Rat gastrocnemius high and low frequency fatigue without metabolic impairment by 31P NMR.

Metabolic status and intracellular pH were investigated using 31-P nuclear magnetic resonance spectroscopy during mechanical fatigue induced in rat gastrocnemius muscle in vivo by continuous stimulation either at low or high frequency. During high frequency stimulations, force decreased to low level (10% of initial in 3-6 min) while phosphocreatine declined abruptly to 28-30% of its initial level and pH fell to 6.36 in 45 seconds. Force then continued to fall but PCr and pH rose again to reach 80-85% of the initial phosphocreatine value and 6.96 (pH) at the end of the stimulation period. The major feature of these results at high frequency was that the muscle could not generate force despite high energy stores and normal pH. During low frequency stimulation, force decreased in 9 min, to 10% of initial level. Phosphocreatine decreased abruptly to become undetectable while pH declined to 6.08 in 90 seconds. But later, phosphocreatine rose again to 35% and pH recovered to 6.84 while force continued to fall. Our results showed that intracellular pH and energy stores are not involved in the development and maintenance of mechanical fatigue.