Background: Cardiac fibrosis, a key contributor to heart failure, is driven by the activation of cardiac fibroblasts (CFs), often induced by angiotensin II (Ang II). Relaxin, a peptide hormone, has been reported to counteract fibrotic processes. This study aims to investigate the antifibrotic effects of relaxin on Ang II-induced CF activation, with a focus on the involvement of the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway.

Methods: Primary CFs were isolated and treated with Ang II to induce fibrotic activation. Relaxin was used to assess its antifibrotic effects. Inhibitors of the NO/cGMP pathway, NG-nitro-L-arginine methyl ester (L-NAME) (a nitric oxide synthase inhibitor) and 1H-(1 ,2,4) -Oxadiazolo-(4, 3-a) quinoxalin-1-one (ODQ) (a guanylyl cyclase inhibitor), were co-administered to examine their effects on relaxin-mediated inhibition. Proliferation and migration were assessed using 5-Ethynyl-2'-de oxyur idine incorporation and Transwell assays. Western blot analysis was conducted to measure the expression of alpha-smooth muscle actin (α-SMA), collagen I, and collagen III, key markers of fibroblast activation. Nitric oxide, cGMP, total nitric oxide synthase (TNOS), and inducible nitric oxide synthase (iNOS) levels were measured in the culture media.

Results: Ang II significantly increased CF proliferation, migration, and the expression of fibrosis markers α-SMA, collagen I, and collagen III. Relaxin treatment markedly reduced these effects. Inhibition of the NO/cGMP pathway by L-NAME or ODQ partially reversed relaxin’s suppressive effects on CF proliferation and migration. Relaxin restored Ang II-induced reductions in NO, cGMP, and TNOS levels, while iNOS levels remained largely unchanged, except for a reduction in the L-NAME group.

Conclusion: Relaxin attenuates Ang II-induced cardiac fibroblast activation and fibrosis primarily through the NO/cGMP signaling pathway.