Calcium-dependent Regulation of Chromaffin Granule Movement, Membrane Contact, and Fusion during Exocytosis

Excerpt

Exocytosis is a general and dominant motif in the lives of most eukaryotic cell types, perhaps occurring at a minimum of 10¹² times per second in the human body. It underlies many important processes such as neurotransmission, hormone and enzyme secretion, blood clotting, and the release of many serum proteins. Yet, the biochemical mechanisms underlying the event remain obscure, except for the fact that elevation of the intracellular free-calcium concentration clearly occurs as a preliminary event in many well-studied systems. We also know that secretory vesicle membranes undergo a well-defined fusion with either plasma membranes or with previously fused vesicle membranes; the latter event is called compound or “piggyback” exocytosis. However, it is just this relationship between calcium and the subsequent dramatic remodeling of membranes during exocytosis that has been most difficult to study experimentally, kinetic arguments indicating only that wherever Ca^{+ +} acts, it must be close to the...