Postcontractile force depression in humans is associated with an impairment in SR Ca(2+) pump function

Area of Science:

• Exercise Physiology
• Muscle Biology
• Biochemistry

Background:

• Postcontractile depression (PCD) is a phenomenon observed after fatiguing exercise.
• The role of intrinsic changes in sarcoplasmic reticulum (SR) function in PCD remains unclear.
• Investigating SR Ca(2+)-sequestration is crucial for understanding muscle fatigue mechanisms.

Purpose of the Study:

• To test the hypothesis that intrinsic changes in SR Ca(2+)-sequestration contribute to PCD in human muscle.
• To examine alterations in maximal SR Ca(2+) uptake and Ca(2+)-ATPase activity during and after fatiguing exercise.

Main Methods:

• Muscle tissue (vastus lateralis) was obtained from eight untrained females before and after a 30-minute isometric exercise protocol designed to induce PCD.
• Maximal Ca(2+) uptake and Ca(2+)-ATPase activity were measured in muscle homogenates.
• Muscle mechanical performance was assessed via evoked activation at various time points.

Main Results:

• Fatiguing exercise progressively reduced evoked muscle force, with significant depression observed at 30 minutes.
• Maximal SR Ca(2+) uptake and Ca(2+)-ATPase activity were significantly depressed by 15 minutes of exercise and remained low throughout the 60-minute recovery period.
• The coupling ratio between Ca(2+)-ATPase and Ca(2+) uptake was maintained, suggesting the reduction in uptake is due to alterations in the Ca(2+) pump itself.

Conclusions:

• Intrinsic alterations in the SR Ca(2+) pump, leading to reduced Ca(2+) uptake, are implicated in postcontractile depression of force in humans.
• These findings highlight the importance of SR function in muscle fatigue.
• Further research is needed to elucidate the precise role of altered Ca(2+) sequestration in Ca(2+) release and cytosolic calcium dynamics during PCD.