The elusive role of store depletion in the control of intracellular calcium release.

The contractile cycle of striated muscles, skeletal and cardiac, is controlled by a cytosolic [Ca2+] transient that requires rapid movements of the ion through channels in the sarcoplasmic reticulum (SR). A functional signature of these channels is their closure after a stereotyped time lapse of Ca2+ release. In cardiac muscle there is abundant evidence that termination of release is mediated by depletion of the Ca2+ store, even if the linkage mechanism remains unknown. By contrast, in skeletal muscle the mechanisms of release termination are not understood. This article reviews measurements of store depletion, the experimental evidence for dependence of Ca2+ release on the [Ca2+] level inside the SR, as well as tests of the molecular nature of putative intra-store Ca2+ sensors. Because Ca2+ sparks exhibit the basic release termination mechanism, much attention is dedicated to the studies of store depletion caused by sparks and its relationship with termination of sparks. The review notes the striking differences in volume, content and buffering power of the stores in cardiac vs. skeletal muscle, differences that explain why functional depletion is much greater for cardiac than skeletal muscle stores. Because in skeletal muscle store depletion is minimal and reduction in store [Ca2+] does not appear to greatly inhibit Ca2+ release, it is concluded that decrease in free SR [Ca2+] does not mediate physiological termination of Ca2+ release in this type of muscle. In spite of the apparent absence of store depletion and its putative channel closing effect, termination of Ca2+ sparks is faster and more robust in skeletal than cardiac muscle. A gating role of a hypothetical "proximate store" constituted by polymers of calsequestrin and associated proteins is invoked in an attempt to preserve a role for store depletion and unify mechanisms in both types of striated muscle.