Objectives 7,8-Dihydroxyflavone (7,8-DHF) is a new potent agonist for TrkB receptors. 7, 8-DHF exerts protecting effects on neurons in Parkinson’s and Alzheimer’s diseases and stroke, improves memory and antagonises depression. Its potentiality of becoming a health product or a clinical medicine has made us think about its roles in other systems (e.g. cardiovascular system) beyond the nervous system. In this study, we selected the rat aorta to examine the effect of 7,8-DHF on arterial functions, and spontaneously hypertensive rats (SHR) to observe its effect on the blood pressure.
Methods Rat aortic ring preparation and measurement of aortic tension changes. Rings were prepared with normal Wistar rats. The rings bathed in the gassed (95% O2/5% CO2) K-H solution at 37ºC were mounted to a force-displacement transducer and PowerLab data acquisition system (ADInstruments). The prepared rings were preconstricted with phenylephrine (PE, 1 μM). The presence of functional endothelium was assessed by ACh (1 μM). The effect of 7, 8-DHF on the PE-preconstriction, was measured by the tension change (g).
Protein extraction and Western blotting were performed to study the effect of 7,8-DHF on eNOS expression and its phosphorylation.
Ca imaging:Primary culture of aortic smooth muscle cells was performed to find the effect of 7,8-DHF on the cellular Ca signalling detected with fura-3 and with confocal.
Measurement of rat blood pressure (BP) by tail cuff method. Normal and SHR rats were given 7,8-DHF orally or intravenously. The changes of systolic (SBP), diastolic (DBP) and mean (MBP) blood pressure were monitored by the tail cuff method (BP-98A, Softron).
Results With the isolated aortic rings from normal rats, we found that 7, 8-DHF dose-dependently dilated phenylephrine (PE)-preconstricted endothelia-intact aortic rings with EC50 = ∼24 µM. Both L-NAME (a NOS inhibitor) and ODQ (a soluble guanylyl cyclase blocker) significantly reduced the vasorelaxant effect of 7, 8-DHF. Western blotting showed that 7, 8-DHF increased the aortic eNOS protein expression and phosphorylation. The mechanism of 7, 8-DHF dilating effect seemed to be mediated by NO/cGMP pathway, instead of TrkB receptors because ANA-12, its specific antagonist, did not show any effect against 7, 8-DHF. With endothelia removed, 7,8-DHF also dilated the PE-preconstricted rings but with EC50 = ∼104 µM, probably through blocking Ca2+signalling pathway because 7, 8-DHF blocked Ca influx detected with Ca imaging. 7, 8-DHF, when administered intravenously or orally, significantly reduced the blood pressure of the spontaneously hypertensive rats without affecting the heart rates of the animals.
Conclusions 7, 8-DHF dilated the aortic rings via NO/cGMP and Ca signalling pathways. 7, 8-DHF in vivo could significantly reduce the blood pressure of the spontaneously hypertensive rats.