Effect of Electrical Field Stimulation (EFS) on PVC Ca²⁺ signals.

(A) Line-scan analysis of spontaneous Ca²⁺ signals and responses to 1 Hz EFS in two neighboring cells. The cells are outlined in the 2-photon fluorescence image of PVCs in a lung slice shown in (B). In (A), the cell border is depicted by the dashed line. The spontaneous Ca²⁺ activity in Cell 1 shows a bidirectional wave (origin indicated by an asterisks, arrowheads indicate direction). The timing of the Ca²⁺ increases in both cells is independent of each other, but did occasionally coincide. EFS caused whole-cell Ca²⁺ increases, which are shown as vertical straight lines. EFS timing and pulse numbers are indicated by the top bar. The ability of Cell 1 or 2 to respond to EFS is indicated in the table; ‘+’ indicates full response, while ‘±’ indicates an incomplete response. (C) Summary, showing that the latency from an EFS-induced Ca²⁺ signal to the next spontaneous Ca²⁺ transient is significantly longer for PVCs with infrequent spontaneous Ca²⁺ activity (left). In contrast, the latency from a spontaneous Ca²⁺ transient to the next EFS-induced Ca²⁺ signal did not depend on the frequency of the spontaneous activity (right) (n >166 events in 10 cells, 3 slices).