The release of catecholamines to the cytosol and the exocytosis of secretory vesicles triggered by IP ₃ 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.<b>NEW & NOTEWORTHY</b> 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.