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Anaerobic Metabolism During Electrical Stimulation of Aged Rat Skeletal Muscle

JF Hopp, PhD, PT, is Associate Professor and Director of Physical Therapy, Carroll College, 100 N East Ave, Waukesha, WI 53186-9988 (USA) (jhopp@carroll1.cc.edu). She was Assistant Professor, Department of Physical Therapy and the Section of Geriatric Medicine, Department of Medicine, University of Illinois at Chicago, Chicago, IL 60612, at the time this study was conducted.

Background and Purpose. During brief, high-intensity activities, skeletal muscle adenosine triphosphate (ATP) is rapidly resynthesized. Little is known, however, about the effect of age on ATP synthesis during such activities. In this study, aged muscle anaerobic metabolism during supramaximal, intermittent electrical stimulation was investigated. Subjects. The flexor digitorum brevis (FDB) muscles from virgin 12-month-old (adult) and 32-month-old (aged) F1 hybrid Fisher 344 X Norway Brown male rats (12 rats per age group) were studied. There were three experimental groups (eight muscles per experimental group): control (no incubation or electrical stimulation), incubation (no electrical stimulation), and electrical stimulation (combined with incubation). Methods. Incubation and electrical stimulation muscles were incubated in vitro in physiological medium for 15 minutes. Electrical stimulation muscles were stimulated intermittently, 10 seconds on:20 seconds off, using direct-current monophasic waveforms (intensity=80 V, pulse duration=200 milliseconds, and frequency=5.0 Hz). Muscle peak tension was measured, and intracellular ATP, creatine phosphate, glycogen, and lactate concentrations were determined. Results. The ATP, creatine phosphate, and glycogen contents in adult and aged muscles were similar in control preparations. There were also no age-related differences in the muscles' utilization of intramuscular ATP and creatine phosphate during stimulation. A greater depletion of glycogen and accumulation of lactate occurred, however, in the aged muscles compared with the adult muscles during stimulation. Conclusion and Discussion. Aged rat FDB muscle appears to retain its capacity to rapidly resynthesize ATP through anaerobic metabolism during brief, high-intensity activity. Inadequate anaerobic resynthesis of ATP does not appear to contribute to the earlier and greater decline in peak train tension in the aged muscle.

Key Words: Age • Anaerobic • Electrical stimulation • Metabolism • Skeletal muscle

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