AJP - Cell Physiology, Vol 232, Issue 5 185-C190, Copyright © 1977 by American Physiological Society
Fatigue and posttetanic potentiation in single muscle fibers of the frog
J. L. Vergara, S. I. Rapoprot and V. Nassar-Gentina
At 15 degrees C, direct stimulation of frog single muscle fibers at a
frequency of 20 Hz produced a tetanic tension that remained constant for 20
s and then declined. The decline was reversed during 1-s interruptions of
the stimulus train in the first 50 s of stimulation, but not with longer
stimulation. Posttetanic potentiation (PTP), characterized by prolonged
twitch relaxation and contraction times and elevation of twitch height,
remained for 10-40 min after a 10-s tetanus and for at least 90 min after a
50- to 150-s tetanus. Posttetanic fatigue appeared only after at least 50s
of tetanic stimulation. Fatigue was manifested invariably by a reduction in
the height of a 200-ms tetanic contraction and usually by a reduction in
twitch height after PTP. Fatigued fibers recovered normal contractile
responses in 40-160 min. Hypertonic solutions, which blocked contraction in
response to tetanic stimulation, prevented posttetanic fatigue but not PTP.
The observations suggest that fatigue is caused by a failure in
excitation-contraction coupling, probably in relation to consumption of
metabolic substrates. Even 10-s tetani which do not produce fatigue can
affect muscle contractile function for up to 40 min.
