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

1. The effect of muscle length on the myoplasmic free Ca2+ concentration ([Ca2+]i) and tetanic force production of intact single muscle fibres was investigated in the mouse flexor brevis muscle. Muscle fibres were analysed at 100 microns intervals from 100 microns shorter than the optimum length (L(zero)) to 300 microns longer than L(zero). The fluorescent Ca2+ indicator indo-1 was used to measure [Ca2+]i. 2. Changes in muscle length did not significantly affect tetanic or resting [Ca2+]i. However, at lower stimulus frequencies (30-50 Hz) the force-length relation was shifted to longer muscle lengths. 3. Force-tetanic [Ca2+]i curves were constructed at each muscle length. A comparison of these curves revealed that the Ca2+ sensitivity was length dependent. Ca2+ sensitivity increased over the ascending limb and plateau phase of the force-length curve, but over the descending limb Ca2+ sensitivity reached a plateau and then started to decrease once more. 4. These results show that length-dependent changes in Ca2+ sensitivity of single muscle fibres cannot simply be related to variations in sarcomere length. It is proposed that other factors, such as cross-bridge attachment and/or developed force, have a role in determining the Ca2+ sensitivity at a particular muscle length.

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