Residual sarcoplasmic reticulum Ca2+ concentration after Ca2+ release in skeletal myofibers from young adult and old mice

Abstract

Contrasting information suggests either almost complete depletion of sarcoplasmic reticulum (SR) Ca2+ or significant residual Ca2+ concentration after prolonged depolarization of the skeletal muscle fiber. The primary obstacle to resolving this controversy is the lack of genetically encoded Ca2+ indicators (GECI) targeted to the SR that exhibit low-Ca2+ affinity, a fast biosensor:Ca2+ off-rate reaction, and can be expressed in myofibers from adult and older adult mammalian species. This work used the recently designed low-affinity Ca2+ sensor (Kd=1.66 mM in the myofiber) CatchER (calcium sensor for detecting high concentrations in the ER) targeted to the SR, to investigate whether prolonged skeletal muscle fiber depolarization significantly alters residual SR Ca2+ with aging. We found CatchER a proper tool to investigate SR Ca2+ depletion in young-adult and older adult mice, consistently tracking SR luminal Ca2+ release in response to brief and repetitive stimulation. We evoked SR Ca2+ release in whole-cell voltage-clamped flexor digitorum brevis (FDB) muscle fibers from young and old FVB mice and tested the maximal SR Ca2+ release by directly activating the ryanodine receptor (RyR1) with 4-Chloro-m-cresol (4-CmC) in the same myofibers. Here, we report for the first time, that the Ca2+ remaining in the SR after prolonged depolarization (2s) in myofibers from aging (~220 µM) was larger than young (~132µM) mice. These experiments indicate that SR Ca2+ is far from fully depleted under physiological conditions throughout life, and support the concept of excitation-contraction uncoupling in functional senescent myofibers.

Contractile properties and sarcoplasmic reticulum calcium content in type I and type II skeletal muscle fibres in active aged humans

“…Interestingly, our finding appears quite analogous to that of a recent study in mouse muscle fibres where it was found that at the end of prolonged voltage steps significantly more Ca 2+ remained within the SR in the fibres of old mice compared to young mice (Wang et al . ). Such a non‐releasing pool of Ca 2+ could not contribute in any direct way to force production by the muscle fibre, irrespective of whether it was located specifically in pockets of isolated SR or instead was distributed uniformly throughout the whole SR.…”

Nonmyofilament‐associated troponin T3 nuclear and nucleolar localization sequence and leucine zipper domain mediate muscle cell apoptosis

“…Muscle fiber depolarization leads to sarcoplasmic reticulum calcium release (Tang et al 2011; Wang et al 2012), which causes conformational changes in the troponin (Tn) complex to expose binding sites for myosin on the actin filaments, and initiating the interaction between myosin and actin, causing muscle contraction (Geeves and Holmes 1999; Jimenez-Moreno et al 2008; Jimenez-Moreno et al 2010). The Tn complex is composed of three subunits: the calcium-binding TnC; the inhibitory TnI; and the Tm-binding TnT.…”

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2+

“…To fill in this gap, our lab developed a fast-kinetic Ca 2+ reporter CatchER by designing a single Ca 2+ -binding site into enhanced green fluorescent protein [51]. CatchER was able to record the SR luminal Ca 2+ in flexor digitorum brevis (FDB) muscle fibers during voltage stimulation due to its unprecedented fast off rate [52]. …”