Biology subjects

Ripplinger, C. M.

Publications and source records attributed to Ripplinger, C. M..

7 recordsLinked to original sources

bioRxiv2026

Sex-specific electrophysiology and cholinergic responses underlie differential mechanisms of arrhythmia vulnerability in rabbit atria

BackgroundSex differences in the epidemiology of atrial fibrillation are well-documented; however, the underlying mechanisms remain poorly understood. This gap in knowledge is compounded by limited data on sex-specific atrial electrophysiology in the absence of disease. ObjectivesThe aim of this study was to investigate sex differences in atrial electrophysiology and arrhythmia susceptibility in a translationally-relevant rabbit model. MethodsDual optical mapping of transmembrane voltage and Ca2+ was performed on intact atria of young (3.5-5 months) male and female rabbit hearts. Baseline atrial electrophysiology and arrhythmia susceptibility were investigated using rapid pacing and premature stimulation and further tested with the parasympathomimetic carbachol. Sex and regional differences in gene expression were assessed using qPCR. ResultsFemales exhibited similar action potential duration (APD), but greater APD heterogeneity across the atria at slower rates, along with longer Ca2+ transient durations compared to males. Greater APD heterogeneity in females was rate-dependent and comparable to males at faster pacing frequencies; however, it was associated with greater susceptibility to transient reentrant arrhythmias with premature stimuli. After carbachol application, males had heightened vulnerability to arrhythmia. This was associated with cholinergic-mediated APD shortening in both atria in males, but only in the right atrium in females. Sex differences in carbachol responses were linked to variations in muscarinic receptor and acetylcholine-activated potassium channel gene expression. ConclusionsThese findings demonstrate sex and regional differences in atrial electrophysiology at baseline and in response to cholinergic stimulation in the healthy heart that may contribute to sex-specific mechanisms of arrhythmia.

physiology↗

bioRxiv2025

Role of Dynamical Instability in QT Interval Variability and Early Afterdepolarization Propensity

Beat-to-beat variability of the QT interval (QTV) is a well-established marker of cardiac health, with increased QTV (> 5 ms) often associated with a higher risk of arrhythmias. However, the underlying mechanisms contributing to this phenomenon remain poorly understood. Recently, we showed that cardiac instability is a major cause of QTV. Early afterdepolarizations (EADs) are abnormal electrical oscillations that occur during the plateau phase of the cardiac action potential (AP), often arising when the membrane potential becomes unstable. In this study, we use a physiologically detailed computational model of rabbit ventricular myocytes with stochastic ion channel gating to investigate the relationship between QTV and EAD propensity. We found that increased AP duration (APD) variability, which serves as a surrogate for QTV on the ECG at the single-cell level, can arise even in the absence of apparent EADs, driven by intrinsic dynamical instability. As the cellular state approaches the threshold for EAD generation, small perturbations in membrane voltage are amplified, leading to increased APD variability. The phase-plane analysis in the voltage-calcium channel inactivation space demonstrates that proximity to the EAD-generating basin of attraction strongly influences repolarization variability, establishing a mechanistic link between QTV and EAD propensity. Furthermore, we observed that QTV increases at longer pacing cycle lengths (PCLs), distinguishing it from alternans-associated APD variability, which increases at shorter PCLs. These findings suggest that increased QTV may serve as an early indicator of arrhythmic risk before the manifestation of EADs, potentially offering a critical window for preventive intervention. Our results provide novel insights into the fundamental mechanisms underlying QTV and its potential role in arrhythmia prediction. Significance statementThis study investigates the relationship between beat-to-beat variability of the QT interval (QTV) and the propensity for early afterdepolarizations (EADs), abnormal electrical oscillations linked to life-threatening arrhythmias. Using a computational model, we show that increased QTV can precede apparent EADs, driven by inherent dynamical instability of cardiac cells. As cells approach an arrhythmia-prone state, small membrane voltage fluctuations are amplified, increasing repolarization variability and thus QTV. Furthermore, QTV increases with slower heart rates, distinguishing it from alternans, another type of instability arising at faster rates. Thus, increased QTV may serve as an early warning signal for arrhythmia risk, potentially enabling preventative interventions. Our results provide novel insights into the fundamental mechanisms underlying QTV and its potential role in arrhythmia prediction.

biophysics↗

bioRxiv2025

Multi-parametric imaging of spatio-temporal cAMP signaling, transmembrane potential, and intracellular calcium in the Langendorff-perfused heart

The spatiotemporal dynamics of intracellular second messengers and signaling molecules, including cyclic adenosine monophosphate (cAMP), have been studied extensively in isolated cardiomyocytes using Forster resonance energy transfer (FRET)-based reporters. However, little is known about how molecular signaling events affect tissue-level function. Optical mapping of transmembrane potential (Vm) and intracellular calcium (Ca2+i) is frequently performed in isolated intact hearts to reveal tissue-level electrophysiological function but cannot reveal molecular underpinnings. Here, we developed a novel multi-parametric optical imaging system that enables multi-parametric recording of four wavelengths to visualize cAMP and real-time electrophysiological responses (Vm and Ca2+i) by utilizing concurrent FRET imaging and dual optical mapping (FRE-DOM) in the intact heart from a cardiac-specific FRET-based cAMP reporter mouse. We showed that cAMP is strongly and heterogeneously activated throughout the heart (atria and ventricles) in response to {beta}-AR stimulation, the time course of which matches heart rate, action potential, and Ca2+ responses. This novel imaging system will provide insight into the relationship between cAMP signaling, electrophysiology, and arrhythmogenesis at high spatio-temporal resolution.

bioengineering↗

bioRxiv2024

Sympathetic structural and electrophysiological remodeling in a rabbit model of reperfused myocardial infarction

Chondroitin sulfate proteoglycans (CSPGs) inhibit sympathetic reinnervation in rodent hearts post myocardial infarction (MI), causing regional hypo-innervation that is associated with supersensitivity of {beta}-adrenergic receptors and increased arrhythmia susceptibility. To investigate the role of CSPGs and hypo-innervation in the heart of larger mammals, we used a rabbit model of reperfused MI and tested electrophysiological responses to sympathetic nerve stimulation (SNS). Innervated hearts from MI and sham rabbits were optically mapped using voltage and Ca2+-sensitive dyes. SNS was performed with electrical stimulation of the spinal cord and {beta}-adrenergic responsiveness was tested using isoproterenol. Sympathetic nerve density and CSPG expression were evaluated using immunohistochemistry. CSPGs were robustly expressed in the infarct and border zone of all MI hearts, and the presence of CSPGs was associated with reduced sympathetic nerve density in the infarct vs. remote region. Action potential duration (APD) dispersion and susceptibility to ventricular tachycardia/fibrillation (VT/VF) were increased with SNS in MI hearts but not in sham. SNS decreased APD80 in MI but not sham hearts, while isoproterenol decreased APD80 in both groups. Isoproterenol also shortened Ca2+ transient duration (CaTD80) in both groups but to a greater extent in MI hearts. Our data suggest sympathetic remodeling post-MI is similar between species, with CSPGs associated with sympathetic hypo-innervation. Despite a reduction in sympathetic nerve density, the infarct region of MI hearts remained responsive to both physiological SNS and isoproterenol, potentially through preserved or elevated {beta}-adrenergic responsiveness, which may underly increased APD dispersion and susceptibility for VT/VF. NEW & NOTEWORTHYHere we show that CSPGs are present in the infarcts of rabbit hearts with reperfused MI, where they are associated with reduced sympathetic nerve density. Despite hypo-innervation, sympathetic responsiveness is maintained or enhanced in MI rabbit hearts, which also demonstrate increased APD dispersion and tendency for arrhythmias following sympathetic modulation. Together, this study indicates that the mechanisms of sympathetic remodeling post-MI are similar between species, with hypo-innervation likely associated with enhanced {beta}-adrenergic sensitivity.

physiology↗

bioRxiv2023

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component in tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. However, the underlying mechanisms are unclear. To address this, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days prior to performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the 1st - 3rd thoracic vertebrae, and {beta}-adrenergic responsiveness was additionally evaluated as changes in heart rate (HR) following norepinephrine (NE) perfusion. Baseline ex vivo HR and SNS stimulation threshold were increased in NIC vs. CT (P = 0.004 and P = 0.003 respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC vs. CT at baseline (P = 0.002) and during SNS (P = 0.0003), with similar results obtained for Ca2+ transient alternans. SNS reduced the PCL at which alternans emerged in CT but not NIC hearts. NIC exposed hearts also tended to have slower and reduced HR responses to NE perfusion. While fibrosis was unaltered, NIC hearts had lower sympathetic nerve density (P = 0.03) but no difference in NE content vs. CT. These results suggest both sympathetic hypo-innervation of the myocardium and diminished {beta}-adrenergic responsiveness with NIC. This autonomic remodeling may underlie the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with continued long-term usage. NEW & NOTEWORTHYHere we show that chronic nicotine exposure was associated with increased heart rate, lower threshold for alternans and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to the sympathetic hypo-innervation of the myocardium and diminished {beta}- adrenergic responsiveness observed following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this pro-arrhythmic remodeling.

physiology↗

bioRxiv2020

Quantitative cross-species translators of cardiac myocyte electrophysiology: model training, experimental validation, and applications

Animal experimentation is key in the evaluation of cardiac efficacy and safety of novel therapeutic compounds. However, inter-species differences in the mechanisms regulating excitation-contraction coupling can limit the translation of experimental findings from animal models to human physiology, and undermine the assessment of drugs efficacy and safety. Here, we built a suite of translators for quantitatively mapping electrophysiological responses in ventricular myocytes across species. We trained these statistical operators using a broad dataset obtained by simulating populations of our biophysically detailed computational models of action potential and Ca2+ transient in mouse, rabbit, and human. We then tested our translators against experimental data describing the response to stimuli, such as ion channel block, change in beating rate, and {beta}-adrenergic challenge. We demonstrate that this approach is well suited to predicting the effects of perturbations across different species or experimental conditions, and suggest its integration into mechanistic studies and drug development pipelines.

systems biology↗

bioRxiv2020

Systemic Bone Loss Following Myocardial Infarction in Mice is Mitigated by Treatment with a β3 Adrenergic Receptor Antagonist

Myocardial infarction (MI) and osteoporotic fracture (Fx) are leading causes of morbidity and mortality worldwide, and there is epidemiological evidence linking their incidence, suggesting possible crosstalk. MI can exacerbate underlying atherosclerosis through sympathetic nervous system activation and {beta}3 adrenoreceptor-mediated release of hematopoietic stem cells (HSCs), leading to monocytosis. We hypothesized that this same pathway may initiate systemic bone loss following MI, since osteoclasts differentiate from monocytes. In this study, MI was performed in 12-week-old male mice (n=24), and mice were randomized to treatment with a {beta}3-adrenergic receptor antagonist (SR 59230A) or no treatment for 10 days post-operatively; additional mice (n=21, treated and untreated) served as un-operated controls. Bone mineral density (BMD), bone mineral content (BMC), and body composition were quantified at baseline and 10 days post-MI using DXA; at 10 days post-MI circulating monocyte and neutrophil levels were quantified, and the L5 vertebral body and femur were analyzed with micro-computed tomography. We found that MI led to increases in circulating monocyte and neutrophil levels, but contrary to our initial hypothesis, {beta}3-antagonist treatment further increased circulating monocytes compared to untreated mice. BMD and BMC of MI mice decreased at the femur and lumbar spine (-6.9% femur BMD, -3.5% lumbar BMD); {beta}3-antagonist treatment diminished this bone loss response (-5.3% femur BMD, -1.2% lumbar BMD). Similarly, trabecular bone volume was decreased in MI mice compared to control mice, and this bone loss was partially attenuated by {beta}3-antagonist treatment. These results suggest that MI leads to bone loss at both the axial and appendicular skeleton, and that the sympathetic nervous system may be a modulator of this response. This study is the first to mechanistically show bone loss following an ischemic injury; these findings suggest that bone loss and increased fracture risk may be important clinical co-morbidities following MI or other ischemic injuries.

physiology↗