Age-Related Changes in Cardiac Autonomic Modulation and Heart Rate Variability in Mice

Abstract

Objective: The aim of this study was to assess age-related changes in cardiac autonomic modulation and heart rate variability (HRV) and their association with spontaneous and pharmacologically induced vulnerability to cardiac arrhythmias, to verify the translational relevance of mouse models for further in-depth evaluation of the link between autonomic changes and increased arrhythmic risk with advancing age.

Methods: Heart rate (HR) and time- and frequency-domain indexes of HRV were calculated from Electrocardiogram (ECG) recordings in two groups of conscious mice of different ages (4 and 19 months old) (i) during daily undisturbed conditions, (ii) following peripheral β-adrenergic (atenolol), muscarinic (methylscopolamine), and β-adrenergic + muscarinic blockades, and (iii) following β-adrenergic (isoprenaline) stimulation. Vulnerability to arrhythmias was evaluated during daily undisturbed conditions and following β-adrenergic stimulation.

Results: HRV analysis and HR responses to autonomic blockades revealed that 19-month-old mice had a lower vagal modulation of cardiac function compared with 4-month-old mice. This age-related autonomic effect was not reflected in changes in HR, since intrinsic HR was lower in 19-month-old compared with 4-month-old mice. Both time- and frequency-domain HRV indexes were reduced following muscarinic, but not β-adrenergic blockade in younger mice, and to a lesser extent in older mice, suggesting that HRV is largely modulated by vagal tone in mice. Finally, 19-month-old mice showed a larger vulnerability to both spontaneous and isoprenaline-induced arrhythmias.

Conclusion: The present study combines HRV analysis and selective pharmacological autonomic blockades to document an age-related impairment in cardiac vagal modulation in mice which is consistent with the human condition. Given their short life span, mice could be further exploited as an aged model for studying the trajectory of vagal decline with advancing age using HRV measures, and the mechanisms underlying its association with proarrhythmic remodeling of the senescent heart.

Keywords: aging; arrhythmias (cardiac); autonomic cardiac modulation; heart rate variability (HRV); mouse model.

FIGURE 1

Long-term evaluation of HR and HRV in 4- and 19-month-old freely moving mice. (A) Time course of heart rates (HR) recorded during the 12-h light and 12-h dark phases of six consecutive days in 4-month-old (n = 10, filled diamonds) and 19-month-old (n = 9, empty circles) freely moving mice. (B) Mean heart rate and heart rate variability parameters evaluated from ECG traces recorded from 4-month-old and 19-month-old mice during the daily cycle (dark and light phases). Data are reported as mean ± SEM. SDNN, standard deviation of beat-to-beat intervals; RMSSD, root mean square of successive beat-to-beat interval differences; LF, low frequency; HF, high frequency; LF (n.u.), low frequency in normalized units; HF (n.u.), high frequency in normalized units. ^∗p < 0.05, two-way ANOVA for repeated measures followed by Fisher’s LSD post hoc test.

FIGURE 2

Age-related differences in the autonomic modulation of heart rate. Heart rate changes (ΔHR) after i.p. injection of vehicle (Saline) and autonomic modulator drugs (Methylscopolamine, Atenolol and Methylscopolamine + Atenolol) in 4 month-old (n = 10) and 19 month-old (n = 9) freely moving mice. The left-hand panels represent the time courses of ΔHR recorded from the moment of the injection; each point represents the mean ΔHR of a 2-min period. Saline was employed to identify the time span which is needed by the animals to recover from the handling-induced stress (60 min after the injection the HR returned similar to the mean basal HR recorded for 60 min before drugs administration, Basal). The right-hand panels report mean ΔHR values recorded during the second hour post-injection. Data are reported as mean ± SEM. ^∗p < 0.05, one-way ANOVA for repeated measures followed by Fisher’s LSD post hoc test.

FIGURE 3

Heart rate variability parameters response to pharmacological autonomic blockade in 4-month-old mice. Changes in the heart rate variability parameters calculated from ECG traces recorded from 60 to 120 min after i.p. injection of vehicle (saline) and autonomic modulator drugs (Methylscopolamine, Atenolol, Methylscopolamine + Atenolol) in 4-month-old freely moving mice (n = 10). The dotted line (reference level) indicates mean basal HRV indexes recorded 60 min before drug administration. In the box chart, middle line indicates the mean value, delimiting lines of the box represent SEM and whiskers are confidence intervals at 95%. ^∗p < 0.05, one-way ANOVA with repeated measures followed by Fisher’s LSD post hoc test.

FIGURE 4

Heart rate variability parameters response to pharmacological autonomic blockade in 19-month-old mice. Changes in the heart rate variability parameters calculated from ECG traces recorded from 60 to 120 min after i.p. injection of vehicle (saline) and autonomic modulator drugs (Methylscopolamine, Atenolol, Methylscopolamine + Atenolol) in 19-month-old freely moving mice (n = 9). The dotted line (reference level) indicates mean basal HRV indexes recorded 60 min before drug administration. In the box chart, middle line indicates the mean value, delimiting lines of the box represent SEM and whiskers are confidence intervals at 95%. ^∗p < 0.05, one-way ANOVA with repeated measures followed by Fisher’s LSD post hoc test.

FIGURE 5

Analysis and classification of spontaneous and induced arrhythmic events occurred in 4- and 19-month-old mice. (A) sample ECG traces of four different types of arrhythmic events: sinus pause, atrioventricular block, supraventricular ectopic beat and ventricular ectopic beat. (B) mean values of the spontaneous (top) and isoprenaline-induced (bottom) total and type-specific (sinus pauses, atrioventricular block, supraventricular ectopic beats and ventricular ectopic beats) arrhythmic events. Data are reported as mean ± SEM. Comparisons were evaluated by Student’s t-test, ^∗p < 0.05.