Mechanisms underlying the relaxation induced by isokaempferide from Amburana cearensis in the guinea-pig isolated trachea

Abstract

The present study examines possible mechanisms by which the flavonoid isokaempferide (IKPF; 5,7,4′-trihydroxy-3-methoxyflavone) from Amburana cearensis, a Brazilian medicinal plant popularly used as bronchodilator, induces relaxation of guinea-pig isolated trachea. In the trachea (with intact epithelium) contracted by carbachol, IKPF (1–1000 μM) caused a graded relaxation, and the epithelium removal increased the sensitivity of the airway smooth muscle to IKPF (EC50, in intact tissue: 77.4 [54.8–109.2] μM; in denuded epithelium: 15.0 [11.3–20.1] μM). The IKPF-induced relaxation was inhibited in 41% by the nitric oxide (NO) synthase inhibitor l-NAME (100 μM); in 31% and 50% by the soluble guanylate cyclase (sGC) inhibitor ODQ (3 and 33 μM); by propranolol (31%) and also by capsaicin (37%). In the trachea pre-contracted by 40 mM KCl the pre-incubation with glibenclamide (33 μM) or iberiotoxin (IbTX, 0.1 μM), selective K + channel inhibitors, inhibited the IKPF-induced relaxation by 39% and 38%, respectively. On the other hand, 4-aminopyridine (100 μM), a nonselective K + channel antagonist, did not significantly influence the effect of IKPF, while IbTX induced a rightward displacement of the IKPF concentration–response curve. However, in muscle pre-contracted with 120 mM KCl the relaxant effect of IKPF was significantly reduced and not affected by glibenclamide. In conclusion, these results indicate a direct and epithelium-independent relaxant effect of IKPF on smooth muscle fibers. Although this IKPF relaxant action seems to be multi-mediated, it occurs via both Ca 2+ and ATP-sensitive K + channels, but some other possible mechanisms unrelated to K + channels cannot be excluded.