School of Biomedical Sciences, The University of Queensland, Brisbane, QLD, 4072, Australia.
School of Biomedical Sciences, The University of Queensland, Brisbane, QLD, 4072, Australia
J Physiol. 2014 Sep 1;592(17):3727-46. doi: 10.1113/jphysiol.2014.274274. Epub 2014 Jun 27.
Skeletal muscle fibres are large and highly elongated cells specialized for producing the force required for posture and movement. The process of controlling the production of force within the muscle, known as excitation-contraction coupling, requires virtually simultaneous release of large amounts of Ca(2+) from the sarcoplasmic reticulum (SR) at the level of every sarcomere within the muscle fibre. Here we imaged Ca(2+) movements within the SR, tubular (t-) system and in the cytoplasm to observe that the SR of skeletal muscle is a connected network capable of allowing diffusion of Ca(2+) within its lumen to promote the propagation of Ca(2+) release throughout the fibre under conditions where inhibition of SR ryanodine receptors (RyRs) was reduced. Reduction of cytoplasmic [Mg(2+)] ([Mg(2+)]cyto) induced a leak of Ca(2+) through RyRs, causing a reduction in SR Ca(2+) buffering power argued to be due to a breakdown of SR calsequestrin polymers, leading to a local elevation of [Ca(2+)]SR. The local rise in [Ca(2+)]SR, an intra-SR Ca(2+) transient, induced a local diffusely rising [Ca(2+)]cyto. A prolonged Ca(2+) wave lasting tens of seconds or more was generated from these events. Ca(2+) waves were dependent on the diffusion of Ca(2+) within the lumen of the SR and ended as [Ca(2+)]SR dropped to low levels to inactivate RyRs. Inactivation of RyRs allowed re-accumulation of [Ca(2+)]SR and the activation of secondary Ca(2+) waves in the persistent presence of low [Mg(2+)]cyto if the threshold [Ca(2+)]SR for RyR opening could be reached. Secondary Ca(2+) waves occurred without an abrupt reduction in SR Ca(2+) buffering power. Ca(2+) release and wave propagation occurred in the absence of Ca(2+)-induced Ca(2+) release. These observations are consistent with the activation of Ca(2+) release through RyRs of lowered cytoplasmic inhibition by [Ca(2+)]SR or store overload-induced Ca(2+) release. Restitution of SR Ca(2+) buffering power to its initially high value required imposing normal resting ionic conditions in the cytoplasm, which re-imposed the normal resting inhibition on the RyRs, allowing [Ca(2+)]SR to return to endogenous levels without activation of store overload-induced Ca(2+) release. These results are discussed in the context of how pathophysiological Ca(2+) release such as that occurring in malignant hyperthermia can be generated.
骨骼肌纤维是大型的、高度伸长的细胞，专门用于产生维持姿势和运动所需的力量。控制肌肉内力量产生的过程，即兴奋-收缩偶联，需要肌肉纤维中每个肌节水平的肌浆网（SR）几乎同时释放大量的 Ca(2+)。在这里，我们对 SR、管状（t-）系统和细胞质内的 Ca(2+) 运动进行成像，观察到骨骼肌的 SR 是一个连通的网络，能够允许腔内的 Ca(2+) 扩散，以促进纤维内 Ca(2+) 释放的传播，在抑制 SR ryanodine 受体（RyRs）的情况下。细胞质中Mg(2+)的减少会导致 Ca(2+) 通过 RyRs 泄漏，从而降低 SR Ca(2+) 缓冲能力，这被认为是由于 SR calsequestrin 聚合物的破裂，导致 SR 内 [Ca(2+)] 的局部升高。SR 内局部 Ca(2+) 的升高，即 SR 内 Ca(2+) 瞬变，会引起细胞质内 Ca(2+) 局部扩散上升。这些事件产生了持续数十秒或更长时间的长 Ca(2+) 波。Ca(2+) 波依赖于 SR 腔中的 Ca(2+) 扩散，并且当 SR 内 [Ca(2+)] 下降到低水平以失活 RyRs 时，Ca(2+) 波结束。如果可以达到 RyR 打开的阈值 [Ca(2+)]SR，则在低 [Mg(2+)]cyto 的持续存在下，RyRs 的失活允许 [Ca(2+)]SR 的再积累和继发性 Ca(2+) 波的激活。在没有 Ca(2+)-诱导的 Ca(2+) 释放的情况下，发生继发性 Ca(2+) 波。这些观察结果与通过 [Ca(2+)]SR 或储存过载诱导的 Ca(2+) 释放降低细胞质抑制来激活 RyR 导致的 Ca(2+) 释放和波传播一致。在没有 Ca(2+)-诱导的 Ca(2+) 释放的情况下，发生 Ca(2+) 释放和波传播。这些观察结果与通过 [Ca(2+)]SR 或储存过载诱导的 Ca(2+) 释放降低细胞质抑制来激活 RyR 导致的 Ca(2+) 释放和波传播一致。在没有 Ca(2+)-诱导的 Ca(2+) 释放的情况下，发生 Ca(2+) 释放和波传播。这些观察结果与通过 [Ca(2+)]SR 或储存过载诱导的 Ca(2+) 释放降低细胞质抑制来激活 RyR 导致的 Ca(2+) 释放和波传播一致。这些结果在恶性高热等病理生理 Ca(2+) 释放发生的情况下进行了讨论。
