The role of calcium stores in fatigue of isolated single muscle fibres from the cane toad

Intracellular calcium ([Ca2+](1)) and tension were measured from single mus cle fibres dissected from the cane toad (Bufo marinus). The amount of Ca2which could be released from the sarcoplasmic reticulum (SR) was estimated by brief (similar to 20 s) exposures to 4-chloro-m-cresol (4-CmC) or caffei ne. 2. Muscle fatigue was produced by repeated tetani at 4s or shorter interval s and continued until tension had fallen to 50% of the control. The intrace llular free calcium concentration during a tetanus (tetanic [Ca2+](1))first increased and then steadily declined to 43 +/- 2% of control by the time t ension 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+]( 1) recovered to 93 +/- 3 % and 100 +/- 4% of the control values after 20 mi n of rest. Over the same period rapidly releasable SR Ca2+ recovered to 98 +/- 12 %. 3. when 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 t etanic [Ca2+](1) did not change significantly. Under these conditions the r apidly releasable SR Ca2+ did not change significantly. 4. The recovery of rapidly releasable SR Ca2+ after fatigue was unaffected by removal of extracellular calcium but did not occur when oxidative phosph orylation was inhibited with cyanide. 5. These results suggest that an important cause of the decline of tetanic [Ca2+](1) 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 consisten t with the hypothesis that Ca2+ precipitates with phosphate in the SR durin g fatigue.