On the mechanisms of the antispasmodic action of some hindered phenols in rat aorta rings

The antispasmodic effects of 3-t-butyl-4-hydroxyanisole (BHA) and some stru cturally related compounds were investigated in endothelium-intact rat aort a rings. Nordihydroguaieretic acid (NDGA), BHA, 3,5-di-t-butyl-4-hydroxyani sole (DTBHA), 2,6-di-isopropyl phenol (propofol) and 2,2'-dihydroxy-3,3'-di -t-butyl-5,5'-dimethoxydiphenyl (DIBHA) did not cause relaxation when added at the plateau of phenylephrine-evoked contraction, nor did they affect th e concentration-relaxation curve for acetylcholine in precontracted rings. In rings depolarised with physiological salt solution (PSS) containing 40 m M K+, NDGA, BHA, DTBHA, 2,5-di-t-butyl-1,4-benzohydroquinone (BHQ), propofo l and nifedipine, but not DIBHA, inhibited the contraction induced by cumul ative addition of Ca2+ (0.05-10 mM) in a concentration-dependent manner; th is inhibition was inversely related to the Ca2+ concentration. In 40 mM KPSS, 25 nM nifedipine blocked the 1 mM Ca2+-induced contraction, whereas 50 mu M DTBHA, NDGA, BHA, BHQ and propofol significantly antagonised it by 84 .4%, 73.0%, 52.8%, 45.6% and 35.7%, respectively. In the presence of 1 mu M methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridi ne -5-carboxylate (Bay K 8644), the response to Ca2+ did not differ from co ntrol values with nifedipine and BHQ, was partially restored with DTBHA and NDGA, and was not affected with BHA and propofol. Nifedipine markedly inhi bited (85.2%) the Ba2+-induced contraction and this effect was totally reve rsed by Bay K 8644. BHA and DTBHA showed antispasmodic activity (45.3% and 43.1%, respectively) which was partly reversed by Bay K 8644. In contrast, Bay K 8644 did not affect the inhibition exerted by BHQ, NDGA and propofol (69.5%, 53.3% and 46.1%, respectively). Nifedipine, BHA, DTBHA, propofol an d NDGA inhibited the contractile response to 1 mM Ca2+ of aorta rings depol arised with 40 or 80 mM K+ PSS to a similar extent. Cromakalim inhibited th e Ca2+-evoked contraction only in 30 mM K+ PSS and BHQ only in 80 mM K+ PSS . DIBHA had no effect on this model. Cromakalim, but not BHA, stimulated Rb -86(+) efflux from ring preparations. In 80 mM K+ PSS containing 1 mu M nif edipine, only papaverine affected the phenylephrine-induced contraction. Mo reover, when the rings were preincubated with 1 mM Ni2+, the response to ph enylephrine in the presence of BHQ was significantly reduced. In conclusion ; we propose that BHA may non-specifically inhibit Ca2+ influx at the plasm alemma level rather than affect the function of K+ channels, Ca2+ release f rom intracellular stores or endothelium-dependent relaxation. (C) 2000 Else vier Science B.V. All rights reserved.