Dual Roles of Extracellular Calcium in Excitation-Contraction Coupling of Mouse Skeletal Muscle

Abstract

Ca2+ ions have four established roles in EC coupling: trigger for contraction, mediators of release (CICR), controllers of RyR channels from the SR-lumen and agents of Ca2+-dependent inactivation. Additionally, in frog muscle extracellular Ca2+ protects the t-tubule voltage sensor (DHPR) from voltage-dependent inactivation (Brum et al. J Physiol 1988). This “priming” role of Ca2+ results in rapid DHPR inactivation in solutions with low [Ca2+], unless Ca2+ is replaced by suitable cations (Pizarro et al. JGP 1989). In mammalian muscle, zero extracellular [Ca2+] is imposed frequently to assess the contribution of SOCE to Ca2+ homeostasis, under the assumption that any effects will be mediated by depletion of Ca2+ in the SR. In mouse FDB muscle fibers under voltage clamp we combined measurements of SR [Ca2+] ([Ca2+]SR), Ca2+ release flux and intramembranous charge movement (Qm). Upon long-lasting depolarization both the net amount of Ca2+ released and the measured decay in [Ca2+]SR (consequence of SR depletion) stabilize after 200-500 ms, reflecting the attainment of balance between release through RyR channels and reuptake by SERCA. In “low (25 μM) Ca” external solutions net Ca2+ release becomes negative -uptake dominates- and concomitantly [Ca2+]SR starts to recover from depletion during the depolarizing pulse. The available Qm is somewhat reduced at rest, and decays to near 0 during the pulse. These observations demonstrate that the priming role of Ca2+ is also present in mammalian muscle. The conclusion calls into question the interpretation of earlier studies on mouse muscle (including our own; Royer et al. JGP 2010), which attribute the depressing effects of low extracellular [Ca2+] exclusively to SR depletion. In mice as in frogs, low extracellular [Ca2+] also promotes voltage-dependent inactivation of the t-tubule voltage sensor. Supported by NIAMS/NIH(ER) and the Physiological Society (GB).