Multiple kinetic components and the Ca2+ requirements of exocytosis.
The use of caged-Ca2+ 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-Ca2+ compounds have also indicated that the Ca2+ 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-affinity Ca2+ indicators (due to dye saturation), of the local increases in cytosolic Ca2+ concentration that trigger the exocytosis of individual vesicles. The effects of local saturation of such indicators are explored by means of a simple theory.
Kasai
H
H
Department of Physiology, Faculty of Medicine, University of Tokyo, Japan. hkasai@m.u-tokyo.ac.jp
Takahashi
N
N
D000818:Animals; D002118:Calcium / Q000502:physiology*; D005089:Exocytosis / Q000502:physiology*; D007202:Indicators and Reagents; D007700:Kinetics; D008954:Models, Biological; D010782:Photolysis
D007202:Indicators and Reagents; D002118:Calcium