THE CALCIUM SENSITIVITY OF INDIVIDUAL SECRETORY VESICLES IS INVARIANTWITH THE RATE OF CALCIUM DELIVERY

Differences in the calcium sensitivity of individual secretory vesicle s can explain a defining feature of calcium-regulated exocytosis, a gr aded response to calcium The role of the time dependence of calcium de livery in defining the observed differences in the calcium sensitivity of sea urchin egg secretory vesicles in vitro was examined. The calci um sensitivity of individual secretory vesicles (i.e., the distributio n of calcium thresholds) is invariant over a range of calcium delivery rates from faster than micromolar per millisecond to slower than micr omolar per second. Any specific calcium concentration above tl:thresho ld triggers subpopulations of vesicles to fuse, and the size of these subpopulations is independent of the time course required to reach tha t calcium concentration. All evidence supports the hypothesis that the magnitude of the free calcium is the single controlling variable that determines the fraction of vesicles that fuse, and that this fraction is established before the application of calcium. Submaximal response s to calcium cannot be attributed to alterations in the calcium sensit ivity of individual secretory vesicles arising from the temporal prope rties of the calcium delivery. Models that attempt to explain the cess ation of fusion using changes in the distribution of calcium threshold s arising from the rate of calcium delivery and/or adaptation are not applicable to this system, and thus cannot be general.