Role of the carboxylic group in the antispasmodic and vasorelaxant action displayed by kaurenoic acid

The present work describes the investigation of the role of the carboxylic group in the structure‐activity relationship of the diterpene ent‐kaur‐16‐en‐19‐oic acid (kaurenoic acid, KA) in inhibiting rat aorta contraction. For this purpose the methylation of the C‐19 carboxyl group of KA was carried out. The effects of the obtained ent‐methyl‐kaur‐16‐en‐19‐oate (KAMe) were compared with those induced by KA. Vascular reactivity experiments showed that KA (50 and 100 μm) concentration‐dependently inhibited KCl‐induced contraction in both endothelium‐intact and denuded rat aortic rings. On the other hand, KAMe attenuated KCl‐induced contraction at 100 μm, but not at 50 μm. KA also reduced CaCl2‐induced contraction in Ca2+‐free solution containing KCl (30 mM). Again, KAMe produced a less accentuated reduction in CaCl2‐induced contraction than that induced by the acid KA. KAMe (1–450 μm) concentration‐dependently relaxed KCl‐pre‐contracted rings (percentages of relaxation 82.57±1.65 and 70.55±4.71, respectively) with denuded endothelium. Similarly, the relaxation induced by KA on phenylephrine (Phe)‐pre‐contracted rings (73.06±3.68%) was more pronounced than that found for KAMe (53.68±4.75%). Pre‐incubation of denuded rings for different periods with KA and KAMe showed that the equilibrium periods required by each compound to achieve its maximal inhibitory response on KCl‐induced contraction are different. Collectively, our results provide functional evidence that methylation of the C‐19 carboxyl group of KA reduces but does not abolish the antispasmodic activity displayed by KA. Additionally, we showed that the equilibrium period is a critical step for the inhibitory effect displayed by kaurane‐type diterpenes.