Abstract 84: Atrial Fibrillation Causes a Progressive, Time-Dependent Reduction in Myocardial Neuronal NOS in Humans and Goats: Implications for AF-Induced Electrical Remodeling

Abstract

Rationale: Nitric oxide (NO) plays a key role in the regulation of cardiac and endothelial function and thrombogenesis. Atrial fibrillation (AF) has been associated with reduced NO availability but the mechanisms and implications of this finding remain to be fully investigated. Methods and Results: We evaluated NO synthase (NOS) activity and localization in right atrial (RA) tissue from 30 patients with permanent AF (vs. 65 controls in sinus rhythm, SR), and in the RA and left atrial (LA) tissue of 48 goats after 2 weeks (2W) and 6 months (6M) of pacing-induced AF. NOS activity was uncoupled in RA tissue from patients and goats in 6M-AF, and was caused by a reduction in BH4 tissue concentration and by an increase in arginase activity (HPLC). Although BH4 and arginine supplementation re-coupled NOS, it did not abolish the difference in NOS activity between AF and SR. Immunoblotting and immunolocalization revealed a progressive reduction in bi-atrial neuronal NOS (nNOS) protein with the duration of AF (by 65% at 2W, 86% at 6M in goats and by 62% in patients with AF) and a reduction in eNOS in long-standing AF. nNOS was reduced in atrial myocytes but not in neuronal tissue. The mRNA expression of NOS (qRT-PCR) was unaltered; however, the reduction in nNOS protein in AF was associated with an increase in nNOS ubiquitination which was partially reversed by inhibition of proteosomal activity with MG132; inhibition of the autophagy-lysosomal pathway with bafilomycin A1 did not restore nNOS protein. To investigate the electrophysiological consequences of a reduced nNOS in LA and RA myocytes, we compared electrical properties of the isolated atrial myocytes from nNOS-/- mice (n=18) and their wild type (WT) littermates after nNOS inhibition with SMTC. Both nNOS gene deletion and inhibition impaired myocytes' relaxation in both RA and LA, and result in a slower rate of decay of [Ca2+]i transient in the LA myocytes only. Conclusions: A reduction in bi-atrial nNOS activity and protein level is an early event in the natural history of AF that results in a chamber-specific effect on electrical properties of the myocytes.