Currents through the fusion pore that forms during exocytosis of a secretory vesicle

Abstract

Exocytosis, or the fusion of cytoplasmic vesicles with the cell membrane, occurs in nearly all eukaryotic cells, but its mechanism is not understood. Morphological and electrophysiological studies have suggested that membrane fusion begins with the formation of a 'fusion pore', a narrow channel across the closely adjacent membranes of vesicle and cell that forms the first connection of the vesicle lumen with the cell exterior and later dilates to allow release of vesicle contents. We used the patch clamp technique to study exocytosis of single giant secretory vesicles in mast cells of beige mice. The first opening of the fusion pore was found to generate a brief current transient, whose size and direction indicated an initial pore conductance of about 230 pS and a lumen-positive vesicle membrane potential. In time-resolved a.c. admittance measurements, the pore conductance was found to increase to much larger values within milliseconds, as if the pore dilated soon after opening. We conclude that the earliest fusion event may be the formation of a structure similar to an ion channel. Its conductance is of the same order of magnitude as that of a single gap junction channel, the only other known channel that spans two membranes.

Dual effects of G‐protein activation on Ca‐dependent exocytosis in bovine lactotrophs

“…The granule fusion event itself is virtually instantaneous in mast cells [23] and in bovine pituitary lactotrophs (Zorec, Sikdar and Mason, manuscript in preparation). The slow time course of Cm increase illustrated in fig.…”

Exocytotic catecholamine release is not associated with cation flux through channels in the vesicle membrane but Na+ influx through the fusion pore

“…The current carried by ion S through a cylindrical fusion pore of length l and cross-sectional area a is given by (1) where D S is the diffusion coefficient for ion S, z S its valency, [S V ] its intravesicular free concentration and [S E ] its extracellular free concentration. V is the voltage present across the fusion pore and F,R,T have their usual meaning.…”

“…Neurotransmitter and hormone release occurs by exocytosis, which begins with the formation of a narrow fusion pore 1,2 . Fusion pores in mast cells and chromaffin cells have typically an initial conductance of ~330 pS 1,3 through which vesicular serotonin and catecholamines are released, respectively 4,5 .…”

“…Fusion pores in mast cells and chromaffin cells have typically an initial conductance of ~330 pS 1,3 through which vesicular serotonin and catecholamines are released, respectively 4,5 . The initial fusion pore of a synaptic vesicle is typically >280 pS and release of neurotransmitter should occur with a time constant <500 μs due to the small vesicle volume 6 .…”

Secretion machinery at the cell plasma membrane

“…Electrophysiological measurements of mast cells [23][24][25][26][27] as well as adrenal chromaffin cells [28], suggested the presence of fusion pores at the cell plasma membrane as "dynamic entities", originating following a secretory stimulus [29]. Results from earlier studies on cell capacitance and conductance measurements, suggested that following stimulation of secretion, the "exocytotic fusion pore" abruptly appears as a 1-2 nm in diameter pore at the cell plasma membrane, with conductance similar to a large ion channel [29].…”