Biological activities of InsP5 and InsP6 in neuronal cells : Analysis by single cell amperometory.

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Inositol polyphosphates (InsPPs) such as Inosito 11,3,4,5,6-pentakisphosphate (InsP5) and inositol hexakisphosphate (InsP6) bind to C2B domain of synaptotagmins I and II (Syt), and inhibit transmitter release. In adrenal chromaffln cells, we have shown that binding of Ca2+ to the C2A domain reverses the inhibition of exocytosis by InsPPs probably dissociating InsPPs ftom Syt. A rapid accumulation of endogenous InsP5 and InsP6 after depolarizing stimuli have been reported in adrenal chromaffin cells, NIE-1 15 cells and rat cerebellar granule cells. Such a rapid accumulation of InsPPs might be a result from their dissociation from C2B domain of Syt. To test this hypothesis, we examined the effects of antibodies against C2A and C2B domains (anti-C2A Ab, anti-C2B Ab) on the accumulation of InsP5 and InsP6 induced by Ca2+ in digitoninpermeabilized adrenal chromafrln Cells. Stimulation with Ca2+ induced a rapid and transient increase of InsPPs in the cytosolic components. Anti-C2B Ab by itself caused a significnt accumulation of InsPPs in the cytosolic components, and increased spontaneous release of catecholamines (CA). Anti-C2A Ab abolished Ca2+-induced increase of InsP5 and InsP6 in cytosohc component. In contrast to anti-C2B Ab, anti-C2A Ab inhibited Ca2+ -evoked release of CA with little effect on the spontaneous release. Moreover, microinjection of InsP6 but not inositol hexakissulfate (InsS6) into the intact chromaffin cells remarkably inhibited both spontaneous and nicotine- evoked exocytotic events. These results suggest that binding of Ca2+ to the C2A domain induces dissociation of endogenous InsPPs from the C2B domain of Syt.