Fluo-3 signals associated with potassium contractures in single amphibian muscle fibres.

1. The calcium-sensitive dye fluo-3 AM was used to obtain fluorescence signals and calcium transients associated with K+ contractures, twitches and tetani, in intact single muscle fibres of the tropical toad Leptodactylus insularis. 2. The changes in free calcium concentration in the myoplasm ([Ca2+]i) were calculated using the values of the 'off' (k- = 33.5 s-1) and 'on' (k+ = 13.1 microM-1 s-1) rate constants for the binding of calcium to the dye (dissociation constant, Kd = k-/k+). The mean (+/- S.E.M., n = 7) peak [Ca2+]i value during twitches or tetani was 3.9 +/- 0.3 or 4.1 +/- 0.3 microM, respectively, while during maximal K+ contractures, it was 10.3 +/- 0.8 microM. The threshold [Ca2+]i for tension development was about 1 microM. 3. For responses elicited with high [K+]o (80-190 mM), the calcium transients decayed faster than tension. At lower [K+]o (30-70 mM), the decay was slower, and relaxation was complete when [Ca2+]i was still above contractile threshold values. 4. Following a K+ contracture, recovery of the calcium transients associated with twitches occurred before recovery of tension, indicating an apparent dissociation between [Ca2+]i and tension output. This apparent dissociation between calcium and tension output could be attributed to the desensitization of the contractile proteins to calcium, or, more probably, to the non-uniform behaviour of calcium release and/or uptake sites, leading to an unhomogeneous distribution of active sarcomeres along the fibre length and localized sarcomere relaxation.