Heartbeat music

The heart is a central player within a hierarchical system of clocks operating within an autonomic neurovisceral axis that creates and synchronizes rhythmic functions ranging from milliseconds to days and beyond.¹

Neurons within the ganglia embedded on the epicardial surface—“the Little Brain of the Heart”²—extend into to the sinoatrial node (SAN), encountering and embracing SAN pacemaker cells. Afferent electrochemical and mechanical “vibrations” emerge from the SAN to communicate with other components of the neurovisceral axis on a beat-to-beat basis informing on neurotransmitter input required for the heart-brain grand symphony (H-BGS) that emerges when the heart beats.^1,2 The H-BGS is composed of variations that differ in tempo and timing; owing to variable degrees of synchronization of brain-heart clock periods,^3,4 these variations are broadcast to the body surface on time, frequency, nonlinear, and heart rate (HR) fragmentation electrocardiogram “channels.” D’Souza et al⁵ of Mark R. Boyett Lab in the United Kingdom as well as numerous European collaborators tuned into “H-BGS” Circadian Variations (“Eine Kleine Nachtmusik”) to discover whether the brain or the heart conducts the Circadian Variations: the vigilant, hypothalamic “clock genes” or “cardiac clock genes”, coding for proteins that operate the SAN cell ultradian “coupled-clock system variations” (C-CSVs).^3,4,6–9

C-CSVs emerge when the sarcoplasmic reticulum, a “Ca²⁺ clock”, that continuously cycles Ca²⁺ via a criticality mechanism,^6–10 couples to an ensemble of surface membrane ion channels (“M clock”, operating on a limit-cycle mechanism¹⁰ to produce current oscillations that underlie action potentials (APs). C-CSVs, are the most beautiful variations of H-BGS¹¹ and kinetic transitions in Ca²⁺ and membrane potential (MP) during each AP cycle are the most basic motifs within the C-CSVs.^8,9 The AP firing rate and rhythm informon the degree to which Ca²⁺ and MP transitions are synchronized during AP cycles.^8,9 These Ca²⁺ and voltage transitions throughout AP cycles are cues that not only regulate the activation-inactivation kinetics of clock proteins in a time-dependent manner, but are also regulated by the activation states of the very proteins they regulate. In other terms, C-C proteins “dance to their own music” during each AP cycle. The presence of beat-to-beat (and diurnal) variability of AP cycle intervals in vitro, or in HR in vivo, indicates that C-C system functions never achieve true steady states from one beat to the next or throughout the day and night.⁹

Although heart rates (HRs) of species from mice to humans differ by 10-fold, kinetics of Ca²⁺ and MP transitions throughout AP cycles are self-similar to each other across species.⁸ And because the kinetics of these Ca²⁺ and MP transitions determine the mean AP cycle interval,^3,4,6–9 kinetic transitions in SAN cells across species and AP cycle intervals in vitro, and HRs in vivo, are self-similar to each other, that is, they obey a power law.⁸

A recent discovery has added an entire new layer of complexity to C-C variations that extends well beyond individual isolated SAN cells.¹² Specifically, local oscillatory Ca²⁺ signals that are heterogeneous in phase, amplitude, and frequency occur within a meshwork of HCN4 expressing cells that extends nearly the entire length and depth of the central mouse SAN.¹²

“Neuronal Variations” via vagal and sympathetic “motifs” modulate the tempo and timing of the SAN C-CSVs on millisecond timescales by affecting the degree to which criticality and limit-cycle mechanisms of “Ca²⁺ and M clocks”¹⁰ are synchronized or coupled during AP cycles and, on this basis, modulate the SAN cell AP firing rate and rhythm.^3,4,6–9

D’Souza et al⁵ used pharmacological, surgical, and genetic tools in order to ablate Neuronal Variations within the H-BGS, thereby exposing the SAN C-CSVs. They measured diurnal expression of SAN C-CS genes and confirmed marked diurnal expression of genes coding for ion channels of the “M clock” (for review, see reference 5) and discovered that both HCN4 channels of the M clock, activated by cyclic-AMP (cAMP),¹³ and calcium-calmodulin activated kinase II (CAMKII), which phosphorylates proteins within both “M and Ca²⁺ clocks”,^3,4,6–9 also manifest circadian variability. D’Souza et al demonstrated that the hyperpolarization-activated inward current, (I_f), which they believed to be the “hallmark “of the C-CSVs, also manifests circadian variability, and that ablation of the cardiac βmal1 gene abolished circadian variability of HCN4 and I_f.⁵ Similar links between cardiac circadian genes and functions intrinsic to the ultradian C-CS of SAN cells had been noted previously (for review, see reference 4).

So, what does I_f contribute to the C-CSVs? One predominant view had been that I_f was the pacemaker¹³ of the C-CS system: acceleration of the HR in response to β-adrenergic receptor stimulation was attributed to an increase in I_f activation by cAMP,¹³ suggesting that I_f plays on the C-CS “offense.” But knock-in of HCN channels that are insensitive to cAMP,¹⁴ pharmacological inhibition of I_f, or genetic ablation of HCN4 (for review, see reference 15) and I_f reduction in parametric sensitivity analyses in numerical models¹⁶ all result in sinus bradycardia, or sinus arrest, and chronotropic incompetence,¹⁴ but do not have much of an effect on the b-adrenergic receptor stimulation-induced increase in rate at which the C-CS fires APs.^14–16 These data^14–16 point to the SAN cell “Ca²⁺ clock” as the C-CS “quarterback” and to I_f as a indenspensible “middle linebacker” of the C-CS “defense.”

So, in reality, to achieve nighttime bradycardia, nature arranges that circadian variations occur within both “M and Ca²⁺ clocks” of the C-CS intrinsic to SAN cells.^3,4,6–9 These circadian changes partially uncouple^3,4,6–9 the criticality and limit-cycle mechanisms¹⁰ within the C-CS. The reduction in I_f facilitates nighttime bradycardia by weakening the C-CS “defense” against clock uncoupling.^14–16 Concurrent shifts in “vagal motifs” within Neuronal Variations of H-BGS effected by input from hypothalmic clock genes creates additional desynchronization within the C-CSV to further ensure nighttime bradycardia.⁴ It is likely that nature, in using mechanisms intrinsic to the SAN in conjunction with hypothalmic input, also partially desynchronizes “neighborhoods” of cells with the SAN^12,14 to guarantee nighttime bradycardia.