Biology subjects

Eisner, D. A.

Publications and source records attributed to Eisner, D. A..

4 recordsLinked to original sources

bioRxiv2026

Increased calcium spark frequency and variability of action potential duration precede early after depolarisations in isolated ventricular myocytes

Drug-induced inhibition of the delayed rectifier potassium (IKr) current predisposes to early afterdepolarisations (EADs) and cardiac arrhythmias. Here, we sought to determine the contribution of action potential duration (APD), APD variability and spontaneous calcium release from the sarcoplasmic reticulum (SR) in the formation of EADs. In isolated sheep ventricular myocytes, EADs were induced by combined inhibition of IKr with dofetilide and {beta}-adrenergic stimulation. The onset of EADs was preceded by increased beat-to-beat variability of APD. To isolate the role of APD in EAD initiation, the sarcoplasmic reticulum (SR) was depleted of calcium with caffeine. The first beat post-caffeine was associated with prolonged APD but not an EAD. During {beta}-AR stimulation, increasing ryanodine receptor open probability had no effect on APD but increased APD variability and induced both EADs and delayed afterdepolarisations (DADs). Targeting RyR open probability with K201 reversibly abolished afterdepolarisations. APD variability was a better predictor of EADs than APD alone. During an EAD, changes in [Ca2+]i preceded those of membrane depolarisation and the changes in [Ca2+]i were in the form of calcium sparks. In silico modelling demonstrated that membrane time constant effects account for the delay between changes in [Ca2+]i and membrane potential. In summary, using a drug-induced model of action potential prolongation with {beta}-AR stimulation, EADs are preceded by increased APD variability and an increase in Ca2+ sparks. Targeting SR function abolishes EADs. These results suggest a key role for SR Ca2+ overload in the formation of EADs and indicate that EADs and DADs share common mechanisms. Key PointsO_LIDrugs that prolong the cardiac action potential and ECG QT interval are a major cause of early afterdepolarisations and dangerous ventricular arrhythmias initiated by early afterdepolarisations. C_LIO_LIProlongation of the action potential is widely assumed to be the primary driver of these events. C_LIO_LIWe show that early afterdepolarisations are instead preceded by increased beat-to-beat variability of action potential duration and that this variability has better sensitivity and specificity for early afterdepolarisations than action potential duration. C_LIO_LISmall, spontaneous calcium release events known as calcium sparks occur before membrane depolarisation driving early afterdepolarisations. C_LIO_LISuppressing calcium release from the sarcoplasmic reticulum abolishes early afterdepolarisations, identifying calcium handling instability as potentially a key mechanism of drug-induced arrhythmia. C_LI

physiology↗

bioRxiv2023

Atrial t-tubules adopt a specialist developmental state while alterations to Ca2+ buffering maintain systolic Ca2+ during postnatal development

Transverse (t)-tubules ensure a uniform rise in calcium (Ca2+) and thus contraction in cardiac cells. Though more extensively studied in the ventricle, t-tubules also play a key role in the atria of large mammals, such as human, and their loss in heart failure is associated with impaired Ca2+ release and thus contractility. T-tubule restoration is therefore an ideal therapeutic target but the process of t-tubule formation is not understood. The aim of this study was to determine how t-tubules develop in the healthy atria and the impact this has on Ca2+ handling. Postnatal development was assessed in sheep from newborn through to adulthood. Atrial t-tubules were present at birth in the sheep atria and increased in density up until 3 months of age. In the latter part of development (3 months to adult) a lack of t-tubule growth but increase in cell width results in t-tubule density decreasing. In the newborn, despite reduced t-tubule density, we found the amplitude of the Ca2+ transient was maintained and this was associated with increases in the L-type Ca2+ current (ICa-L) and the Ca2+ content of the sarcoplasmic reticulum (SR). We suggest these changes are sufficient to overcome the elevated cytosolic Ca2+ buffering in the newborn and the decreased t-tubule density. We have shown the neonate atria is highly specialised to negate reduced central Ca2+ release through enhanced surface ICa-L and SR load. This maintains atrial function despite immature t-tubules highlighting important differences in Ca2+ handling in the newborn and heart failure atria where t-tubules are sparse.

physiology↗

bioRxiv2021

Disordered yet functional atrial t-tubules on recovery from heart failure

Transverse (t)-tubules drive the rapid and synchronous Ca2+ rise in cardiac myocytes. The virtual complete loss of atrial t-tubules in heart failure (HF) decreases Ca2+ release. It is unknown if or how atrial t-tubules can be restored and if restored t-tubules are functional. Sheep were tachypaced to induce HF and recovered when pacing was stopped. Serial block face Scanning Electron Microscopy and confocal imaging were used to understand t-tubule ultrastructure and function. Candidate proteins involved in atrial t-tubule recovery were identified by western blot and causality determined using expression studies. Sheep atrial t-tubules reappeared following recovery from HF. Despite being disordered (branched, longer and longitudinally arranged) recovered t-tubules triggered Ca2+ release and were associated with restoration of systolic Ca2+. Telethonin and myotubularin abundance correlated with t-tubule density and altered the density and structure of BIN1-driven tubules in neonatal myocytes. Myotubularin had a greater effect, increasing tubule length and branching, replicating that seen in the recovery atria. Recovery from HF restores atrial t-tubules and systolic Ca2+ and myotubularin facilitates this process. Atrial t-tubule restoration could present a new and viable therapeutic strategy. Brief SummaryThe loss of atrial transverse (t)-tubules and the associated dysfunction in heart failure is reversible and the protein myotubularin plays an important role.

physiology↗

bioRxiv2018

PDE5 inhibition improves symptom-free survival and restores transverse tubule loss and catecholamine responsiveness in heart failure

Heart failure is characterized by poor survival, a loss of catecholamine reserve and cellular structural remodeling in the form of disorganization and loss of the transverse tubule network. Indeed, survival rates for heart failure are worse than many common cancers and have not improved over time. Tadalafil is a clinically relevant drug that blocks phosphodiesterase 5 with high specificity and is used to treat erectile dysfunction. Using a sheep model of advanced heart failure, we show that tadalafil treatment results in a marked improvement in symptom-free survival, reverses transverse tubule loss and restore cardiac myocyte systolic calcium transient amplitude and the hearts chronotropic and contractile response to catecholamines. These effects are independent of changes in myocardial cGMP content and are associated with upregulation of both monomeric and dimerized forms of protein kinase G and of the cGMP hydrolyzing phosphodiesterase 2 and 3. We propose that the molecular switch for the loss of transverse tubules in heart failure and their restoration following tadalafil treatment involves the BAR domain protein Amphiphysin II (BIN1) and the restoration of catecholamine sensitivity is through reductions in G-protein receptor kinase 2, protein phosphatase 1 and protein phosphatase 2A abundance following phosphodiesterase 5 inhibition.

physiology↗