Circadian regulation of sinoatrial nodal cell pacemaking function: Dissecting the roles of autonomic control, body temperature, and local circadian rhythmicity

Abstract

Strong circadian (~24h) rhythms in heart rate (HR) are critical for flexible regulation of cardiac pacemaking function throughout the day. While this circadian flexibility in HR is sustained in diverse conditions, it declines with age, accompanied by reduced maximal HR performance. The intricate regulation of circadian HR involves the orchestration of the autonomic nervous system (ANS), circadian rhythms of body temperature (CRBT), and local circadian rhythmicity (LCR), which has not been fully understood. Here, we developed a mathematical model describing ANS, CRBT, and LCR in sinoatrial nodal cells (SANC) that accurately captures distinct circadian patterns in adult and aged mice. Our model underscores how the alliance among ANS, CRBT, and LCR achieves circadian flexibility to cover a wide range of firing rates in SANC, performance to achieve maximal firing rates, while preserving robustness to generate rhythmic firing patterns irrespective of external conditions. Specifically, while ANS dominates in promoting SANC flexibility and performance, CRBT and LCR act as primary and secondary boosters, respectively, to further enhance SANC flexibility and performance. Disruption of this alliance with age results in impaired SANC flexibility and performance, but not robustness. This unexpected outcome is primarily attributed to the age-related reduction in parasympathetic activities, which maintains SANC robustness while compromising flexibility. Our work sheds light on the critical alliance of ANS, CRBT, and LCR in regulating time-of-day cardiac pacemaking function and dysfunction, offering insights into novel therapeutic targets for the prevention and treatment of cardiac arrhythmias.

Fig 1. Model schematic for ANS, CRBT and LCR in mouse SANC.

During each heartbeat, SANC FR is determined by coupled interactions between the MO linked to sarcolemmal ionic currents, and the CO associated with SR Ca²⁺ release (J_Rel) and uptake (J_up). Over the course of a day and night cycle, the properties of MO and CO in SANC are tightly regulated by the ANS, CRBT, and LCR to generate circadian variations in FR. The ANS is regulated by the master circadian clock—SCN. Simultaneous SNA (orange dots) and PNA (blue dots) co-modulate a diverse range of subcellular targets in SANC and exhibit non-additive effects via cAMP-PKA dependent or independent pathways [6]. CRBT regulates the autonomic balance between SNA and PNA, influencing the kinetics and/or conductance of ion channels, exchangers, and pumps. LCR (green dots) within the SANC nucleus leads to circadian variations in the expression levels of ion channels, e.g., I_HCN. In aged mice, SANC pacemaking function is disrupted by aging-dependent ion channel remodeling, intracellular Ca²⁺ cycling alternation, PNA impairment, and CRBT disruption (red dots) [,,–41]. I_Na1.1, Na⁺ channel isoform Na_v1.1 current; I_Na1.5, Na⁺ channel isoform Na_v1.5 current; I_st, sustained inward Na⁺ current; I_Nab, background Na⁺ current; I_to, transient component of 4-Aminopyridine-sensitive current; I_sus, sustained component of 4- Aminopyridine-sensitive current; I_Kr, rapid delayed rectifying K⁺ current; I_Ks, slow delayed rectifying K⁺ current; I_K1, inward rectifier K⁺ current; I_Kb, background K⁺ current; I_KACh, muscarinic K⁺ current; I_CaL, L-type Ca²⁺ channel current; I_CaT, T-type Ca²⁺ channel current; I_NCX, Na⁺/Ca²⁺ exchanger current; I_Cab, background Ca²⁺ current; I_NaK, Na⁺/K⁺ pump current; I_HCN, hyperpolarization-activated cyclic nucleotide–gated channel current; JSR, junctional sarcoplasmic reticulum; NSR, network sarcoplasmic reticulum; J_diff, Ca²⁺ diffusion flux from subspace to cytoplasm compartment; J_Rel, Ca²⁺ release from JSR to subspace compartment; J_tr, Ca²⁺ transfer flux from NSR to JSR; J_up, SERCA Ca²⁺ pump flux. The CRBT icon is derived from https://openclipart.org/detail/231080/thermometer, while the circadian clock icon is modified based on https://openclipart.org/detail/198766/mono-tool-timer.

Fig 2. Quantitative reconstruction of diverse circadian patterns in SANC FR under various conditions.

(A) Averaged circadian HR fluctuations in anesthetized mice (BT = 37°C) with 12-h light cycles before (control; grey dashed line) and after SNA blockade (blue dashed line), PNA blockade (yellow dashed line), and ANS blockade (green dashed line) [38]. Circadian fluctuations were fitted using a sine function as described in [38] and normalized to HR at ZT6 with ANS blockade. (B) Simulated circadian FR patterns in a single mouse SANC (BT = 37°C; normalized to FR at ZT6 with ANS blockade) closely recapitulate experimental findings [38]. (C-D) Model simulations accurately reproduce minimal and maximal time-of-day FR (C), and normalized circadian amplitudes of FR (D) under control, SNA blockade, PNA blockade, and ANS blockade conditions. The normalized amplitude values are obtained by dividing circadian FR amplitudes in beats per minute (BPM) by the mean time-of-day FR. (E) Either additive or unidirectional modulation effects alone are insufficient to accurately reproduce time-of-day FRs under the control conditions. (F-G) Circadian rhythmicity of BT (36~38°C) enhances the circadian amplitude of FR under control conditions (grey dots) (F) with simulated Q₁₀ = 2 in agreement with experimental studies (G) (15). (H-I) In aged model, simulated maximal FR reduction (25%; red dots) (H) is consistent with experimental findings (I) [25].

Fig 3. The alliance of ANS, CRBT and LCR is essential to achieve circadian flexibility and performance while preserving robustness in SANC automaticity.

(A) CO-MO parameter space maps color-coded by FR in BPM at ZT6 and ZT18 under control conditions in adult model. (B) Steady-state SANC membrane potential oscillations using parameter settings sampled from panel (A; labeled 1 to 6). (C-G) CO-MO parameter space maps at ZT6 and ZT18 under (CRBT-/-) (C), (CRBT-/-; LCR-/-) (D), (CRBT-/-; ANS-/-) (E), (CRBT-/-; SNA-/-) (F), and (CRBT-/-; PNA-/-) (G) conditions. (H-J) Quantification of SANC flexibility (H), performance (I), and robustness (J) under various conditions. (K) Normalized changes in SANC flexibility, performance, and robustness compared to the control conditions in adult model.

Fig 4. Quantitative dissection of SANC pacemaking dysfunction in aging.

(A-E) CO-MO parameter space maps at ZT6 and ZT18 under aged (A), aged MO (B), aged PNA (C), aged CO (D), and aged MO+PNA+CO (E) conditions. (F-H) Quantitative differences in flexibility (F), performance (G), and robustness (H) under various aging conditions. (I) Normalized changes in SANC flexibility, performance, and robustness compared to the control conditions in adult model.

Fig 5. Distinct mechanisms underlying circadian patterns of SANC FR in adult and aged mice.

(A-B) Relative contributions to SANC flexibility and performance are quantified by selective inhibition of each module in the adult (A) and aged (B) models. (C) Relative contributions to aging-dependent reduction in SANC flexibility and performance are quantified by selective inhibition of each aging-dependent change in the aged model (red box).

Fig 6. Illustrative trajectories in the parameter space of SANC flexibility, performance and robustness.

(A) Uphill trajectories show the transition from baseline (grey circle) to adult (CRBT + ANS + LCR; grey dot) model states. (B) Downhill trajectories demonstrate the transition from adult (grey dot) to aged model states (red dot).