Fusion Pore Selectivity in Chromaffin Cells

Abstract

Healthy physiological conditions depend on dynamic release of neurotransmitters and hormones by exocytosis, which is initiated by the fusion pore. Previous studies show that reduction of extracellular [NaCl] decreases the amperometric foot current, indicating no co-release of anions with transmitter. It is unknown if this is due to cation selectivity of the fusion pore or due to negatively charged ions, such as ATP, being bound to the granular matrix and unavailable for release. Such selectivity might be expected if the fusion pore is lined by negative charges from lipids and protein. If the initial fusion pore is selective for cations, then its selectivity will affect the rate of transmitter release via the reversal potential across the fusion pore. If the fusion pore is non-selective, then the flux of anions from extracellular space into the vesicle is expected to reduce the rate of release of positively charged catecholamines by making the vesicle lumen more negative. Using amperometry we investigate in chromaffin cells if the transmitter release rate through the fusion pore changes when extracellular chloride is exchanged by the less mobile D-glutamate anion. Electrodiffusion calculations predict that for a non-selective anion permeable fusion pore the average foot current amplitude should increase by ∼30% when chloride is exchanged by D-glutamate with a mobility of 0.25 relative to chloride, whereas no change in foot current is expected for a cation selective fusion pore. Preliminary analysis of all amperometric events in chromaffin cells, results in a mean amperometric foot current of 1.14±0.139 pA in the presence of chloride, and 1.04±0.07 pA for D-glutamate. Further sample collections and analysis will reveal if there is a significant difference in foot current amplitude depending on anion mobility and reveal if the early narrow fusion pore is cation selective.