Changes of myoplasmic calcium concentration during fatigue in single mouse muscle fibers

Measurements of the intracellular free concentration of Ca2+ (Ca2+]i) were performed during fatiguing stimulation of intact, single muscle fibers, which were dissected from a mouse foot muscle and loaded with fura-2. Fatigue, which was produced by repeated 100-Hz tetani, generally occurred in three phases. Initially, tension declined rapidly to approximately 90% of the original tension (0.9 Po) and during this period the tetanic Ca2+]i increased significantly (phase 1). Then followed a lengthy period of almost stable tension production and tetanic Ca2+]i (phase 2). Finally, both the tetanic Ca2+]i and tension fell relatively fast (phase 3). The resting Ca2+]i rose continuously throughout the stimulation period. A 10-s rest period during phase 3 resulted in a significant increase of both tetanic Ca2+]i and tension, whereas a 10-s pause during phase 2 did not have any marked effect. Application of caffeine under control conditions and early during phase 2 resulted in a substantial increase of the tetanic Ca2+]i but no marked tension increase, whereas caffeine applied at the end of fatiguing stimulation (tension depressed to approximately 0.3 Po) gave a marked increase of both tetanic Ca2+]i and tension. The tetanic Ca2+]i for a given tension was generally higher during fatiguing stimulation than under control conditions. Fatigue developed more rapidly in fibers exposed to cyanide. In these fibers there was no increase of tetanic Ca2+]i during phase 1 and the increase of the resting Ca2+]i during fatiguing stimulation was markedly larger. The present results indicate that fatigue produced by repeated tetani is caused by a combination of reduced maximum tension-generating capacity, reduced myofibrillar Ca2+ sensitivity, and reduced Ca2+ release from the sarcoplasmic reticulum. The depression of maximum tension-generating capacity develops early during fatiguing stimulation and it is of greatest importance for the force decline at early stages of fatigue. As fatigue gets more severe, reduced Ca2+ sensitivity and reduced Ca2+ release become quantitatively more important for the tension decline.