The release of catecholamines to the cytosol and the exocytosis of secretory vesicles triggered by IP3 in chromaffin cells.

The aim of the present study was to investigate the secretory responses elicited by inositol 1,4,5-trisphosphate (IP3) and their regulation by Ca2+ from different sources. Fura-2, carbon fiber amperometry, and plasma membrane capacitance recordings were performed in mouse chromaffin cells to evaluate cytosolic Ca2+ changes, catecholamine release, and exocytosis, respectively. Amperometric recordings revealed that IP3 triggered the continuous release of catecholamines to the cytosol with a plateau shape, either applied independently or in combination with the V-ATPase blocker bafilomycin A1, without exhibiting additive effects, which suggests that V-ATPase blockade might be a potential mechanism of action. The catecholamine release elicited by IP3 can take place in the absence of cytosolic Ca2+; however, it may be also regulated by it through a bell-shaped mechanism, with the contribution of Ca2+ stored in intracellular organelles. Furthermore, plasma membrane capacitance recordings showed that IP3 could also elicit exocytosis of secretory vesicles with the participation of intracellular organelle Ca2+ stores. This exocytosis could be regulated by vesicular or cytosolic Ca2+, as shown in experiments with bafilomycin A1 or the Ca2+ chelator BAPTA-AM, respectively, and by kaempferol, an activator of the mitochondrial Ca2+ uniporter, suggesting that mitochondria may exert physiologically this Ca2+ regulatory mechanism. Therefore, in the IP3-mediated secretion, Ca2+ from different sources control the different steps of catecholamine release from the secretory vesicle to the cytosol and then finally to the extracellular space.NEW & NOTEWORTHY Inositol 1,4,5-trisphosphate (IP3) triggers the release of catecholamines from secretory vesicles to the cytosol through a process that may occur in the absence of cytosolic Ca2+, it is biphasically regulated by it and is dependent on Ca2+ from intracellular organelles. Additionally, IP3 triggers the exocytosis of secretory vesicles through a cytosolic and vesicular Ca2+ regulatory mechanism that may be physiologically modulated by mitochondria.