RESUMO
Fatigue was studied in intact tibialis anterior muscle of anesthetized mice. The distal tendon was detached and connected to a force transducer while blood flow continued normally. The muscle was stimulated with electrodes applied directly to the muscle surface and fatigued by repeated (1 per 4 s), brief (0.4 s), maximal (100-Hz stimulation frequency) tetani. Force declined monotonically to 49 ± 5% of the initial value with a half time of 36 ± 5 s and recovered to 86 ± 4% after 4 min. Intracellular phosphate concentration ([P(i)]) was measured by (31)P-NMR on perchloric acid extracts of muscles. [P(i)] increased during fatigue from 7.6 ± 1.7 to 16.0 ± 1.6 mmol/kg muscle wet wt and returned to control during recovery. Intracellular Ca(2+) was measured with cameleons whose plasmids had been transfected in the muscle 2 wk before the experiment. Yellow cameleon 2 was used to measure myoplasmic Ca(2+), and D1ER was used to measure sarcoplasmic reticulum (SR) Ca(2+). The myoplasmic Ca(2+) during tetani declined steadily during the period of fatigue and showed complete recovery over 4 min. The SR Ca(2+) also declined monotonically during fatigue and showed a partial recovery with rest. These results show that the initial phase of force decline is accompanied by a rise in [P(i)] and a reduction in the tetanic myoplasmic Ca(2+). We suggest that both changes contribute to the fatigue. A likely cause of the decline in tetanic myoplasmic Ca(2+) is precipitation of CaP(i) in the SR.
