Amperometric characterization of exocytotic events from single mast cells: dependence on external and internal Ca++ sources.

Mast cells exocytotically release histamine/serotonin in response to different secretagogues. We have used substance P and compound 48/80 to study the Ca++ dependency of serotonin exocytosis from peritoneal mast cells using carbon fiber amperometric techniques. The exocytotic release pattern consists of a burst of events superimposed on a slow, transient, amperometric current baseline increase. Cellular re lease parameters (number, frequency and total charge of amperometric events) and individual event characteristics (charge integral, half width and peak amplitude) were similar for the two secretagogues used. Zero Ca++ conditions greatly reduced, without completely abolishing,cellular release parameters. Cyclopiazonic acid, an inhibitor of the endoplasmatic Ca++ ATPase, reduced the cellular exocytotic capacity and diminished the amplitude of individual exocytotic events more effectively than the 0 Ca++ condition. The cyclopiazonic acid effects occurred in the presence of external Ca++, indicating that this condition is not sufficient for maintaining full exocytotic capacity. The results confirm the importance of intracellular Ca++ for exocytotic activation. For the first time evidence is presented that the integrity of intracellular Ca++ pools determines the amplitude and frequency of individual exocytotic events. Saponin, a non-specific detergent, also induced quantal release similar to that obtained with substance P and compound 48/80. This release was not dependent on extracellular Ca++, but cyclopiazonic acid significantly reduced individual exocytotic release.