Spark‐ and ember‐like elementary Ca²⁺ release events in skinned fibres of adult mammalian skeletal muscle

Abstract

Using laser scanning confocal microscopy, we show for the first time elementary Ca2+ release events (ECRE) from the sarcoplasmic reticulum in chemically and mechanically skinned fibres from adult mammalian muscle and compare them with ECRE from amphibian skinned fibres. Hundreds of spontaneously occurring events could be measured from individual single skinned mammalian fibres. In addition to spark‐like events, we found ember‐like events, i.e. long‐lasting events of steady amplitude. These two different fundamental release types in mammalian muscle could occur in combination at the same location. The two peaks of the frequency of occurrence for ECRE of mammalian skeletal muscle coincided with the expected locations of the transverse tubular system within the sarcomere, suggesting that ECRE mainly originate at triadic junctions. ECRE in adult mammalian muscle could also be identified at the onset of the global Ca2+ release evoked by membrane depolarisation in mechanically skinned fibres. In addition, the frequency of ECRE was significantly increased by application of 0.5 mm caffeine and reduced by application of 2 mm tetracaine. We conclude that the excitation‐contraction coupling process in adult mammalian muscle involves the activation of both spark‐ and ember‐like elementary Ca2+ release events.

Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes

“…In resting skeletal fibers, inhibitory physical DHPR-RyR1 interactions prevent the development of spontaneous Ca 2ϩ sparks (46,47,59). Adult skeletal fibers show abundant sparks only after permeabilization, which collapses the membrane potential and weakens these physical DHPRRyR1 interactions (24,25,46,47,58,59). Supporting the functional evidence, electron microscopy (EM) studies (3,5,18,19,52) found a pattern of linkage between skeletal RyR channels (RyR1) at the terminal cisternae and DHPRs in T tubules of fast-twitch muscle.…”

Altered Ca2+ sparks in aging skeletal and cardiac muscle

“…Such activity in these muscle preparations were attributed to the action of the type 3 RyR (Ward et al, 2000), an RyR isoform highly expressed in skeletal muscle during fetal development but only found in small quantities in adult muscle (Sutko et al, 1991). Until recently, significant numbers of Ca 2+ sparks were only observed in adult mammalian skeletal fibers following permeabilization of their sarcolemmal membrane with various physical or chemical methods (Kirsch et al, 2001;Zhou et al, 2003). Because of these intrinsic difficulties with monitoring CICR activity in intact mammalian skeletal muscle fibers, the cellular and molecular mechanisms underlying the regulation of CICR in skeletal muscle and the adaptive changes of CICR in muscle aging remain largely unknown.…”

Ca2+ sparks are initiated by Ca2+ entry in embryonic mouse skeletal muscle and decrease in frequency postnatally

“…5B). Some time courses displayed a repetitive firing pattern, whereas others showed a sustained fluorescence, reminiscent of long-duration Ca 2ϩ sparks seen previously (19,32), often preceded by a graded fluorescence increase and/or followed by a graded decrease in fluorescence.…”