Analysis of the mechanisms underlying the vasorelaxant action of kaurenoic acid in the isolated rat aorta

The present work describes the mechanisms involved in the vasorelaxant effect of the diterpene ent-kaur-16-en-19-oic acid (kaurenoic acid). Kaurenoic acid (10, 50 and 100 microM) concentration-dependently inhibited phenylephrine and KCl-induced contraction in either endothelium-intact or -denuded rat aortic rings. Kaurenoic acid also reduced CaCl(2)-induced contraction in CA(2+)-free solution containing KCl (30 mM). The diterpene did not interfere with CA(2+) release from intracellular stores mediated by either phenylephrine (1 microM) or caffeine (30 mM). Kaurenoic acid (1-450 microM) concentration dependently relaxed phenylephrine-pre-contracted rings with intact (72.27+/-3.79%) or denuded endothelium (73.28+/-5.91%). The diterpene also relaxed KCl-pre-contracted rings with intact (80.44+/-3.68%) or denuded endothelium (78.12+/-1.26%). Pre-incubation of denuded aortic rings with N(G)-nitro-l-arginine methyl ester (l-NAME, 100 microM), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 1 microM) and 7-nitroindazole (100 microM) reduced kaurenoic acid-induced relaxation (percentage of relaxation: 49.12+/-3.26%, 53.10+/-6.72% and 51.74+/-4.76%, respectively). Indomethacin (10 microM) did not affect kaurenoic acid-induced relaxation. In endothelium-intact rings, 7-nitroindazole and N(pi)-nitro-l-arginine (l-NNA, 100 microM) displaced the curves for the diterpene to the right. Tetraethylammonium (5 mM), 4-amynopiridine (1 mM) and charybdotoxin (0.1 microM) caused a rightward displacement of the concentration-response curve for kaurenoic acid. Conversely, neither apamin (1 microM) nor glibenclamide (3 microM) affected kaurenoic acid-induced relaxation. Collectively, our results provide functional evidence that the effects elicited by kaurenoic acid involve extracellular CA(2+) influx blocked. Its effects are also partly mediated by the activation of NO-cGMP pathway and the opening of K(+) channels sensitive to charybdotoxin and 4-amynopiridine. Additionally, the activation of the endothelial and neuronal NO Synthase isoforms are required for the relaxant effect induced by kaurenoic acid.