Myocardial ischemia selectively depletes cardiolipin in rabbit heart subsarcolemmal mitochondria.

Mitochondria contribute to myocyte injury during ischemia. After 30 and 45 min of ischemia in the isolated perfused rabbit heart, subsarcolemmal mitochondria (SSM), located beneath the plasma membrane, sustain a decrease in oxidative phosphorylation through cytochrome oxidase. In contrast, oxidation through cytochrome oxidase in interfibrillar mitochondria (IFM), located between the myofibrils, remains unaffected. Cytochrome oxidase activity in the intact membrane requires an inner mitochondrial membrane lipid environment enriched in cardiolipin. During ischemia, the content of cardiolipin decreased only in SSM, whereas the content of other phospholipids was preserved. Ischemia did not alter the composition of the cardiolipin that remained in SSM. Cardiolipin content was preserved in IFM during ischemia. Thus cardiolipin is a relatively early target of ischemic mitochondrial damage, leading to loss of oxidative phosphorylation through cytochrome oxidase in SSM.

Aging selectively decreases oxidative capacity in rat heart interfibrillar mitochondria.

Mitochondrial-derived oxidative injury contributes to cellular aging as well as to reperfusion-induced tissue damage. While the aging-heart suffers greater tissue damage following ischemia and reperfusion than the adult heart, the occurrence of aging-related alterations in mitochondrial oxidative metabolism in the elderly heart has remained uncertain. We determined if aging altered oxidative metabolism in either of the two populations of cardiac mitochondria, subsarcolemmal mitochondria (SSM) that reside beneath the plasma membrane or interfibrillar mitochondria (IFM) located between the myofibrils. SSM and IFM were isolated from 6-month adult and 24- and 28-month elderly Fischer 344 rat hearts. Aging-related alterations were limited to IFM, while SSM remained unaffected. Aging decreased the rate of oxidative phosphorylation in IFM, including when stimulated by electron donors specific for cytochrome oxidase. Cytochrome oxidase enzyme activity was decreased in IFM from aging hearts, while activity in SSM remained similar to adult controls. These findings allow future studies of aging-related decrements in oxidative function to focus upon IFM, while SSM provide an inherent control group of mitochondria that are free of aging-related alterations in oxidative function. The selective alteration of IFM during aging raises the possibility that the consequences of aging-induced mitochondrial dysfunction will be enhanced in specific subcellular regions of the senescent myocyte.

Neurofibromin colocalizes with mitochondria in cultured cells.

Mutations in neurofibromatosis type 1 target the gene coding for neurofibromin. While neurofibromin is able to accelerate the rate of GTP hydrolysis by cellular Ras proteins, its biological function is not well understood. To gain information regarding its function, the intracellular localization of neurofibromin was analyzed in cultured cell lines using polyclonal antisera raised against four neurofibromin-specific peptides, three from the carboxyl terminus and one from the amino terminus. In methanol-fixed cells distinct rod-like structures distributed throughout the cytoplasm were recognized by the antisera. Similar structures were seen with each antiserum, including affinity-purified antibodies, and in each of the cultured cell lines tested. Similar structures were seen in paraformaldehyde-fixed cells. Double staining experiments showed that these structures colocalize with mitochondria, but not with actin, beta-tubulin, or endoplasmic reticulum. When actin or tubulin structures within the cell were disrupted by separate antimitotic drugs, these stained structures retained their shape. Neurofibromin association with mitochondria was confirmed biochemically when highly purified mitochondrial fractions from bovine heart tissue were shown in Western analysis to contain neurofibromin. This association might be helpful in predicting identification of some of the cellular proteins with which neurofibromin interacts.

Myocardial ischemia decreases oxidative phosphorylation through cytochrome oxidase in subsarcolemmal mitochondria.

The effect of myocardial ischemia on mitochondrial oxidative phosphorylation was investigated using isolated, buffer-perfused rabbit hearts. After 45 min of global ischemia, oxidative phosphorylation was decreased only in the subsarcolemmal population of mitochondria with all substrates tested. The oxidation of N,N,N',N' tetramethyl p-phenylenediamine-ascorbate, an electron donor to cytochrome oxidase via cytochrome c, was decreased in subsarcolemmal mitochondria [ischemia (n = 6): 76 +/- 3 vs. control (n = 5): 105 +/- 6 nanoatoms O.min-1.mg-1, P < 0.01] but not in interfibrillar mitochondria. Only minor morphological changes were observed by electron microscopy in the isolated mitochondria after ischemia. Neither cytochrome oxidase activity measured under conditions for maximal activity nor the apparent Michaelis constant and maximum velocity values of the two cytochrome c binding sites were different in subsarcolemmal mitochondria isolated from ischemic and control hearts. The cytochrome c content was decreased in subsarcolemmal mitochondria after ischemia (ischemia: 0.111 +/- 0.013 vs. control: 0.156 +/- 0.007 nmol/mg protein, P < 0.05). Thus ischemia decreased the rate of oxidative phosphorylation through cytochrome oxidase selectively in intact subsarcolemmal mitochondria. Ischemic damage to the terminal segment of the electron transport chain involves a decrease in the content of cytochrome c, whereas the expressible catalytic activity of cytochrome oxidase remains unchanged.