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Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham and Women's Hospital, Boston, MA 02115-6195.

1. The purpose of this study was to determine, with high temporal resolution, the relationship between the intracellular Ca2+ transient (ICT) and the mechanical responses of intact, single skeletal muscle fibres of frogs following stimulation by a single, brief depolarization. 2. The time course of the ICT was monitored using the Ca(2+)-sensitive fluorescent dyes mag-fura-2 (furaptra) and mag-fura-5. The mag-fura dyes have a low affinity for Ca2+ and have been shown to track the ICT with no appreciable kinetic delay. Continuous records of mag-fura fluorescence, tension and stiffness responses were obtained simultaneously at high time resolution at a sarcomere length of 2.9 microns. Experimental temperature was 3 degrees C. 3. When a delay of 0.4 ms due to the low-pass filter associated with the photodetector was included, the onset of the fluorescence response preceded the onset of latency relaxation (the small fall in tension that precedes positive tension generation) by 3.1 +/- 0.2 ms (mean +/- S.E.M., n = 8). After its onset, the mag-fura fluorescence signal continued to change rapidly (indicating increasing intracellular [Ca2+]) to an extreme level that occurred 1.5 +/- 0.5 ms (mean +/- S.E.M., n = 7) before tension had recovered to its resting level following latency relaxation. The time delay from the extreme of the fluorescence signal to the peak of the tension signal was 239 +/- 27 ms (mean +/- S.E.M., n = 6).(ABSTRACT TRUNCATED AT 250 WORDS)

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