Vasorelaxant And Antihypertensive Effects Of7,8-Dihydroxyflavone

7,8-Dihydroxyflavone (7,8-DHF) has recently been found to be a potent agonist for tyrosine kinases B (TrkB) receptors.7,8-DHF activates TrkB receptors and produces important biological effects. Its biological effects not only include protecting effects on neurons in the neurodegenerative diseases, e.g. Parkinson’s and Alzheimer’s disease, but also the effects on some neural behaviors, such as memory improvement and anti-depression. Since TrkB receptors are extensively distributed in the body, there is a possibility that DHF can act on the receptors expressed in other systems than nervous system and play some biological roles. However, so far, to our acknowledgement, there have been no publications on the biological roles of DHF in other systems at tissue or organ level, such as arteries except for nervous system. Some flavonoid compounds play an important role in cardiovascular function adjustment.7,8-DHF, as a kind of isoflavone compound, has exhibited various functions. But research in cardiovascular function has not been reported yet.Therefore, in this study, we have performed the following experiments to explore effects of7,8-DHF on vascular function.1. By vascular perfusion, the study tested vasodilatating roles of the7,8-DHF in intact and endothelia-destroyed rat thoracic aortic rings. Dose-dependent relaxation by7,8-DHF was done through a cumulative dosing method.2. ANA-12(an antagonist specific for TrkB receptors) and other inhibitors were used to preincubat the intact or endothelia-destroyed aortas to test mechanisms of the7,8-DHF vasodilatating function3. We further studied the expression of eNOS and its phosphorylation levels after7,8-DHF treatment using Western blotting method; with primarily cultured vascular smooth muscle cell (VSMC), the calcium signaling pathways were further tested.4.7,8-DHF is administrated to spontaneously hypertensive rats (SHR) by intragastric administration and tail vein injection, we observed antihypertensive effects of7,8-DHF. Results1. In intact and endothelia-destroyed aortic rings,7,8-DHF could dose-dependently realax phenylephrine (PE)-precontracted thoracic aortic ring. Half maximum effect concentration (EC50) is24μmol/L in the intact aortic rings and104μmol/L in the endothelia-destroyed rings. The results suggest that7,8-DHF vasodilatating function partly depends on endothelial cells.2. In intact and endothelia-destroyed aortic rings,10and100μmol/L ANA-12could not weaken the vasodilatating function of the7,8-DHF, which suggested7,8-DHF vasodilatation function was not dependent on TrkB receptor pathway.3. In intact aortas, after preincubation with NOS inhibitor L-NAME and sGC inhibitor ODQ, the vasodilatation by7,8-DHF was significantly reduced. While cyclooxygenase inhibitor indomethacin did not reduce the function of7,8-DHF. This suggests that7,8-DHF vasodilatating function is dependent on the NO-cGMP pathway, rather than PG. In order to further verify the the role of the NO-cGMP pathway, with Western blotting method we found that the7,8-DHF could cause the expression eNOS and its phosphorylation level was significantly enhanced in rat thoracic aorta.4. In endothelia-destroyed aortas30,100and300μmol/L7,8-DHF could cause significant decrease of contraction induced by1umol/L PE at calcium-free solution. After the concentration of Ca2+in bath solution gradually increased, aortic rings further contracted correspondingly. Compared with the control group, three concentrations of7,8-DHF could cause contraction intensity to be lowered significantly suggesting that7,8-DHF may have blocked the intracellular calcium release and external calcium influx. It is known that PE contracts blood vessels mainly through receptor-operated calcium channels (ROCC). Therefore, it is possible that7,8-DHF caused vasodilatation thorugh blocking ROCC.7,8-DHF could also weaken PE-induced cellular Ca2+-sensitive fluorescent intensity in primary cultured VSMC.When60mmol/L KC1was used to study the voltage-dependent calcium channels (VDCC), we found that100and300μimol/L7,8-DHF could significantly inhibit KC1-induced contraction. Therefore, it is possible that7,8-DHF caused vasodilatation by blocking VDCC. However, in endothelia-destroyed aortas, various K+channel blockers could not inhibit7,8-DHF-caused vasodilatation.5. When7,8-DHF was administered to SHR intragastrically, the diastolic blood pressure (DBP)was significantly lowered after2h, while the systolic blood pressure (SBP) showed no significant difference. When administered intravenously,7,8-DHF significantly reduced SBP and DBP of the SHR within10to20min. In summary,7,8-DHF could dose-denpendently cause relaxation of rat thoracic aortas. This relaxing effect was mediated by stimulating NO/cGMP and blocking Ca2+signaling pathways, instead of TrkB receptor and K+channels.7,8-DHF significantly reduced SBP and DBP of SHR when administered intravenously, while DBP was significantly reduced after intragastric administration. This study has provided a good foundation for further application of7,8-DHF to hypertension.