The calcium sensitivity of individual secretory vesicles is invariant with the rate of calcium delivery

The role of the time dependence of calcium delivery in defining the differences in the calcium sensitivity of sea urchin egg secretory vesicles in vitro was studied. The calcium sensitivity of individual secretory vesicles was found to be invariant over a range of calcium delivery rates from faster than micromolar per millisecond to slower than micromolar per second. Any specific calcium concentration above threshold triggers subpopulations of vesicles to fuse, and the size of these subpopulations is independent of the time needed to attain that calcium concentration.