Background Early-afterdepolarizations (EADs) are triggers of cardiac arrhythmia driven by L-type Ca2+ current (ICaL) reactivation or sarcoplasmic reticulum (SR) Ca2+ release and Na+/Ca2+ exchange. In large mammals the positive action potential (AP) plateau promotes ICaL reactivation, and the current paradigm holds that cardiac EAD dynamics are dominated by interaction between ICaL and the repolarizing K+ currents. However, EADs are also frequent in the rapidly repolarizing mouse AP, which should not readily permit ICaL reactivation. This suggests that murine EADs exhibit unique dynamics, which are key for interpreting arrhythmia mechanisms in this ubiquitous model organism. We investigated these dynamics in myocytes from arrhythmia-susceptible CaMKIIδC-overexpressing mice (Tg), and via computational simulations. Methods and Results In Tg myocytes, β-adrenergic challenge slowed late repolarization, potentiated SR Ca2+ release, and initiated EADs below the ICaL activation range (−47±0.7 mV). These EADs were abolished by caffeine and tetrodotoxin (but not Ranolazine), suggesting that SR Ca2+ release and Na+ current (INa), but not late INa, are required for EAD initiation. Simulations suggest that potentiated SR Ca2+ release and Na+/Ca2+ exchange triangulate late AP repolarization, which permits non-equilibrium reactivation of INa, and thereby drives the EAD upstroke. AP clamp experiments suggest that lidocaine eliminates virtually all inward current elicited by EADs, and that this effect occurs at concentrations (40-60 μM) for which lidocaine remains specific for inactivated Na+ channels. This strongly suggests that previously inactive channels are recruited during the EAD upstroke, and that non-equilibrium INa dynamics underlie murine EADs. Conclusions Non-equilibrium reactivation of INa drives murine EADs.

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Angiogenesis, the sprouting of new blood vessels from existing vessels, is one of six known mechanisms employed by solid tumors to recruit blood vessels necessary for their initiation, growth, and metastatic spread. The vascular network within the tumor facilitates the transport of nutrients, oxygen, and immune cells and is regulated by pro- and anti-angiogenic factors. Nearly four decades ago, vascular endothelial growth factor (VEGF) was identified as a critical factor promoting vascular permeability and angiogenesis, followed by identification of VEGF family ligands and their receptors (VEGFRs). Since then, over a dozen drugs targeting the VEGF/VEGFR pathway have been approved for ~20 solid tumor types, usually in combination with other therapies. Initially designed to starve tumors, these agents transiently “normalize” tumor vessels in preclinical and clinical studies, and in the clinic, increased tumor blood perfusion or oxygenation in response to these agents is associated with improved outcomes. Nevertheless, the survival benefit has been modest in most tumor types, and there are currently no biomarkers in routine clinical use for identifying which patients are most likely to benefit from treatment. However, the ability of these agents to reprogram the immunosuppressive tumor microenvironment into an immunostimulatory milieu has rekindled interest, and has led to the FDA-approval of 7 different combinations of VEGF/VEGFR pathway inhibitors with immune checkpoint blockers for many solid tumors in the past 3 years. In this review, we discuss our understanding of the mechanisms of response and resistance to blocking VEGF/VEGFR, and potential strategies to develop more effective therapeutic approaches.