Estimating the time course of pore expansion during the spike phase of exocytotic release in mast cells of the beige mouse.

Our objective is to determine the time course of exocytotic fusion pore opening (P) in mast cells of the beige mouse from the measured efflux of the spike phase of exocytotic release (J). We show that a pore whose meridian or radius grows linearly with time cannot reproduce the efflux. We also show that a pore that opens very quickly [relative to the diffusivity of 5-hydroxytryptamine (5-HT)] and completely (P = pi) also does not mimic the experimental efflux, and estimate maximum pore angles of 70 (+/- 20) degrees. We show that a larger class of opening functions reproduces the rising phase and part of the decay phase and calculate pore expansion rate, pore radius and pore angle, none of which can be readily measured. In the initial stages of the spike phase (50-200 ms) when the gel matrix has not expanded significantly, this model suggests that the pore radius increases exponentially with a time constant of 82(+/- 62) ms with pore expansion reaching its maximum velocity of 20 (+/- 7) nm ms-1. We conclude that the release process is dynamic and suggest that the velocity of pore opening (V) and the diffusivity of 5-HT (D), in addition to the size of the vesicle (R, radius), vary with time. We discuss assumptions and improvements to the model and propose that this methodology is applicable for determining P from measured J in other endocrine cells and neurons when D within the secretory vesicle is much less than D within the pore neck.