FSK potentiates Ca²⁺-evoked exocytosis. (A and C) Cells, loaded sequentially with dopamine for amperometry and then with indo-1 Ca²⁺-sensitive dye, were pretreated with 1 μM ionomycin in Ca²⁺-free solution (0 Ca²⁺). All test solutions contained 1 μM ionomycin in a Ca²⁺-free solution. Increases of [Ca²⁺]_i and of exocytosis were induced by switching to 2 mM Ca²⁺ (2 Ca²⁺). Top traces are representative amperometric current recordings. The lower traces show optically measured [Ca²⁺]_i (green line) and the simultaneous rate of exocytosis (filled circles, number of spikes per 30 s) in the absence (A) or presence (C) of FSK. (B and D) Average time course of [Ca²⁺]_i (green line) and normalized rate of exocytosis (symbols) in the absence (B; n = 6) or presence (D; n = 12) of 1 μM FSK.