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IN VITRO
JOURNAL ARTICLE
RESEARCH SUPPORT, NON-U.S. GOV'T
Changes in tetanic and resting [Ca2+]i during fatigue and recovery of single muscle fibres from Xenopus laevis.
Journal of Physiology 1991 Februrary
1. Single muscle fibres were dissected from the toe muscles of Xenopus laevis and microinjected with Fura-2 to measure myoplasmic calcium concentration ([Ca2+]i). Injected fibres were illuminated at 340 and 380 nm and the ratio of the resulting fluorescence at 505 nm (the Fura-2 ratio) was taken as a measure of [Ca2+]i. Fibres were fatigued at 21 degrees C by repeated tetani until developed tension had fallen to 50% of control. 2. Tetanic tension declined monotonically during fatiguing stimulation, whereas the tetanic Fura-2 ratio first increased and then declined. At the 10th tetanus, tension was 87% of control whereas the Fura-2 ratio was 106% of control. At the end of fatiguing stimulation, where tension was around 50% of control, the tetanic Fura-2 ratio was reduced to 71%. The rate of decline of both tension and the Fura-2 ratio after a tetanus slowed during fatigue. During recovery, the tension and the tetanic Fura-2 ratio recovered in parallel. 3. The resting Fura-2 ratio increased throughout fatigue reaching 237% of control when tension had declined to 50%. There was a rapid phase of recovery, complete within 1 min, by which time the resting Fura-2 ratio was 198% of control. Subsequent recovery was slower and took 20-30 min to reach a stable level which was 121% of control. 4. The resting Fura-2 ratio towards the end of fatiguing stimulation was greater than the tetanic Fura-2 ratio in the early part of recovery although there was no detectable increase of resting tension during fatiguing stimulation. This observation suggests that the Ca2+ sensitivity of the contractile proteins was reduced at the end of fatiguing stimulation. 5. Plots of the tetanic tension against tetanic Fura-2 ratios throughout fatiguing stimulation and recovery also suggested that Ca2+ sensitivity was reduced during fatiguing stimulation when compared to recovery. 6. The increases in resting [Ca2+]i caused by raised [K+]o (from 2.5 to 10 mM) and/or by application of 15% CO2 were much less than those produced by fatiguing stimulation. Much of the elevated [Ca2+]i in fatigue could be reversed by application of dantrolene (25 microM). 7. The results suggest that both reduced tetanic [Ca2+]i and reduced Ca2+ sensitivity contribute to the decline of tension during fatigue.(ABSTRACT TRUNCATED AT 400 WORDS)
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