OPPOSITE MECHANICAL RESPONSES OF TONIC MUSCLES TO ACETYLCHOLINE STIMULATION IN NON-IONIC AND IONIC SOLUTIONS

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Abstract
In ion-free, isotonic solutions, the frog rectus abdominis and other "tonic" muscles undergo a slow contracture. When the muscles are then stimulated with acetylcholine (ACh), they exhibit a rapid relaxation in which the tension decreases by as much as 2 g. The relaxation response, like contraction, is blocked by tubocurarine and increased by ACh-potentiating drugs. Returning Na+ to the external fluid immediately restores the capacity of the rectus to contract to ACh. None of the other cations or anions of Ringer''s solution in physiological concentrations restores this contraction. However, muscles will contract to KC1 stimulation when Na+ is not present in the extracellular fluid. If the relaxation response of the rectus in ion-free solutions is the reverse of normal contraction, ideas about the relation between chemical and physical events and muscle contraction can be easily tested in this simple experimental preparation.