Intrinsic diurnal variations in cardiac metabolism and contractile function

It was already known that cardiac function fluctuates with the time of day, largely due to external influences like sympathetic activity. However, whether these fluctuations stem from intrinsic heart properties remained unclear. Prior research has shown that circadian rhythms affect various physiological systems, but the heart’s own metabolic and contractile rhythms had not been directly studied. This gap motivated the investigation of whether the heart itself exhibits diurnal changes in function and metabolism. No prior work had resolved whether these changes are intrinsic or solely driven by external factors. The study aimed to determine if the heart’s metabolic and contractile behavior varies over a 24-hour cycle without external influences. Researchers also sought to explore how these rhythms relate to the expression of metabolic genes. The findings could clarify whether the heart’s daily adaptations are lost in pathological states like hypertrophy.

The aim of the study was to examine whether the heart exhibits intrinsic diurnal variations in metabolism and contractile function. The researchers focused on the isolated working rat heart to eliminate external influences like sympathetic activity. They sought to determine if metabolic flux and contractile performance change over a 24-hour period. The study also aimed to investigate how these changes correlate with the circadian expression of metabolic genes. Researchers wanted to assess whether these rhythms persist in a hypertrophied heart. The motivation came from the lack of prior studies on intrinsic cardiac rhythms. By isolating the heart, they could observe its natural behavior without external modulation. The findings could help understand how cardiac function adapts to environmental changes and what happens when this adaptability is lost.

The researchers used isolated working rat hearts perfused with a physiological solution to maintain function. They measured contractile performance, carbohydrate oxidation rates, and oxygen consumption at different times of day. Fatty acid oxidation levels were also monitored to compare with carbohydrate metabolism. The expression of metabolic genes was analyzed using RNA sequencing at various time points. Hearts were tested under normal conditions and after inducing pressure overload to simulate hypertrophy. The study compared metabolic gene expression patterns between normal and hypertrophied hearts. Researchers tracked rhythmic changes in gene activity related to carbohydrate and fatty acid metabolism. The experimental design allowed them to assess intrinsic cardiac rhythms without external influences.

Contractile performance peaked in the middle of the night, with significant variation in carbohydrate oxidation and oxygen consumption. Fatty acid oxidation remained relatively stable throughout the day. Metabolic gene expression showed diurnal variation, with most genes peaking at night. Genes regulating carbohydrate utilization and mitochondrial function exhibited rhythmic patterns. Pressure overload-induced hypertrophy eliminated these diurnal gene expression patterns. Hypertrophied hearts showed no rhythmic fluctuations in metabolic gene activity. The loss of rhythmicity suggests a diminished ability to adapt to physiological changes. These findings indicate that normal hearts anticipate and respond to daily environmental changes.

The study suggests that the normal heart exhibits intrinsic diurnal variations in metabolism and contractile function. These changes appear to be driven by circadian gene expression patterns. The heart’s ability to anticipate and adapt to physiological changes is lost in hypertrophy. The loss of metabolic gene rhythmicity in hypertrophied hearts may contribute to contractile dysfunction. The findings highlight the importance of circadian regulation in cardiac physiology. Researchers propose that the hypertrophied heart loses plasticity, which could worsen dysfunction. The study does not suggest that these rhythms are essential for normal function but that their disruption may be harmful. The results emphasize the need to consider circadian factors in cardiac disease models.