Despite the similarities there are some differences between

Despite the similarities, there are some differences between the augmentation by hypoxia and that by thymoquinone. While in porcine coronary arteries they both depend on calcium sensitization mediated by ROCK12, 32, 33, L-type calcium channels are involved only in thymoquinone-induced augmentation. It is unknown whether or not T-type voltage-dependent calcium influx also plays a role in hypoxic vasoconstriction (Table 1).
Importance of the quinone moiety Comparing the responses to thymoquinone with those to compounds with a similar chemical structure, namely 1,4-benzoquinone (Fig. 12) in rat arteries suggests that the quinone moiety is essential for evoking endothelium-dependent contractions that depend on activation of eNOS and sGC. Quinones are substrates of NADPH:quinone oxidoreductase (NQO-1), an enzyme expressed abundantly in the vascular wall both at the endothelial and at the smooth muscle levels. The enzyme, by detoxifying endogenous and xenobiotic quinones and their derivatives, prevents the participation of these substances in redox cycling, hence playing an important role in the defense against oxidative stress37, 38, 39 NQO-1 metabolizes thymoquinone, likely because of its structural resemblance to ubiquinone, the natural electron carrier in mitochondria. Indeed, thymoquinone acts as an electron acceptor during the oxidation of nicotinamide Nelfinavir dinucleotide (NADH; an essential cofactor for NQO-1) to NAD+. NQO-1 regulates the NAD+/NADH ratio, the increase of which initiates a signalling cascade involving cluster of differentiation and cyclic adenosine diphosphate ribose (cADPR), resulting in calcium mobilization that in endothelial cells can activate eNOS by a protein kinase B/adenosine monophosphate-activated protein kinase-dependent mechanism or increase contractility of smooth muscle. Therefore, it is possible that the necessary eNOS-activation for the endothelium-dependent augmentation by thymoquinone to occur is caused at least in part by activation of endothelial NQO-1 upon binding with its substrate (Fig. 13). Whether or not, and how, activated NQO-1 in the vascular smooth muscle cells is involved in the biased activation of sGC remains to be resolved.
Conclusions Ex vivo studies in rat and porcine arteries reveal for quinones a novel and unique mechanism of action, favouring the occurrence of endothelium-dependent, NO- and sGC-mediated contractions, that depend on interference with calcium homeostasis, similar to that observed during hypoxic vasoconstriction32, 33. These contractions require a biased activation (by endogenous NO or synthetic activators) of sGC with subsequent production of cyclic IMP, as also demonstrated with acute hypoxia33, 42, 43 Quinone-induced augmentations likely involve activation of NQO-1, their metabolizing enzyme. The increase in NAD+/NADH increases calcium mobility, favouring production of NO by eNOS which then diffuses to the underlying vascular smooth muscle cells to activate the biased sGC producing cyclic IMP, in turn facilitating contraction. In the absence of experiments performed with purified sGC, it is still uncertain whether or not the biased activity of the enzyme is due to a direct target effect of thymoquinone. Likewise, although the quinone moiety seems essential for thymoquinone-induced augmentations to occur, the link between NQO-1 activation by quinones and the alteration in sGC-activity resulting in contraction remains to be identified. Moreover, the importance (or not) of the augmenting effect of thymoquinone in vivo remains to be determined. Hypoxic contractions may contribute to the increased prevalence of cardiovascular complications in cardiac patients with sleep apnea44, 45. By identifying the mechanisms underlying the endothelium-dependent augmentation caused by quinones and possibly other agents causing biased activity of sGC, the cellular targets involved in cardiovascular complications due to hypoxia can be studied, facilitating the development of novel strategies to prevent coronary hypoxic vasospasm.