Magnesium and manganese modulate the vasoconstrictor effect of thymoquinone

Abstract

Background and Purpose:Thymoquinone (2‐methyl‐5‐isopropyl‐1,4‐benzoquinone) is a natural product known to induce vasodilatation.However, previous studies in isolated rat mesenteric arteries demon-strated unexpected augmentations of vasoconstrictor responses by thymoquinone, at concentrations (10−6to 10−4M) lower than thosecausing relaxation; such augmentations were associated with therelease of inosine 3′,5′‐cyclic monophosphate (cIMP), a non‐canonicalcyclic nucleotide product of soluble guanylyl cyclase (sGC). The maxi-mal rate of cIMP‐formation by purified sGC is greater than that ofguanosine 3′,5′‐cyclic monophosphate (cGMP) with magnesium(Mg2+) as cofactor; the opposite is true when manganese (Mn2+)isthe cofactor. The present study was designed to examine whether ornot thymoquinone‐induced contractions in blood vessels can bereversed by reductions in Mg2+concentration. Experimental Approach:Male Sprague–Dawley rats, 12–16 weeksold, were anaesthetized with an overdose of pentobarbital(100 mg·kg−1, i.p.). Their aortae were isolated and cut into rings for iso-metric tension recording in conventional organ chambers, in the pres-ence of different concentrations of Mg2+or Mn2+ions (1.2 × 10−3or3×10−3M) [by equimolar replacement of MgSO4(1.2 × 10−3M) inthe incubating Krebs–Henseleit solution with MnSO4without or withfurther addition of MnSO4(1.8 × 10−3M)], and contracted with phen-ylephrine. They were then exposed to increasing concentrations ofthymoquinone or sodium nitroprusside (NO‐donor which to activatessGC to produce relaxation). Data are given as mean ± SEM (nanimals),and analysis was performed using two‐way ANOVA followed by thepost hoc Bonferroni test or the unpaired Student'sttest, asappropriate. Key Results:Thymoquinone induced concentration‐dependent aug-mentations of contraction in rat aortae with endothelium: at 10−4M,it caused an augmentation of 88 ± 11% (n= 4). The augmentationwas abolished by the sGC inhibitor ODQ (10−5M;n= 6). It was notaffected by increasing the extracellular concentration of Mg2+to3×10−3M but was reduced when Mg2+was replaced by Mn2+; theaugmentations by 10−4M thymoquinone averaged 85 ± 8% and58 ± 7%, respectively (n= 4). Higher Mn2+concentration(3 × 10−3M) did not further inhibit thymoquinone‐induced augmenta-tions (57 ± 7% at 10−4M;n= 4). Relaxations to sodium nitroprussidein phenylephrine‐contracted aortic rings were also inhibited by replac-ing Mg2+with Mn2+in the incubating Krebs–Henseleit solution(116 ± 14% vs. 65 ± 15%;n= 4).Conclusions and Implications:Manganese and/or the lack of magne-sium inhibit sGC‐mediated vasoconstrictor and vasodilator responses,respectively, to thymoquinone and sodium nitroprusside.