Role of Ca(2+) in changing active force during intermittent submaximal stimulation in intact, single mouse muscle fibers

Fatigue of single mouse fibers during repeated high-frequency stimulation results initially from decreased Ca(2+) sensitivity while free myoplasmic calcium concentration ([Ca(2+)](m)) increases, followed by decreasing [Ca(2+)](m). Recovery of active force with low-frequency stimulation is slow and persistent fatigue results from low [Ca(2+)](m). However, the consequences of intermittent submaximal contractions are not known. The aim of the present study was to investigate the changes in [Ca(2+)](m) and active force during intermittent submaximal contractions and subsequent recovery. Single fibers of mouse flexor digitorum brevis muscles at 32 °C were stimulated with 40 or 50 Hz, for 350 ms every 2 s for 2 min and then every 1 s until < 40% of initial force. Values obtained during the intermittent stimulation were compared with a control force-[Ca(2+)](m) relationship. A “P”-shaped pattern in the force-[Ca(2+)](m) relationship was observed during intermittent stimulation. Early in the intermittent stimulation, [Ca(2+)](m) increased while active force decreased. Subsequent force potentiation was accompanied by increased Ca(2+) sensitivity. Later, as active force declined, [Ca(2+)](m) decreased significantly (p < 0.001). This was followed, in the final phase, by a significant decrease in Ca(2+) sensitivity determined by [Ca(2+)](m) at half-maximal force (Ca(50)) (p = 0.001). Low-frequency fatigue persisted during recovery while Ca(50) was not significantly different from prefatigue (p > 0.5). In conclusion, the main mechanism of fatigue is due to decreases in both [Ca(2+)](m) and Ca(2+) sensitivity following the initial force potentiation. The intermittent submaximal contractions resulted in persistent low-frequency fatigue seen during recovery, which was explained by depressed [Ca(2+)](m) with no change in Ca(2+) sensitivity.