Rapid Ca2+ release from the sarcoplasmic reticulum (SR) is essential for normal contraction of muscle. To study Ca2+ release in muscle we simultaneously imaged mag-indo-1 trapped in SR with cytosolic rhod-2 in skinned skeletal muscle fibres of frog using confocal microscopy. [Ca2+] depletion inside the SR measured during spontaneous Ca2+ sparks, termed “skraps”, did not simply follow the Ca2+ release time course observed in the cytosolic Ca2+ image, but showed a ∼20 ms delay between the peak of the spark and the nadir of the skrap. A similar result was observed when Ca2+ release was induced by an action potential. This suggests that depletion continues even after Ca2+ release channels have closed. A further intriguing observation was an intra-SR Ca2+ transient during prolonged Ca2+ release induced by lowering [Mg2+]cyto. Such an increase in [Ca2+]SR during a decrease in total SR [Ca] would violate mass conservation laws. However, these observations can be explained in the framework of the properties of calsequestrin (CSQ), a Ca2+-buffering protein attached to the lumenal side of Ca2+ release channels (Dulhunty et al., 2006): (i) CaCSQ represents a third compartment, a proximate source of Ca2+ for release and is invisible to the monitoring dye, thus explaining the apparent delay between cytosolic release and lumenal Ca2+ depletion; and (ii) CSQ depolymerizes as the total SR [Ca] falls. Thus as CSQ breaks into dimers and monomers, the capacity of CSQ for Ca2+ drops, resulting in a Ca2+ transient within the SR during prolonged release. The strategic location and reduction in dimensionality of Ca2+-adsorbed linear polymers of CSQ could deliver Ca2+ more efficiently to the release channels than lumenal Ca2+.
Dulhunty, A.F., Varsanyi, M., Wei, L. & Beard, N.A. (2006) Proceedings of the Australian Physiological Society, 37: 26P