AJP - Cell Physiology, Vol 263, Issue 4 C803-C809, Copyright © 1992 by American Physiological Society
Muscle fatigue in the frog semitendinosus: role of the high-energy phosphates and Pi
L. V. Thompson and R. H. Fitts
Biology Department, Marquette University, Milwaukee, Wisconsin 53233.
The purpose of this study was to determine the concentration of ATP,
phosphocreatine (PC), Pi, lactate, and glycogen in single frog skeletal
muscle fibers and assess their role in the etiology of muscle fatigue. The
frog semitendinosus (ST) muscle was fatigued, quick frozen at selected time
points of recovery, and freeze-dried, and single fibers were dissected,
weighed, and assayed for ATP, PC, lactate, Pi, and glycogen. The fatigue
protocol reduced peak tetanic force (Po) to 8.5% of initial, while ATP and
PC decreased from 45.18 to 33.16 and 128.90 to 28.76 mmol/kg dry wt,
respectively. Lactate and Pi increased from 29.36 to 100.84 and 33.04 to
142.50 mmol/kg dry wt, respectively. It is doubtful that the small decline
in ATP limited cross-bridge force production. Although a significant
correlation between the recovery of PC and Po was demonstrated (r = 0.994),
the time period showing the fastest rate of force recovery coincided with
little change in PC. A significant correlation was demonstrated between the
recovery of both total and the H2PO4- form of Pi and Po. In conclusion, the
results of this study are incompatible with the hypothesis that the
high-energy phosphates (ATP and PC) mediate muscle fatigue. The large
increase in Pi with stimulation and the high correlation between the
recovery of both total and the H2PO4- form of Pi and Po support a role for
Pi in the production of skeletal muscle fatigue.
