Prediction of local pH variations during amperometric monitoring of vesicular exocytotic events at chromaffin cells.

Electrochemical monitoring of the exocytosis process is generally performed through amperometric oxidation of the electroactive messengers released by single living cells. Herein, we consider the vesicular release of catecholamines by chromaffin cells. Each exocytotic event is thus detected as a current spike whose morphology (intensity, duration, area, etc.) features the efficiency of the secretion process. However, the electrochemical oxidation of catechols produces quinone derivatives and protons. As a consequence, unless specific mechanisms may be adopted by a cell to regulate the pH near its membrane, the local pH between the cell membrane and the electrode necessarily drops within the electrode-cell cleft. Though this consequence of amperometric detection is generally ignored, it has been investigated in this work through simulation of the local pH drop created during the amperometric recording of a sequence of exocytotic events. This was performed based on frequencies and magnitudes of release detected at chromaffin cells. The corresponding acidification was shown to severely depend on the microelectrode radius. For usual 10 μm diameter carbon fiber electrodes, pH values below six were predicted to be reached within the electrode-cell cleft after monitoring a few current spikes.