Maurocalcin phosphorylated at threonin 26 maintains its activity on ryanodine receptor-mediated Ca2+ release in intact muscle fibers

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Role du courant calcique dans le couplage excitation-contraction de la fibre musculaire squelettique rapide de grenouille

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Nitric oxide synthase inhibition affects sarcoplasmic reticulum Ca(2+) release in skeletal muscle fibres from mouse

Nitric oxide (NO) generated by skeletal muscle is believed to regulate force production but how this is achieved remains poorly understood. In the present work we tested the effects of NO synthase (NOs) inhibitors on membrane current and intracellular calcium in isolated skeletal muscle fibres from mouse, under voltage-clamp conditions. Resting [Ca(2+)] and [Ca(2+)] transients evoked by large depolarizations exhibited similar properties in control fibres and in fibres loaded with tenth millimolar levels of the NOs inhibitor N-nitro-l-arginine (l-NNA). Yet the voltage dependence of calcium release was found to be shifted by ∼15 mV towards negative values in the presence of l-NNA. This effect could be reproduced by the other NOs inhibitor S-methyl-l-thiocitrulline (l-SMT). Separate experiments showed that the voltage dependence of charge movement and of the slow calcium current were unaffected by the presence of l-NNA, ruling out an effect on the voltage sensor. A negative shift in the voltage dependence of calcium release with no concurrent alteration in the properties of charge movement was also observed in fibres exposed to the oxidant H(2)O(2) (1 mm). Conversely the reducing agent dithiothreitol (10 mm) had no obvious effect on Ca(2+) release. Overall, the results indicate that physiological levels of NO exert a tonic inhibitory control on the activation of the calcium release channels. Changes in the voltage dependence of Ca(2+) release activation may be a ubiquitous physiological consequence of redox-related modifications of the ryanodine receptor

Régulation du couplage excitation-contraction par le cholestéro et l'oxyde nitrique dans la fibre musculaire squelettique de souris

Le couplage excitation-contraction (EC) du muscle squelettique s articule sur les interactions entre le détecteur de potentiel membranaire (récepteur des dihydropyridines, DHPR), et le canal calcique du réticulum (récepteur de la ryanodine, RyR). Le DHPR est localisé dans les tubules transverses et les cavéoles, deux structures sarcolemmales enrichies en cholestérol. De plus, les cavéoles contiennent la synthase de l oxyde nitrique (NO). Le travail présenté apporte des éléments nouveaux concernant la modulation fonctionnelle du couplage EC par le cholestérol et le NO, à l aide d une approche d électrophysiologie cellulaire combinée à des mesures de fluorescence. La teneur membranaire en cholestérol régule les fonctions de canal calcique et de détecteur de potentiel du DHPR. Le NO cible spécifiquement le RyR. À des niveaux physiologiques, il module l activation du canal lors d une dépolarisation ; en excès, il maintient certains RyR en configuration activéeExcitation-contraction (EC) coupling of skeletal muscle depends upon interactions between the membrane voltage-sensor (dihydropyridine receptor, DHPR), and the sarcoplasmic reticulum calcium release channel (ryanodine receptor, RyR). The DHPR stands within two membrane structure enriched in cholesterol, the transverse tubule system and the caveolae. Caveolae also contain the nitric oxide (NO) synthase. The present work provides new insights into the functional modulation of EC coupling by cholesterol and NO, using electrophysiological techniques and fluorescence measurements on isolated muscle cells. The membrane cholesterol content regulates the voltage-sensing and calcium channel function of the DHPR. NO specifically affects the RyR function. At physiological levels, NO modulates the channel activation by membrane depolarization; excess NO maintains some RyR within an activated stateLYON1-BU.Sciences (692662101) / SudocSudocFranceF

Sustained release of calcium elicited by membrane depolarization in ryanodine-injected mouse skeletal muscle fibers.

The effect of micromolar intracellular levels of ryanodine was tested on the myoplasmic free calcium concentration ([Ca(2+)](i)) measured from a portion of isolated mouse skeletal muscle fibers voltage-clamped at -80 mV. When ryanodine-injected fibers were transiently depolarized to 0 mV, the early decay phase of [Ca(2+)](i) upon membrane repolarization was followed by a steady elevated [Ca(2+)](i) level. This effect could be qualitatively well simulated, assuming that ryanodine binds to release channels that open during depolarization and that ryanodine-bound channels do not close upon repolarization. The amplitude of the postpulse [Ca(2+)](i) elevation depended on the duration of the depolarization, being hardly detectable for pulses shorter than 100 ms, and very prominent for duration pulses of seconds. Within a series of consecutive pulses of the same duration, the effect of ryanodine produced a staircase increase in resting [Ca(2+)](i), the slope of which was approximately twice larger for depolarizations to 0 or +10 mV than to -30 or -20 mV. Overall results are consistent with the "open-locked" state because of ryanodine binding to calcium release channels that open during depolarization. Within the voltage-sensitive range of calcium release, increasing either the amplitude or the duration of the depolarization seems to enhance the fraction of release channels accessible to ryanodine