Multiple kinetic components and the Ca2+ requirements of exocytosis

Abstract

The use of caged-Ca 2+ compounds to stimulate Ca 2+ -dependent exocytosis has substantially increased our understanding of this complex process. By this approach, the existence of multiple kinetic components of exocytosis has been established. These components may correspond to a series of sequential steps that lead to a single fusion-ready state (sequential mechanism) or, alternatively, to heterogeneity in secretory vesicles or in fusion-ready states (parallel mechanism). It is suggested that both of these mechanisms can underlie exocytosis of a single type of vesicle (mixed sequential^parallel mechanism). Studies with caged-Ca 2+ compounds have also indicated that the Ca 2+ requirement for exocytosis is substantially greater than that suggested by conventional methodologies. This discrepancy is mainly attributable to the underestimation, by imaging studies with high-a¤nity Ca 2+ indicators (due to dye saturation), of the local increases in cytosolic Ca 2+ concentration that trigger the exocytosis of individual vesicles. The e¡ects of local saturation of such indicators are explored by means of a simple theory.

Kinetic Control of Multiple Forms of Ca2+ Spikes by Inositol Trisphosphate in Pancreatic Acinar Cells

“…The preferential localization of type-3 IP 3 receptors in the trigger zone (Nathanson et al 1994) is possibly responsible for the large increases in [Ca 2+ ] i in this region, given the small inhibitory effect of Ca 2+ on these receptors (Hagar et al 1998). It is therefore suggested that the type-3 IP 3 receptor plays a specific role in cellular processes such as exocytosis that require high [Ca 2+ ] i (Ito et al 1997; Kasai and Takahashi 1999). The Ca 2+ release in the trigger zone exhibited a similar sensitivity (Fig.…”

“…We found that this was indeed the case for local Ca 2 ϩ spikes, but not for global Ca 2 ϩ spikes. Ca 2 ϩ imaging was performed with a low-affinity Ca 2 ϩ indicator, benzothiazole coumarin (BTC), that minimizes the effects of changes in intrinsic Ca 2 ϩ buffering in the cells and allowed us to quantify large increases in [Ca 2 ϩ ] i without the problem of dye saturation (Ito et al, 1997;Kasai and Takahashi, 1999).…”

Stimulation of Glutamate, Aspartate and Gamma-Aminobutyric Acid Release from Synaptosomes by Hyperforin

“…Thus, although stimulation of Glu-release was always preceded by a lag-period, the increases in synaptosomal [Ca 2+ ] i or pH ilowering and neutralization of pH gradients in vesicles were observed immediately after adding hyperforin to the incubation medium. It is now well known that synaptic neurotransmitter release (and/or uptake) is not only a calcium-dependent process, but is also regulated by various other interdependent mechanisms [18,29,32]. Our observations suggest that hyperforin could modulate a few of them before stimulating amino acid release.…”