Fig. 8.

Changes in intracellular Na⁺ concentration ([Na⁺]_i), intracellular Ca²⁺ concentration ([Ca²⁺]_i) transient, and contraction amplitudes with pacing and Iso treatment in KO-GFP and KO-S68E myocytes. A: increases in [Na⁺]_i (Δ[Na⁺]_i) were measured at 37°C in sodium-binding benzofuran isophthalate (SBFI)-loaded myocytes (methods). After resting [Na⁺]_i was obtained (at −1 min), pacing (2 Hz) was started at “time 0.” Iso (1 μM) was added at ∼2 min after initiation of pacing when both [Ca²⁺]_i transient (9) and contraction amplitudes (unpublished observations) have reached steady state. There are 15 KO-GFP (●) and 11 KO-S68E (○) myocytes. For comparison, Δ[Na⁺]_i from 5 WT (□) myocytes (26) are also shown. B: [Ca²⁺]_i transient amplitudes were measured at 37°C in fura-2-loaded myocytes paced at 2 Hz (methods). After ≥2 min of pacing, Iso (1 μM) was added. To facilitate comparison between myocytes, [Ca²⁺]_i transient amplitudes are normalized to the maximum values observed for each myocyte. There are 17 KO-GFP (●) and 20 KO-S68E (○) myocytes. For comparison, [Ca²⁺]_i transients from 8 WT (□) myocytes (26) are also shown. C: contraction amplitudes were measured at 37°C in myocytes paced at 2 Hz and normalized to the maximum values observed for each myocyte. After ≥2 min of pacing, Iso (1 μM) was added. There are 9 KO-GFP (●) and 14 KO-S68E (○) myocytes. For comparison, normalized contraction amplitudes from 6 WT (□) myocytes (26) are also shown. Error bars are not shown if they fall within the boundaries of the symbol.