AMPK is constitutively activated in old, but not in young, mice. (A–D) Whole-cell recordings of I_f in 3-mo-old (young) (A and B) and 24-mo-old (old) (C and D) mouse SAN cells in control (black) and after 4-h treatment with AMPK inhibitor (Compound C 30 µM for 4 h) (blue). Data are obtained from N = 3 young and N = 5 old mice. (A and C) Representative current traces (left) and activation curves (right). Best fitting yielded the following values (mV): (A) V1/2 = −86.2 ± 1.30 (n = 18) and −86.4 ± 1.42 (n = 14) (n.s.); s = 8.3 ± 0.4 and 10.2 ± 0.7; (C) V1/2 = −83.6 ± 1.04 (n = 24) and −82.5 ± 1.19 (n = 17) (n.s.); s = 8.7 ± 0.3 and 9.6 ± 0.4, for control and Compound C–treated cells, respectively. (B and D) I/V relations; bar graphs on the left show the distributions of current densities at −125 mV. Mean ± SE values (pA/pF) were as follows: (B, young) −45.3 ± 6.94 (n = 19) and −44.1 ± 7.24 (n = 16) (P = 0.9986, n.s.); and (D, old) −29.1 ± 3.55 (n = 27) and −47.1 ± 6.37 (n = 20) (P = 0.0110, *), for control and Compound C–treated cells, respectively. Current densities from young mice in control conditions and from old mice after Compound C treatment did not differ significantly (P = 0.9975, n.s.). Details of statistical analysis are shown in Table S1.