Endothelium-Independent and Endothelium-Dependent Vasorelaxation by a Dichloromethane Fraction from Anogeissus Leiocarpus (DC) Guill. Et Perr. (Combretaceae): Possible Involvement of Cyclic Nucleotide Phosphodiesterase Inhibition

Many traditional medicinal herbs from Burkina Faso are used to treat arterial hypertension (HTA). Among them, Anogeissus leiocarpus (A. Leiocarpus) which is well known and widely used in Burkina traditional medicine. Herein we assess the effects of dichloromethane fraction from A. leiocarpus stem bark (ALF), selected as the most active on cyclic nucleotide phosphodiesterases (PDEs) and characterized its specificity towards purified vascular PDE1 to PDE5 isoenzymes and study its effects on a vascular model. ALF potently and preferentially inhibits (IC₅₀=1.6 ± 0.6 µg/mL) the calmodulin-dependent phosphodiesterase PDE1, being mainly present in vascular smooth muscle and preferentially hydrolyses cGMP. In the same range (IC₅₀ =2.8 ±0.2 µg/ml) ALF inhibits PDE2, a cGMP-activated enzyme that is only present in endothelial cells and hydrolyses both cAMP and cGMP. PDE5, which specifically hydrolyses cGMP and which mainly contributes to cGMP hydrolysis is also potently inhibited by ALF (IC₅₀=7.6±3.5 µg/ml). The potencies of ALF on cAMP hydrolyzing isoenzymes was lesser, being more effective on PDE4 (IC₅₀= 17.6±3.5 µg/ml) than on PDE3 (60.9 ± 1.8 µg/ml). Since the major effect of ALF were against cGMP hydrolysis and since cGMP is implicated in endothelium-dependent relaxation, the endothelium-dependent vasorelaxation was studied on isolated porcine coronary arteries rings pre-contracted with U46619. The endothelium-dependent vasorelaxation is significantly inhibited by N^ω-nitro-L-arginine (LNA 300 µmol/L, an inhibitor of endothelial NO synthase), but not affected by charybdotoxin (CTX, 100nM) plus apamin (APA, 100nM) (two inhibitors of EDHF-mediated responses). The combination of 4-aminopyridine (4-AP, 1 mmol/L, inhibitor of voltage-dependent potassium channels, K_v) plus baryum (Ba²⁺, 30 µmol/L, inhibitor of the potassium channels with entering correction, K_ir) plus ouabain (3 µmol/L, inhibitor of ATPase Na⁺/K⁺ channels) partially inhibits endothelium-independent vasorelaxant effect. This endothelium-independent relaxant effect was also sensitive to combination of 1H-1,2,4]-oxadiazole-4,3-α]-quinoxalin1-one (ODQ, 10 µM, soluble guanylyl cyclase inhibitor) and N-2-(p-Bromocinnamylamino)ethyl]-5-isoquinoline sulfonamide dihydrochloride (H89, 100 nM, Protein Kinase A inhibitor). Taken together, these results indicate that ALF is a powerful vasodilator modulated by the formation of NO from endothelium, but also act by directly relaxing the vascular smooth muscle cells, by inhibiting cGMP hydrolyzing PDEs (PDE1, PDE2 and PDE5) and to a lesser extend on cAMP degradation (PDE3 and PDE4), cAMP and cGMP being second messengers involved in vascular relaxation.