7,8-dihydroxyflavone enhanced cholinergic contraction of rat gastric smooth muscle via augmenting muscarinic M3 receptor expression.

Although 7,8-dihydroxyflavone (7,8-DHF), a synthetic agonist specific for tyrosine kinase receptor B (TrkB), has been reported to promote intestinal dynamics, its effect on gastric dynamics has not been studied as yet. In this study, we explored how 7,8-DHF affected the carbachol (CCh)-induced contraction of rat gastric muscle by way of measuring the contractile tension of muscular strips. We found that although 7,8-DHF did not directly cause contraction of gastric muscle, it enhanced CCh-induced, instead of substance P- or high K+ -induced, contraction. The enhancing role of 7,8-DHF was partially blocked by ANA-12, a blocker specific for TrkB the activation of which in the gastric strips was evidenced by its phosphorylation. Although 7,8-DHF alone did not activate : phospholipase C (PLC)-γ in gastric muscle, CCh did, and importantly, the combined treatment with CCh + 7,8-DHF activated more PLC-γ. U73122, an antagonist to PLC-γ blocked both the CCh-induced and the 7,8-DHF-enhanced/CCh-induced contraction by ~30%. To pursue how 7,8-DHF could augment CCh-activated PLC-γ phosphorylation, we first examined the effect of 7,8-DHF on the expression of muscarinic receptors in gastric muscle and found that 7,8-DHF specifically increased M3 but not M2 receptor expression possibly through TrkB/Akt (protein kinase B) pathway because the Akt antagonist, LY294002 significantly suppressed the 7,8-DHF-augmemted M3 expression and completely blocked the 7,8-DHF-enhanced cholinergic contraction. Supporting the result, Akt phosphorylation in the gastric muscle was enhanced by 7,8-DHF treatment. The in vivo experiment showed that orally fed 7,8-DHF increased gastric emptying rate. The results imply a possibility that 7,8-DHF may be developed into a drug in the future for enhancing gastric dynamics.