Intravesicular calcium transient during calcium release from sarcoplasmic reticulum

Abstract

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 tetramethylmurexide (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.

Facilitation of Membrane Fusion During Exocytosis and Exocytosis-Coupled Endocytosis and Acceleration of ``Ghost' Detachment in Paramecium by Extracellular Calcium. A Quenched-Flow/Freeze-Fracture Analysis

“…2 values of ∼50 mM [58,59], assuming 20% dry weight. Similar variation ranges were reported for SR, i.e., between 6 mM [24], 15 [23], 20 [22] and 33 mM [60], or for ER in different cells, i.e., between 1 mM [6], 1.5 to 2.5 [11], 3 [36], and 9.5 mM [7]. In sum, if one disregards excessively high values reported in older references for all of these systems, it appears reasonable to assume a total of ∼5 mM Ca 2+ in alveolar sacs.…”

Fast kinetics of calcium dissociation from calsequestrin

“…In response to RyR activation by submaximal concentrations of releasing agents, CSQ unbinds Ca 2+ (and presumably undergoes a significant change in conformation, as determined here) before Ca 2+ is released from the SR (Ikemoto et al, 1991). These results raise the possibility that the E-C coupling signal transmitted from the transverse tubule voltage sensors to RyR channels also may be sensed by CSQ, resulting in fast Ca 2+ dissociation from CSQ.…”

“…Hence, it becomes relevant to determine the kinetics of Ca 2+ dissociation from CSQ, and also to determine how fast CSQ undergoes conformational changes following Ca 2+ dissociation, since these changes may affect RyR activity. In fact, in response to RyR activation by caffeine or polylysine, CSQ releases Ca 2+ to the SR lumen before Ca 2+ is released from the SR (Ikemoto et al, 1991). These results suggest that RyR activation is sensed by CSQ and raise the intriguing possibility that CSQ may transmit to RyR conformational changes associated with Ca 2+ dissociation, modifying their activity.…”

Depletion “skraps” and dynamic buffering inside the cellular calcium store

“…The evolution of [Ca 2ϩ ] SR upon stimulation of prolonged cell-wide Ca 2ϩ release revealed other properties of this proximate store. An analogous transient, described in suspensions of SR vesicles upon stimulation by RyR-opening drugs was traced to intravesicular Ca 2ϩ release from CSQ (19). The intrastore buffer is clearly involved in the present transients as well.…”