Functional interaction of H2-receptors and 5HT4-receptors in atrial tissues isolated from double transgenic mice and from human patients

In the past, we generated transgenic mice that overexpress the human histamine 2 (H2)-receptor (H2-TG) or that overexpress the human serotonin 4 (5-HT4)-receptor (5-HT4-TG) in the heart. Here, we crossbred these lines of mice to generate double transgenic mice that overexpress both receptors (DT). This was done to study a conceivable interaction between these receptors in the mouse heart as a model for the human heart. When in left atria, initially, force of contraction was elevated maximally with 1 µM serotonin, and subsequently, histamine was cumulatively applied; a biphasic effect of histamine was noted: the force of contraction initially decreased, maximally at 10 nM histamine, and thereafter, the force of contraction increased again at 1 µM histamine. Notably, functional interaction between 5-HT and histamine was also identified in isolated electrically stimulated trabeculae carneae from human right atrium (obtained during cardiac surgery). These functional and biochemical data together are consistent with a joint overexpression of inotropically active H2-receptors and 5-HT4-receptors in the same mouse heart. We also describe an antagonistic interaction on the force of contraction of both receptors in the mouse atrium (DT) and in the human atrial muscle strips. We speculate that via this interaction, histamine might act as a “brake” on the cardiac actions of 5-HT via inhibitory GTP-binding proteins acting on the activity of adenylyl cyclase.

Abstract 284: Disruption of Nrf2 Promotes Atrial Remodeling and Fibrillation on Aging

Background: Heart failure is a leading health problem with over 500,000 new cases and 275,000 deaths annually in the United States. Recent reports indicate that atrial fibrillation (AF) is linked to various forms of ventricular dysfunction (VD). Over 2 million Americans have AF, which is more prevalent in people over 65 years of age. However, the molecular “cause and effect” relationship between AF and VD has not been elucidated. We hypothesize that abrogation of nuclear erythroid-2 like factor-2 (Nrf2), a master antioxidant transcriptional regulator, induces atrial remodeling and fibrillation on aging. Methods: Age and sex matched WT and Nrf2-/- mice were used in this study. Atrial mass, remodeling, antioxidants and molecular redox signaling were studied at 2 and >20 months of age. Echocardiography, immunoblotting, quantitative real-time PCR and immunofluorescence (fibrosis) analyses were performed in the atrial tissue. Results: At 2 months of age, WT and Nrf2-/- mice had comparable levels of ROS in the atrium. On aging, the ROS levels were significantly increased in atria of Nrf2- null when compared to wild-type (WT) mice at 20 months of age. While decreased Nrf2-antioxidant signaling in response to increased ROS generation in atria of Nrf2- null mice was observed, ventricular function appeared to be normal at 20 months of age. However, upon endurance exercise stress (EES), hypertrophy markers including ANF, BNF, PLN and SERCA2A were significantly (p<0.05) altered in Nrf2- null when compared to age-matched WT mice. Further, activation of fibrotic process was evident in the Nrf2- null mouse atrium as indicated by significantly (p<0.05) increased markers of tissue remodeling (i.e. MMP2/9) on aging. These results indicating an early onset of atrial hypertrophy/remodeling due to age-induced oxidative stress, which cause AF in Nrf2- null mice. Age-dependent decline in Nrf2 and sustained progression of AF could lead to ventricular dysfunction and heart failure. Conclusion: Our findings indicate that atria are primary targets to age-associated oxidative stress and exhibit fibrosis, which promotes atrial fibrillation and remodeling. Thus, activation of Nrf2 signaling to prevent oxidative stress could be a potential therapeutic target for AF.