Heart mitochondrial dysfunction in diabetic rats

Diabetic cardiomyopathy,i.e.the ventricular dysfunction in the absence of hypertension or coronary arterial disease,is a common complication of diabetes mellitus that leads to a heightened risk of heart failure and death among diabetic patients.This contractile dysfunction could be associated to mitochondrial dysfunction,in which mitochondrial biogenesis could emerge as a compensatory mechanism triggered in response to hyperglycemia.It has been proposed that nitric oxide synthase activities with enhanced NO production are involved in this process.Alterations in the contractile response and lusitropic reserve were observed in streptozotocin diabetic rats afterβ-adrenergic stimuli.Additionally,tissue O_(2) consumption was declined.A condition of mitochondrial dysfunction with decreased mitochondrial state 3 O_(2) consumption,respiratory control ratio,mitochondrial respiratory complexes activities and ATP production were present in hearts of diabetic animals.We observed an increase in NO production by heart mitochondria and in cytochrome oxidase activity in heart homogenates.The latter suggests an increase of newly formed mitochondria.Thus,the impairment of mitochondrial function with increased mitochondrial biogenesis may precede the onset of diabetic cardiomyopathy.However,mitochondrial biogenesis does not necessarily imply that the resultant mitochondria are functional,which might explain the changes in cardiac energy metabolism occurring in hearts of diabetic rats.

Superoxide and hydrogen peroxide productions by NO-inhibited complex Ⅲ

Complex Ⅲ plays a central role in the mitochondrial respiratory chain transferring electrons from ubiquinol to cytochrome c and pumping protons to the intermembrane space,contributing to the protonmotive force.Furthermore,complex Ⅲ can act as a source of O_(2^(·-))in the presence of ubiquinol and antimycin,an expermiental condition in which the oxidation of the cytochrome b hemes is blocked.The O_(2^(·-))dismutation catalyzed by superoxide dismutase produces H2O2,a known second messenger in redox signalling.Results from our laboratory have shown that NO,released from GSNO or from SPER-NO or generated by mtNOS,inhibits electron transfer at ubiquinone-cytochrome b area producing antimycin-like effects.Thus,both antimycin-and NO-inhibited complex Ⅲ showed a high content of cytochromes b in the reduced state(79 and 71%,respectively)and an enhancement in the ubisemiquinone EPR signal at g=1.99(42 and 35%,respectively).As consequence,O_(2^(·-))and H2O2 productions were increased,being the O_(2^(·-))/H_(2)O_(2) ratio equal to 1.98 in accordance with the stoichiometry of the O_(2^(·-))disproportionation.The interruption of the oxidation of cytochromes b by NO leads to an enhancement of the steady-state concentration of UQH·,allowing cytochrome bc1 complex to act as a source of reactive oxygen species in physiological conditions.

Nitric oxide metabolism in heart mitochondria

Normal cardiac function is accomplished through a continuous energy supply provided by mitochondria.Heart mitochondria are the major source of reactive oxygen and nitrogen species:superoxide anion(O_(2)^(-))and nitric oxide(NO).NO production by mitochondrial NOS(mtNOS)is modified by metabolic state and shows an exponential dependence on Δψ.The interaction between mtNOS and complexes I and IV might be a mechanism involved in the regulation of mitochondrial NO production.NO exerts a high affinity,reversible and physiological inhibition of cytochrome c oxidase activity.A second effect of NO on the respiratory chain is accomplished through its interaction with ubiquinol-cytochrome c oxidoreductase.The ability of mtNOS to regulate mitochondrial O_(2) uptake and O_(2)^(-)and H_(2)O_(2) productions through the interaction of NO with the respiratory chain is named mtNOS functional activity.Together,heart mtNOS allows NO to optimize the balance between cardiac energy production and utilization,and to regulate the steady-state concentrations of other oxygen and nitrogen species.