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The objective of this study was to determine the vasodilating effect of 3β-hydroxy-5-spirostene (diosgenin), a phytoestrogen found in wild yams, using porcine resistance left anterior descending coronary artery. In 5-hydroxytryptamine (3 μM) pre-contracted preparation, diosgenin caused a concentration-dependent (0.01 to 1 μM), endothelium-independent relaxation, with a maximum relaxation of ∼72% at 1 μM. No apparent effect was observed with 17β-oestradiol and progesterone with concentrations ≤0.3 μM, and a relaxation of ∼15% and ∼23% caused by 17β-oestradiol (1 μM) and progesterone (1 μM), respectively. Diosgenin-elicited relaxation was not altered by 7α,17β-[9[(4,4,5,5,5-pentafluoropentyl)sulfinyl]nonyl]estra-1,3,5(10)-triene-3,17-diol (ICI 182,780), mifepristone, (+)-bicuculline, cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL 12330A), glibenclamide and scavengers of reactive oxygen species. The iberiotoxin-sensitive, Ca2+-activated K+ (BKCa) current of single vascular myocytes recorded, using patch-clamp techniques, was markedly enhanced by diosgenin, 17β-oestradiol and progesterone. Application of (9S, 10R, 12R)-2,3,9,10,11,12-hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3′,2′,1′-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid methyl ester (KT 5823, 300 nM) eradicated the enhancement of BKCa amplitude. Diosgenin, 17β-oestradiol and progesterone did not affect whereas phloretin, biochanin A and zearalanone (1 μM each) significantly suppressed [Ca2+]o-induced contraction. In oestrogen competition essay using human breast cancer cell (MCF-7 cells), diosgenin (0.001 nM to 10 μM) did not interact with oestrogen receptor-α, and no displacement of [3H]17β-oestradiol was observed. In oestrogen receptor α- and β-fluorescence polarization competitor assay, diosgenin (100 μM) demonstrated a greater competition with the β-isoform of oestrogen receptor. These results suggest that diosgenin caused an acute, endothelium-independent coronary artery relaxation via protein kinase G signalling cascade and an activation of BKCa channel of arterial smooth muscle cells. The oestrogen receptor (α and β-isoforms) and progesterone receptor are probably not involved. [Copyright &y& Elsevier]

Narirutin is one of the most common flavanones found in citrus fruits. The vascular effects of its analogues naringenin and naringin have been reported but its effects on the cardiovascular system are largely unknown. In this study, relaxation effect of narirutin and its mechanisms of action were investigated by measuring isometric tension in rat mesenteric arteries. Patch-clamping was also used to study the effect of narirutin on potassium channels in vascular smooth muscle cells. Moreover, its effects on phosphorylation of endothelial nitric oxide synthase, cAMP level and phosphodiesterase activity in rat mesenteric arteries were studied by Western blot and biochemical assays. The results showed that pre-incubation of rat mesenteric arteries with narirutin had no influence on acetylcholine-induced endothelial-dependent relaxation. However, narirutin caused a direct concentration-dependent relaxation in rat mesenteric arteries. This relaxation effect was comparable to that of narirutin's structural analogue naringenin. Narirutin-induced relaxation was reduced by the removal of endothelium, N G-nitro-L-arginine methyl ester (a nitric oxide synthase inhibitor), and 4-aminopyridine (a voltage-gated potassium channel blocker). In addition, narirutin increased the phosphorylation of endothelial nitric oxide synthase and increased the voltage-dependent potassium current in mesenteric arterial smooth muscle cells. These effects were abolished by protein kinase A inhibitor. Furthermore, narirutin could increase cAMP level and inhibit phosphodiesterase activity in rat mesenteric arteries. In conclusion, narirutin has vasorelaxing effect and the mechanism involves the inhibition of phosphodiesterase, which increases intracellular cAMP, thereby stimulating the endothelial nitric oxide synthase and activating the voltage-gated potassium channels in vascular smooth muscle cells. [ABSTRACT FROM AUTHOR]