FLUO-3 SIGNALS ASSOCIATED WITH POTASSIUM CONTRACTURES IN SINGLE AMPHIBIAN MUSCLE-FIBERS

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 mu(-1)s(-1)) rate constants for mean the binding of calcium to the dye (dissociation constant, K-d = 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 mu M, respectively, while during maximal K+ contractures, it was 10.3 +/- 0.8 mu M. The threshold [Ca2+](i) for tension development was about 1 mu M. 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.