Impaired Capacity of Mice with Cardiac Specific AC8 Overexpression to Adapt to Dietary Caloric Manipulation
Authors: Mackey Chelsea Denise , Mattson Mark P. , Chakir Khalid , Lakatta Edward Denise 
Published: April 2020

Background

Cardiac specific overexpression of adenylyl cyclase 8 (TGAC8) leads to increased protein kinase A activity and widespread post‐translational protein modifications that chronically increase cardiac contractility and heart rate (30%) and create a markedly robust and coherent heart rhythm that is largely dissociated from autonomic control1. Metabolic profiling indicates that cardiac energy metabolism also shifts distinctly in the TGAC8 mouse in order to meet high energy demands. This disruption in heart‐brain crosstalk and ‐‐shift in energy metabolism in TGAC8 prompted us to investigate the effects of intermittent fasting, a dietary manipulation that is known to affect both metabolism and autonomic signaling to the heart, the latter leading to a reduction in resting heart rate (HR) and increase in heart rate variability (HRV). These effects are facilitated, in part, by enhanced parasympathetic tone, which involves an increase in brain‐derived neurotropic factor signaling (BNDF).

Objectives

We hypothesize that the effects of alternate‐day fasting (ADF) on HR and HRV in the TGAC8 mouse would differ to that in wild‐type (WT) littermate mice.

Methods

We assessed HR and HRV in vivo via telemetric ECG recordings in awake, freely moving young (6 months) WT and TGAC8 mice, prior to and following an ADF dietary regimen, consisting of a 24‐hr eating period alternated with a 24‐hr fasting period for 4‐weeks. HRV was analyzed using a combination of Labchart 7.37 and a custom‐built python software. Segments containing at least 2048 accurate RR intervals were transformed using a fast Fourier transform. RNA sequencing (RNA Seq) was performed on tissue isolated from the left ventricle of the heart. Body weight was measured weekly on both feeding and fasting days. Results were analyzed via repeated measures (mixed) ANOVA.

Results

ADF resulted in no net weight gain nor weight loss in either group. Mean HR became markedly reduced following 4‐weeks of ADF, but this reduction was statistically significant only in WT (Table
). HRV in both time‐ and frequency‐domains in WT displayed increased variability after ADF compared to an ad libitum diet (Table
), including a two‐fold increase in the root mean square of the successive differences (RMSSD) and total power spectral density. HR and HRV were not significantly altered in TGAC8 following 4‐weeks of ADF. RNA Seq revealed a downregulation of BDNF by 39% in the left ventricle of TGAC8 mice.

Conclusions

Our results provide the first evidence that adrenergic‐like activity intrinsic to the heart, due to cardiac‐specific overexpression of AC8, in addition to affecting heart rate and HRV, also affects dietary modulation by ADF suggesting that the effect of diet that are modulated by parasympathetic tone are blunted in TGAC8. This may involve reduced BDNF levels in the left ventricle. Future research on the altered response of HR and HRV to ADF in TGAC8 will investigate specific heart‐brain crosstalk in order to elucidate mechanisms of altered neuronal input to the TGAC8 heart or the altered response of the heart to this input.

Support or Funding Information

This work was funded by the Intramural Research Program of the National Institutes of Health, National Institute on Aging

Heart Rate and Heart Rate Variability Analysis Following Alternate‐Day Fasting

WT(n = 5)

TGAC8(n = 3)

Heart Rate

Ad Libitum

4‐Weeks ADF

Ad Libitum

4‐Weeks ADF

Mean HR (bpm)

535.28

409.73
+

631.15
¥

579.71
*

HRV: Time Domain

Ad Libitum

4‐Weeks ADF

Ad Libitum

4‐Weeks ADF

SDNN

8.37

18.50
+

3.76

6.50
*

Coefficient of Variation (CV)

7.34

12.37
+

3.85

6.14
*

RMSSO

4.69

14.69
+

3.22

3.72
*

HRV: Frequency Domain

Ad Libitum

4‐Weeks ADF

Ad Libitum

4‐Weeks ADF

Total Power PSD

133.79

460.35
+

58.10

73.23
*

HF PSD

49.48

163.74
+

30.83

36.32
*

LF PSD

31.93

91.36
+

11.35

15.82
*

VLF PSD

52.38

205.25
+

15.92

21.09
*

+p < 0.05 effect of diet within each genotype

¥p < 0.05 effect of genotype at ad libitum

*p < 0.05 effect of genotype at 4‐Weeks ADF