Celebrating Physiology in Oxford, June 2026

The biophysical basis of intracellular homeostasis

Cellular physiology operates at the theoretical limits set by physical laws – for example, ion channels are sensitive to the passage of a single elementary charge, and retinae can detect one photon. Eukaryotic cells also exploit every form of energy storage mechanism available – biochemical (e.g., solute concentrations and chemical bonds), electrical (e.g., capacitive charge storage in the cell membrane), mechanical (e.g., the elastic compliance of cellular membranes), and thermal (the heat storage essential for maintaining body temperature). Since cells appear to have evolved to maximally exploit the laws of physics within their particular environmental niche, we should try to explain cell behaviour in the light of those laws.

This talk will present an analysis of cell homeostasis using a ‘bond graph’ modelling approach that ensures that the conservation laws of physics (i.e., conservation of mass, charge, and energy, respectively) are satisfied for the interdependent biochemical, electrical, mechanical, and thermal energy storage mechanisms operating within the cell. The bond graph approach is applied to several cell membrane transport mechanisms and then used to consider how physics constrains intracellular electrolyte homeostasis for enterocytes (the epithelial absorptive cells in the lining of the intestinal mucosa). The model includes the electrogenic sodium-potassium ATPase pump (NKA) and an inwardly rectifying potassium channel (Kir) in the basolateral membrane, the electrogenic sodium-driven glucose transporter (SGLT1) in the apical membrane, and the glucose transporter (GLUT2) expressed in both membranes.

Glycolysis converts the imported glucose to ATP to drive NKA. For specified levels of sodium (Na⁺), potassium (K⁺), and glucose in the blood, the model demonstrates how enterocytes absorb Na⁺ and glucose from the lumen and transfer glucose to the blood while maintaining the membrane potential and homeostasis of intracellular Na⁺ and K⁺. The Gibbs free energy available from the ATP hydrolysis ensures that the cell operates as a ‘sodium battery’ with a high external to internal ratio of Na⁺ concentration in order to provide the energy for many other cellular transport processes. We show balanced homeostasis of Na⁺, K⁺, glucose, ATP, and membrane potential under varying levels of glucose in the lumen. We demonstrate that this balance is due to the 3:2 stoichiometry of Na⁺/K⁺ exchange in NKA, the 2:1 Na⁺/glucose cotransport in SGLT1, the 1:2:2 ratio between glucose consumption, ATP, and water production in glycolysis, the K⁺ efflux through Kir, and the glucose transport via GLUT2. We analyse the energy costs of varying the ratio of GLUT2 to SGLT1 in the apical membrane of the enterocyte. We also show how the flux for SLC transporters, ATPase pumps and ion channels can be expressed in a consistent and thermodynamically valid expression.

A bond graph approach captures the laws of physics at the appropriate scale for understanding physiological mechanisms. In the example presented here, a biophysical analysis of enterocyte homeostasis provides a quantitative explanation for the variable and transient expression of GLUT2 transporters, against the background of SGLT1 expression, under varying post-prandial levels of glucose and sodium in the intestinal lumen.

Artificial Intelligence-Enabled Reconstruction of Beat-by-Beat Cardiomyocyte Nano-Dynamics Reveals Mechanical Auto-Regulation of Cardiac Function

The external appearance of uniformity of cardiomyocyte contractions emerges from coordination of numerous, spatio-temporally heterogeneous processes, involving specialised sub-cellular nano-domains. Through omnidirectional coupling of electrical activity, calcium fluxes, and mechanical action, cardiomyocytes rapidly adjust to ever-changing physiological demand. It remains unknown how dynamic ultrastructural cross-talk contributes to the marked cardiac auto-regulatory capacity.

I propose that beat-by-beat auto-regulation of heart function is governed at the nano-scale by a process unique to cardiomyocytes: the rhythmic cycle of cardiomyocyte stretch, contraction, and relaxation. In this concept, the heartbeat is not merely the consequence of cardiomyocyte activity, but an intrinsic regulator that continually modifies nano-domain structure and function in preparation for the next beat.

In order to understand beat-by-beat cardiomyocyte nano-dynamics, we combine time-resolved 3D electron tomography, sub-cellular functional imaging, and artificial intelligence-assisted segmentation of cardiomyocyte sub-cellular ultrastructure, where deep learning-based segmentation enables automated reconstruction of complex cardiomyocyte ultrastructural components. Finally, we use artificial intelligence-based generative models and computational modelling for data synthesis, and for testing and generation of new hypotheses.

We have shown that cardiomyocyte contraction and stretch are associated with beat-by-beat deformation of cardiomyocyte organelles: transverse-axial tubular system (TATS), sarcoplasmic reticulum, mitochondria, microtubules, caveolae, and nuclei. In order to visualise cardiomyocyte contraction on the nano-scale (something that would be impossible experimentally) we used generative adversarial models to synthesise biophysically constrained ‘motion pictures’ of cardiomyocyte ultrastructure with nanometre-resolution, creating a powerful hypothesis-generation platform linking nanoscale structural deformation to electro-mechanical function.

As an example of a functional relevance of organelle deformation, we have shown that TATS (a network of surface membrane invaginations that enables close structural coupling of plasma membrane and extracellular fluid with sarcoplasmic reticulum) cyclically deforms during cardiomyocyte stretch and contraction in rabbit (all investigations conformed to national and international animal welfare laws). Cyclic, beat-by-beat TATS deformation which is associated with faster luminal content exchange with the bulk extracellular space, an effect that scales up with contraction amplitude and frequency. TATS ‘pumping’ serves to abolish intraluminal calcium depletions that would otherwise develop as a consequence of transmembrane calcium fluxes, and – if left unrectified by deformation-driven pumping – would lead to the loss of the contractile force.

In conclusion, our studies represent an innovative framework that can provide unique insights into the nanoscopic dynamics of cardiomyocytes. Cardiac organelle mechano-sensitivity likely represents a fundamental principle of cardiac physiology, where mechanical modulation of nano-domain structure and function can help us understand how cardiomyocytes adapt rapidly to changes in circulatory demand.

Glycocalyx Integrity Associates with Diastolic Function and is Therapeutically Targetable in Cardiometabolic Diseases

Introduction:

Damage to the coronary microvascular endothelial cell (CMVEC) glycocalyx has been implicated in cardiometabolic disease and may contribute to microvascular dysfunction. The glycocalyx is a complex and dynamic structure composed of proteoglycans and glycoproteins and plays a key role in regulating vascular permeability. Given the sensitivity of the myocardium to alterations in microvascular barrier function, disruption of the glycocalyx may be linked to impaired cardiac relaxation. However, its relationship with diastolic dysfunction across cardiometabolic conditions and its responsiveness to therapeutic intervention remain incompletely understood.

We aimed to determine whether glycocalyx integrity is associated with diastolic function in experimental models of cardiometabolic disease and to assess whether pharmacological interventions that preserve the glycocalyx are accompanied by improvements in cardiac function.

Methods:

In vitro: Human CMVEC were treated with 10ng/ml of TNF-α for 6 hours and treated with the MMP2/9-specific inhibitor, SB-3CT (10mg/ml) or finerenone (3mM). Quantitative PCR arrays, immunofluorescence, and ELISAs were used for investigation.

In vivo: Type 1 diabetes was induced in male FVB mice by injection of streptozotocin (STZ). Diabetic mice were treated with SB-3CT (10mg/kg), daily for 2 weeks from 7 weeks post STZ. In a second model, aged female mice (~17–20 months), were fed on a high fat diet or control low fat diet for 12 weeks. After 8 weeks, mice were given finerenone at 2 mg/kg/day orally for 4 weeks. Echocardiography was utilised to assess diastolic function. Lectin staining was conducted on heart tissue to give an indication of glycocalyx depth.

Results:

TNF-α significantly increased MMP-9 and heparanase mRNA expression (p<0.01–0.001) and promoted shedding of syndecan-4 (SDC4) and glycosaminoglycans into the conditioned media (p<0.05–0.001), alongside reduced cell surface SDC4 expression (p<0.05). Treatment with the MMP-2/9 inhibitor SB-3CT attenuated SDC4 shedding and restored surface expression (p<0.01), while both SB-3CT and finerenone reduced glycosaminoglycan shedding (p<0.05–0.01). Finerenone also significantly suppressed TNF-α-induced heparanase expression (p<0.001), supporting a protective effect on glycocalyx integrity under inflammatory conditions.

In vivo, streptozotocin-induced diabetic mice exhibited diastolic dysfunction, evidenced by a reduced E/A ratio (n=10 for controls and 5 for diabetes; control: 2.0±0.12 vs diabetes: 1.5±0.09; p<0.05), accompanied by reduced glycocalyx depth (n=5, control: 279.4±11.7 nm vs diabetes: 158.7±30.2 nm; p<0.05), increased cardiac MMP-9 activity, and enhanced vascular permeability as indicated by albumin extravasation. Treatment with SB-3CT restored glycocalyx depth (n=5 for SB-3CT + diabetes; 270.3±31.3 nm; p<0.05), improved diastolic function (E/A: 2.2±0.15; p<0.05), reduced MMP-9 activity (p<0.01), and decreased albumin leakage. Glycocalyx depth was positively correlated with diastolic function, while cardiac MMP-9 activity was negatively correlated (p<0.05).

In a complementary model, aged female mice subjected to high-fat diet feeding developed diastolic dysfunction, which was significantly improved following finerenone treatment (n=6 for all groups, p<0.05) without alterations in blood pressure.

Conclusion:

Glycocalyx integrity is closely associated with diastolic function across distinct cardiometabolic models and is responsive to pharmacological intervention. These findings support the glycocalyx as a central marker of coronary microvascular health and a potential therapeutic target in diastolic dysfunction.

Investigating the role of nerve growth factor in neuro-cardio-vascular co-development in a human 3D innervated cardiac muscle model

Nerve growth factor (NGF) is a neurotrophin essential for the differentiation and survival of sympathetic neurons (SNs). In the developing heart, NGF released by perivascular cells orchestrates sympathetic innervation. To model this process in vitro, we established a 3D innervated engineered heart muscle (iEHM) by fusing human iPSC-derived sympathetic neuronal organoid (SNO) with engineered heart muscle (EHM). Optogenetic SYN-f-Chrimson SNO fused to the EHM increased beating rate upon light stimulation, demonstrating functional connectivity between SNs and cardiomyocytes (n=41 iEHMs, 3 independent differentiations).

To investigate the cellular composition of this system, single-nuclei RNA sequencing (snRNA-seq) was performed, revealing co-development of vascular and perivascular cells. These findings were validated by immunofluorescence that showed a dense vascular network (PECAM1⁺), supported by pericytes (PDGFRβ⁺) interlaced with neurons and sympathetic varicosities (TH⁺/SYN1⁺), suggesting close neuro-vascular interactions. Quantitative analysis using machine learning–based orientation and proximity analysis demonstrated a strong directional correlation between SNs and vessels. Furthermore, snRNA-seq identified perivascular cells as the main source of NGF, consistent with in vivo data.

To dissect the specific role of NGF in neuro-cardio-vascular development, we engineered a doxycycline-inducible NGF (iNGF) hiPSC line using CRISPR/Cas9. The TET-ON iNGF system, integrated at the AAVS1 locus, couples a NGF coding sequence with a GFP reporter. iNGF-EHMs were fused with optogenetic SNOs and cultured for 4 weeks under NGF induction of 0, 1, 2, or 3 weeks (n=10-11 iEHMs per condition). Light stimulation over 3 weeks NGF induction significantly increased the beating rate, while contractile performance significantly decreased in 2- and 3-week iNGF tissues. Immunofluorescence further showed hyperinnervation (TH⁺/NF⁺) of cardiomyocytes (CTNT⁺) after 1–2 weeks of NGF induction, which declined in 3-week iNGF tissue. Orientation/proximity analysis across doxycycline-treated versus control tissues (n=7-13 imaging fields) revealed reduced neuro–vascular alignment after prolonged NGF induction, supporting the concept that pericyte-derived NGF attracts neuronal axons.
Together, the iNGF-iEHM represents a robust in vitro human model to dissect NGF-dependent mechanisms of sympathetic innervation and neuro-cardio-vascular development. Our results demonstrate acute NGF induction enhances neuro-cardiac connectivity, while chronic exposure disrupts its contractile performance. These findings highlight that balanced neurotrophic signaling is essential for coordinated cardiac development and establish a platform to investigate NGF-driven mechanisms underlying cardiac dysfunction.

Redefining neurocardiac interactions in the human heart

The heart is innervated by a sympathetic neuronal network that regulates its function, especially during conditions of increased physiological demand such as stress or exercise. Increased sympathetic activation is a hallmark of several cardiovascular diseases and arrhythmias. However, little is known about the physiological mechanisms underlying sympathetic control of the heart, hyperactivation or hyperinnervation. Although significant progress has been made in understanding heart function, including the neurohormonal effects in the cardiomyocytes, understanding direct neuro-cardiac coupling mechanisms and the influence of neuronal density, contact and remodelling has been challenging. The aim of this study was to investigate neuro-cardiac interactions for the first time human primary cardiac neurons and myocytes .

Human atrial discarded tissue was obtained during cardiac surgery from 42 patients. Tissue samples were cleared of adipose tissue and only myocardium sections were used for cells isolation. Cells were then transduced with adenoviral vectors, encoding for the fluorescence resonance energy transfer (FRET) biosensor Epac1-camps, for real-time live-cell measurement of cytosolic cyclic adenosine monophosphate (cAMP)[1], and cultured during 48h as previously described[2]. Traditionally, neuronal somas are thought to be restricted to the central nervous system and the ganglia. However, we found a large population of functional resident neurons spread through the myocardium. We then co-cultured intracardiac neurons and myocytes and compared the FRET response to increasing concentrations of isoprenaline (1, 10 and 100 nM), a β-adrenergic agonist. Additionally, the cAMP-degrading capacity was assessed by measuring the response to 100 µM 3-isobutyl-1-methylxanthine (IBMX), a non-specific phosphodiesterase inhibitor, under isoprenaline stimulation and basal conditions. cAMP increase in response to 1 nM isoprenaline was not significantly different in isolated myocytes compared to those in contact with neurons (2.52 ± 0.63% vs. 6.40 ± 1.30%, p=0.126) but it was significantly greater at 10 nM (7.97 ± 1.63% vs. 14.36 ± 2.43%, p = 0.036) and 100 nM (14.08 ± 2.20% vs. 25.40 ± 4.20%, p=0.004). Importantly, cAMP increase to 1 nM isoprenaline in myocytes that were in contact to neurons was similar to the maximal response of 100 nM isoprenaline in myocytes alone. IBMX response was also stronger in myocytes in contact with neurons both in isoprenaline stimulation (21.58 ± 3.45% vs. 30.05 ± 5.09%, p=0.028) and basal conditions (15.39 ± 4.98% vs. 51.17 ± 5.97%, p<0.001), an effect that can only be explained by sympathetic neurotransmission. Normal distribution was confirmed by Shapiro-Wilk test. Data are expressed as mean ± standard error of the mean. Linear mixed models were used employing estimated marginal means as post-hoc test to obtain the p-values of pairwise comparisons.

These findings suggest that efficient sympathetic regulation of cardiac function occurs via direct coupling between myocytes and intracardiac neurons. The existence of this neuronal network and extensive cell-cell interactions could resolve the precise, specific and temporally tuning of heart function and provide the mechanistic framework for some cardiac arrhythmias.

Association between sympathetic nervous system dysregulation and ventricular arrhythmogenesis in a progressive murine model of chronic kidney disease

Introduction

Chronic kidney disease (CKD) drives adverse cardiac remodeling, characterized by interstitial fibrosis, diastolic dysfunction, and increased arrhythmic susceptibility, even in early disease stages. Early-stage CKD is associated with a six-fold elevation in sudden cardiac death risk, predominantly due to ventricular arrhythmia. A hallmark of CKD is persistent sympathetic nervous system (SNS) over activation, which perturbs cardiac electrophysiological and mechanical properties. However, the mechanistic link between renal dysfunction, SNS dysregulation, and ventricular arrhythmogenesis remains incompletely understood.

Aim

To investigate CKD-induced SNS dysregulation and its contribution to ventricular arrhythmia susceptibility in murine hearts.

Materials and Methods

Female C57BL/6J mice were fed either 0.15% adenine (n = 11) or control chow (n = 13) for 7 weeks to induce CKD. Renal dysfunction was quantified using serum BUN, creatinine, and normalised kidney weight. Cardiac electrophysiology was assessed via in vivo ECG and ex vivo optical mapping, with ventricular arrhythmia susceptibility evaluated using programmed stimulation. SNS activity was determined by heart rate variability (HRV) and circulating noradrenaline levels (ELISA). Cardiac structure and blood pressure were assessed by echocardiography and haemodynamic measurements. Results are presented as control versus adenine.

Results

Adenine-fed mice demonstrated increased serum BUN (8.5±0.9mmol/L vs 23.4±6.0mmol/L, p<0.0001), elevated creatinine (14.6±3.7umol/L vs 39.4±23.9umol/L, p<0.0001), and reduced kidney weight (251.8±26.90mg/g vs 188.6±33.9mg/g, p<0.0001), confirming renal impairment.

Adenine diet reduced HRV (figure 1) (2.1±1.0ms vs 1.2±0.6ms, p = 0.0007) and markedly elevated circulating noradrenaline (figure 2) (326.7±12.6pg/ml vs 1275±25pg/ml, p < 0.0001), despite reduced heart rate (736.6±22.56bpm vs 703.4±33.0bpm, p=0.0002), suggesting impaired chronotropic responsiveness and autonomic dysfunction.

Electrophysiological alterations included P-duration shortening (12.3±0.7 vs 11.2±0.9ms, p=0.0007) and PR-interval prolongation (33.0±2.0ms vs 35.3±2.5ms, p=0.0014). Ex vivo optical mapping showed action potential duration prolongation. For example, at 120ms pacing cycle length, there was a significant action potential duration prolongation at 30% (14.4±1.6 vs 18.7±3.9, p=0.002), 50% (27.6±4.9 vs 35.6±8.2, p=0.006) and 80% repolarisation (49.6±6.7 vs 58.4±12.4, p=0.04).

Furthermore, adenine mice displayed amplitude alternans, indicating beat-to-beat electrical instability, across pacing cycle lengths (p <0.0001). 58% of adenine hearts displayed pacing-induced arrythmias, primarily ventricular tachycardia (75%) and ventricular fibrillation (25%), compared to 0% of control. These changes occurred without overt structural remodelling or hypertension.

Conclusion

Adenine-induced CKD in mice leads to significant SNS dysregulation, alongside bradycardia, PR prolongation, prolonged action potential duration, electrical instability, and increased ventricular arrhythmias. The coexistence of sympathetic activation with bradycardia suggests impaired autonomic control and neuro-cardiac remodelling contributing to arrhythmogenesis.

The effect of unilateral carotid body resection on beat-to-beat blood pressure variability in resistant hypertension

Background

The carotid chemoreceptors play an important role in cardiovascular regulation. In animal models of heart failure, heightened carotid chemoreflex activity is associated with increased blood pressure variability (BPV), which normalises following carotid body (CB) denervation (Del Rio, R. et al., 2013). In humans, higher beat-to-beat BPV indicates poor prognosis for cardiovascular mortality as it predicts increased risk for recurrent stroke and major cardiac events such as cardiovascular death and myocardial infarction (Webb, A.J.S. et al. 2018). Unilateral removal of CBs was shown to significantly reduce systolic blood pressure (SBP) in essential hypertensives, which was sustained in the long-term (mean ±SEM 3.1±0.6 years) follow-up investigation (Fudim, M. et al. 2015). However, whether beat-to-beat BPV is influenced by the carotid chemoreflex in humans is unclear. Therefore, the aim of this study was to retrospectively examine whether unilateral CB excision reduced BPV in people with drug resistant hypertension.

Methods

The data are from that published by Narkiewicz, K. et al. (2016), which complied with the Declaration of Helsinki and was approved by the Central Bristol research ethics committee. Fifteen people (7 males and 8 females) age 52 ±1 years with drug resistant hypertension participated in the study. Participants underwent unilateral carotid chemoreceptor excision. Beat-to-beat blood pressure was recorded during a 10-minute resting period (semi-supine) at baseline (pre-operative) and 3, 6 and 12 months post-operation. Beat-to-beat BPV was measured using average real variability (ARV) and the coefficient of variation (CoV) for both SBP and diastolic blood pressure (DBP). Only SBP ARV was normally distributed, ascertained by the Shapiro-Wilks test (alpha = 0.05), therefore all other data underwent logarithmic transformation. Data were then compared using a repeated measures one-way ANOVA with Dunnet’s correction post-hoc comparisons. Data are presented as mean ± standard deviation (SD), unless otherwise stated.

Results

Resting SBP and DBP at baseline (pre-operative) were 165 ±38 mmHg and 81±24 mmHg. There was an effect of CB excision on the ARV for DBP (Figure 1, p = 0.411). The ARV of DBP was reduced from median 1.67 mmHg (interquartile range (IQR) 0.64) at baseline to median 1.26 mmHg (IQR 0.86 ) at 6 months post CB excision only (p = 0.0411). There were no differences in other measures of BPV from baseline to 3, 6 and 12 months post excision (p > 0.05).

Figure 1: Average real variability of DBP at baseline vs post CB excision. The ARV of DBP at baseline 0 months (2±1mmHg) is compared to 3 months (2±1mmHg); 6 months (2±1mmHg); and 12 months (2±1mmHg) post CB excision. Following logarithmic transformation, a repeated measures one-way ANOVA and post-hoc Dunnet’s correction, only baseline vs 6 months post-CB excision was significant (p = 0.041) whereas baseline vs 3 months (p > 0.05) and baseline vs 12 months (p > 0.05) were not significant.

Conclusions

There was a reduction of DBP variability in resistant hypertensives at 6 months post-CB excision, suggesting that the CBs may contribute to BPV in humans. However, this needs further evaluation in a larger cohort to determine whether targeting the carotid body will improve BPV and prognosis in resistant hypertension.

The role of the carotid chemoreflex in the resting hyperventilation experienced by patients with long-COVID

Introduction

Long-COVID is a debilitating condition in which people continue to have health problems for months or years following COVID-19 infection. Long-COVID affects nearly 2 million people in the UK and costs ~£5.7 billion per year in productivity loss^[1]. Many people with long-COVID experience ongoing breathing difficulties, despite having normal lung function and no evidence of gas exchange abnormalities. Evidence indicates that patients with long-COVID, who were not hospitalised for their initial infection, experience hyperventilation at rest and during exercise. The carotid chemoreflex plays an important role in the control of breathing at rest and during exercise^[2] and its sensitivity was recently shown to be increased in people with long-COVID^[3]. However, whether this hypersensitive carotid chemoreflex drives some of the hyperventilation experienced by patients with long-COVID remains unknown. Dopamine is used to inhibit carotid body afferent discharge into the brainstem, which inhibits the carotid chemoreflex response to hypoxia. However, some evidence suggests that dopamine signalling may be impacted in patients with long-COVID^[4]. Thus, whether dopamine inhibits the carotid chemoreflex in patients with long-COVID is also unclear.

Objective

Dopamine will reduce carotid chemoreflex sensitivity in response to hypoxia and improve hyperventilation at rest in people with long-COVID, versus saline.

To determine whether inhibiting the carotid chemoreflex with low-dose dopamine can reduce heightened carotid chemoreflex sensitivity during hypoxia in people with long-COVID and improve their hyperventilation at rest.

Hypothesis

Methods

Six (2M, 4F, age; 50±9 years, BMI; 29.6±3.2 kg/m²) participants with long-COVID completed resting ventilation measurements (during normoxia) and hypoxic ventilatory response (HVR) testing (measure of carotid chemoreflex sensitivity) during intravenous infusion of low-dose dopamine (2 mcg/kg/min) or saline (control) at rest. HVR was evaluated as the slope of the linear regression relating the minute ventilation to the nadir of oxygen saturation for each nitrogen exposure. Data were compared using Wilcoxon matched-pairs rank test. Data are reported as median (IQR).

Results

Intravenous dopamine reduced carotid chemoreflex sensitivity to hypoxia in people with long-COVID compared to saline (-0.15 (-0.17 to -0.09) L/min/SpO₂% versus -0.36 (-0.79 to -0.20) L/min/SpO₂%, P=0.0312, Cohen’s D=1.061). Resting partial pressure of the end-tidal CO₂ (P_ETCO₂) tended to be increased with dopamine compared to saline (43 (41-44) mmHg versus 40 (38-42) mmHg), P=0.0312, Cohen’s D=1.5), indicating a reduction in excessive hyperventilation at rest with dopamine.

Conclusion

Inhibiting the carotid chemoreflex with low-dose dopamine normalised HVR and improved hyperventilation at rest in people with long-COVID, versus saline. Tempering the carotid chemoreflex may be a viable treatment for people with long-COVID and unexplained dysregulation of ventilation.

Occurrence and prevention of a novel class of ischaemia-reperfusion arrhythmias

Myocardial infarction is one of the leading causes of death and incapacity worldwide. Reopening the culprit artery to restore oxygenation is essential to prevent irreversible myocardial damage, but can also lead to deleterious events including arrhythmias. In previous work with rabbits, we identified a new type of ischaemia-reperfusion arrhythmia occurring in the myocardium along the main branch of the reperfused vessel (“perivascular excitation tunnelling”, PVET) and showed that a 2-stage reperfusion method could prevent subsequent re-entrant arrhythmias.

Computational modelling of reperfusion using openCARP was performed. A simplified ventricular model was generated with an ischaemic area and a central channel representing the recovered myocardium around the reperfused vessel. With this model we could replicate PVET-induced arrhythmias and predicted that the size of the ischaemic area is a major determinant of arrhythmia likelihood. Prior work comparing small versus large rabbit hearts showed that larger hearts were indeed more likely to have re-entrant arrhythmias.¹

To assess possible clinical relevance, as well as to identify whether risk scales with heart size across species, we performed experiments in pig hearts. All investigations conformed to German animal welfare laws and were performed with ethical approval by the local Institutional Animal Care and Use Committee (Regierungspräsidium Freiburg, X-21/03B). Explanted pig hearts were Langendorff perfused with HEPES-buffered physiological solution and optical mapping of transmembrane voltage performed. A cannula was inserted into a coronary artery and local ischaemia was achieved by perfusion of a simulated ischaemic solution (high K⁺/low O₂ solution). Reperfusion was performed either by reperfusing the whole ischaemic area at once (1-stage), or in a sequential manner (2-stage, with the cannula initially pushed deep into the artery for the first reperfusion step, and subsequently returned to the original position).

We confirmed the presence of PVET in pig hearts, characterised by the preferential recovery of myocardial excitability along the main branch of the reperfused vessel. In hearts reperfused with the 1-stage method, PVET occurred in 11 of the 12 hearts, resulting in re‑entrant arrhythmias in 7 cases (N=12). The likelihood of PVET-driven re-entrant arrhythmias in the pig heart was no higher than in the rabbit heart, indicating that ischaemic size alone is not a sufficient parameter to predict arrhythmia. With the 2-stage protocol, PVET occurred in 4 of 6 hearts, but no re-entrant arrhythmias were observed. Thus, the two-stage protocol did not significantly alter PVET likelihood but did reduce the likelihood of re-entrant arrhythmias (p-value: 0.038; Fisher’s exact test). We are currently optimising the computational model to incorporate realistic vasculature geometries and species differences. Finally, we are testing a new catheter, developed together with OSYPKA AG, to assess the efficacy of a 2-zone reperfusion method in a preclinical setting, which involves two separately controlled flow beds, enabling simultaneous reperfusion with different solutions preventing a delay in tissue re-oxygenation. In the future, we hope to use imaging approaches and computational modelling for risk stratification to identify which patients would benefit from use of this catheter during percutaneous coronary intervention following myocardial infarction and thereby improve patient outcomes.

Lessons learned from pacemaking: more players for a single function, more functions for a single player

Pacemaking, a most fundamental cardiac function, has long occupied a central place in physiological research. What makes the heart “beat”? Spontaneous activity originates in SAN pacemaker cells thanks to a distinctive phase of their action potentials, the diastolic depolarization (DD). In the late 70s, a new ionic current (“funny”, I_f current), was identified in pacemaker tissue. Its unusual property (an inward current activated on hyperpolarization) introduced a novel concept in pacemaker physiology in strong contrast with existing models. Subsequent studies of I_f and its molecular correlates, the HCN channels, established their key role in DD generation and in autonomic modulation of heart rate. Pacemaking is however a complex cellular process, and other players are known to contribute, including the Ca-clock mechanism, the sustained inward current, the ACh-activated K current, the Cav1.3 channel; this latter player, expressed exclusively in pacemaker cells, has distinctive properties and according to recent data fully controls, together with HCN4 channels, adrenergic rate modulation. Although specific functional aspects may require fewer mechanisms, the whole pacemaking function emerges from the coordinated action of many players. This raises two questions: 1) why several players for a single function? 2) if one function needs several players, how extensive must the repertoire be to support the extraordinarily large number of cellular functions?

The answer to 1) is “robustness”: redundancy ensures that backup mechanisms compensate when one or more components fail. The answer to 2) is “multifunctionality”: many molecular players serve diverse roles across tissues. f/HCN channels, for example, are expressed far beyond cardiomyocytes and participate in processes as varied as sensory transduction, hormone secretion, gastrointestinal motility, osteoclastogenesis, immune cell activation, mitochondrial respiration, even stem‑cell cycle control and early embryonic development, and others.

The lessons learned from these studies illustrate a general principle: biological functions arise from combinatorial partnerships among mechanisms, rather than from a simplistic “one gene–one function” paradigm.

“A Device for Dynamic FAD Measurements in the Human Heart Ex Vivo Model and an In Vivo Large Animal Model”

Introduction:

We have developed an implantable device for dynamic flavin adenine dinucleotide (FAD) measurements in cardiac tissue. FAD fluorescence provides a label-free optical readout and can be used to assess metabolic changes during ischemia, reperfusion, and pharmacologically induced alterations in coronary perfusion. The device is designed for direct placement on the heart surface, enabling real-time monitoring of relative FAD levels in both ex vivo and in vivo cardiac models.

Aims:

The aim of this study was to perform dynamic FAD measurements from the heart using an implantable, suturable device format. This approach was designed to enable continuous monitoring of changes in myocardial FAD levels under controlled ischemic and perfusion-modifying conditions.

Methods:

FAD measurements were performed in one canine model, two porcine models, and one declined donor human heart maintained in an ex vivo perfusion system. In the canine model, the obtuse marginal branch was ligated to mimic localized myocardial ischemia. In the porcine models, acetylcholine injections were performed to induce coronary vascular responses and assess corresponding changes in FAD signals. In the ex vivo human heart model, perfusion was transiently interrupted for defined intervals to evaluate changes in FAD levels during ischemia and reperfusion. The implantable LED sensor array was positioned on the epicardial surface to record dynamic relative FAD signals under these experimental conditions.

Results:

Dynamic changes in relative FAD levels were successfully recorded in both in vivo large animal models and the ex vivo human heart model. In the canine ischemia model, local coronary ligation produced measurable alterations in FAD signal consistent with regional metabolic changes. In the porcine models, acetylcholine administration was associated with detectable changes in relative FAD levels during pharmacologically induced vascular responses. In the ex vivo human heart, transient interruption and restoration of perfusion resulted in dynamic FAD signal changes during ischemia-reperfusion intervals.

Conclusions:

This study demonstrates the feasibility of an implantable, suturable cardiac sensor array for dynamic measurement of relative FAD levels on the heart surface. The device enables real-time assessment of metabolic changes in both in vivo large animal models and an ex vivo human heart preparation. These findings support further development of implantable optical technologies for monitoring myocardial metabolism during ischemia, reperfusion, and altered coronary perfusion.

The animal studies were performed in compliance with animal protocols. The human donor heart was declined for transplant and hereby considered discarded tissue.

https://www.science.org/doi/full/10.1126/sciadv.ads8608 – features the previous version of the device.

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Registration opens	17 February 2026
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Abstract submission closes	31 March 2026
Early bird registration deadline	4 May 2026
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Meeting dates	18 – 19 June 2026

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Advancing Physiology Together: Showcasing Research, Enhancing Careers, and Building Communities

Thursday 18 June - Friday 19 June 2026

Celebrating Physiology in Oxford

Date

Location

15:00 SA01 The biophysical basis of intracellular homeostasis

16:30 Welcome

16:35 The history of the Physiological Society

16:50 The work of Sir John Eccles

17:15 Plaque Unveiling Ceremony to celebrate Sir John Eccles

08:45 Welcome

08:50 Humpty Dumpty: How advanced engineering helps to decipher the heartbeat

09:30 Re-assembly: AI at the cross-roads of integration and reduction

10:10 C01 Artificial intelligence-enabled reconstruction of beat-by-beat cardiomyocyte nano-dynamics reveals mechanical auto-regulation of cardiac function

10:18 C02 Glycocalyx integrity associates with diastolic function and is therapeutically targetable in cardiometabolic diseases

10:26 C03 Investigating the role of nerve growth factor in neuro-cardio-vascular co-development in a human 3D innervated cardiac muscle model

10:34 C04 Redefining neurocardiac interactions in the human heart

11:15 SA02 Adaptation to stress: Why reduction ain’t the answer

11:55 Neural-cardiac communication in organoids

12:35 C05 Association between sympathetic nervous system dysregulation and ventricular arrhythmogenesis in a progressive murine model of chronic kidney disease

12:43 C06 The effect of unilateral carotid body resection on beat-to-beat blood pressure variability in resistant hypertension

12:51 C07 The role of the carotid chemoreflex in the resting hyperventilation experienced by patients with long-COVID

12:59 C08 Occurrence and prevention of a novel class of ischaemia-reperfusion arrhythmias

13:45 SA03 Lessons learned from pacemaking: More players for a single function, more functions for a single player

14:25 Mechanics and pacemaking

14:40 Patient specific modelling and simulation: fact or fiction?

15:20 SA04 Linking wet and dry lab to control rhythm disturbances

15:35 SA05 A device for dynamic FAD measurements in the human heart ex vivo model and an in vivo large animal model

16:30 The cell is swell, but the issue is tissue

17:10 Slaves or masters: Interstitium vs. stroma

17:50 Closing Remarks

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