Divergence and convergence in regulated exocytosis: the characteristics of cAMP-dependent enzyme secretion of parotid salivary acinar cells.

The process of membrane fusion is separated into three steps: docking, priming and fusion. The last fusion step of most regulated exocytosis is triggered by cytosolic free calcium (Ca2+). However, enzyme secretion from the parotid salivary glands is regulated by the accumulation of intracellular cAMP, although Ca2+ does augment the cAMP-induced secretion. The difference of the regulatory mechanisms is thought to be due to the brake points that will be passed upon stimulations. Vesicles of Ca2+-regulated exocytosis such as neurotransmission and norepinephrine release from chromaffin cells are waiting for the stimulation docked to the plasma membrane, and Ca2+ triggers the membrane fusion after the priming. In contrast, secretory granules of parotid acinar cells begin exocytosis with the docking step that may be regulated by cAMP. After the start of the docking, the exocytotic process of enzyme release runs a similar course to that of the neurotransmission: the priming and the Ca2+-enhanced fusion steps. Therefore, there are probably some common mechanisms involving the SNARE proteins both in Ca2+-regulated exocytosis and cAMP-dependent secretion.