Model simulation in response to pacemaker function changes occurring in response to IBMX in adult and aged mice. In response to maximal concentration of IBMX, AP firing rate (A), cAMP (B), and PKA (C) activities are restored; the flux of Ca²⁺ existing 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.