Pannexin (Px) channels, first been described in 2000, have been mostly studied in the brain where they reportedly play a role in synaptic function, propagation of Ca waves and regulation of vascular tone. Much less is known about their role in the heart. In general, three isoforms have been described (Px1, 2, 3). Px channels from large pores permeable to molecules up to 1 kDa and can open at physiological extracellular Ca. In ventricular cardiomyocytes Px channels are expressed in very low density, but single channel conductance is so large (>300 pS) that it can be resolved in whole cell patch clamp. The objective of the proposal is to test the novel hypothesis that Px channel activation during diastolic Ca release facilitates triggered heart beats. Diastolic Ca releases from the sarcoplasmic reticulum (SR) Ca store and triggered beats are hallmarks of inherited catecholaminergic polymorphic ventricular tachycardia (CPVT) and are also found as acquired defect after myocardial infarction (MI). Diastolic Ca releases cause a transient inward current (Iti) that depolarizes the cell membrane (termed delayed after depolarization's, DADs), which, when large enough to activate the Na channel, trigger an action potential (AP) and a propagated beat. It is generally accepted that Iti is foremost generated by the Na Ca exchanger (NCX) transporting Ca out of the cell. However, it remains unclear how NCX can generate enough current to trigger an AP in intact tissue, where ventricular myocytes are electrically coupled and effectively stabilized by a large current sink. Our preliminary data indicate that Px channel opening during diastolic Ca release contributes to Iti in cardiomyocytes from a CPVT mouse model, the calsequestrin KO mice (Casq2-/-). Moreover, pharmacologic inhibition of Px channels or genetic KO of Px1 effectively reduced triggered arrhythmia in Casq2-/- mice. Thus, our central hypothesis is that Px channels importantly increase DADs caused by premature SR Ca release and elevate arrhythmia susceptibility. The proposed experiments will investigate Px channel activation and contribution to Iti in isolated cardiomyocytes (Aim 1) and test arrhythmia susceptibility in vivo using Casq2-/- mice (Aim 2) and mice with coronary artery ligation (Aim 3). Accomplishing these aims will provide new mechanistic information on Px channels in arrhythmia and may lead to a new paradigm for treating Ca triggered arrhythmia in common heart diseases such as MI.
Annually more than 300,000 cases of sudden cardiac death (SCD) occur in the United States alone, accounting for 10-20% of all deaths in adults in the US. Triggered arrhythmias importantly contribute to acquired and inherited conditions associated with SCD. The proposed research will investigate if pannexin channels importantly facilitate triggered arrhythmias and thus can be targeted therapeutically to prevent deadly arrhythmias.
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