Figure 19. Ca²⁺ fluxes and the SR Ca²⁺ load.

Simulated major integral Ca²⁺ fluxes during one cardiac cycle in the isolated ventricular cell model for control conditions (Panels A and C) and after application of 1 µM isoproterenol (Panels B and D). Pacing frequencies are 1 Hz (Panels A and B) and 5 Hz (Panels C and D). Major integral Ca²⁺ fluxes are shown after 300 s of stimulation. In Panels B and D 1 µM isoproterenol is applied at time t = 0 s. Here, J_CaL is the Ca²⁺ entering the cell through L-type Ca²⁺ channels; J_up – J_leak is the uptake Ca²⁺ from the cytosol to the network SR with subtracted Ca²⁺ leak from the SR to the cytosol; J_NaCa is the Ca²⁺ outflux from the cytosol through the Na⁺/Ca²⁺ exchanger; and J_pCa is the Ca²⁺ outflux through the sarcolemmal Ca²⁺ pump. Panel E: Increase in the SR Ca²⁺ load after 300-s application of isoproterenol. Model data are shown for stimulation frequencies 1, 2, 3, 4, and 5 Hz; experimental data of Liu et al. [129] and Fernandez-Velasco et al. [132] are obtained at 1 µM isoproterenol; data of Knollmann et al. [128] are obtained at 0.5 µM isoproterenol.