Differential effects of lysophospholipids on exocytosis in rat PC12 cells

Secretory phospholipase A₂ (SPLA2) activity is present in the CNS and the SPLA₂-IIA isoform has been shown to induce exocytosis in cultured hippocampal neurons. However, little is known about possible contributions of various lysophospholipid species to exocytosis in neuroendocrine cells. This study was therefore carried out to examine the effects of several lysophospholipid species on exocytosis on rat pheochromocytoma-12 (PC 12) cells. An increase in vesicle fusion, indicating exocytosis, was observed in PC 12 cells after external infusion of Iysophosphatidylinositol (LPI), but not lysophosphatidylcholine or lysophosphatidylserine by total internal reflection microscopy. Similarly, external infusion of LPI induced significant increases in capacitance, or number of spikes detected at amperometry, indicating exocytosis. Depletion of cholesterol by pre-incubation of cells with methyl beta cyclodextrin and depletion of Ca²⁺ by thapsigargin and incubation in zero external Ca²⁺ resulted in attenuation of LPI induced exocytosis, indicating that exocytosis was dependent on the integrity of lipid rafts and intracellular Ca²⁺. Moreover, LPI induced a rise in intracellular Ca²⁺ suggesting that this could be the trigger for exocytosis. It is postulated that LPI may be an active participant in sPLA₂mediated exocytosis in the CNS.