KLEE, V.; WEINSTEIN, R.B.; Univ. of Arizona, Tucson: Fatiguing stimulation of ghost crab extensor carpopodite is not accompanied by an accumulation of inorganic phosphate in muscle homogenates

Ghost crabs exercising at a speed faster than their maximum aerobic speed (MAS) fatigue within several minutes. We examined whether high intensity stimulation of the extensor capopodite, an important locomotor muscle, was correlated with a rise in inorganic phosphate, a postulated causative agent of musclular fatigue. Electromyogram (EMG) signals were recorded from the extensor carpopodite in trailing legs of ghost crabs moving at a speed of 0.3 m/s (approximately 160% MAS) on a motorized treadmill. The mean stride frequency for a 38 g crab was 3.0 Hz and the duty factor was 23% at a body temperature of 24C. In a separate set of experiments, the extensor carpopodite was stimulated in an in vivo preparation to mimic the natural muscle stimulation patterns. The peak isometric force (740 mN) declined by 50% within the first minute of stimulation and by approximately 90% within 3 to 4 minutes. Increasing the stimulating current did not restore muscle force. Fatigued muscles were rapidly freeze-clamped at the end of 4 minutes of stimulation. Muscle homogenates were assayed by enzymatic analysis and by 31P-magnetic resonance spectroscopy. In samples analyzed by both methods, inorganic phosphate, ATP, and arginine phosphate were similar in resting and fatigued muscles. The results of our muscle homogenate assays suggest that fatigue from exercise at 160% MAS is not correlated with an accumulation of inorganic phosphate in the extensor carpopodite. Supported by University of Arizona Prop. 301/Optics and Undergraduate Biology Research Program.