Abstract

Preliminary experiments on isolated rat arteries demonstrated that thymoquinone, a compound widely used for its antioxidant properties and believed to facilitate endothelium-dependent relaxations, as a matter of fact caused endothelium-dependent contractions. The present experiments were designed to determine the mechanisms underlying this unexpected response. Isometric tension was measured in rings (with and without endothelium) of rat mesenteric arteries and aortae and of porcine coronary arteries. Precontracted preparations were exposed to increasing concentrations of thymoquinone, which caused concentration-dependent, sustained further increases in tension (augmentations) that were prevented by endothelium-removal, L-NAME [NO synthase inhibitor] and ODQ [sGC inhibitor]. In L-NAME-treated rings, the NO-donor diethylenetriamine NONOate restored the thymoquinone-induced augmentations; YC-1 (sGC activator) and inosine 3',5'-cyclic monophosphate (cIMP) caused similar restorations. By contrast, in ODQ-treated preparations, the cell-permeable guanosine 3',5'-cyclic monophosphate (cGMP) analogue did not restore the augmentation by thymoquinone. The compound augmented the content (measured with UPLC-MS/MS) of cIMP, but not that of cGMP; these increases in cIMP content were prevented by endothelium-removal, L-NAME and ODQ. The augmentation of contractions caused by thymoquinone was prevented in porcine arteries, but not in rat arteries, by HA-1077 and Y-27632 (Rho-kinase inhibitors); in the latter, but not in the former, it was reduced by ML-218 (T-type calcium channel inhibitor), demonstrating species/vascular bed differences in the impact of cIMP on calcium handling. Thymoquinone is the first pharmacological agent that causes endothelium-dependent augmentation of contractions of isolated arteries which requires endothelium-derived NO and biased sGC activation, resulting in the augmented production of cIMP favoring the contractile process.