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 structurally related compounds were investigated in endothelium-intact rat aorta rings. Nordihydroguaieretic acid (NDGA), BHA, 3,5-di-t-butyl-4-hydroxyanisole (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 the concentration-relaxation curve for acetylcholine in precontracted rings. In rings depolarised with physiological salt solution (PSS) containing 40 mM K(+), NDGA, BHA, DTBHA, 2, 5-di-t-butyl-1,4-benzohydroquinone (BHQ), propofol and nifedipine, but not DIBHA, inhibited the contraction induced by cumulative addition of Ca(2+) (0.05-10 mM) in a concentration-dependent manner; this inhibition was inversely related to the Ca(2+) concentration. In 40 mM K(+) PSS, 25 nM nifedipine blocked the 1 mM Ca(2+)-induced contraction, whereas 50 microM 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 microM methyl-1,4-dihydro-2, 6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5-carboxylate (Bay K 8644), the response to Ca(2+) did not differ from control values with nifedipine and BHQ, was partially restored with DTBHA and NDGA, and was not affected with BHA and propofol. Nifedipine markedly inhibited (85.2%) the Ba(2+)-induced contraction and this effect was totally reversed 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 and NDGA inhibited the contractile response to 1 mM Ca(2+) of aorta rings depolarised with 40 or 80 mM K(+) PSS to a similar extent. Cromakalim inhibited the Ca(2+)-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 86Rb(+) efflux from ring preparations. In 80 mM K(+) PSS containing 1 microM nifedipine, only papaverine affected the phenylephrine-induced contraction. Moreover, when the rings were preincubated with 1 mM Ni(2+), the response to phenylephrine in the presence of BHQ was significantly reduced. In conclusion, we propose that BHA may non-specifically inhibit Ca(2+) influx at the plasmalemma level rather than affect the function of K(+) channels, Ca(2+) release from intracellular stores or endothelium-dependent relaxation.