THE INTRACELLULAR CA2-MUSCLE FIBERS MEASURED WITH HIGH TEMPORAL RESOLUTION( TRANSIENT AND TENSION IN FROG SKELETAL)

1. The purpose of this study was to determine, with high temporal reso lution, the relationship between the intracellular Ca2+ transient (ICT ) and the mechanical responses of intact, single skeletal muscle fibre s of frogs following stimulation by a single, brief depolarization. 2. The time course of the ICT was monitored using the Ca2+-sensitive flu orescent 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 fluoresc ence, tension and stiffness responses were obtained simultaneously at high time resolution at a sarcomere length of 2.9 mum. Experimental te mperature was 3-degrees-C. 3. When a delay of 0.4 ms due to the low-pa ss filter associated with the photodetector was included, the onset of the fluorescence response preceded the onset of latency relaxation (t he 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-fur a fluorescence signal continued to change rapidly (indicating increasi ng intracellular [Ca2+]) to an extreme level that occurred 1.5 + 0.5 m s (mean +/- S.E.M., n = 7) before tension had recovered to its resting level following latency relaxation. The time delay from the extreme o f the fluorescence signal to the peak of the tension signal was 239 +/ - 27 ms (mean +/- S. E. M., n = 6). 4. It is concluded that the intrac ellular concentration of Ca2+ begins to rise before any detectable mec hanical changes occur during a twitch contraction and begins to decrea se before the onset of positive tension development.