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. Author manuscript; available in PMC: 2023 Apr 1.
Published in final edited form as: Chronobiol Int. 2021 Dec 7;39(4):525–534. doi: 10.1080/07420528.2021.2011307

Timing of Food Intake in Mice Unmasks a Role for the Cardiomyocyte Circadian Clock Mechanism in Limiting QT-interval Prolongation

Elizabeth A Schroder 1,2,†, Don E Burgess 1,†, Sidney R Johnson 1, Makoto Ono 1, Tanya Seward 1, Claude S Elayi 3, Karyn A Esser 4, Brian P Delisle 1,†
PMCID: PMC8989643  NIHMSID: NIHMS1766890  PMID: 34875962

Abstract

Cardiac electrophysiological studies demonstrate that restricting the feeding of mice to the light cycle (time restricted feeding or TRF) causes a pronounced change in heart rate and ventricular repolarization as measured by the RR- and QT-interval, respectively. TRF slows heart rate and shifts the peak (acrophase) of the day/night rhythms in the RR- and QT-intervals from the light to the dark cycle. This study tested the hypothesis that these changes in cardiac electrophysiology are driven by the cardiomyocyte circadian clock mechanism. We determined the impact that TRF had on RR- and QT-intervals in control mice or mice that had the cardiomyocyte circadian clock mechanism disrupted by inducing the deletion of Bmal1 in adult cardiomyocytes (iCSΔBmal1−/− mice). In control and iCSΔBmal1−/− mice, TRF increased the RR-intervals measured during the dark cycle and shifted the acrophase of the day/night rhythm in the RR-interval from the light to the dark cycle. Compared to control mice, TRF caused a larger prolongation of the QT-interval measured from iCSΔBmal1−/− mice during the dark cycle. The larger QT-interval prolongation in the iCSΔBmal1−/− mice caused an increased mean and amplitude in the day/night rhythm of the QT-interval. There was not a difference in the TRF-induced shift in the day/night rhythm of the QT-interval measured from control or iCSΔBmal1−/− mice. We conclude that the cardiomyocyte circadian clock does not drive the changes in heart rate or ventricular repolarization with TRF. However, TRF unmasks an important role for the cardiomyocyte circadian clock to prevent excessive QT-interval prolongation, especially at slow heart rates.

Keywords: restricted feeding, cardiac electrophysiology, heart rate, ventricular repolarization, QT-interval, Bmal1, circadian rhythms, circadian clock

Introduction

Organisms evolved circadian rhythms to prepare the body for daily changes in the environment and activity (Reppert et al. 2002). In mammals, these endogenous physiological rhythms that cycle with periodicity of ~24 are generated at the cellular level by the circadian clock mechanism, a transcription-translation feedback loop (Lowrey et al. 2004; Mohawk et al. 2012). The positive limb of the circadian clock mechanism is driven by the transcription factors BMAL1 and CLOCK. BMAL1 and CLOCK heterodimerize to activate the transcription of Period (Per) and Cryptochrome (Cry) genes. PER and CRY proteins negatively feedback on BMAL1 and CLOCK activity. The circadian clock mechanism is present in most cells and regulates the expression of many genes and proteins. The circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus entrains to the light cycle, and neurohumoral signaling from the SCN drives daily patterns in physiology and behavior to align the circadian clock mechanism in cells throughout the body (Stephan et al. 1972; Rusak et al. 1982; Kalsbeek et al. 2006; Hastings et al. 2018). Data suggest that the neuronal activity from the SCN modifies autonomic output to most of the organs in the body including the heart (Meijer et al. 1996; Buijs et al. 1998; Meijer et al. 1998; Buijs et al. 1999; Scheer et al. 2001). The autonomic output from the SCN is thought to regulate the day/night rhythm in heart rate because either lesioning the SCN or autonomic nervous system blockade causes a loss in the day/night rhythm of heart rate (Tong et al. 2013).

The time of feeding behavior is an important synchronizing cue for the phase of the circadian clock in the heart (Damiola et al. 2000; Pickel et al. 2020). For example, mouse studies that quantify the mRNA expression of circadian clock output genes in different tissues show that restricting the time of eating to the light cycle (when mice normally sleep/rest) inverts the phase in the circadian expression of clock controlled genes in peripheral tissues (e.g., liver, heart, kidney, pancreas) but not the SCN (Ogawa et al. 1997; Balsalobre et al. 2000; Stokkan et al. 2001; Bray et al. 2013; Schibler et al. 2015). Time restricted feeding to the light cycle (TRF) in mice also causes a realignment in the day/night rhythm of heart rate as measured by the RR-interval on an electrocardiogram (ECG) (Schroder et al. 2014). The TRF impact on heart rate is surprising because it suggests that autonomic output from the SCN does not drive the day/night rhythm in heart rate.

Recent studies suggest that the day/night rhythm in heart rate involves the circadian clock mechanism in the heart (D’Souza et al. 2021; Hayter et al. 2021). In this study we tested the hypothesis that the circadian clock mechanism in the heart is responsible for the TRF-induced changes in RR-intervals and QT-intervals using ECG telemetry (Mitchell et al. 1998; Postema et al. 2014; Schroder et al. 2014). The QT-interval is a measure of cardiac depolarization and repolarization and encompasses the QRS complex and JT interval (Postema et al. 2014). We measured the QT-interval because it changes with the RR-interval and is an important biomarker for arrhythmogenicity (Mitchell et al. 1998; Shah et al. 2005). To selectively disrupt the cardiomyocyte circadian clock, we used a transgenic mouse model that allowed us to induce cardiomyocyte specific deletion of Bmal1 (iCSΔBmal1) in adult cardiomyocytes (Schroder et al. 2013; Schroder et al. 2015). We determined how TRF impacted the day/night rhythms in the RR- and QT-intervals in control mice or mice that had Bmal1 deleted from cardiomyocytes.

Materials and Methods

Animals.

All animal procedures were conducted in compliance with the guidelines of the Association for Assessment and Accreditation of Laboratory Animal Care and were approved by the Institutional Animal Care and Use Committee at the University of Kentucky. The mice used in these studies were male C57BL/6 mice that were 11–12 months old. The iCSΔBmal1 mice were generated by crossing the floxed Bmal1 (Bmal1f/f) mouse and the cardiac-specific, Myh6-MerCreMer recombinase mouse (Storch et al. 2007; Schroder et al. 2013). The control mice in this study consisted of vehicle injected iCSΔBmal1+/+ mice. The iCSΔBmal1−/− mice were generated by activating Cre-recombination with intraperitoneal injections of tamoxifen (2 mg/day) for 5 consecutive days. The tamoxifen concentration and duration of the injections used causes effective cardiomyocyte-specific recombination without any obvious long-term effects as assessed by changes in the structure, function, and the ECG (Eckardt et al. 2004; Andersson et al. 2009; Hall et al. 2011; Schroder et al. 2013). Studies were performed ~6 months after the last injection and mice were housed in 12 h light and dark cycles.

In vivo Telemetry.

We used telemetry to measure activity and ECG. Briefly, male mice were anesthetized with isoflurane and TA11ETA-F10 (Data Science International) transmitter units were implanted in the peritoneal cavity. Activity is measured as the number of counts at 1 Hz. Counts are arbitrary units (AU) derived from the signal strength of the telemetry probe. ECG telemetry was performed as described previously (Schroder et al. 2013). ECG telemetry data were recorded at 1000 Hz. The method for RR- and QT-interval analysis was done as previously described (Schroder et al., 2014). To measure the QT-interval in mice, the ECG traces recorded for each hour were aligned to the peak of the R wave to generate an average trace, the start of the QRS complex was defined as the base of the QRS complex (where the slope of the profile changed from negative to positive). The end of the T-wave was defined at the point where the ECG returned 75% of the way from the minima of the T wave to the isoelectric level (Batchvarov et al. 1998; Schroder et al. 2014).

We quantified day/night rhythms in RR- and QT-intervals by fitting 4 consecutive days to the nonlinear sinusoidal model: Interval = A·cos[2π(t − τ)/T] + m, to calculate the period (T), the time between the peak amplitudes; phase (τ), timing of the peak rhythm in reference to the onset of the light cycle (ZT = 0); the amplitude (A), one-half of the peak to trough levels, and rhythm adjusted mean (m) (Schroder et al. 2014; Schroder et al. 2015).

We assessed the dependence of the QT- and RR-interval by calculating the slope of the linear relation between the log-transformed hourly averages in the QT- and RR100-interval (RR100 = RR interval/100 ms) (Mitchell et al. 1998). This approach is similar to that previously used to determine the exponential used to calculate the heart rate corrected QT-interval using ECG telemetry in mice. A steeper slope indicates a greater dependence of the QT-interval on the RR-interval.

Time Restricted Feeding.

Mice are nocturnal and eat ~80% of their caloric intake during the dark cycle in ad libitum feeding conditions (ALF) (Damiola et al. 2000; Arble et al. 2009). We used an established light cycle time restricted feeding (TRF) protocol to uncouple the circadian clock mechanism in the SCN and the heart (Damiola et al. 2000; Schroder et al. 2014). On the first day of the TRF protocol, food was removed at 10 h after the beginning of the light cycle (Zeitgeber time or ZT = 9). On the next day, the presentation of the food was restricted for 7 h during the light cycle between ZT 2–9. The mice were acclimated to TRF for 2-weeks before quantifying the ECG. Based on previous reports, 2-weeks is sufficient time to allow the re-alignment of peripheral circadian clocks to achieve a new steady state (Damiola et al. 2000; Schroder et al. 2014). Previous studies show mice undergoing a similar TRF protocol do not show significant differences in food consumption over the 24-hour cycle (Bray et al. 2013). Similarly, we performed a TRF study in 12 week old C57/Bl6 mice. Mice were divided into two cohorts; one part was maintained under ALF conditions and the other underwent TRF. Food intake was not different between the two cohorts (3.7±0.13 ALF; 3.8±0.20 TRF; n=32 mice/cohort) after 2 weeks.

Statistical analysis.

The statistical JTK_CYCLE package was used to confirm that the day/night rhythms in the RR- and QT-intervals of the individual mice cycled with a periodicity of ~24 h (Hughes et al. 2010). ECG data were analyzed using a two-way ANOVA to identify significant interactions (PRISM, MathWorks). We used the Šidák correction method to correct for multiple comparisons. Activity data were analyzed using a one-way ANOVA and multiple comparisons were corrected for using the Tukey correction. Data in the figures are presented as the mean ± SEM.

Results

TRF for 2-weeks does not alter body mass but causes a loss in the activity difference between dark and light cycle

Two weeks after TRF, there was no difference in the average 24-hour food consumption or body mass between iCSΔBmal1+/+ and iCSΔBmal1−/− mice. The daily food consumption in iCSΔBmal1+/+ and iCSΔBmal1−/− mice during TRF was 4.0 ± 0.2 and 4.1 ± 0.2 grams, respectively (p>0.05, n = 4–5). The average body mass of iCSΔBmal1+/+ and iCSΔBmal1−/− mice before TRF was 42.0 ± 0.9 g and 41.1 ± 1.2 g, respectively, and 44.2 ± 0.9 g and 42.6 ± 1.4 g during TRF, respectively (p>0.05, n = 4–5). We measured activity in conscious free moving mice using telemetry. Compared to ALF, TRF did not impact total activity in iCSΔBmal1+/+ or iCSΔBmal1−/− mice measured over the 24-hour cycle. During ALF, iCSΔBmal1+/+ and iCSΔBmal1−/− mice were more active during the dark cycle (light cycle iCSΔBmal1+/+ activity = 3.9 ± 0.6 AU; dark cycle iCSΔBmal1+/+ activity = 6.4 ± 0.9 AU, p<0.05, n = 4; light cycle iCSΔBmal1−/− activity = 2.5 ± 0.4 AU, dark cycle iCSΔBmal1−/− activity = 4.5 ± 0.3 AU, p<0.0.5 n = 5 mice). These differences in activity between the light and dark cycle were lost during TRF (light cycle iCSΔBmal1+/+ activity = 4.6 ± 0.7 AU, dark cycle iCSΔBmal1+/+ activity = 4.7 ± 0.1 AU, p>0.05, n = 4; light cycle iCSΔBmal1−/− activity = 3.8 ± 0.7 AU, dark cycle iCSΔBmal1−/− activity = 3.4 ± 0.7 AU, p>0.0.5 n = 5 mice).

TRF prolongs the dark cycle RR-interval in iCSΔBmal1+/+ and iCSΔBmal1−/− mice.

We performed in vivo ECG telemetry in iCSΔBmal1+/+ and iCSΔBmal1−/− mice, and calculated the daily RR-interval by averaging the RR-interval measured for 4 consecutive days in ALF or TRF. (Figure 1A). The average in the RR-interval measured from iCSΔBmal1+/+ mice was not different from the average in the RR-interval measured from iCSΔBmal1−/− mice. However, the average in the RR-interval measured from iCSΔBmal1+/+ or iCSΔBmal1−/− mice was longer during TRF compared to ALF (Figure 1B).

Figure 1. Time restricted feeding prolongs the RR-interval during the dark cycle in iCSΔBmal1+/+ and iCSΔBmal1−/− mice.

Figure 1.

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A. Raw ECG traces recorded from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF during the light or dark cycle (n = 4–5). Horizontal dashed lines are the zero potential line B. The graphs show the average daily RR-intervals or C. average RR-interval measured during the light or dark cycle from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (*p<0.05, **p<0.01, ***p<0.001, ****p<0.0001).

We measured the average light cycle (ZT 0–12 h) and dark cycle (ZT 12–0h) RR-interval from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (Figure 1C). The light or dark cycle RR-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice was not different. In ALF, the light cycle RR-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice was longer than the dark cycle RR-interval. TRF reversed this. TRF caused the dark cycle RR-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice to become longer than the light cycle RR-interval. Together the data show that TRF prolonged the average RR-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice by selectively increasing the RR-interval measured during the dark cycle.

iCSΔBmal1−/− mice have longer QT-intervals and TRF prolongs the dark cycle QT-interval in iCSΔBmal1−/− mice more than iCSΔBmal1+/+ mice.

We calculated the average in QT-intervals by averaging the hourly QT-interval measured for 4 consecutive days from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (Figure 2A). The average QT-interval measured from iCSΔBmal1−/− mice was longer than the average QT-interval measured from iCSΔBmal1+/+ mice. For iCSΔBmal1+/+ mice, the average QT-interval measured in ALF or TRF was not different (Figure 2B). In contrast, the average QT-interval measured from iCSΔBmal1−/− mice during TRF was longer than the average QT-interval measured from mice in ALF.

Figure 2. Time restricted feeding to the light cycle augments the QT-interval prolongation during the dark cycle in iCSΔBmal1−/− mice.

Figure 2.

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A. Hourly averaged ECG signals generated from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (n = 4–5). The start of the Q wave was defined as the base of the QRS complex and the T point was defined as where the ECG returned 75% of the way from the minima of the T wave to the isoelectric level (vertical dashed lines). Horizontal dashed lines are the zero potential line and the scale bar is 0.2 mV and 25 ms. B. The data show the average daily QT-intervals or C. average QT-interval measured during the light or dark cycle from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (*p<0.05, **p<0.01, ***p<0.001, ****p<0.0001).

We measured the average QT-intervals during the light and dark cycle for iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF (Figure 2C). The light and dark cycle QT-intervals measured from iCSΔBmal1−/− mice were longer than the corresponding light and dark cycle QT-intervals measured from iCSΔBmal1+/+ mice. For both iCSΔBmal1+/+ or iCSΔBmal1−/− mice, the light cycle QT-interval was longer than the respective dark cycle QT-interval measured in ALF. For iCSΔBmal1+/+ mice, TRF eliminated the difference in the QT-interval between the light and dark cycle. For iCSΔBmal1−/− mice, TRF caused a larger increase in the dark cycle QT-interval than the dark cycle QT-interval measured from iCSΔBmal1+/+ mice. Together the data show the QT-intervals measured from iCSΔBmal1−/− mice in ALF or TRF are longer than the QT-intervals measured from iCSΔBmal1+/+ mice. TRF increased the dark cycle QT-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice, but TRF caused a larger increase in the dark cycle QT-interval for iCSΔBmal1−/− mice to increase the mean QT-interval during TRF.

The phase shift in the day/night rhythm of the RR- and QT-intervals with TRF is not disrupted in iCSΔBmal1−/− mice.

To better understand how the RR- and QT-interval changed with respect to one another, we quantified the day/night rhythms in the RR- and QT-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice by plotting hourly RR- and QT-intervals as a function of ZT for 4 days (Figure 3A). Individual mouse data were fit to a nonlinear sinusoidal model to calculate the average period, rhythm adjusted mean, amplitude, and acrophase.

Figure 3. Light cycle-restricted feeding realigns the peak day/night rhythms in the RR- and QT-intervals of iCSΔBmal1+/+ and iCSΔBmal1−/− mice to the dark cycle.

Figure 3.

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A. Hourly averages in RR- or QT-intervals measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice (n=4–5) in ALF or TRF conditions (solid bars bars). Averages are plotted as a function of ZT for 4 days. Shaded regions denote the dark cycle. B. The individual mouse data were fit with a nonlinear sinusoidal model to calculate the average rhythm adjusted mean, amplitude and acrophase. The bar graphs show the mean (left), amplitude, and acrophase for the RR-and C. QT-intervals (*p<0.05, **p<0.01, ***p<0.001, ****p<0.0001).

The day/night rhythms in the RR- and QT-interval measured from iCSΔBmal1+/+ and iCSΔBmal1−/− mice followed an ~24 h period in all the conditions tested (data not shown). Compared to ALF, TRF increased the rhythm adjusted mean and amplitude in RR-interval for both iCSΔBmal1+/+ and iCSΔBmal1−/− mice. TRF also shifted the acrophase of the day/night RR-interval rhythm measured for iCSΔBmal1+/+ and iCSΔBmal1−/− mice by ~16 h (Figure 3B). Compared to ALF, TRF in iCSΔBmal1+/+ mice did not change the rhythm adjusted mean or amplitude in the QT-interval, but it shifted the acrophase by ~16 h. Compared to ALF, TRF in iCSΔBmal1−/− mice increased the rhythm adjusted mean and amplitude in the QT-interval, and it shifted the acrophase by ~16 h. Compared to iCSΔBmal1+/+, TRF selectively increased the amplitude of the QT-interval in iCSΔBmal1−/− mice.

The loss of Bmal1 in cardiomyocytes increases QT-interval prolongation at slow heart rates

The data suggest that compared to iCSΔBmal1+/+ mice, the QT-interval in iCSΔBmal1−/− is more sensitive to changes to the RR-interval. We calculated the power relation in the hourly QT- and RR-interval for iCSΔBmal1+/+ and iCSΔBmal1−/− mice. We plotted the hourly LN QT-interval as a function of the respective hourly LN RR100-interval measured during ALF, TRF only, or ALF and TRF together for each mouse and performed linear regression to calculate the slope factor (Figure 4). Linear regression analysis showed that there was not a difference in the slope factors for iCSΔBmal1+/+ and iCSΔBmal1−/− mice in ALF or TRF alone. However, when the data from the ALF and TRF conditions were combined linear regression analysis showed that the iCSΔBmal1−/− mice had a steeper slope factor compared to iCSΔBmal1+/+ mice. The data suggest that TRF-induced slowing of the RR-interval and caused the QT-interval measured from iCSΔBmal1−/− mice to increase more than iCSΔBmal1+/+ mice. Therefore, we conclude that an important function of Bmal1 in cardiomyocytes is to limit the QT-interval prolongation at slower heart rates.

Figure 4. The hourly QT-RR100 relations are steeper in iCSΔBmal1−/− mice.

Figure 4.

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The graphs on the left shows the LN of the hourly QT-interval plotted as a function of the corresponding LN of the RR100-intervals for all the data measured from the iCSΔBmal1+/+ and iCSΔBmal1−/− mice in A. ALF conditions only, B. TRF conditions only, or C. ALF and TRF conditions together (n = 4–5). The individual mouse LN QT-interval vs. LN RR100-interval were fit with a linear regression to quantify the slope factor. The right graphs show the average slopes calculated from iCSΔBmal1+/+ and iCSΔBmal1−/− mice in A. ALF conditions only, B. TRF conditions only, or C. ALF and TRF conditions together (**p<0.01).

Discussion

This study showed that TRF prolonged the RR-interval during the dark cycle which increased the mean/amplitude and shifted the phase of the day/night rhythm in the RR-interval. TRF caused only a modest prolongation in the QT-interval during the dark cycle. TRF did not alter the day/night rhythm in the QT-interval mean or amplitude, but it shifted the phase similar to that seen with the RR-interval. Inducing the deletion of Bmal1 in adult cardiomyocytes did not prevent the phase shifts in the day/night rhythms in RR- or QT-intervals with TRF. However, it increased the prolongation of the QT-interval during the dark cycle. The increased QT-interval prolongation during the dark cycle caused an increase in the mean and amplitude of the QT-interval day/night rhythm. We conclude that the cardiomyocyte circadian clock does not drive changes in the day/night rhythm in heart rate or ventricular repolarization with TRF, but TRF unmasked an important role for the cardiomyocyte circadian clock in limiting QT-interval prolongation at slower heart rates.

Time of feeding is an important modifier of the day/night rhythm of heart rate independent of the cardiomyocyte circadian clock

It is postulated that the day/night rhythm in heart rate is controlled, in part, by the SCN regulating autonomic signaling to the heart (Scheer et al. 2001; Tong et al. 2013). Recent studies also suggest a possible role for the cardiomyocyte circadian clock (D’Souza et al. 2021; Hayter et al. 2021). TRF shifts the day/night rhythm in heart rate to align with feeding, but it does not impact the phase of the circadian clock mechanism in the SCN (Damiola et al. 2000; Schroder et al. 2014). Our data show TRF shifts the day/night rhythm in heart rate in mice after disrupting the cardiomyocyte circadian clock mechanism by inducing the deletion of Bmal1. We conclude that the circadian clock mechanism in the SCN or circadian clock mechanisms in cardiomyocytes are not the primary drivers for changes in the day/night rhythm in heart rate with TRF, and TRF modifies the day/night rhythm in heart rate by other mechanisms sensitive to timing of feeding. These mechanisms could include, but are not limited to, signaling from other nuclei in the brain that respond to changes in timing of feeding behavior and regulate autonomic signaling to the heart, changes in metabolism, or alterations in body temperature. (Damiola et al. 2000; Sheward et al. 2010; Schroeder et al. 2011; Bray et al. 2013; Schroder et al. 2014; Schibler et al. 2015).

The cardiomyocyte circadian clock limits QT-interval prolongation at slow heart rates

Previous studies show inducing the deletion of Bmal1 in adult cardiomyocytes can decrease mean heart rate without altering the phase or amplitude (Schroder et al. 2013; Schroder et al. 2021). The absolute decrease in heart rate in these mice is small. In this study we did not observe a significant difference in the heart rates between control mice or mice after inducing the deletion of Bmal1. A possible reason for this discrepancy might be the mice used in this study were older than the previous studies (3–4 months vs. 11–12 months) and this difference might decrease with age. Other ECG telemetry studies show that the non-inducible heart specific Bmal1 deletion mouse model (αMHCCREBmal1Fl/Fl) show substantial lengthening and variability in the RR-interval compared to control mice (Gottlieb et al. 2021; Hayter et al. 2021). These differences in mouse models suggest Bmal1 might have impacts on cardiac development resulting in a more pronounced prolongation in the RR-interval.

Several studies show that that the deletion of Bmal1 in adult cardiomyocytes increases the QT-interval (Schroder et al. 2015; Gottlieb et al. 2021; Schroder et al. 2021). The mechanism for this is likely secondary to changes in cardiac ion channel expression (Delisle et al. 2021). Bmal1 directly or indirectly regulates the functional expression of several ion channels important for cardiac repolarization (Schroder et al. 2015; Schroder et al. 2021). Most recently, Gottlieb and colleagues (2021) show that deletion of Bmal1 in cardiomyocytes prolongs the ventricular action potential duration (a cellular surrogate for the QT-interval) (Gottlieb et al. 2021). Consistent with these previous studies, the QT-intervals measured from mice that had Bmal1 deleted from cardiomyocytes in this study were longer than the QT-intervals recorded from control mice. The deletion of Bmal1 in adult cardiomyocytes showed a more pronounced prolongation of the QT-interval measured at the slower heart rates caused by TRF. The data suggest that an important function of the cardiomyocyte clock is to limit QT-interval prolongation at slow heart rates.

Abnormal prolongations of the QT-interval are associated with an increased risk for ventricular arrhythmias and sudden cardiac death (Postema et al. 2014). If the cardiomyocyte circadian clock function is similar in mice and humans, then disruptions of the cardiomyocyte circadian clock mechanism in people could lead to abnormal QT-interval prolongation at slower heart rates. An increase in QT-interval prolongation at slow heart rates is likely to be pro-arrhythmic. Consistent with this concept, people with the deadly congenital long QT syndrome show disproportionate prolongation in the QT-interval at slower heart rates (Merri et al. 1992).

A limitation of this study is we did not measure the impact that the timing of feeding had on the heart rate and QT-intervals in constant darkness. A recent study by Zhang and colleagues (2021), using blood pressure telemetry, showed that the timing of food intake drives the day/night rhythm in heart rate in constant darkness (Zhang et al. 2021). In their study, the mice were acclimated to constant darkness and mice in ALF were compared to mice presented with food between circadian time 0 and 12. The peak in the 24-hour rhythm of heart rate aligned to the timing of food access. These data suggest the day/night rhythm in heart rate aligns to feeding behavior independent of the light cycle.

In summary, our data suggest feeding behavior causes large changes in day/night heart rate and ventricular repolarization in mice. The changes in heart rate and ventricular repolarization do not depend on the cardiomyocyte circadian clock mechanism. However, these studies demonstrate that the cardiomyocyte circadian clock mechanism limits excessive QT-interval prolongation at slow heart rates. It should be noted that, even though mouse models are practical for testing how genetic and behavioral manipulations impact circadian clocks and cardiac electrophysiology, there are clear species-specific differences in cardiac electrophysiology between mice and humans (Sabir et al. 2008; Nerbonne 2014; Dobrev et al. 2018). Whether or not the disruption in the cardiomyocyte circadian clock mechanism or restricted feeding shows similar impacts in day/night rhythms of heart rate or ventricular repolarization in people will require further investigation.

Acknowledgements

This work was supported by National Heart Lung and Blood Institute grants R01HL153042 and R01HL141343.

Footnotes

Declaration of interest statement

None.

Data availability

Data will be made available by the authors upon request.

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