Juris Galvanovskis and Bo Söderberg
The Probability of Exocytosis: The Dependence on Ca2+ Sensing Latency Times, Ca2+ Channel Kinetic Parameters and Clustering

The release of secrete from secretory vesicles and granules requires a transient increase of intracellular Ca2+ concentration in the vicinity of the fusion site. Usually there is a short delay to the onset of the actual fusion of membranes, that is described as a latency time of the Ca2+ sensing system of the secreting machinery. The delay time affects in an essential way the probability for a certain granule to fuse with the plasmalemma and to release its contents into the extra-cellular space. We have investigated here, theoretically and numerically, the extent of this influence and its dependence on the parameters of Ca2+ channels, channel clustering, the Ca2+ sensing system, and the length of depolarizing pulses. We used a linear probabilistic model for a random opening and closing of channels, that yields an explicit expression for the Laplace transforms of the waiting time distributions for at least one channel to be open during the latency time. This allows one in principle to calculate the probability that a vesicle will fuse with plasmalemma during the action potential. We used the theoretical results to test our numerical simulations.

LU TP 01-18