOUNDS

¹ National Institute of Neurological Diseases and Blindness, National Institutes of Health, Public Health Service, U. S. Department of Health, Education and Welfare, Bethesda, Maryland

When stimulated directly with a supramaximal voltage, the twitch tension developed by the gastrocnemius muscle of the rat is reduced an average of 50% by doses of succinylcholine or decamethonium which abolish neuromuscular transmission for 30 minutes. In contrast to the failure of neuromuscular transmission, which at the doses used occurs in 2 minutes or less, the partial block to direct stimulation develops gradually during the first 20 minutes following the administration of the drug. When neuromuscular transmission is abolished for a similar length of time by d-tubocurarine, gallamine, or fatigue through the nerve, the tension developed remains at or near control levels. A partial block of directly stimulated muscle also occurs ing the administration of benzoquinonium or neostigmine.

As long as the absolute refractory period of the muscle is not exceeded, an increase in the duration of the stimulus does not restore the contraction toward the control value.

Doses of succinylcholine or decamethonium, which normally abolish neuromuscular transmission for 30 minutes, do not affect the contraction of directly stimulated muscle when injected after transmission has been abolished by d-tubocurarine or gallamine. This protective effect is markedly less pronounced in denervated than in innervated muscle.

In denervated muscle, the partial block to direct stimulation develops more rapidly, progresses to a greater degree and has a longer duration than in innervated muscle.

Stimulus response curves are significantly different after the administration of succinylcholine or decamethonium from those recorded after d-tubocurarine or gallamine. At the point of maximum activity of succinylcholine or decamethonium, the rate of development of twitch tension and relaxation is the same as in untreated muscle in which tension is reduced a comparable amount by a decrease in stimulus voltage.