ABSTRACT
The goal of the present study was to discern the cellular mechanism(s) that contributes to the age-associated decrease in skeletal muscle aerobic capacity. Skeletal muscle mitochondrial content, a parameter of oxidative capacity, was significantly lower (25 and 20% calculated on the basis of citrate synthase and succinate dehydrogenase activities, respectively) in 24-mo-old Fischer 344 rats compared with 6-mo-old adult rats. Mitochondria isolated from skeletal muscle of both age groups had identical state 3 (ADP-stimulated) and ADP-stimulated maximal respiratory rates and phosphorylation potential (ADP-to-O ratios) with both nonlipid and lipid substrates. In contrast, mitochondria from 24-mo-old rats displayed significantly lower state 4 (ADP-limited) respiratory rates and, consequently, higher respiratory control ratios. Consistent with the tighter coupling, there was a 68% reduction in uncoupling protein-3 (UCP-3) abundance in mitochondria from elderly compared with adult rats. Congruent with the respiratory studies, there was no age-associated decrease in carnitine palmitoyltransferase I and carnitine palmitoyltransferase II activities in isolated skeletal muscle mitochondria. However, there was a small, significant decrease in tissue total carnitine content. It is concluded that the in vivo observed decrease in skeletal muscle aerobic capacity with advanced age is a consequence of the decreased mitochondrial density. On the basis of the dramatic reduction of UCP-3 content associated with decreased state 4 respiration of skeletal muscle mitochondria from elderly rats, we propose that an increased free radical production might contribute to the metabolic compromise in aging.
Subject(s)
Aging , Carrier Proteins/metabolism , Mitochondria, Muscle/metabolism , Muscle, Skeletal/ultrastructure , Adenosine Diphosphate/pharmacology , Animals , Carnitine O-Palmitoyltransferase/metabolism , Citrate (si)-Synthase/metabolism , Ion Channels , Kinetics , Male , Mitochondria, Muscle/enzymology , Mitochondrial Proteins , Oxidation-Reduction , Oxidative Phosphorylation , Oxygen Consumption , Phosphorylation , Rats , Rats, Inbred F344 , Succinate Dehydrogenase/metabolism , Uncoupling Protein 3ABSTRACT
The aging heart sustains greater injury during ischemia and reperfusion compared to adult hearts. Aging decreases oxidative function in interfibrillar mitochondria (IFM) that reside among the myofibers, while subsarcolemmal mitochondria (SSM), located beneath the plasma membrane, remain unaltered. Aging decreases complex III activity selectively in IFM via alteration of the cytochrome c binding site. With 25 min of global ischemia, complex III activity decreases in SSM and further decreases in IFM in the aging heart. Ischemia leads to a marked decrease in the electron paramagnetic resonance signal of the iron-sulfur protein (ISP) in both SSM and IFM, despite a preserved content of ISP peptide. Thus, ischemia results in a functional decrease in the iron-sulfur center in ISP without subunit peptide loss. In the aging heart, at the onset of reperfusion, IFM contain two tandem defects in the path of electron flow through complex III, providing a likely mechanism for enhanced oxidant production and reperfusion damage.
Subject(s)
Electron Transport , Iron/chemistry , Reperfusion Injury/metabolism , Sulfur/chemistry , Age Factors , Aging , Animals , Binding Sites , Cell Membrane/metabolism , Cytochrome c Group/chemistry , Cytochrome c Group/metabolism , Electron Spin Resonance Spectroscopy , Electron Transport Complex IV/metabolism , Electrophoresis, Polyacrylamide Gel , Immunoblotting , Ischemia , Male , Mitochondria/metabolism , Mitochondria/ultrastructure , Myocardium/metabolism , Myocardium/pathology , Myocardium/ultrastructure , Oxygen/metabolism , Rats , Rats, Inbred F344 , Reactive Oxygen Species , Sarcolemma/metabolism , Sarcolemma/ultrastructureABSTRACT
Aging alters cardiac physiology and structure and enhances damage during ischemia and reperfusion. Aging selectively decreases the rate of oxidative phosphorylation in the interfibrillar population of cardiac mitochondria (IFM) located among the myofibers, whereas subsarcolemmal mitochondria (SSM) located beneath the plasma membrane remain unaffected. Aging decreased the rate of oxidative phosphorylation using durohydroquinone, an electron donor to complex III, in IFM only. Complex III activity was decreased in IFM, but not SSM. Aging did not alter the content of catalytic centers of complex III (cytochromes b and c(1)and iron-sulfur protein). Complex III activity measured at physiologic ionic strength in IFM from aging hearts was decreased by 49% compared to IFM from adults, whereas activity measured at low ionic strength was unchanged, localizing the aging defect to the cytochrome c binding site of complex III. Subunits VIII and X of the cytochrome c binding site were present in complex III with the aging defect, indicating that loss of subunits did not occur. Study of aging damage to complex III will help clarify the contribution of altered electron transport in IFM to increased oxidant production during aging, formation of the aging cardiac phenotype, and the relationship of aging defects to increased damage following ischemia.
