The protective and toxic effects of rhubarb tannins and anthraquinones in treating hexavalent chromium-injured rats: The Yin/Yang actions of rhubarb

Chromium nephrotoxicity (CrNT) is thought to occur through the oxidant lesion mechanism. There is still a lack of specific remedies against CrNT. We primarily screened Chinese herbal medicines with a potential protective effect against CrNT, e.g., rhubarb (Rheum palmatum L.). However, the active constituents in rhubarb and its mechanisms remain unclear. In this study, the total rhubarb extract (TR) was successively separated into three parts: total anthraquinone extract (TA), total tannin extract (TT) and remaining component extract (RC). The effects of each extract on the potassium dichromate (K2Cr2O7)-induced nephrotoxicity in rats were comparatively assessed. The results showed that only the administration of TT protected the kidney function in K2Cr2O7-injured rats. Besides, TT showed significant activity to scavenge hydroxyl radicals, which is considered to be the dominant lesion product generated by hexavalent chromium. TT also showed a reduced ability to transform toxic high valence chromium ions into non-toxic low valence ions. And TT was able to further precipitate chromium ions. These results suggested that rhubarb tannins treat CrNT as a free radical scavenger, reductant, and metal precipitant. The multiple protective routes of the plant tannins reveal a superior option for development into a promising natural remedy against CrNT. In addition, the opposite effects of rhubarb anthraquinones in treating CrNT were observed compared to rhubarb tannins, which suggested the duo-directional effects (Yin and Yang) of herbal medicines should be addressed.