Enhancement of inward Ca²⁺ currents in bovine chromaffin cells by green tea polyphenol extracts

Green tea contains four major polyphenol compounds: they are (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epicatechin (EC). Although all four polyphenol compounds are known to affect tumor suppression, little is known about whether they alter membrane properties. In this study, we examined the effects of ECG and EGCG on ionic currents and secretion. Membrane capacitance changes were used to monitor secretion in bovine chromaffin cells. ECG had the ability to reversibly enhance the inward Ca²⁺ current by 21%, and inhibited the peak sodium current by 34%. EGCG had no effect on Ca²⁺ current even though it differs from ECG by just a hydroxyl group. The EC₅₀ of ECG in enhancing Ca²⁺ current was 7.6 μM. The maximum enhancement of Ca²⁺ current was observed at 0 mV and the maximum current was shifted ∼10 mV in the hyperpolarizing direction. When cells were stimulated by trains of depolarizations, the exocytosis elicited was enhanced by ECG treatment and the largest enhancement of secretion was observed in later stimulations. EGCG, although it had no significant effect on Ca²⁺ current, enhanced exocytosis and slowed endocytosis. These results suggest that green tea polyphenol compounds modulate stimulus-secretion coupling in bovine chromaffin cells.