Figure 8.

A flexible and robust pacemaker model with minimal set of components (44) features a small steady-state influx of Na⁺ and is expected to be more energy-efficient in terms of Na⁺ balance regulation (i.e. requires less ATP-dependent Na⁺ extrusion) vs. prior models Kurata et al. (66) and Maltsev and Lakatta (ML) (10). A: Top panels show simulations of trans-membrane Na⁺ currents via different mechanisms (I_f,Na, I_st, I_b,Na, I_NCX,Na) and their sum (dotted lines) in different models for one AP cycle (MDP-to-MDP, bottom panels). Middle panels: respective time-dependent integrals of Na⁺ currents of the top panels reflecting Na⁺ accumulation (during one AP cycle) in the absence of Na⁺ extrusion by ATPases. B: Respective integrals of Na⁺ currents in panel A calculated for one AP cycle. All three models have cell electric capacitance of 32 pF. C: Cell energy budget to maintain Na⁺ homeostasis [ATP_NaK] and to pump Ca²⁺ to the SR [ATP_SERCA] estimated by three models of SANC. From (44).