Abstract 13196: TRPC3 Channels Regulate Nuclear Ca2+ in Atrial Fibrillation
Background: Recent studies reported that Ca2+-permeable transient receptor potential canonical-3 (TRPC3) channels underlie cardiac hypertrophy, but the mechanisms are poorly understood. Atrial fibrillation (AF) activates signaling pathways that alter gene transcription and cause extensive remodeling. We hypothesized that TRPC3 channels expressed in the nuclear envelope may regulate nuclear Ca2+entry and gene transcription.
Methods: Dogs kept in AF for 1 week by pacing at 600 beats/min (n=14) were compared to controls (n=13, CTL). Freshly-isolated left atrial cardiomyocytes (LA CMs) were used for immunostaining, Western blotting and confocal line-scan Ca2+ imaging (fluo-4 AM). Isolated nuclei were used for fluorescent Ca2+ imaging (fura-2 AM). Differential centrifugation was used to purify nuclear and cytosolic cell fractions.
Results: TRPC3-subunit protein expression increased in AF by 61%* (*p<0.05 vs. CTL). Confocal images of LA CMs stained with TRPC3 antibody and TOPRO-3 (nuclear marker) showed perinuclear expression of TRPC3 subunits, which was enhanced in AF. The TRPC3 protein expression in nuclear fractions purified from CMs (relative to cytosolic fractions) increased in AF by 66%*, indicating increased nuclear TRPC3 localization. Cytosolic and nuclear [Ca2+] in LA CMs were simultaneously measured with confocal line scanning during 1 Hz pacing. Nuclear [Ca2+] transient amplitude (versus cytosolic) decreased in AF, whereas nuclear/cytosolic diastolic [Ca2+] ratios increased (values 79% and 130% of CTL, respectively), suggesting nuclear Ca2+ accumulation; treatment with Pyr3 (a TRPC3-selective blocker) returned the values towards CTL (96%+ and 114%+ of CTL, respectively, +p<0.05 vs. AF). In isolated nuclei, angiotensin-II-induced nuclear Ca2+ entry increased in AF (141%* of CTL); Pyr3 suppressed the increased Ca2+ entry in the nucleus (to 101%+ of CTL).
Conclusion: Nuclear TRPC3 channel protein expression, nuclear diastolic [Ca2+] levels and angiotensin-II induced nuclear Ca2+ entry increase in AF CMs. TRPC3 blockade reduces AF-induced nuclear Ca2+ accumulation. Therefore, TRPC3 channels are a potential regulator of nuclear Ca2+ homeostasis, and may contribute to cardiac remodeling in AF and other disease states.
- © 2012 by American Heart Association, Inc.