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Fluo‐3 signals associated with potassium contractures in single amphibian muscle fibres.
Author(s) -
Caputo C,
Bolaños P
Publication year - 1994
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1994.sp020423
Subject(s) - sarcomere , calcium , chemistry , biophysics , dissociation constant , caffeine , muscle contraction , muscle contracture , potassium , anatomy , biochemistry , myocyte , endocrinology , biology , receptor , organic chemistry
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.