Exocytosis in adrenal chromaffin cells

Recent advances have led to an increased understanding of the Ca2+-signalling pathway leading to exocytosis in bovine adrenal chromaffin cells. Video-imaging studies have allowed the temporal and spatial aspects of the Ca2+ signal to be investigated in detail. Ca2+ entry at the plasma membrane appears to be crucial for the activation of exocytosis. Ca2+ can enter through the nicotinic channel or characterised voltage-activated channels, or through other poorly defined pathways due to a variety of agonists. Emptying of internal Ca2+ stores is sufficient to activate a Ca2+ entry pathway. The elevation of cytosolic Ca2+ concentration leads to a reorganisation of the cortical actin network and to the triggering of exocytosis. Studies on permeabilised chromaffin cells have resulted in the identification of some of the proteins that control Ca2+-dependent exocytosis. These include the peripheral plasma membrane protein annexin II and the cytosolic proteins, protein kinase C and 14-3-3 proteins (Exol).