Effects of time-restricted exercise on activity rhythms and exercise-induced adaptations in the heart

Abstract

Circadian rhythms play a crucial role in the regulation of various physiological processes, including cardiovascular function and metabolism. Exercise provokes numerous beneficial adaptations in heart, including physiological hypertrophy, and serves to shift circadian rhythms. This study investigated the impact of time-restricted exercise training on exercise-induced adaptations in the heart and locomotor activity rhythms. Male mice (n = 45) were allocated to perform voluntary, time-restricted exercise in the early active phase (EAP), late active phase (LAP), or remain sedentary (SED) for 6 weeks. Subsequently, mice were allowed 24-h ad libitum access to the running wheel to assess diurnal rhythms in locomotor activity. Heart weight and cross-sectional area were measured at sacrifice, and cardiac protein and gene expression levels were assessed for markers of mitochondrial abundance and circadian clock gene expression. Mice rapidly adapted to wheel running, with EAP mice exhibiting a significantly greater running distance compared to LAP mice. Time-restricted exercise induced a shift in voluntary wheel activity during the 24-h free access period, with the acrophase in activity being significantly earlier in EAP mice compared to LAP mice. Gene expression analysis revealed a higher expression of Per1 in LAP mice. EAP exercise elicited greater cardiac hypertrophy compared to LAP exercise. These findings suggest that the timing of exercise affects myocardial adaptations, with exercise in the early active phase inducing hypertrophy in the heart. Understanding the time-of-day dependent response to exercise in the heart may have implications for optimizing exercise interventions for cardiovascular health.

Figure 1

Protocol for time-restricted exercise (TRE) implementation. Mice were housed with wireless running wheels available during the early active period (EAP) or late active period (LAP), or remained sedentary (SED), for a 6-week training period (n = 10 mice/group). At the conclusion of the training period, mice were given ad libitum access to the wheel for a 24-h period to determine the effects of time-restricted exercise training on activity rhythms. Hearts were excised to examine the effects of TRE on circadian clock genes and cardiac hypertrophy.

Figure 2

Activity patterns and wheel running volume during 6 weeks of TRE. Actogram representing the 6-week training period (n = 9–10 mice/group; A). Wheel data was summed in 10-min bins for EAP (yellow) and LAP (gray) mice. Shaded areas represent the dark cycle (ZT12-ZT24), wheels were locked during the light cycle (ZT0–ZT12). Average daily running distance was calculated as an average daily distance for each week of the 6-week training period for EAP and LAP mice (n = 9–10 mice/group; B). Distance represents only wheel activity during the 6-h period of wheel access. ^#Different from LAP, p < 0.05; *different from Week 1, p < 0.05.

Figure 3

Time-restricted exercise leads to a shift in activity acrophase between EAP and LAP mice. Actogram of a single 24-h period of free access to wheels after 6 weeks of TRE where activity was averaged into 1-h bins (n = 9–10 mice/group; A). Acrophase of activity in EAP and LAP mice determined via Cosinor analysis of activity data recorded in 10-min bins (n = 9–10; B). Total wheel running distance of EAP and LAP mice during the 24-h period of free wheel access (n = 9–10 mice/group; C). Distance run during the first 6 h of the active phase (ZT12–18) and the last 6 h of the active phase (ZT18–24) during ad libitum wheel access of EAP and LAP mice (n = 9–10 mice/group; D). Percent of total activity completed in the first 6 h of the active phase of EAP and LAP mice (n = 9–10 mice/group; E). ^#Different from LAP, p < 0.05; ^‡different from ZT12–ZT18, p < 0.05.

Figure 4

Clock gene expression in the heart was modestly impacted by time-restricted exercise. Expression of myocardial Bmal1, Clock, Per1, Per2, Reverb-α, Cry1, and Cry2 in RNA isolated from hearts of mice sacrificed at ~ ZT13.5, after 6 weeks of TRE (n = 4–5 mice/group). *Different from SED, p < 0.01; ^¥different from EAP, p < 0.05.

Figure 5

EAP exercise preferentially elicits cardiac hypertrophy. Absolute heart weight (A), heart weight standardized to body weight (B), heart weight standardized to tibia length (C) was determined in hearts isolated from mice after 6 weeks of TRE (n = 8–9 mice/group). Gene expression for insulin-like growth factor 1 (IGF-1) was elevated in both exercise groups (n = 4–5 mice/group; D). Cardiomyocyte cross-sectional area was increased in EAP and LAP hearts compared to SED (n = 5 mice/group; E). Scale bar is 20 µm. *Different from SED, p < 0.05.

Figure 6

Oxidative phosphorylation complex protein expression is modestly impacted by voluntary wheel running. Western blot quantification OXPHOS complexes I–V and representative western blot image (n = 9–10 mice/group). ^¥Different from EAP, p < 0.05.