Ca2+ influx through {alpha}1S DHPR may play a role in regulating Ca2+ release from RyR1 in skeletal muscle

Differentiated primary myotubes isolated from wild-type mice exhibit ryanodine-sensitive, spontaneous global Ca²⁺ oscillations as well as spontaneous depolarizations in the plasma membrane. Immunolabeling of these myotubes showed expression of both _1S dihydropyridine receptors (DHPRs) and ryanodine-sensitive Ca²⁺-release channel 1 (RyR1), the two key proteins in skeletal excitation-contraction (E-C) coupling. Spontaneous global Ca²⁺ oscillations could be inhibited by addition of 0.1 mM CdCl₂/0.5 mM LaCl₃ or 5 µM nifedipine to the extracellular bathing solution. After either treatment, Ca²⁺ oscillations could be restored upon extensive washing. Although exposure to DHPR antagonists completely blocked Ca²⁺ oscillations, normal orthograde signaling between DHPRs and RyRs, such as that elicited by 80 mM KCl depolarization, was still observed. In addition, we showed that spontaneous Ca²⁺ oscillations were never present in cultured mdg myotubes, which lack the expression of _1SDHPRs. These results suggest that under physiological conditions in conjunction with the mechanical coupling between the _1SDHPRs and RyR1, the initiation of Ca²⁺ oscillations in myotubes may be facilitated, in part, by the Ca²⁺ influx through the _1s-subunit of the DHPR. calcium-induced calcium release; dihydropyridine receptors; excitation-contraction coupling; ryanodine receptors; skeletal muscle