- 1Intracellular calcium ([Ca2+]i) and tension were measured from single muscle fibres dissected from the cane toad (Bufo marinus). The amount of Ca2+ which could be released from the sarcoplasmic reticulum (SR) was estimated by brief (≈20 s) exposures to 4-chloro-m-cresol (4-CmC) or caffeine.
- 2Muscle fatigue was produced by repeated tetani at 4 s or shorter intervals and continued until tension had fallen to 50 % of the control. The intracellular free calcium concentration during a tetanus (tetanic [Ca2+]i) first increased and then steadily declined to 43 ± 2 % of control by the time tension had fallen to 50 %. Over the period of fatigue the rapidly releasable Ca2+ from the SR fell to 46 ± 6 % of control. Tension and tetanic [Ca2+]i recovered to 93 ± 3 % and 100 ± 4 % of the control values after 20 min of rest. Over the same period rapidly releasable SR Ca2+ recovered to 98 ± 12 %.
- 3When a similar number of tetani (200) were repeated at longer intervals (10 s), fibres showed only a small reduction in tension (to 85 ± 1 %) and tetanic [Ca2+]i did not change significantly. Under these conditions the rapidly releasable SR Ca2+ did not change significantly.
- 4The recovery of rapidly releasable SR Ca2+ after fatigue was unaffected by removal of extracellular calcium but did not occur when oxidative phosphorylation was inhibited with cyanide.
- 5These results suggest that an important cause of the decline of tetanic [Ca2+]i during fatigue is an equivalent decline in the amount of rapidly releasable SR Ca2+. The results show that the decline of rapidly releasable SR Ca2+ is related to a metabolic consequence of fatigue and are consistent with the hypothesis that Ca2+ precipitates with phosphate in the SR during fatigue.