Table 1.

Age-dependent changes in HRs and SAM AP FRs

	Intrinsic			Maximum			βAR response (maximum/intrinsic)
	2–3 mo	21–24 mo	32+ mo	2–3 mo	21–24 mo	32+ mo	2–3 mo	21–24 mo	32+ mo
HR, bpm	537.3 ± 14.3 (5)	478.7 ± 10.7* (12)	414.9 ± 21.6* (5)	737.3 ± 10.3 (5)	619.1 ± 7.4* (12)	562.9 ± 24.0* (5)	1.37 ± 0.04	1.30 ± 0.04	1.36 ± 0.05
FR rate, AP/min	414.3 ± 19.3 (10)	318.5 ± 14.7* (12)	268.6 ± 21.2* (10)	626.4 ± 41.7 (10)	478.4 ± 20.9* (12)	438.4 ± 33.1* (10)	1.51 ± 0.06	1.50 ± 0.03	1.46 ± 0.04

Data are averages (±SEM) of 30-s recording windows for HRs from ECG recordings or SAM FRs from perforated-patch recordings. Numbers of mice or cells are shown in parentheses. The βAR response is reported as the average ratio of maximum/intrinsic for both HR and FR.

^*P < 0.05 vs. animals/cells aged 2–3 mo (one-way ANOVA with a Holm–Sidak posttest).