Apelin Inhibits Angiotensin II-Induced Atrial Fibrosis and Atrial Fibrillation via TGF-β1/Smad2/α-SMA Pathway

FIGURE 1. Plasma Ang II and apelin levels. Ang II plasma levels were significantly higher in both Ang II (n = 16) and Ang II + apelin groups (n = 16) compared with the corresponding groups before administration. Apelin plasma levels were significantly higher in Ang II + apelin group than predose. ∗P < 0.001.

FIGURE 2. Atrial fibrosis of control, Ang II and Ang II + apelin groups (n = 16 in each group). (A) Representative images of HE, Masson’s trichrome staining and Picrosirius Red stained atrial sections. (B–D) Quantification of fibrotic areas of HE, Masson’s trichrome and Picrosirius Red stained atrial sections, respectively. Ang II group showed a higher extent of atrial fibrosis, while concomitant apelin treatment led to a significant reduction of atrial fibrotic areas in Ang II + apelin group. Scale bar indicates 10 μm. ∗P < 0.001.

FIGURE 3. Representative images of epicardial multi-electrode activation mapping. The arrow indicates the conduct direction. The images of epicardial multi-electrode activation mapping and velocity-time curve both demonstrated significant inhomogeneity increase in Ang II group compared with control group (inhomogeneity index of Ang II group vs. control group = 4.21 ± 0.32 vs 1.73 ± 0.26, P < 0.001), which was completely abolished in Ang II + apelin group (inhomogeneity index of Ang II group vs Ang II + apelin group = 4.21 ± 0.32 vs 2.16 ± 0.35, P < 0.001) (n = 16 in each group).

FIGURE 4. Apelin inhibits fibrosis by inhibiting TGF-β1/Smad2/α-SMA pathway. (A) DDR2/a-SMA co-staining of murine atrial sections showed a significant higher amount of fibrosis in the atrial tissue of Ang II group compared with the control group, while apelin treatment reduced the fibrosis to the level of animals in the control group. Scale bar indicates 50 μm. (B) Apelin reduces TGF-β1-induced phosphorylation of Smad2, as detected by western blot. ∗P < 0.001 (n = 16 in each group).

FIGURE 5. Apelin attenuatation elevated AF vulnerability. (A) Representative figures of ECGs with and without atrial fibrillation episodes after stimulation. ECG interference caused by stimulus current could not be ruled out. (B,C) Ang II treatment led to a distinct elevation of inducibility of AF and the total time of AF episodes, whereas in the Ang II + apelin group, the inducibility of AF and total time of AF episodes are significantly decreased. ∗P < 0.001 (n = 16 in each group).

FIGURE 6. The mechanism of apelin reduces fibrosis in the atrium of Ang II-treated rat through TGF-β1/Smad2/α-SMA signaling pathway.