Figure 5. [Ca²⁺]_i transient characteristics in SAN cells.

(A) Examples of line-scan confocal images showing [Ca²⁺]_i transients (shown as peak, F/F₀, where F is the fluorescence peak and F₀ the fluorescence during the diastolic period) from WT (left) and KI (right) SAN cells. (B) [Ca²⁺]_i transient amplitude is unchanged in KI SAN. (C) KI SAN cells have similar decay time constant (obtained by fitting the decay portion of the fluorescence trace to a single exponential). (D) Time to peak (duration between the beginning of the fluorescence [Ca²⁺]_i transient and its peak) is longer in KI SAN. (E) The maximum value of the derivative of the fluorescence [Ca²⁺]_i transient over time is smaller in KI cells, reflecting a slowing of the Ca²⁺ release. (F) Example of a ramp (late diastolic Ca²⁺ release, left), and the percentage of cells that present at least one ramp (right), which is higher in KI SAN cells (χ²). Ramp identification in the fluorescence trace is indicated by the black arrow. Bar graphs display mean value ± SEM of SAN cells with individual data shown on the columns, while each SAN value is averaged from at least 4 cells recorded in the same SAN. WT, white bars, n = 13 SAN cells; KI, red bars, n = 13 SAN cells. *P < 0.05, **P < 0.01.