Anomalous Behavior of Isolated Muscle Fibers Toward Certain Chemical Stimuli

Abstract

The rhythmic twitching induced in skeletal muscle by sodium chloride solutions is well known. It has also been known for a long time that the sodium salts of the calcium-precipitating anions induce even stronger twitching and that potassium chloride, citrate and oxalate are also effective, although potassium salts in addition cause a characteristic contracture. It was rather surprising, therefore, to find that the single isolated muscle fiber, when immersed in any of these solutions, did not twitch at all nor did it undergo contracture. The behavior of the isolated fiber was so different from that of the whole muscle that an attempt was made to discover the cause of this paradox.

Single muscle fibers were isolated from the adductor magnus muscle of the frog (Rana pipiens) in the usual frog Ringer's solution, buffered with di-sodium phosphate. Ordinarily, no attempt was made to preserve the nerve ending and the fiber was cut at both ends in the process of dissection. The isolated fiber then differs from the fiber in the intact muscle in that its nerve ending may be lacking, that is, it may have been located in a part of the fiber which was discarded; and also in that its ends are cut. This fiber responds quite normally to electrical and ultraviolet stimulation. It twitches in response to a condenser discharge or an induction shock and shortens on faradic stimulation or ultraviolet irradiation. However, its reaction to chemical stimulation is lacking in so far as the chloride, citrate and oxalate of sodium and potassium are concerned. Moreover, it does not respond to acetylcholine in Ringer's solution. In this respect my results corroborate those of Keil and Sichel. 1

By more careful dissection, it was possible to isolate muscle fibers with the nerve ending intact. In the frog, the motor nerve ending is rather diffuse, so that it was not assumed to be present unless a portion of the myelinated fiber was identifiable also. Such preparations were treated with isosmotic sodium citrate or with acetylcholine in Ringer's solution (1:10,000).