Calcium induced fusion of isolated secretory vesicles from the islet of Langerhans

Isolated endocrine pancreatic secretory vesicles were exposed to series of cations. As revealed by freeze cleaving only Ca²⁺ was able to cause fusion of these vesicles forming a common lumen. Ca²⁺ specific fusion was preceded by an aggregation of membrane associated particles. Particle aggregation also occurred in vesicles incubated with the other cations (Ba²⁺, Sr²⁺, Mn²⁺, Mg²⁺, La³⁺) tested in 10^-4 M concentrations. Fusion occurred in the assumed physiological range of free intracellular Ca²⁺ concentration. Fusion of secretory vesicles can already be observed at 10^-6 M Ca²⁺ and is maximum at 10^-5 M Ca²⁺, indicating that small variations of intracellular free calcium concentration may trigger fusion of secretory vesicles. As the Ca²⁺ specific fusion of isolated secretory vesicles takes place with the same morphological changes observed in intact cells during fusion of the vesicle membrane with the plasma membrane in the exocytotic process, it is concluded that Ca²⁺ may act as the final intracellular trigger in stimulus secretion coupling.