Fig. 4.

Effect of intracellular Na⁺ concentration ([Na⁺]_i) clamp at fast pacing (CL = 300 ms) to slow rate values during the final paced beat (Na⁺ at CL = 2,000 ms, Na2K). The AP, the difference between slow and fast rate APs (ΔV_m), Na⁺/K⁺ pump current (I_NaK), and Na⁺/Ca²⁺ exchange current (I_NaCa,i and I_NaCa,ss, myoplasm and subpace components) are shown from top to bottom, respectively. Inset: [Na⁺]_i. Solid black, CL = 2,000 ms, no [Na⁺] clamp; solid gray, CL = 300 ms, no [Na⁺] clamp; dashed black, CL = 300 ms, [Na⁺] clamp.