Circadian control of intrinsic heart rate via a sinus node clock and the pacemaker channel

Abstract

29In the human, there is a circadian rhythm in the resting heart rate and it is higher during the day in 30 preparation for physical activity. Conversely, slow heart rhythms (bradyarrhythmias) occur primarily 31 at night. Although the lower heart rate at night is widely assumed to be neural in origin (the result 32 of high vagal tone), the objective of the study was to test whether there is an intrinsic change in 33 heart rate driven by a local circadian clock. In the mouse, there was a circadian rhythm in the heart 34 rate in vivo in the conscious telemetrized animal, but there was also a circadian rhythm in the 35 intrinsic heart rate in denervated preparations: the Langendorff-perfused heart and isolated sinus 36 node. In the sinus node, experiments (qPCR and bioluminescence recordings in mice with a Per1 37 luciferase reporter) revealed functioning canonical clock genes, e.g. Bmal1 and Per1. We identified 38 a circadian rhythm in the expression of key ion channels, notably the pacemaker channel Hcn4 39 (mRNA and protein) and the corresponding ionic current (funny current, measured by whole cell 40 patch clamp in isolated sinus node cells). Block of funny current in the isolated sinus node 41 abolished the circadian rhythm in the intrinsic heart rate. Incapacitating the local clock (by cardiac-42 specific knockout of Bmal1) abolished the normal circadian rhythm of Hcn4, funny current and the 43 intrinsic heart rate. Chromatin immunoprecipitation demonstrated that Hcn4 is a transcriptional 44 target of BMAL1 establishing a pathway by which the local clock can regulate heart rate. In 45 conclusion, there is a circadian rhythm in the intrinsic heart rate as a result of a local circadian 46 clock in the sinus node that drives rhythmic expression of Hcn4. The data reveal a novel regulator 47 of heart rate and mechanistic insight into the occurrence of bradyarrhythmias at night. 483 Living things including humans are attuned to the 24 h day-night cycle and many biological 49 processes exhibit an intrinsic ~24 h (i.e. circadian) rhythm. In the human, the resting heart rate (in 50 the absence of physical activity) shows a circadian rhythm and is higher during the day when we 51 are awake 1,2 . The heart is therefore primed, anticipating the increase in demand during the awake 52 period. Conversely, bradyarrhythmias primarily occur at night 1,3 . Previously, the circadian rhythm in 53 heart rate in vivo has been attributed to the autonomic nervous system: to high sympathetic nerve 54 activity accompanying physical activity during the awake period and high vagal tone during the 55 sleep period 4 . This is partly based on heart rate variability as a surrogate measure of autonomic 56 tone 5-7 ; however, we have shown that heart rate variability cannot be used in any simple way as a 57 measure of autonomic tone 8 . Therefore, the mechanism underlying the circadian rhythm in heart 58 rate is still unknown. We asked the question whether there is a circadian rhythm in the intrinsic 59 heart rate set by the pacemaker of the he...