Gnaphaliin A and B relax smooth muscle of guinea-pig trachea and rat aorta via phosphodiesterase inhibition.

OBJECTIVES: To explore the relaxant mechanism of action of gnaphaliin A and gnaphaliin B in guinea-pig trachea and rat aorta, and to investigate the theoretical and experimental phosphodiesterase (PDE) inhibitory activity of these flavones.

METHODS: The relaxant effect and the inhibition of calcium chloride induced contractions of both flavones were evaluated on guinea-pig trachea and rat aorta rings. The PDE inhibitory activity was evaluated using a cyclic nucleotide PDE colorimetric assay kit with cAMP and cGMP as substrates. The docking analysis was carried out with AutoDock4 software and X-ray structure of PDE type 5. The activity of both gnaphaliins was compared with the activity of sildenafil, rolipram, aminophylline, IBMX and enoximone.

KEY FINDINGS: Gnaphaliin A and B were more actives as relaxants on rat aorta than guinea-pig trachea. They were less potent in the relaxation of guinea-pig trachea and rat aorta than sildenafil, but they were equal or more potent than the other PDE inhibitors tested. The relaxant effect of these flavones was potentiated by nitroprusside and forskolin, and blocked by 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one but not by 2',5'-dideoxyadenosine in guinea-pig trachea. L-NAME did not modify the relaxant effect of gnaphaliins. Gnaphaliins were more potent as PDE inhibitors when cGMP was used as substrate. Docking analysis revealed that gnaphaiins bind to the same binding site of sildenafil at PDE type 5.

CONCLUSIONS: The results suggest that the main relaxant mechanism of action of gnaphaliin A and B is inhibition of PDEs with a preference to inhibit the degradation of cGMP. The docking study suggested that these flavones bind with high specificity to the same binding site of sildenafil at PDE type 5.