Two modes of exocytosis from synaptosomes are differentially regulated by protein phosphatase types 2A and 2B

The inhibitors okadaic acid (OA), fostriecin (FOS) and cyclosporin A (CsA), were used to investigate the roles of protein phosphatases in regulating exocytosis in rat brain synaptosomes by measuring glutamate release and the release of the styryl dye FM 2‐10. Depolarization was induced by 30 mm KCl, or 0.3 mm or 1 mm 4‐aminopyridine (4AP). OA and FOS produced a similar partial inhibition of KCl‐ and 0.3 mm 4AP‐ evoked exocytosis in both assays, but had little effect upon exocytosis evoked by 1 mm 4AP. In contrast, CsA had no effect upon KCl‐ and 0.3 mm 4AP‐evoked exocytosis, but significantly enhanced glutamate release but not FM 2‐10 dye release evoked by 1 mm 4AP. None of the phosphatase inhibitors changed calcium signals from FURA‐2‐loaded synaptosomes either before or after depolarization. Pretreatment with 100 nm phorbol 12‐myristate 13‐acetate abolished the inhibitory effect of OA on exocytosis induced by 0.3 mm 4AP. Taken together, these results show that exocytosis from synaptosomes has a phosphatase‐sensitive and phosphatase‐insensitive component, and that there are two modes of phosphatase‐sensitive exocytosis that can be elicited by different depolarization conditions. Moreover, these two modes are differentially sensitive to phosphatase 2A and 2B.