Effects of linopirdine and XE991 on [Ca²⁺]_i and their dependence on activation of Na⁺ channels. Effects of linopirdine and XE991 on [Ca²⁺]_i in sympathetic neurons were tracked by fura-2 imaging in the presence (triangles) and absence (squares) of TTX.A, Linopirdine (20 μm) alone caused a gradual increase in [Ca²⁺]_i(n = 12 cells); the [Ca²⁺]_i increase was prevented by coapplied TTX (100 nm; n = 14).B, [Ca²⁺]_i was raised by XE991 (10 μm; n = 10), and the effect was blocked by TTX (100 nm; n = 7).