Dihydropyridine receptors actively control gating of ryanodine receptors in resting mouse skeletal muscle fibres.
Robin, Gaelle 1; Allard, Bruno 1
[Miscellaneous Article]
Journal of Physiology.
590(23):6027-6036, December 1, 2012.
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Key points:
* Depolarization of the skeletal muscle membrane elicits a change in the configuration of dihydropyridine receptors that in turn triggers sarcoplasmic reticulum (SR) Ca2 release through ryanodine receptors.
* At rest, it is assumed, but never demonstrated in adult muscle fibres, that dihydropyridine receptors exert a repressive action on ryanodine receptors that keeps them in a closed state.
* By measuring Ca2 changes in the SR in voltage-clamp conditions, we report that any interventions designed to alter the conformation of dihydropyridine receptors at rest induce an SR Ca2 efflux.
* These results show that dihydropyridine receptors maintain a strict control upon ryanodine receptors in resting skeletal mouse muscle fibres.
Contraction of skeletal muscle is triggered by the release of Ca2 from the sarcoplasmic reticulum (SR) in response to depolarization of the muscle membrane. Depolarization is known to elicit a conformational change of the dihydropyridine receptor (DHPR) in the tubular membrane that controls in a time- and voltage-dependent manner the opening of the ryanodine receptor (RyR), the SR Ca2 release channel. At rest, it is assumed that RyRs are kept in a closed state imposed by the repressive action of DHPRs; however, a direct control of the RyR gating by the DHPR has up to now never been demonstrated in resting adult muscle. In this study, we monitored slow changes in SR Ca2 content using the Ca2 indicator fluo-5N loaded in the SR of voltage-clamped mouse muscle fibres. We first show that external Ca2 removal induced a reversible SR Ca2 efflux at -80 mV and prevented SR Ca2 refilling following depolarization-evoked SR Ca2 depletion. The dihydropyridine compound nifedipine induced similar effects. The rate of SR Ca2 efflux was also shown to be controlled in a time- and voltage-dependent manner within a membrane potential range more negative than -50 mV. Finally, intracellular addition of ryanodine produced an irreversible SR Ca2 efflux and kept the SR in a highly depleted state following depolarization-evoked SR Ca2 depletion. The fact that resting SR Ca2 efflux is modulated by conformational changes of DHPRs induced by external Ca2 , nifedipine and voltage demonstrates that DHPRs exert an active control on gating of RyRs in resting skeletal muscle.
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