EFFECTS OF POTASSIUM ON THE ENDPLATE POTENTIAL AND NEUROMUSCULAR TRANSMISSION IN THE CURARIZED SEMITENDINOSUS OF THE FROG

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Abstract
The excised ventral head of the semitendinosus muscle of the frog (Rana pipiens) was curarized by soaking in Ringer''s soln. containing curare. The desired concn. of KC1 for intra-arterial injn. was obtained by placing 1% KC1 in Ringer''s-curare soln. taken from the bath. It was found that K first facilitated neuromuscular transmission and increased the size of the endplate potential (e.p.p.) and subsequently blocked neuromuscular transmission and markedly reduced the e.p.p. Facilitation occurred before an increase in size of the e.p.p. appeared, and neuromuscular transmission ceased before the height of the e.p.p. was reduced to the pre-injn. level. It was suggested that the facilitating and depressant actions of K may involve alterations of both the excitability of the muscle fibers and the size of the e.p.p. During the period of depression and subsequent recovery after KC1 injn., the response to the first of 2 stimuli reduced the responsiveness to a second stimulus. It was pointed out that K diminishes restoration in some manner not revealed by the electrical signs of repolarization because marked reduction of responsiveness to a second stimulus persisted several msec, after the e.p.p. produced by the first of 2 stimuli had returned to zero.

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