Store-operated Ca²⁺ release in skeletal muscle: tailored for a specialized system

Store-operated Ca²⁺ entry (SOCE) is observed in numerous cell types and involved not only in Ca²⁺ store refilling but also in diverse functions such as exocytosis, motility and gene expression (Lewis, 2007). There seems to be many variations on the basic theme and it would make sense that the SOCE mechanism is appropriately tailored to the needs and constraints of each particular cell type. In this issue of The Journal of Physiology, Launikonis & Ríos (2007) describe the characteristics of SOCE in adult mammalian skeletal muscle fibres, which is a particularly interesting case of SOCE specialization.

Skeletal muscle fibres are extremely specialized cells, with a highly ordered structure and unique molecular mechanisms that enable powerful contractions to be synchronously started and stopped on a time scale of tens of milliseconds. The contractile proteins are arranged in an almost crystalline array, repeated in parallel and in series, with each sarcomeric unit surrounded by a high capacity Ca²⁺ store, the sarcoplasmic reticulum (SR). Action potentials (APs) rapidly spread along the surface (sarcolemmal) membrane of a muscle fibre and down into the transverse tubular (t-) system, which abuts the end chambers of the SR for > 90% of its length. Vertebrate skeletal muscle has evolved a unique activation mechanism whereby the APs in the t-system are sensed by a variant of the L-type Ca²⁺ channel (dihydropyridine receptors, DHPRs) which directly activates the Ca²⁺ release channels (ryanodine receptors, RyRs) in the adjacent SR without the need for any Ca²⁺ influx. Indeed, it appears that the Ca²⁺ influx function of the DHPRs is greatly down-regulated. A single AP causes the release of a large amount of Ca²⁺ (> 200 micromoles per litre total cell volume) over the course of only a few milliseconds, as is required for initiating rapid force development. Equally important, the SR also avidly takes up large amounts of Ca²⁺ very rapidly, which is necessary for fast relaxation and the ability to perform rapid repeated movements. In fact, a skeletal muscle fibre seems normally to function as an almost fully closed system, where virtually all of the Ca²⁺ involved in activating contraction is cycled out of and back into the SR (which normally contains ∼1 millimole per litre fibre volume; Fryer & Stephenson, 1996), with very little exchange with extracellular Ca²⁺, at least in the short term.

So what is happening with SOCE in skeletal muscle fibres? In general, SOCE is triggered by depletion of intracellular Ca²⁺ stores, and involves at least two molecular elements, one in the store to sense the level of Ca²⁺ depletion and another in the surface membrane to act as the SOC channel that allows the influx of extracellular Ca²⁺. In Jurkat cells, activation of SOCE involves a redistribution of the intrastore Ca²⁺ sensors and physical rearrangement of the junctions between the store and surface membranes. Launikonis & Ríos (2007) used mechanically skinned muscle fibres, where the sarcolemmal membrane was pealed away and the t-system sealed off to become a closed compartment. Using confocal microscopy to image fluorescent Ca²⁺ indicators present in the sealed t-system and in the cytoplasmic space, they showed that SOCE from the t-system lumen into cytoplasmic space started less than a second after the initiation of store depletion. This is far too rapid to involve substantial structural rearrangement of the junctions and sensor elements, indicating that the elements are already prepositioned for facilitating SOCE. Moreover, the constraints involved in having rapid, controlled release of SR Ca²⁺ for muscle contraction also are likely to preclude there being substantial alterations in the structure of the t-system–SR junctions. This all raises interesting questions about what molecules act as the SR Ca²⁺ sensors (is it the RyRs themselves?), and what molecules act as the SOC channels and how do they fit into the highly ordered coupling between the DHPRs and RyRs that occurs along virtually the entire length of the t-system?

Launikonis & Ríos further show that SOCE is controlled locally, being activated deeper in the t-system when the adjacent SR is depleted of Ca²⁺. This further fits with the SOCE elements being prepositioned in an ordered manner all down the length of the t-system. It also fits with SOCE regulation being designed to ensure that all regions of the SR throughout a muscle fibre are adequately loaded with Ca²⁺, an essential requirement for proper muscle function, as all parts of a muscle fibre must contract uniformly and synchronously. Perhaps this might explain the report that there is little if any SOCE across the sarcolemma in muscle fibres (Allard et al. 2006); instead, the t-system may be the conduit for extracellular Ca²⁺ to access the SR locally, so that it isn't all sequestered by the SR immediately under the sarcolemma.

Launikonis & Ríos also show that there is rapid efflux of Ca²⁺ from the cytoplasm back into the t-system during prolonged periods of substantial SR Ca²⁺ release. However, this Ca²⁺ efflux, just like the total of SOCE and other Ca²⁺ influx, is very small in absolute terms compared to the fluxes across the SR. The net Ca²⁺ flux into or out of the fibre of course is even smaller. SOCE would appear to be relatively inconsequential to muscle function in the short term, particularly given that the amount of Ca²⁺ in the SR of fast-twitch fibres can be increased threefold above its normal level without altering the amount of Ca²⁺ release to an AP. Where SOCE is likely to be important is in preventing any net loss of SR Ca²⁺ that might otherwise occur with very prolonged muscle activity, because depleting SR Ca²⁺ below its normal level would cause substantial reductions in evoked Ca²⁺ release and tetanic force responses (Dutka & Lamb, 2005).