Kaempferol enhances endothelium-independent relaxation: the role of cAMP pathway and K+ channels

Abstract

OBJECTIVE: There is an increasing interest in the use of Chinese herbal medicine and compounds isolated from these herbs. One of them is the dried flower of Carthamus tincturius. This traditional Chinese herb is used for the management of various cardiovascular disorders, including thrombosis, hypertension and coronary artery disease. It possesses a high content of biological active flavonoid derivatives including kaempferol. The aim of this study was to assess the effect of acute exposure to kaempferol on vasomotor changes in isolated porcine coronary arterial rings and the signalling pathway(s) for its action. METHODS: Isometric tension was recorded in isolated porcine coronary arterial rings incubated in 37o C oxygenated modified Krebs-Henseleit solution. Rings were contracted with the thromboxane A2 analogue, U46619 (30 nM) before the addition of increasing concentrations of kaempferol (1 nM-100 µM). In another study, arterial rings were pretreated with low concentrations of kaempferol (0.1-10 µM) for 20 minutes before being contracted with U46619 (30 nM). Responses to sodium nitroprusside (an endothelium-independent vasodilator, 1 nM - 100 µM) were determined in endothelium-denuded and intact rings. In order to investigate the mechanism of action of kaempferol, blockers of the cAMP and cGMP pathways, and various K+ channel blockers were used. An enzyme immunoassay kit was used to measure the level of cAMP and cGMP in the porcine coronary arteries after incubation with kaempferol (10, 50 and 100 µM). RESULTS: Kaempferol only caused a significant relaxation at high doses (>10 µM) and the relaxation was concentration-dependent. Kaempferol, at a concentration without any relaxing action (<10 µM), significantly enhanced the vasorelaxation response to sodium nitroprusside in endothelium-intact and -denuded porcine coronary arteries. Our data also showed that the potentiating effect of kaempferol could be blocked by Rp-8-BrcAMP (a cAMP blocker, 20 µM) but not by Rp-8-Br-cGMP (a cGMP blocker, 10 µM) and ODQ (a guanylyl cyclase inhibitor, 10 µM). The enhanced relaxation of kaempferol was also partly reversed by various K+ channel blockers (TEA, 10 mM, iberotoxin, and apamin + charybdotoxin). Enzyme immunoassay indicated that different concentrations of kaempferol did not increase the cAMP and cGMP levels. CONCLUSION: Kaempferol caused direct relaxation and potentiated vascular reactivity to sodium nitroprusside in endothelium-intact and denuded porcine coronary arteries. Our results suggest that kaempferol enhances sodium nitroprusside-induced relaxation via the cAMP pathway but not cGMP pathway. Since kaempferol did not affect the totel cAMP content, it suggests that a specific pool of cAMP might be involved. Furthermore, K+ channels also play a role in this event. The interplay between cAMP and K+ channels requires further investigations.