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Department of Physiology, University of Sydney F13, NSW, Australia.

1. Intracellular calcium concentration ([Ca2+]i) and force were measured from isolated single fibres of mouse skeletal muscle. The effects of 5 mM caffeine on muscle fibres at rest and during short tetani were examined. 2. Caffeine increased tetanic tension and slowed the rate of relaxation. [Ca2+]i was increased in the presence of caffeine both in the resting muscle and during tetani. The time course of decline of [Ca2+]i after a tetanus is complex with a large, early, rapid phase followed by a smaller and slower phase. Caffeine accelerated the early phase but slowed the later phase. 3. The sensitivity of the myofibrillar proteins to Ca2+ measured in the intact fibre was increased in the presence of caffeine, confirming earlier findings on skinned muscle fibres. 4. Analysis of the late phase of the decline of [Ca2+]i after a tetanus provides information about the properties of the sarcoplasmic reticulum (SR) Ca2+ pump. Caffeine slowed the pump to 60-70% of the control value at a given [Ca2+]i but had no effect on the Ca2+ leak from the SR. 5. Analysis of relaxation made use of the Ca(2+)-derived force in which the [Ca2+]i during relaxation was converted to the Ca(2+)-derived force by means of the steady-state relation between [Ca2+]i and force. The Ca(2+)-derived force fell more slowly in the presence of caffeine but the lag between Ca(2+)-derived force and measured force was unaffected. Thus, the slowed relaxation was caused by changes in Ca2+ handling and not by slowed cross-bridge kinetics. 6. A model of the Ca2+ movements and force production of muscle was used to examine independently the effects of increased Ca2+ sensitivity, slowing of the SR Ca2+ pump and increased SR Ca2+ permeability. The effects of caffeine on [Ca2+]i, tetanic force and relaxation could be explained by a combination of these three effects.

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