Model simulation in response to pacemaker function changes occurring in response to ISO in adult and aged mice. In response to maximal concentration of ISO, AP firing rate (A), cAMP (B), and PKA activities (C) are restored; the flux of Ca²⁺ in the SR (j_SRCarel; D) and Ca²⁺ concentration in the junctional sarcoplasmic reticulum compartment (Ca_jSR; E) are compensated for; and the Na⁺-Ca²⁺ exchanger current (I_NCX; F), HCN current (I_f; G), L-type current (I_CaL; H), and T-type Ca²⁺ current (I_CaT; I) are completely or partly restored.