Effect of sustained hypobaric hypoxia during maturation and aging on rat myocardium. II. mtNOS activity

Mitochondrial nitric oxide ( NO) production was assayed in rats submitted to hypobaric hypoxia and in normoxic controls (53.8 and 101.3 kPa air pressure, respectively). Heart mitochondria from young normoxic animals produced 0.62 and 0.37 nmol NO center dot min(-1) center dot mg protein(-1) in metabolic states 4 and 3, respectively. This production accounts for a release to the cytosol of 29 nmol NO center dot min(-1) center dot g heart(-1) and for 55% of the NO generation. The mitochondrial NO synthase (mtNOS) activity measured in submitochondrial membranes at pH 7.4 was 0.69 nmol NO center dot min(-1) center dot mg protein(-1). Rats exposed to hypobaric hypoxia for 2 - 18 mo showed 20 - 60% increased left ventricle mtNOS activity compared with their normoxic siblings. Left ventricle NADH-cytochromec reductase and cytochrome oxidase activities decreased by 36 and 12%, respectively, from 2 to 18 mo of age, but they were not affected by hypoxia. mtNOS upregulation in hypoxia was associated with a retardation of the decline in the mechanical activity of papillary muscle upon aging and an improved recovery after anoxia-reoxygenation. The correlation of left ventricle mtNOS activity with papillary muscle contractility ( determined as developed tension, maximal rates of contraction and relaxation) showed an optimal mtNOS activity ( 0.69 nmol center dot min(-1) center dot mg protein(-1)). Heart mtNOS activity is regulated by O-2 in the inspired air and seems to play a role in NO-mediated signaling and myocardial contractility.