Decreased pacemaker activity in aged sinoatrial node

National Institute On Aging

Investigators

Linked publications & trials

Abstract

Multiscale approach/ bridging in vivo and ex vivo approaches to uncover pacemaker aging mechanisms: To directly study intrinsic pacemaker variability with age, we developed from the ground up a set of analytical tools and workflows to handle video-based calcium imaging of intact sinoatrial node (SAN) tissue. This high-resolution ex vivo approach goes beyond simple frequency measurements, enabling detailed spatiotemporal mapping of calcium wave initiation and propagation across the SAN. Using these custom-built pipelines, we uncovered age-associated alterations, including shifts in the site of origin, conduction heterogeneity, and progressive fragmentation of calcium waves. To quantitatively assess these dynamics, we adapted and extended heart-rate variability (HRV) style analyses to calcium imaging, applying time-, frequency-, and non-linear-domain metrics—including SDRR, DFA α₂, and fragmentation indices—to extract measures of beat-to-beat variability and signal disorder at the tissue level. In parallel, in vivo telemetry ECG in freely moving young and old mice (dual-lead, temperature-monitored) provides a whole-animal perspective. By applying double autonomic blockade (atropine + propranolol) in both systems, we isolate intrinsic pacemaker function from autonomic influences. This integrated bridge between in vivo and ex vivo recordings is generating a multidisciplinary dataset that allows us to test whether age-related changes in SAN calcium signaling correspond to a state of 'pacemaker frailty.' Mechanistically, the project targets the coupled-clock system of pacemaker cells, where rhythmic sarcoplasmic reticulum Ca²⁺ release (the calcium clock) interacts with membrane ion channel dynamics (the membrane clock) to generate rhythmic impulses. Using spatiotemporal calcium imaging, we can dissect how these coupled oscillators are altered with age, both at baseline and under pharmacological perturbations. Results of calcium imaging aging studies in the SAN: Calcium imaging revealed clear age-dependent differences in SAN function. In young tissue, calcium waves originated consistently near the superior vena cava, propagated smoothly across the SAN, and maintained stable frequency with low beat-to-beat variability. In contrast, old SANs displayed slower rates, increased variability, and marked fragmentation of the spatiotemporal distribution of calcium signals, with heterogeneous initiation sites and partial conduction failures. These findings point toward an intrinsic “pacemaker frailty” that emerges with age. We are further investigating how structural remodeling—including the distribution of intrinsic cardiac neurons and age-related fibrosis—contributes to these changes. To this end, we apply double autonomic blockade in both young and old preparations to isolate intrinsic mechanisms and complement functional imaging with immunostaining and histological analyses to relate cellular and extracellular remodeling to altered calcium signaling. The sinoatrial node as a control center and pharmacological testing: We also conceptualize the SAN as a distributed network, or 'small brain of the heart,' in which pacemaker cells and supporting cell types integrate multiple inputs to govern rhythm. To probe this complexity, we perform systematic pharmacological interventions, including neurotransmitters and modulators of autonomic tone (e.g., serotonin, endothelin-1, carbachol, isoproterenol, ivabradine), as well as double autonomic blockade to unmask intrinsic properties. Comparing young and old SAN responses to these interventions provides insight into how the aging pacemaker network adapts or fails to adapt to autonomic and humoral signals. Together, these studies position the SAN as both a target and a sensor of aging processes and provide mechanistic insight into how age-related remodeling leads to loss of rhythmic stability. Results of serotonin studies: In ex vivo SAN preparations, serotonin increased the frequency of calcium transients in a dose-dependent manner, with enhanced responsiveness in young tissue and blunted, variable responses in old. Spatiotemporal mapping revealed that serotonin not only accelerated rate but also modulated the organization of calcium wave initiation and propagation. To place these findings in context, we are working in a multidisciplinary team with NIDA collaborators and building on our previous work on phosphodiesterases, which regulate cAMP/PKA signaling within the coupled-clock system. This systems biology framework allows us to move beyond pacemaker cells alone, integrating neuronal, glial-like, and structural components of the SAN to better understand how serotonin and related neuromodulators shape network-level pacemaker dynamics across aging. Previous work: Our earlier studies established the conceptual and methodological basis for the present investigations into SAN aging. We demonstrated that synchronized cardiac impulses emerge not from a single dominant pacemaker, but from heterogeneous, subcellular calcium signals (local Ca²⁺ releases, LCRs) that self-organize within and among HCN4⁺ pacemaker cells to drive full-scale action potentials. This multiscale process, resembling neuronal network dynamics, reframed the SAN as a complex, brain-like structure rather than a simple conduction pathway. We further identified novel interstitial and glial-like cells, including S100B⁺ populations, that interact with pacemaker cells and autonomic innervation, highlighting the SAN as a highly integrated 'little brain of the heart.' Finally, earlier mechanistic work demonstrated that constitutive phosphodiesterase activity critically constrains pacemaker firing by suppressing LCRs, establishing a direct link between cAMP–PKA signaling, the calcium clock, and spontaneous impulse generation. Together, these discoveries advanced the coupled-clock paradigm and revealed new structural and biochemical layers of SAN regulation, setting the stage for our current use of ex vivo calcium imaging, in vivo–ex vivo bridging, and neurotransmitter-related pharmacological interventions to interrogate how these mechanisms remodel with aging.