Vasorelaxant effect of isoliquiritigenin, a novel soluble guanylate cyclase activator, in rat aorta.

1. The vasorelaxant activity of isoliquiritigenin, isolated from Dalbergia odorifera T, was investigated in the phenylephrine-precontracted rat aorta by measuring tension, guanylate and adenylate cyclase activities, guanosine 3':5'-cyclic monophosphate (cyclic GMP) and adenosine 3':5'-cyclic monophosphate (cyclic AMP) levels. 2. Isoliquiritigenin concentration-dependently relaxed rat aorta contracted with phenylephrine, KCl, U-46619, endothelin and 5-hydroxytryptamine, with EC50s of 7.4 +/- 1.6, 10.5 +/- 2.3, 14.3 +/- 3.3, 11.8 +/- 2.0 and 13.6 +/- 3.7 microM, respectively. 3. Isoliquiritigenin caused endothelium-independent relaxation of phenylephrine-precontracted rat aortic rings. Neither NG-monomethyl-L-arginine (L-NMMA) (an inhibitor of the L-arginine-NO pathway) nor oxyhaemoglobin (which binds NO) modified the relaxant effect of isoliquiritigenin. The relaxant action of isoliquiritigenin also persisted in intact aorta in the presence of indomethacin or glibenclamide. However, methylene blue, an inhibitor of soluble guanylate cyclase, abolished relaxation induced by isoliquiritigenin. 4. Incubation of rat aorta with isoliquiritigenin not only increased aortic cyclic GMP content but also caused small increases in aortic cyclic AMP content, and greatly potentiated the increases in cyclic AMP observed in the presence of forskolin. The maximum increase in cyclic GMP by isoliquiritigenin was reached earlier than the increase in cyclic AMP. This result suggests that the increases in cyclic GMP caused by isoliquiritigenin might stimulate the accumulation of cyclic AMP. 5. Concentration-dependent increases in soluble guanylate cyclase activity were observed in isoliquiritigenin (1-100 microM)- or sodium nitroprusside (SNP)-treated rat aortic smooth muscle cells, while adenylate cyclase activity was unchanged in isoliquiritigenin (100 microM)-treated cells. 6. Relaxation and cyclic AMP formation of rat aorta caused by isoliquiritigenin was potentiated in the presence of forskolin (10 nM), which had little effect when given alone. 2',5'-Dideoxyadenosine (DDA,200 microM), an adenylate cyclase inhibitor, diminished the relaxation and cyclic AMP formation of rat aorta by isoliquiritigenin only in the presence of forskolin. DDA did not affect the increases in cyclic GMP formation induced by isoliquiritigenin. These results suggest that elevated levels of cyclic GMP may mediate the majority of the relaxation of the phenylephrine-precontracted aorta induced byisoliquiritigenin, while the synergistic interaction with a low concentration of forskolin depends on an enhanced accumulation of cyclic AMP.7. Relaxation of phenylephrine-precontracted rat aorta and carbachol-precontracted guinea-pig trachea by rolipram (phosphodiesterase, PDE IV inhibitor) was markedly enhanced by isoliquiritigenin, while response to cilostamide (PDE III inhibitor) was not significantly changed by isoliquiritigenin.8. It is concluded that isoliquiritigenin exerts a vasorelaxant effect by activating soluble guanylatecyclase and increasing cyclic GMP. Synergistic effects of isoliquiritigenin and forskolin on muscle relaxation and cyclic AMP accumulation indicate that inhibition of cyclic AMP breakdown by cyclic GMP via the inhibition of PDE III (cyclic GMP-inhibited PDE) is the dominant mechanism.