Implication of cyclic nucleotide phosphodiesterase inhibition in the vasorelaxant activity of the citrus-fruits flavonoid (±)-naringenin

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 mu M - 0.3 mM) did not affect the contractile response induced by okadaic acid (OA, 1 mu M). However, (+/-)-naringenin relaxed, in a concentration-dependent manner, the contractions elicited by phenylephrine (PHE, 1 mu M) or by a high extracellular KCI concentration (60 mM) in intact rat aortic rings. Mechanical removal of endothelium and/or pretreatment of aorta rings with glibenclamide (GB, 10 mu M) or tetraethyl ammonium (TEA, 2 mM) did not significantly modify the vasorelaxant effects of this flavanone. (+/-)-Naringenin (10 mu M-0.1 mM) did not alter the basal uptake of Ca-45(2+) but decreased the influx of Ca-45(2+) induced by PHE and KCI in endothelium-containing and endothelium-denuded rat aorta. (+/-)Naringenin (10 mu M - 0.1 mM) was ineffective to scavenge superoxide radicals (O-2*(-)) 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 mu M) and high KCI (60 mM) in cultured rat aortic myocytes. (+/-)-Naringenin preferentially inhibited calmodulin (CaM)-activated PDE1, PDE4 and PDE5 isolated from bovine aorta with lC(50) values of about 45 mu M, 60 mu M and 68 mu M, respectively. In contrast, the 7-rhamnoglucoside of (+/-)-naringen-in, naringin (1 mu M-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.