Does Altered Retrograde Coupling Between and RyR1 and the DHPR Contribute to Malignant Hyperthermia?

Abstract

In skeletal muscle, intermolecular communication between the dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) is bi-directional; orthograde coupling (skeletal excitation-contraction coupling) is observed as depolarization-induced Ca2+ release via RyR1 and retrograde coupling is manifested by increased L-type Ca2+ current via the DHPR. The initial goal of this study was to determine whether the conformational state of RyR1 regulates DHPR gating. In this regard, we found that exposure of normal myotubes to ryanodine (200 μM, 1 hr, 37°C) caused an increase in L-type current at less depolarized test potentials as a result of a ∼5 mV hyperpolarizing shift in the voltage-dependence of activation. Likewise, charge movements of ryanodine-treated myotubes were shifted ∼13 mV to more hyperpolarizing potentials. The observation that pharmacologically-induced conformational changes in RyR1 affected DHPR gating raised the possibility that mutations in RyR1 that are linked to malignant hyperthermia (MH) may also affect DHPR gating. To this effect, the I-V relationship for L-type currents in cells originating from mice carrying an MH-linked mutation in RyR1 (R163C) were shifted to more hyperpolarizing potentials (∼7 mV for both HET and HOM) in comparison to WT cells. Compared to WT cells, HET and HOM cells both displayed a greater sensitivity to the L-type channel agonist ±Bay K 8644 (10 μM). Interestingly, L-type currents in normal myotubes were inhibited by the anti-MH drug dantrolene. Our present results and similar observations made with mice carrying another MH-linked mutation (Y522S; Chelu et al., 2006; Durham et al., 2008) suggest that altered retrograde coupling interactions may contribute to the aberrant Ca2+ handling associated with MH episodes. Supported by NIH NS24444 and AR44750 to K.G.B., NIH AR052354 to P.D.A. and MDA 4155 to R.A.B.