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Department of Physiology and Pharmacology, Karolinska Institute, Stockholm, Sweden. davida@physiol.usyd.edu.au

1. Single fibres were dissected from mouse flexor brevis muscle and injected with indo-1 and the P3-1 (2-nitrophenyl)ethyl ester of ATP (caged ATP). Myoplasmic calcium concentration ([Ca2+]i) and force were monitored during single tetani or tetani repeated until force was reduced to about 30% of control values. In vitro experiments showed that an intense, brief ultraviolet illumination (a flash) photolysed 12% of the caged ATP to ATP. 2. Fibres that had been injected with caged ATP showed concentration-dependent changes. High concentrations of caged ATP caused a reduction in [Ca2+]i during tetani (tetanic [Ca2+]i), a reduction in force in unfatigued tetani and the fibres fatigued more rapidly when stimulated repeatedly. 3. Photolytic release of ATP in unfatigued fibres caused a concentration-dependent increase in tetanic [Ca2+]i and in force. 4. When ATP was released by photolysis in a fibre fatigued by repeated tetani, it produced a concentration-dependent increase in tetanic [Ca2+]i and force. The increase in tetanic [Ca2+]i was small (63 nM per 100 microM increase in ATP) and could explain some, but not all, the increase in force. However, taking into account the fact that control flashes in the absence of caged ATP caused a small decrease in tetanic [Ca2+]i, we believe that the increase in force may be explained by the increase in tetanic [Ca2+]i. There was no evidence of changes in the sarcoplasmic reticulum Ca2+ pump rate after photolysis of caged ATP. 5. Caged ATP affects some site(s) involved in excitation-contraction coupling and the consequences are similar to muscle fatigue. When a small fraction of this caged ATP is photolysed to ATP, the consequences of fatigue are partially reversed. These observations suggest that site(s) which either bind ATP or depend on ATP hydrolysis have a key role in excitation-contraction coupling and in muscle fatigue.

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