Implication of Cyclic Nucleotide Phosphodiesterase Inhibition in the Vasorelaxant Activity of the Citrus-Fruits Flavonoid (±)-Naringenin

Abstract

The potential vasorelaxant, antioxidant and cyclic nucleotide phosphodiesterase (PDE) inhibitory effects of the citrus-fruit flavonoids naringin and (±)-naringenin were comparatively studied for the first time in this work. (±)-Naringenin (1 μM - 0.3 mM) did not affect the contractile response induced by okadaic acid (OA, 1 μM). However, (±)-naringenin relaxed, in a concentration-dependent manner, the contractions elicited by phenylephrine (PHE, 1 μM) or by a high extracellular KCl concentration (60 mM) in intact rat aortic rings. Mechanical removal of endothelium and/or pretreatment of aorta rings with glibenclamide (GB, 10 μM) or tetraethylammonium (TEA, 2 mM) did not significantly modify the vasorelaxant effects of this flavanone. (±)-Naringenin (10 μM - 0.1 mM) did not alter the basal uptake of ⁴⁵Ca²⁺ but decreased the influx of ⁴⁵Ca²⁺ induced by PHE and KCl in endothelium-containing and endothelium-denuded rat aorta. (±)-Naringenin (10 μM - 0.1 mM) was ineffective to scavenge superoxide radicals (O₂ ^·-) generated by the hypoxanthine (HX)-xanthine oxidase (XO) system and/or to inhibit XO activity. (±)-Naringenin (0.1 mM) significantly increased the production of cGMP and cAMP decreased by PHE (1 μM) and high KCl (60 mM) in cultured rat aortic myocytes. (±)-Naringenin preferentially inhibited calmodulin (CaM)-activated PDE1, PDE4 and PDE5 isolated from bovine aorta with IC₅₀ values of about 45 μM, 60 μM and 68 μM, respectively. In contrast, the 7-rhamnoglucoside of (±)-naringenin, naringin (1 μM - 0.3 mM), was totally inactive in all experiments. These results indicate that the vasorelaxant effects of (±)-naringenin seem to be basically related to the inhibition of PDE1, PDE4 and PDE5 activities.

Abbreviations

CaM:calmodulin

GB:glibenclamide

HX:hypoxanthine

IBMX:3-isobutyl-1-methylxanthine

MLC:myosin light chain

NBT:nitroblue tetrazolium

O₂ ^·-:superoxide radical

OA:okadaic acid

PBS:phosphate-buffered saline

PDE(s):cyclic nucleotide phosphodiesterase(s)

PHE:phenylephrine

SOD:superoxide dismutase

TEA:tetraethylammonium

XO:xanthine oxidase

Key words

(±)-Naringenin - flavanone - cyclic nucleotide phosphodiesterases - ⁴⁵Ca²⁺ uptake - rat aorta - vasorelaxation

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