Correlation between Vesicle Quantal Size and Fusion Pore Release in Chromaffin Cell Exocytosis

Abstract

A significant number of exocytosis events recorded with amperometry demonstrate a prespike feature termed a "foot" and this foot has been correlated with messengers released via a transitory fusion pore before full exocytosis. We have compared amperometric spikes with a foot with spikes without a foot at chromaffin cells and found that the probability of detecting a distinct foot event is correlated to the amount of catecholamine released. The mean charge of the spikes with a foot was found to be twice that of the spikes without a foot, and the frequency of spikes displaying a foot was zero for small spikes increasing to approximately 50% for large spikes. It is hypothesized that in chromaffin cells, where the dense core is believed to nearly fill the vesicle, the expanding core is a controlling factor in opening the fusion pore, that prefusion of two smaller vesicles leads to excess membrane, and that this slows pore expansion leading to an increased observation of events with a foot. Clearly, the physicochemical properties of vesicles are key factors in the control of the dynamics of release through the fusion pore and the high and variable frequency of this release makes it highly significant.

Role of the Vesicular Chloride Transporter ClC-3 in Neuroendocrine Tissue

“…The probability of detecting a foot signal is positively correlated with the quantal size of a vesicle (Amatore et al, 2005). The properties of the spike foot in regards to its height, duration and area were significantly reduced in KO cells (Table 1).…”

The evidence for open and closed exocytosis as the primary release mechanism

“…Careful analysis of these pre-spike events in chromaffin cells has revealed that they are quite common, occurring in approximately 30% of events, and that although their occurrence does not indicate the size of the event, the flux through the corresponding fusion pore correlates with the flux of the entire event (Amatore et al 2009). Feet have been detected both before (Amatore et al 2005) and after (Mellander et al 2012) the spike for a significant number of events leading to a higher total release from the vesicle as measured by the charge released.…”

Theoretical Model of Neurotransmitter Release during In Vivo Vesicular Exocytosis Based on a Grainy Biphasic Nano-Structuration of Chromogranins within Dense Core Matrixes

“…These vesicles are morphologically distinct from dense core vesicles in mast and chromaffin cells, a fact that maybe related to the ability of PC12 to continuously reload their vesicles. [65][66][67] Beyond the very origin of this important morphological difference, it must be noted that the observation of these halos in EM micrographs provides a strong support for the co-existence of different compaction domains of chromogranins-catecholamine cations inside of vesicles.…”

“…250%. [65][66] However, despite these extremely important changes, the time constants of the current spikes descending branches do not vary significantly for L-DOPA-loaded vesicles compared to controls. This suggests that even if (k diff ρ ) halo /k diff ρ is larger than unity, this gain is more or less compensated by the decrease associated to the presence of the factor 1/(1 + k 4 /k 3 ) in the time constant in Equation 17.…”