Major currents in adult and aged mice. Modification in specific membrane and sarcoplasmic proteins (I_CaT, RyR, I_f) modified the other proteins. (A) Membrane voltage (V_m). (B) 1.2 L-type current (I_CaL1.2). (C) 1.3 L-type current (I_CaL1.2). (D) Rapidly activating delayed rectifier K⁺ current (I_Kr). (E) HCN current. (F) Inward rectifying K⁺ current (I_K1). (G) Sodium current (I_Na1.1). (H) Sodium current (I_Na1.5). (I) T-type Ca²⁺ current (I_CaT). (J) Na⁺-Ca²⁺ exchanger current (I_NaCa). (K) Na⁺-K⁺ pump current (I_NaK). (L) 4-aminopyridine–sensitive currents (I_to).