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A repetitive mode of activation of discrete Ca 2+ release events (Ca 2+ sparks) in frog skeletal muscle fibres
Author(s) -
Klein Michael G.,
Lacampagne Alain,
Schneider Martin F.
Publication year - 1999
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 - 10.1111/j.1469-7793.1999.391ac.x
Subject(s) - depolarization , biophysics , gating , chemistry , electrophysiology , triad (sociology) , endoplasmic reticulum , anatomy , neuroscience , biology , biochemistry , psychology , psychoanalysis
1 Ca 2+ release events (Ca 2+ ‘sparks’), which are believed to arise from the opening of a sarcoplasmic reticulum (SR) Ca 2+ release channel or a small cluster of such channels that act as a release unit, have been measured in single, frog ( Rana pipiens ) skeletal muscle fibres. 2 Under conditions of extremely low rates of occurrence of Ca 2+ sparks we observed, within individual identified triads, repetitive Ca 2+ release events which occurred at a frequency more than 100‐fold greater than the prevailing average event rate. Repetitive sparks were recorded during voltage‐clamp test depolarizations after a brief (0.3–2 s) repriming interval in fibres held at 0 mV and in chronically depolarized, ‘notched’ fibres. 3 These repetitive events are likely to arise from the re‐opening of the same SR Ca 2+ release channel or release unit operating in a repetitive gating mode (‘ rep ‐mode’), rather than from the random activation of multiple, independent channels or release units within a triad. A train of rep ‐mode events thus represents a series of Ca 2+ sparks arising from a single location within the fibre. Rep ‐mode events are activated among different triads in a random manner after brief repriming. The frequency of repetitive events among all identified events during voltage‐clamp depolarization to 0 mV after brief repriming was 3.9 ± 1.3%. The occurrence of repetitive events was not related to exposure of the fibre to laser illumination. 4 The events observed within a rep‐ mode train exhibited a relatively uniform amplitude. Analysis of intervals between identified events in triads exhibiting rep‐ mode trains indicated similar variations of fluorescence as in neighbouring, quiescent triads, suggesting there was not a significant number of small, unidentified events at the triads exhibiting rep‐ mode activity. 5 The distribution of rep‐ mode interspark intervals exhibited a paucity of events at short intervals, consistent with the need for recovery from inactivation before activation of the next event in a repetitive train. The mean interspark interval of repetitive sparks during voltage‐clamp depolarizations was 88 ± 5 ms, and was independent of membrane potential. 6 The individual Ca 2+ sparks within a rep ‐mode train were similar in average amplitude and spatiotemporal extent to singly occurring sparks, suggesting a common mechanism for termination of the channel opening(s) underlying both types of events. The average properties of the sparks did not vary during a train. The relative amplitude of a spark within a rep ‐mode was not correlated with its rise time. 7 Repetitive Ca 2+ release events represent a mode of gating of SR Ca 2+ release channels which may be significant during long depolarizations and which may be influenced by the biochemical state of the SR ryanodine receptor Ca 2+ release channels.