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Department of Physiology, University of Sydney, NSW, Australia.

1. The role of the myoplasmic free Ca2+ concentration ([Ca2+]i) in the reduction of muscle force following contractions with stretch was investigated in single fibres from mouse toe muscle. Muscle fibres were either stretched by 25% of their optimum length (Lo) for ten tetani (Protocol I) or stretched by 50% of Lo for between ten and thirty tetani (Protocol II). Indo-1 was used to measure [Ca2+]i. 2. In each protocol the stretch series was compared with isometric controls; the stretch series always resulted in greater changes in muscle properties than in the isometric controls. The observed changes were (i) reduced tetanic force, (ii) reduced tetanic [Ca2+]i, (iii) increased resting [Ca2+]i and (iv) the greater relative reduction in force at low stimulus frequencies (30 and 50 Hz) compared with high (100 Hz). These changes were maintained for up to 60 min. 3. Stretching a resting muscle fibre had no effect on the subsequent [Ca2+]i or force. 4. Following Protocol I 10 mM caffeine restored tetanic force to pre-stretch levels. Tetanic [Ca2+]i vs. force curves were constructed pre- and post-stretch and showed that neither the maximum Ca(2+)-activated force nor the Ca2+ sensitivity of the muscle fibres post-stretch was significantly different from control. The force reduction, therefore, appears to be the result of reduced tetanic [Ca2+]i. 5. The more severe stretching regimen of Protocol II resulted in a much greater reduction in force than Protocol I. Ten millimolar caffeine did not restore control force. Comparison of the [Ca2+]i-force relationships pre- and post-stretch showed that the reduction in tetanic force was caused by a combination of a reduced tetanic [Ca2+]i, reduced maximum Ca(2+)-activated force and reduced Ca2+ sensitivity. 6. Following both protocols the resting [Ca2+]i showed a small rise which persisted for at least 60 min. This elevated [Ca2+]i was associated with a reduction in the pump rate of the sarcoplasmic reticulum Ca2+ pump. 7. This study establishes that reduced Ca2+ release and reduced Ca2+ sensitivity contribute to the reduction in force generating capacity of single mammalian muscle fibres following active stretches.

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