Background: Relaxins are small peptide hormones, which are novel candidate molecules that play important roles in cardiometablic syndrome. Relaxins are structurally related to the insulin hormone superfamily, which provide vasodilatory effects by activation of G-protein-coupled relaxin receptors (RXFPs) and stimulation of endogenous nitric oxide (NO) generation. Recently, relaxin could be demonstrated to activate G(i) proteins and phosphoinositide 3-kinase (PI3K) pathways in cultured endothelial cells in vitro. However, the contribution of the G(i)-PI3K pathway and their individual components in relaxin-dependent relaxation of intact arteries remains elusive. Methods: We used G alpha(i2)- (Gnai2(-/-)) and G alpha(i3)-deficient (Gnai3(-/-)) mice, pharmacological tools and wire myography to study G-protein-coupled signaling pathways involved in relaxation of mouse isolated mesenteric arteries by relaxins. Human relaxin-1, relaxin-2, and relaxin-3 were tested. Results: Relaxin-2 (similar to 50% relaxation at 10(-11) M) was the most potent vasodilatory relaxin in mouse mesenteric arteries, compared to relaxin-1 and relaxin-3. The vasodilatory effects of relaxin-2 were inhibited by removal of the endothelium or treatment of the vessels with N (G)-nitro-L-arginine methyl ester (L-NAME, endothelial nitric oxide synthase (eNOS) inhibitor) or simazine (RXFP1 inhibitor). The vasodilatory effects of relaxin-2 were absent in arteries of mice treated with pertussis toxin (PTX). They were also absent in arteries isolated from Gnai2(-/-) mice, but not from Gnai3(-/-) mice. The effects were not affected by FR900359 (G alpha(q) protein inhibitor) or PI-103 (PI3K alpha inhibitor), but inhibited by TGX-221 (PI3K beta inhibitor) or AS-252424 (PI3K gamma inhibitor). Simazine did not influence the anti-contractile effect of perivascular adipose tissue. Conclusion: Our data indicate that relaxin-2 produces endothelium- and NO-dependent relaxation of mouse mesenteric arteries by activation of RXFP1 coupled to G(i2)-PI3K-eNOS pathway. Targeting vasodilatory G(i)-protein-coupled RXFP1 pathways may provide promising opportunities for drug discovery in endothelial dysfunction and cardiometabolic disease.
