Abstract P081: Voltage-Gated TTX-Sensitive Nav1.1 Brain Sodium Channels Are Important for Cardiac Contractile Function

Abstract

Background: Mutations in voltage gated brain sodium channel Nav1.1 have been linked to many disorders, including Generalized Epilepsy with Febrile Seizures Plus (GEFS+) and Severe Myoclonic Epilepsy of Infancy (SMEI). Recent studies have identified TTX- sensitive Nav1.1 brain sodium channels in the SA node and ventricular T-tubules of the heart, though their role in cardiac function is still controversial. We tested the functional significance of Nav1.1 sodium channels in the heart by creating a novel knock-in of human epilepsy GEFS+ mutation SCN1A-R1648H at the Scn1a locus of a C57BL/6J X 129 mouse. Method: In vivo 2-D echocardiography was performed on 2 week old (juvenile) and 8 week old (adult) wild-type and heterozygote (Scn1aRH/+) mice after extracardiac neuronal block through intraperitoneal injections of atropine and propranolol (2.5mg/kg each). Calcium and contractility studies on adult ventricular cardiomyocytes isolated from the wild type and Scn1aRH/+ mice paced at 0.5Hz were followed by administration of TTX (100nM, a brain sodium channel inhibitor) and pacing at 2Hz. qRT-PCR and Western blot of the isolated cells and whole heart samples was also done. Results: A decrease in Nav1.1 gene expression in the Scn1aRH/+ juvenile (by 31%, 0.69 of 1) and adult (by 60%, 0.4 of 1) whole heart samples and isolated cells (p<0.05) was seen. Echocardiography revealed concentric hypertrophy in the juvenile Scn1aRH/+ mice by a significant increase in LV mass, LV mass/body weight ratio, and relative wall thickness (p<0.05). In the adult Scn1aRH/+ mice, systolic isovolumic contraction time (IVCT) was reduced (p=0.03) and decrease in diastolic function was evident through significant decreases in isovolumic relaxation time (IVRT) and E’/A’ ratio, and increase in E/E’ ratio. Isolated adult ventricular Scn1aRH/+ cardiomyocytes showed significant reduction in percent sarcomere shortening, maximum rate of contraction and relaxation, and time to peak contraction, exaggerated with TTX and pacing at 2Hz (p<0.5). Conclusions: Our study demonstrates the importance of voltage gated TTX-sensitive Nav1.1 brain sodium channels in cardiac contractile function and their possible role in cardiac complications in epilepsy.