Dihydropyridine receptors actively control gating of ryanodine receptors in resting mouse skeletal muscle fibres

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. Abstract 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.