Perimetric [Ca2+]i rise and exocytosis detected by ultraviolet laser scanning confocal microscopy in rat peritoneal mast cells

It has been reported that in dialysed mast cells an increase in mean [Ca2+]i is neither necessary nor sufficient for secretion; however, it is possible that juxtamembranal [Ca2+]i may exceed the mean [Ca2+]i before exocytosis. The present study was carried out to analyse spatial and temporal dynamics of [Ca2+]i and concomitant exocytosis in intact rat peritoneal mast cells using UV laser scanning confocal microscopy. Stimulation with Compound 48/80 (16 microM) increased mean [Ca2+]i, causing an initial rapid elevation from 40 to 200 nM, which lasted for 6 s and was followed by a delayed secondary increase to 600 nM. Exocytotic images were seen in the cell perimeter 16 s after the stimulation. Perimetric and nuclear [Ca2+]i increased to several mumoles per litre, while that in the intermediate region remained low. In a Ca(2+)−deficient environment, Compound 48/80 still increased perimetric [Ca2+]i to micromolar values and induced exocytosis. This study clearly indicates, for the first time, that a perimetric increase in [Ca2+]i to micromolar levels precedes exocytosis, in intact, as opposed to permeabilized cells.