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

Abstract

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.

Keywords: IP3; amperometry; calcium; capacitance; catecholamine.