Vasorelaxing Effect Of 7, 8-dihydroxyflavone In Endothelium-denuded Rat Aorta And Its Underlying Mechanism

7, 8-dihydroxyflavone(7, 8-DHF), a member of flavonoid family, has been identified as a selective tyrosine kinase receptor B(Trk B) agonist and exerts multiple biological functions. Our previous study found that 7, 8-dihydroxyflavone(7, 8-DHF)dilated phenylephrine(PE)-precontracted, endothelium-intact rat aorta through activating NO/c GMP signaling pathway. However, in the endothelium-denuded aorta,the underlying mechanism for 7, 8-DHF is still unknown. In the present study, we further investigated the vasorelaxing mechanism of 7, 8-DHF in KCl or PE-preconstricted, endothelium-denuded aortic rings. The results were as follows:1. 7, 8-DHF(1-300 μM) exerted dose-dependent vasorelaxation in 60 m M KCl or 1μM PE-preconsticted rat aortic rings. However, at the same concentration, 7,8-DHF showed higher relaxation ratio in PE-preconstricted aorta, indicating that the vesorelaxing effect of 7, 8-DHF might be different in two kinds of aortic rings.2. In KCl-preconstricted aortic rings, we had the following findings:(1) During Ca2+ recovery, pretreatment with 7, 8-DHF markedly inhibited KCl-induced contraction, suggesting that 7, 8-DHF might block the KCl-induced extracellular Ca2+ influx.(2) Pretreatment with 7, 8-DHF reduced the aorta contraction induced by VDCC activator FPL64176.(3) Pretreatment with the VDCC blocker nifedipine significantly reduced the vasorelaxation ratio of 7,8-DHF(300 μM).3. In PE-preconstricted aortic rings, we had the following findings:(1) 7, 8-DHF reduced PE-induced aorta contraction both in extracellular Ca2+free buffer and after Ca2+ recovery, indicating its blocking effects on intracellular Ca2+ release and extracellular Ca2+ influx.(2) Ryanidine receptor(Ry R) activator caffeine induced rapid but short contraction, and this effect was attenuated by 7, 8-DHF.(3) In U73122(a PLC inhibitor)- pretreated aorta, the vasorelation ratio of 7,8-DHF was similar to that in the control group. However, pretreatment with 2-APB(an IP3 R blocker) markedly inhibited the vasorelaxing effect of 7,8-DHF.(4) SKF96365(a nonspecific SOCC blocker)attentuated the vasorelaxing effect of 7, 8-DHF. In contrast, nifedipine had no similar effect.(5) Pretreatment with Y27632(a ROCK blocker)did not affect the vasorelaxing effect of 7, 8-DHF.(6) Pretreatment with 7, 8-DHF reduced PE-induced contraction from 30 n M-10μM. The reduced maximal effect of PE indicates that 7, 8-DHF is not a competitive antagonist for α-receptor in vascular smooth muscle cells(VSMCs).In summary, 7, 8-DHF exerted vasorelaxing effect in KCl or PE-preconstricte rat aorta with endothelium removal. In KCl-preconstricted aorta, the dilating effect of 7,8-DHF is due to its blocking VDCC. In PE-preconstricted aorta, the dilating effect of7, 8-DHF might be attributed to the reduced intracellular Ca2+ release through IP3 R and Ry R as well as the reduced extracellular Ca2+ influx through SOCC. However, the vasorelaxing effect of 7, 8-DHF seems to be not mediated by VDCC or ROCK signaling pathway. Furthermore, we found that pretreatment with 7, 8-DHF attenuated the aorta constriction induced by cumulative concentrations of PE.