title: Metabolic correlates of fatigue and of recovery from fatigue in single frog muscle fibers.
abstract: Fatigue and recovery from fatigue were related to metabolism in single fibers of the frog semitendinosus muscle. The fibers were held at a sarcomere length of 2.3 microm in oxygenated Ringer solution at 15 degrees C and were stimulated for up to 150 s by a schedule of 10-s, 20-Hz tetanic trains that were interrupted by 1-s rest periods, after which they were rapidly frozen for biochemical analysis. Two kinds of fatigue were produced in relation to stimulus duration. A rapidly reversed fatigue occurred with stimulation for under 40 s and was evidenced by a decline in tetanic tension that could be overcome by 1 s of rest. A prolonged fatigue was caused by stimulation for 100-150 s. It was evidenced during stimulation by a fall in tetanic tension that could not be overcome by 1 s of rest, and after stimulation by a reduction, lasting for up to 82 min, in the peak tension of a 200-ms test tetanus. Fiber phosphocreatine (PCr) fell logarithmically in relation to stimulus duration, from a mean of 121 +/- 8 nmol/mg protein (SEM, n = 12) to 10% of this value after 150 s of stimulation. PCr returned to normal levels after 90-120 min of rest. Stimulation for 150 s did not significantly affect fiber glycogen and reduced fiber ATP by at most 15%. It is suggested that the prolonged fatigue caused by 100-150 s of tetanic stimulation was caused by long-lasting failure of excitation-contraction coupling, as it was not accompanied by depletion of energy stores in the form of ATP. One possibility is that H+ accumulated in fatigued fibers so as to interfere with the action of Ca2+ in the coupling process.
mesh_terms: D000255:Adenosine Triphosphate / Q000032:analysis; D000818:Animals; D001001:Anura; D006003:Glycogen / Q000032:analysis; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000032:analysis; D009132:Muscles / Q000032:analysis / Q000378:metabolism / Q000502:physiology*; D010725:Phosphocreatine / Q000032:analysis; D005082:Physical Exertion; D011188:Potassium / Q000378:metabolism; D011894:Rana pipiens; D013997:Time Factors
chemical_list: D009124:Muscle Proteins; D010725:Phosphocreatine; D000255:Adenosine Triphosphate; D006003:Glycogen; D011188:Potassium