MICRODOMAIN CA2- SUPERPOSITION OF LOCAL SUBPLASMALEMMAL CALCIUM STOREACTIVATION BY LOCAL CA2+ INFLUX( ACTIVATION DURING EXOCYTOSIS IN PARAMECIUM CELLS )

In Paramecium tetraurelia, polyamine-triggered exocytosis is accompani ed by the activation of Ca2+-activated currents across the cell membra ne (Err leben, C., and H. Plattner. 1994. J. Cell Biol. 127:935-945). We now show by voltage clamp and extracellular recordings that the pro duct of current x time (As) closely parallels the number of exocytotic events. We suggest that Ca2+ mobilization from subplasmalemmal storag e compartments, covering almost the entire cell surface, is a key even t, In fact, after local stimulation, Ca2+ imaging with high time resol ution reveals rapid, transient, local signals even when extracellular Ca2+ is quenched to or below resting intracellular Ca2+ concentration ([Ca2+](e) less than or equal to [Ca2+](i)). Under these conditions, q uenched-flow/freeze-fracture analysis shows that membrane fusion is on ly partially inhibited. Increasing [Ca2+], alone, i.e,, without secret agogue, causes rapid, strong cortical increase of [Ca2+](i) but no exo cytosis, In various cells, the ratio of maximal vs. minimal currents r egistered during maximal stimulation or single exocytotic events, resp ectively, correlate nicely with the number of Ca stores available. Sin ce no quantal current steps could be observed, this is again compatibl e with the combined occurrence of Ca2+ mobilization from stores (provi ding close to threshold Ca2+ levels) and Ca2+ influx from the medium ( which per se does not cause exocytosis). This implies that only the co mbination of Ca2+ flushes, primarily from internal and secondarily fro m external sources, can produce a signal triggering rapid, local exocy totic responses, as requested for Paramecium defense.