Time course of regression of the protection conferred by simulated high altitude to rat myocardium: correlation with mtNOS

Abstract

During acclimatization to sustained hypobaric hypoxia, retardation of age-associated decline in left ventricle mechanical activity and improved posthypoxic recovery were accompanied by upregulation of mitochondrial nitric oxide synthase (mtNOS). To evaluate the time course of regression of these effects on deacclimatization, rats exposed to 53.8 kPa in a hypopressure chamber for 5 mo were returned to 101.3 kPa, whereas controls remained at 101.3 kPa throughout the study. At three time points, contractile function in response to calcium and to hypoxia-reoxygenation (H/R) were determined in papillary muscle, and NOS activity and expression were determined in mitochondria isolated from left ventricle. Developed tension was, before H/R, 65, 58, and 40%, and, after H/R, 129, 107, and 71% higher than in controls at 0.4, 2, and 5 mo of normoxia, respectively. Maximal rates of contraction and relaxation followed a similar pattern. All three parameters showed a linear decline during deacclimatization, with mean half-time (t(1/2)) of 5.9 mo for basal mechanical activity and 5.3 mo for posthypoxic recovery. Left ventricle mtNOS activity was 42, 27, and 20% higher than in controls at 0.4, 2, and 5 mo, respectively (t(1/2) = 5.0 mo). The expression of mtNOS showed similar behavior. The correlation of mtNOS activity with muscle contractility sustained a biphasic modulation, suggesting an optimal mtNOS activity. This experimental model would provide the most persistent effect known at present on preservation of myocardial mechanical activity and improved tolerance to O(2) deprivation. Results support the putative role of mtNOS in the mechanism involved.

Mitochondrial nitric oxide metabolism during rat heart adaptation to high altitude: effect of sildenafil,l-NAME, andl-arginine treatments

“…It has been shown previously that heart mitochondria exhibit immunoreactivity with anti-iNOS antibodies (15,19,47). By means of a similar approach to the one of this study, La Padula et al (27) recently reported a decrease in heart mtNOS expression during the regression of the cardioprotection conferred by hypoxia, using not only anti-nNOS but also anti-iNOS in Western blot experiments. The 51-57% homology reported for nNOS, iNOS, and eNOS, and the cross-reactivity of isoform-specific anti-NOS antibodies could contribute to explain the conflicting results (24,26).…”

Strategic localization of heart mitochondrial NOS: a review of the evidence

“…This upregulation was associated with a preservation of the papillary muscle contractile parameters upon aging and improved tolerance to postischemic contractile dysfunction. Using a similar approach, a decrease in heart mtNOS activity and expression during the regression of the cardioprotection conferred by hypoxia has been reported (79). Other studies showed that liver mtNOS activity is upregulated after acute exposure to hypoxia (80,118).…”

“…In the heart, however, the most convincing data implicate nNOS as the primary candidate for the cytoplasmic NOS izosyme targeted into mitochondria (40,72,142). Evidence in favor of eNOS (10,66,140) and iNOS (45,60,79,129) (Fig. 2B) presence in heart mitochondria has also been published.…”

“…There is a general consensus that iNOS is transiently expressed in the heart during immune responses under stress (8) and, therefore, during pathophysiological conditions of the myocardium such as in ischemia-reperfusion injury (133), septicemia (125), heart failure (146), as well as during aging (138). During the last fifteen years, different groups have used various experimental approaches and reported the presence of mtNOS in the heart (1,10,16,40,45,60,66,72,79,91,115,129,130,140,141,143,147). Regarding the localization of mtNOS, one interesting study provided evidence about the docking of eNOS to the outer mitochondrial membrane in endothelial cells (52).…”

“…2C). Studies of cardiac mtNOS have used various methods including immunohistochemistry (10,66), spectrophotometry (16,30,45,79,115,141,142), radiometry (139,143), fluorometry (40,139,143,147), chemiluminescence (143), and electrochemistry (72). On the other hand, one study reported that the ubiquinone/cyt bc 1 is a site where nitrite is reduced to form NO congeners within mitochondria in a NOS-independent manner (100).…”