Temperature Dependence of Fusion Kinetics and Fusion Pores in Ca2+-triggered Exocytosis from PC12 Cells

The temperature dependence of Ca(2+)-triggered exocytosis was studied using carbon fiber amperometry to record the release of norepinephrine from PC12 cells. Single-vesicle fusion events were examined at temperatures varying from 12 to 28 degrees C, and with release elicited by depolarization. Measurements were made of the initial and maximum frequencies of exocytotic events, of fusion pore lifetime, flux through the open fusion pore, kiss-and-run versus full-fusion probability, and parameters associated with the shapes of amperometric spikes. The fusion pore open-state flux, and all parameters associated with spike shape, including area, rise time, and decay time, had weak temperature dependences and activation energies in the range expected for bulk diffusion in an aqueous solution. Kiss-and-run events also varied with temperature, with lower temperatures increasing the relative probability of kiss-and-run events by approximately 50%. By contrast, kinetic parameters relating to the frequency of exocytotic events and fusion pore transitions depended much more strongly on temperature, suggesting that these processes entail structural rearrangements of proteins or lipids or both. The weak temperature dependence of spike shape suggests that after the fusion pore has started to expand, structural transitions of membrane components are no longer kinetically limiting. This indicates that the content of a vesicle is expelled completely after fusion pore expansion.