Abstract:High-frequency paired stimuli used to initiate a tetanus result in increased force and rate of force development in skeletal muscle. Bakker et al. investigate this mechanism and find that doublet stimulation increases the amount of Ca2+ bound to troponin C, resulting in rapid force development.
“…1a, Methods section), in which we trapped the Ca 2+ -sensitive fluorescent dye rhod-5N in the t-system of skinned fast twitch extensor digitorum longus (EDL) fibres from the rat, as previously described 19 . This preparation preserves intact EC coupling 13,20 and respective Ca 2+ release and force responses are indistinguishable from those in intact fibres 19,21–23 . The removal of the sarcolemma during the skinning process grants access to the cytosol and allows for the measurement of the intracellular Ca 2+ levels with the use of a spectrally separate Ca 2+ -sensitive dye from rhod-5N, fluo-4.Fig.…”
Store-operated calcium (Ca2+) entry (SOCE) in skeletal muscle is rapidly activated across the tubular system during direct activation of Ca2+ release. The tubular system is the invagination of the plasma membrane that forms junctions with the sarcoplasmic reticulum (SR) where STIM1, Orai1 and ryanodine receptors are found. The physiological activation of SOCE in muscle is not defined, thus clouding its physiological role. Here we show that the magnitude of a phasic tubular system Ca2+ influx is dependent on SR Ca2+ depletion magnitude, and define this as SOCE. Consistent with SOCE, the influx was resistant to nifedipine and BayK8644, and silenced by inhibition of SR Ca2+ release during excitation. The SOCE transient was shaped by action potential frequency and SR Ca2+ pump activity. Our results show that SOCE in skeletal muscle acts as an immediate counter-flux to Ca2+ loss across the tubular system during excitation-contraction coupling.
“…1a, Methods section), in which we trapped the Ca 2+ -sensitive fluorescent dye rhod-5N in the t-system of skinned fast twitch extensor digitorum longus (EDL) fibres from the rat, as previously described 19 . This preparation preserves intact EC coupling 13,20 and respective Ca 2+ release and force responses are indistinguishable from those in intact fibres 19,21–23 . The removal of the sarcolemma during the skinning process grants access to the cytosol and allows for the measurement of the intracellular Ca 2+ levels with the use of a spectrally separate Ca 2+ -sensitive dye from rhod-5N, fluo-4.Fig.…”
Store-operated calcium (Ca2+) entry (SOCE) in skeletal muscle is rapidly activated across the tubular system during direct activation of Ca2+ release. The tubular system is the invagination of the plasma membrane that forms junctions with the sarcoplasmic reticulum (SR) where STIM1, Orai1 and ryanodine receptors are found. The physiological activation of SOCE in muscle is not defined, thus clouding its physiological role. Here we show that the magnitude of a phasic tubular system Ca2+ influx is dependent on SR Ca2+ depletion magnitude, and define this as SOCE. Consistent with SOCE, the influx was resistant to nifedipine and BayK8644, and silenced by inhibition of SR Ca2+ release during excitation. The SOCE transient was shaped by action potential frequency and SR Ca2+ pump activity. Our results show that SOCE in skeletal muscle acts as an immediate counter-flux to Ca2+ loss across the tubular system during excitation-contraction coupling.
“…The activation of the t-system voltage-sensor via action potential stimulation causes the temporary removal of Mg 2+ inhibition exerted on the RyR to allow rapid release of Ca 2+ through the transition of the RyR from the closed to the open state (14,15). Note that the rate of Ca 2+ release evoked by action potential in skinned fibers is not distinguishable from that occurring in intact fibers (33,34). (Fig.…”
Malignant hyperthermia (MH) is a clinical syndrome of skeletal muscle that presents as a hypermetabolic response to volatile anesthetic gases, where susceptible persons may develop lethally high body temperatures. Genetic predisposition mainly arises from mutations on the skeletal muscle ryanodine receptor (RyR). Dantrolene is administered to alleviate MH symptoms, but its mechanism of action and its influence on the Ca 2+ transients elicited by MH triggers are unknown. Here, we show that Ca 2+ release in the absence of Mg 2+ is unaffected by the presence of dantrolene but that dantrolene becomes increasingly effective as cytoplasmic-free [Mg 2+ ] (free [Mg 2+ ] cyto ) passes mM levels. Furthermore, we found in human muscle susceptible to MH that dantrolene was ineffective at reducing halothane-induced repetitive Ca 2+ waves in the presence of resting levels of free [Mg 2+ ] cyto (1 mM). However, an increase of free [Mg 2+ ] cyto to 1.5 mM could increase the period between Ca 2+ waves. These results reconcile previous contradictory reports in muscle fibers and isolated RyRs, where Mg 2+ is present or absent, respectively, and define the mechanism of action of dantrolene is to increase the Mg 2+ affinity of the RyR (or "stabilize" the resting state of the channel) and suggest that the accumulation of the metabolite Mg 2+ from MgATP hydrolysis is required to make dantrolene administration effective in arresting an MH episode. malignant hyperthermia | dantrolene | ryanodine receptor | magnesium | skeletal muscle fiber R yanodine receptors (RyRs) are essential regulators of cytoplasmic Ca 2+ in muscle, heart, and brain (1-3). Congenital or acquired mishandling of Ca 2+ by RyRs is associated with organ dysfunction, myopathy, and the increased risk of sudden death (1, 4-7). Consequently, the search for drugs to modulate RyR function is an area of intense research (8-10). An example of a successful drug controlling the Ca 2+ mishandling of the RyR is the muscle relaxant dantrolene (11). It is primarily used to treat malignant hyperthermia (MH), a life-threatening condition in which genetically predisposed individuals adversely react to the exposure of volatile anesthetics (5, 12). Since its approval, the drug has cut the mortality rate associated with MH from more than 80% to below 2% (11). Despite this success, the exact mechanism of how dantrolene antagonizes MH episodes and depresses overactive Ca 2+ release during MH episodes remains unknown.Susceptibility to MH most commonly arises from mutations in the RyR1 gene, the Ca 2+ release channel of skeletal muscle. Mg 2+ is present in the muscle at ∼1 mM and exerts an inhibitory action over the RyR1, which needs to be overcome for normal voltagecontrolled Ca 2+ release (13-16). RyR1 variants have a lowered affinity for Mg 2+ (13,17), making them more prone to opening and resulting in increased sensitivity to RyR agonist. The ensuing abnormal Ca 2+ release in RyR variants (18-24) during an MH event leads to excessive heat production as the muscle attempts to clear the...
“…They are also regulated properties of cardiac and most smooth muscles, in which Ca 2+ delivery is modulated by neural activity and circulating levels of cardioactive and vasoactive hormones. In this issue, Bakker et al examine the mechanism underlying the observation that a short burst of high-frequency action potentials at the onset of tetany results in a greater rate of force development.Previous studies of the initial stages of contraction have shown that tetanic contractions of fast-twitch motor units in both rodents and humans begin with a high-frequency train of two or three action potentials before settling into a lower frequency of continued stimulation (studies cited in the present paper by Bakker et al [2017]). Although this might be the response of an under-damped neural control system or a system of more complex design, long-standing work by several groups (review by Binder-Macleod and Kesar [2005] cited by Bakker et al [2017]) has shown that this initial burst of action potentials contributes to the rate of force development in fast-twitch skeletal muscles.…”
mentioning
confidence: 62%
“…In the context of the study by Bakker et al (2017), a high-frequency doublet stimulus that sustains the initial increase in myoplasmic Ca 2+ would serve to maintain high levels of Ca 2+ binding to troponin C, as confirmed by the authors' modeling of their results and also to sustain high levels of thin filament activation. As consequences of these events, cooperative recruitment of cross-bridges to the thin filament would be reduced or absent, and the rate of force development would be determined principally by the rate of cross-bridge cycling without the slowing effects of cooperative recruitment.…”
mentioning
confidence: 73%
“…A study by Cheng et al (2013), using the Ca 2+ -sensitive dye indo-1, suggested that a pair of high-frequency action potentials at the beginning of a lower-frequency tetanus evoked an initial Ca 2+ transient of significantly greater amplitude. Such an increase would explain the faster rate of rise of force associated with the initial doublet of higher-frequency stimuli and was believed by the authors to be caused by greater Ca 2+ release from the sarcoplasmic reticulum in response to the second stimulus.The present study by Bakker et al (2017) further examines the mechanism by which an initial short burst of action potentials alters the time course of intracellular Ca 2+ in ways that might explain a faster rise of force observed under these same conditions. In this instance, the authors used the Ca 2+ indicator Mag-Fluo-4, which is faster and has lower affinity than indicators used in previous studies, and laser-scanning microscopy to achieve greater temporal resolution in recordings of intracellular Ca 2+ concentration during tetanic stimulation.…”
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