Abstract
In previous works we demonstrated that 2-methyl-1,4-naphthoquinone (menadione) causes a marked increase in the force of contraction of guinea pig and rat isolated atria. This inotropic effect was significantly higher in the guinea pig than in the rat and was strictly related to the amount of superoxide anion (O 2 − ), generated as a consequence of cardiac menadione metabolism through mitochondrial NADH-ubiquinone oxidoreductase. The present study was designed to further elucidate the basis of these quantitatively different positive inotropic responses. To this purpose, we measured O 2 − and hydrogen peroxide (H 2O 2) produced by mitochondria isolated from guinea pig and rat hearts in the presence of 20 μM menadione. Moreover, we evaluated the menadione detoxification activity (DT-diaphorase) and the antioxidant defences of guinea pig and rat hearts, namely their GSH/GSSG content, Cu/Zn- and Mn-dependent superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (Gpx) activities. Our results indicate that DT-diaphorase activity and glutathione levels were similar in both animal species. By contrast, guinea pig mitochondria produced greater amounts of O 2 − and H 2O 2 than those of rat heart. This is probably due to both the higher Mn-SOD activity (2.93±0.02 vs . 1.95±0.06 units/mg protein; P<0.05) and to the lower Gpx activity (10.09±0.30 vs. 32.67±1.02 units/mg protein; P<0.001) of guinea pig mitochondria. A lower CAT activity was also observed in guinea pig mitochondria (2.40±0.80 vs . 6.13±0.20 units/mg protein; P<0.01). Taken together, these data provide a rational explanation for the greater susceptibility of guinea pig heart to the toxic effect of menadione: because of the greater amount of O 2 − generated by the quinone and the higher mitochondrial Mn-SOD activity, guinea pig heart is exposed to more elevated concentrations of H 2O 2 that is less efficiently detoxified, because of lower Gpx and CAT levels of mitochondria.
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