Functional Impact of the Ryanodine Receptor on the Skeletal Muscle L-Type Ca2+ Channel

Abstract

L-type Ca2+ channel (L-channel) activity of the skeletal muscle dihydropyridine receptor is markedly enhanced by the skeletal muscle isoform of the ryanodine receptor (RyR1) (Nakai, J., R.T. Dirksen, H.T. Nguyen, I.N. Pessah, K.G. Beam, and P.D. Allen. 1996. Nature. 380:72–75.). However, the dependence of the biophysical and pharmacological properties of skeletal L-current on RyR1 has yet to be fully elucidated. Thus, we have evaluated the influence of RyR1 on the properties of macroscopic L-currents and intracellular charge movements in cultured skeletal myotubes derived from normal and “RyR1-knockout” (dyspedic) mice. Compared with normal myotubes, dyspedic myotubes exhibited a 40% reduction in the amount of maximal immobilization-resistant charge movement (Qmax, 7.5 ± 0.8 and 4.5 ± 0.4 nC/μF for normal and dyspedic myotubes, respectively) and an approximately fivefold reduction in the ratio of maximal L-channel conductance to charge movement (Gmax/Qmax). Thus, RyR1 enhances both the expression level and Ca2+ conducting activity of the skeletal L-channel. For both normal and dyspedic myotubes, the sum of two exponentials was required to fit L-current activation and resulted in extraction of the amplitudes (Afast and Aslow) and time constants (τslow and τfast) for each component of the macroscopic current. In spite of a >10-fold in difference current density, L-currents in normal and dyspedic myotubes exhibited similar relative contributions of fast and slow components (at +40 mV; Afast/[Afast + Aslow] ∼ 0.25). However, both τfast and τslow were significantly (P < 0.02) faster for myotubes lacking the RyR1 protein (τfast, 8.5 ± 1.2 and 4.4 ± 0.5 ms; τslow, 79.5 ± 10.5 and 34.6 ± 3.7 ms at +40 mV for normal and dyspedic myotubes, respectively). In both normal and dyspedic myotubes, (−) Bay K 8644 (5 μM) caused a hyperpolarizing shift (∼10 mV) in the voltage dependence of channel activation and an 80% increase in peak L-current. However, the increase in peak L-current correlated with moderate increases in both Aslow and Afast in normal myotubes, but a large increase in only Afast in dyspedic myotubes. Equimolar substitution of Ba2+ for extracellular Ca2+ increased both Afast and Aslow in normal myotubes. The identical substitution in dyspedic myotubes failed to significantly alter the magnitude of either Afast or Aslow. These results demonstrate that RyR1 influences essential properties of skeletal L-channels (expression level, activation kinetics, modulation by dihydropyridine agonist, and divalent conductance) and supports the notion that RyR1 acts as an important allosteric modulator of the skeletal L-channel, analogous to that of a Ca2+ channel accessory subunit.