Effects of Na+ Current and Mechanogated Channels in Myofibroblasts on Myocyte Excitability and Repolarization

Mechanistic Insights and Knowledge Gaps in the Effects of Radiation Therapy on Cardiac Arrhythmias

Stereotactic body radiation therapy (SBRT) is an innovative modality for the treatment of refractory ventricular arrhythmias (VAs). Phase 1/2 clinical trials have demonstrated the remarkable efficacy of SBRT at reducing VA burden (by >85%) in patients with good short-term safety. SBRT as an option for VA treatment delivered in an ambulatory nonsedated patient in a single fraction during an outpatient session of 15 to 30 minutes, without added risks of anesthetic or surgery, is clinically relevant. However, the underlying mechanism remains unclear. Currently, the clinical dosing of SBRT has been derived from preclinical studies aimed at inducing transmural fibrosis in the atria. The propitious clinical effects of SBRT appear earlier than the time course for fibrosis. This review addresses the plausible mechanisms by which radiation alters the electrophysiological properties of myocytes and myocardial conduction to impart an antiarrhythmic effect, elucidate clinical observations, and point the direction for further research in this promising area.

Cardiac Mechano-Electric Coupling: Acute Effects of Mechanical Stimulation on Heart Rate and Rhythm

The heart is vital for biological function in almost all chordates, including humans. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops, so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer term changes in gene expression and functional or structural remodeling). This process is known as mechano-electric coupling (MEC). In the current review, we present evidence for, and implications of, MEC in health and disease in human; summarize our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modeling in developing a more comprehensive understanding of ‟what makes the heart tick.ˮ.

Ca2+ signalling in fibroblasts and the therapeutic potential of KCa3.1 channel blockers in fibrotic diseases

The role of Ca²⁺ signalling in fibroblasts is of great interest in fibrosis-related diseases. Intracellular free Ca²⁺ ([Ca²⁺ ]_i ) is a ubiquitous secondary messenger, regulating a number of cellular functions such as secretion, metabolism, differentiation, proliferation and contraction. The intermediate conductance Ca²⁺ -activated K⁺ channel K_Ca 3.1 is pivotal in Ca²⁺ signalling and plays a central role in fibroblast processes including cell activation, migration and proliferation through the regulation of cell membrane potential. Evidence from a number of approaches demonstrates that K_Ca 3.1 plays an important role in the development of many fibrotic diseases, including idiopathic pulmonary, renal tubulointerstitial fibrosis and cardiovascular disease. The K_Ca 3.1 selective blocker senicapoc was well tolerated in clinical trials for sickle cell disease, raising the possibility of rapid translation to the clinic for people suffering from pathological fibrosis. This review after analysing all the data, concludes that targeting K_Ca 3.1 should be a high priority for human fibrotic disease.

Stretch-activated current in human atrial myocytes and Na+ current and mechano-gated channels' current in myofibroblasts alter myocyte mechanical behavior: a computational study

Background

The activation of stretch-activated channels (SACs) in cardiac myocytes, which changes the phases of action potential repolarization, is proven to be highly efficient for the conversion of atrial fibrillation. The expression of Na⁺ current in myofibroblasts (Mfbs) regenerates myocytes’ action potentials, suggesting that Mfbs play an active role in triggering cardiac rhythm disturbances. Moreover, the excitation of mechano-gated channels (MGCs) in Mfbs depolarizes their membrane potential and contributes to the increased risk of post-infarct arrhythmia. Although these electrophysiological mechanisms have been largely known, the roles of these currents in cardiac mechanics are still debated. In this study, we aimed to investigate the mechanical influence of these currents via mathematical modeling. A novel mathematical model was developed by integrating models of human atrial myocyte (including the stretch-activated current, Ca²⁺–force relation, and mechanical behavior of a single segment) and Mfb (including our formulation of Na⁺ current and mechano-gated channels’ current). The effects of the changes in basic cycle length, number of coupled Mfbs and intercellular coupling conductance on myocyte mechanical properties were compared.

Results

Our results indicated that these three currents significantly regulated myocyte mechanical parameters. In isosarcometric contraction, these currents increased segment force by 13.8–36.6% and dropped element length by 12.1–31.5%. In isotonic contraction, there are 2.7–5.9% growth and 0.9–24% reduction. Effects of these currents on the extremum of myocyte mechanical parameters become more significant with the increase of basic cycle length, number of coupled Mfbs and intercellular coupling conductance.

Conclusions

The results demonstrated that stretch-activated current in myocytes and Na⁺ current and mechano-gated channels’ current in Mfbs significantly influenced myocyte mechanical behavior and should be considered in future cardiac mechanical mathematical modeling.