EXOCYTOSIS IN ADRENAL CHROMAFFIN CELLS

Recent advances have led to an increased understanding of the Ca2+-sig nalling pathway leading to exocytosis in bovine adrenal chromaffin cel ls. Video-imaging studies have allowed the temporal and spatial aspect s of the Ca2+ signal to be investigated in detail. Ca2+ entry at the p lasma 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 t o activate a Ca2+ entry pathway. The elevation of cytosolic Ca2+ conce ntration leads to a reorganisation of the cortical actin network and t o the triggering of exocytosis. Studies on permeabilised chromaffin ce lls have resulted in the identification of some of the proteins that c ontrol 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).