Intravesicular Calcium Transient during Calcium Release from Sarcoplasmic Reticulum

The time course of changes in the intravesicular Ca²⁺ concentration ([Ca²⁺]_i) in terminal cisternal sarcoplasmic reticulum vesicles upon the induction of Ca²⁺ release was investigated by using tetra-methylmurexide (TMX) as an intravesicular Ca²⁺ probe. Upon the addition of polylysine at the concentration that led to the maximum rate of Ca²⁺ release, [Ca²⁺]_i decreased monotonically in parallel with Ca²⁺ release. Upon induction of Ca²⁺ release by lower concentrations of polylysine, [Ca²⁺]_i first increased above the resting level, followed by a decrease well below it. The release triggers polylysine, and caffeine brought about dissociation of calcium that bound to a nonvesicular membrane segment consisting of the junctional face membrane and calsequestrin bound to it, as monitored with TMX. No Ca²⁺ dissociation from calsequestrin-free junctional face membranes or from the dissociated calsequestrin was produced by release triggers, but upon reassociation of the dissociated calsequestrin and the junctional face membrane, Ca²⁺ dissociation by triggers was restored. On the basis of these results, we propose that the release triggers elicit a signal in the junctional face membrane, presumably in the foot protein moiety, which is then transmitted to calsequestrin, leading to the dissociation of the bound calcium; and in SR vesicles, to the transient increase of [Ca²⁺]_i, and subsequently release across the membrane.