Cardiac DT-diaphorase contributes to the detoxification system against doxorubicin-induced positive inotropic effects in guinea-pig isolated atria

1. Doxorubicin (DOX), a standard chemotherapeutic anthracycline agent, causes a positive inotropic effect in guinea-pig isolated atria in a concentration-dependent manner with an ED50 of 3.6 mumol/L. This increase in contractility is strictly related to the generation of reactive oxygen species (ROS) as a consequence of quinone metabolism. The ED50 Of DOX is significantly increased (P < 0.05) in the presence of 150 U superoxide dismutase (SOD). In the heart, DOX may be subjected to one- or two-electron reductions catalysed by flavoenzymes in the presence of suitable electron donors. Two-electron reduction is catalysed by NAD(P)H quinone acceptor oxidoreductase (DT-diaphorase; DTD). Whether DOX will be activated or detoxified by two-electron reduction is important for the understanding of the mechanism of both the toxic and antitumour actions of DOX. 2. In order to assess the role of DTD in cardiac responses to DOX, we examined the effect of both a specific inhibitor (dicoumarol) and an inducer (3-methylcholanthrene; MCA) of the enzyme on the inotropic action of DOX. 3. In guinea-pig isolated left atria, 4 μmol/L dicoumarol significantly enhanced the positive inotropic effect of DOX, especially at lower concentrations of DOX. In atria isolated from guinea-pigs treated with MCA (44 mglkg, i.p. for 4 days), DTD activity was enhanced (approximately twice that of the control; P < 0.01), whereas the activity of glutathione S-transferase (GST) was not significantly altered. In these preparations, DOX caused a significantly lower increase in force of contraction than in atria isolated from untreated animals. 4. These results demonstrate that cardiac DTD does not contribute to ROS generation, but represents a detoxification system.