Abstract 178: Volume Overload Induced by Mitral Regurgitation Alters Spatial Regulation of Nitric Oxide/Reactive Nitrogen Species Signaling

Volume-overload (VO), as seen in regurgitant valvular disease, large myocardial infarction, and severe cardiac systolic dysfunction, triggers eccentric hypertrophy. Given that nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) modulate cardiomyocyte hypertrophy, apoptosis, and cardioprotection, differential regulation of these signals may distinguish eccentric from the more commonly studied concentric hypertrophy. We recently showed that pressure-overload (PO) induces relocalization and oxidation of the NO receptor soluble guanylyl cyclase (sGC), thereby diminishing cyclase activity and cGMP cardioprotection. The effects of volume-overload on NO and cGMP signaling are unknown. We tested the hypothesis that VO induces relocalization but not oxidation of sGC, thereby disrupting spatial regulation of NO-cGMP signaling without depressing cyclase activity. Volume-overload was established by chordal rupture-induced mitral regurgitation in dogs. We compared intracellular localization and activity of sGC in VO and control LVs (N=5 per group). Both sGC subunits were detected within and outside of caveolae-enriched lipid raft microdomains (Cav3+LR). In VO hearts, sGCβ1 fell in expression by nearly 50% and relocalized away from Cav3+LR to non-lipid raft microdomains (NLR). Despite VO-induced sGCβ1 changes, overall NO-stimulated sGC activity was preserved. An enhanced response to heme/NO-independent sGC activator BAY 60-2770 suggested that a subset of sGC was heme-oxidized in VO hearts, though to a much lesser degree than in PO hearts. As in PO hearts, Cav3+LR appear to protect sGC from heme-oxidation in VO hearts. Initial study of downstream reactive nitrogen species (RNS) and cGMP signaling supported our theory that VO altered spatial regulation of NO-cGMP signaling. Also, a trend towards increased overall tyrosine-nitration, predominantly within NLR, was observed in VO hearts. Volume-overload shifted cardiac NO-cGMP signaling from Cav3+LR to NLR microdomains without depressing NO/heme-dependent sGC activation. These findings suggest that differential spatial regulation of NO/RNS signaling, rather than simply increased RNS signaling, might drive the distinct molecular pathophysiology of eccentric hypertrophy.