Extracellular ATP Regulates the Vesicular Pore Opening in Chromaffin Cells and Increases the Fraction Released During Individual Exocytosis Events.

Adenosine triphosphate (ATP) is the main energy source for cellular metabolism. Besides that, ATP is a neurotransmitter and a cotransmitter that acts on purinergic receptors present either pre- or postsynaptically. Almost all types of secretory vesicles from any neuron or animal species contain high concentrations of ATP, being an essential factor in the accumulation of neurotransmitters. In this work, we studied the effects of ATP on quantum catecholamine release and vesicular storage in chromaffin cells. We combined three electrochemical methods: conventional amperometry with intracellular vesicle impact electrochemical cytometry and vesicle impact electrochemical cytometry. We found that extracellular ATP increased the released quantal fraction of catecholamine but not its vesicular content. Studying the dynamics of exocytosis events in ATP treated cells showed that ATP affects the release fusion pore. To elucidate the mechanisms of the observed ATP effects, cells and vesicles were pharmacologically treated with suramin (a purinergic blocker) and ARL-67156 (an antagonist of ecto-ATPases). The data indicate that the catecholamine content of vesicles increased compared to control after these drugs. Our data suggest that ATP acting on purinergic receptors increases the quantum releasable size through an increased fusion pore opening and that ARL-67156 and/or suramin protect the vesicle from neurotransmitter leakage by functioning as competitive inhibitors to ATP.