From the Department of General and Environmental Physiology, University of Naples "Federico II," Naples, Italy
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ABSTRACT |
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Increasing age is associated with obesity and insulin resistance not only in humans but also in rodents (15). In fact, ad libitum-fed caged rats are a good animal model that simulates the situation (sedentary lifestyle and unrestricted diet) of people in the western world.
It has been previously shown that age-induced insulin resistance in rats is already detectable at 4 months of age, concomitant to a rapid rise in fat mass, and does not further increase later in life (3). This early decrease in insulin sensitivity could in turn lead to the metabolic disturbance typical of old age.
Our previous results also indicate that in the transition from postpubertal (60 days) to adult (180 days) age, rats display reduced sensitivity to the anorexic effect of leptin (6) and, at the cellular level, hepatic mitochondrial machinery shows some age-related biochemical impairment (7,8). From all the above findings, it can be deduced that adult rats can be considered a good experimental tool for studying the onset of the "aging" phenomenon.
It has been postulated that defects in mitochondrial performance could contribute to the development of insulin resistance (9), and mitochondrial oxidative capacity has been considered a good predictor of insulin sensitivity (10). These suggestions are not surprising since control of mitochondrial respiration and oxidative phosphorylation is fundamental for the maintenance of cellular homeostasis. The above control is exerted at two levels, i.e., regulation of the rate of oxygen consumption and ATP production and regulation of oxidative phosphorylation efficiency (11). The main determinant of oxidative phosphorylation efficiency is represented by the degree of coupling between oxygen consumption and ATP synthesis, which is always lower than 1 and can vary according to the metabolic needs of the cell (12). Among the factors that affect mitochondrial degree of coupling, the permeability of the mitochondrial inner membrane to H+ ions (leak) plays an important role. It is now well known that the mitochondrial inner membrane exhibits a basal proton leak pathway whose contribution to basal metabolic rate in rats has been estimated to be 2025% (13). In addition, it is well known that fatty acids can act as natural uncouplers of oxidative phosphorylation by generating a fatty acid-dependent proton leak pathway (1416), which is a function of the amount of unbound fatty acids in the cell.
Taking into account the above considerations, we considered it of interest to investigate mitochondrial performance and efficiency in 60- and 180-day-old rats. To this purpose, we measured basal and fatty acid-mediated proton leak and the parameters of thermodynamic coupling and efficiency in mitochondria isolated from skeletal muscle. In fact, skeletal muscle is the primary site of insulin action and is thus inherently linked to the development of whole-body insulin resistance (17). Analyses were carried out, taking into account that the skeletal muscle mitochondrial population is heterogeneous for localization, function, and regulation (1821). In fact, mitochondria located beneath the sarcolemmal membrane (subsarcolemmal [SS]) exhibit lower respiratory rates than those located between the myofibrils (intermyofibrillar [IMF]), and aging has been shown to selectively affect IMF mitochondria in heart (22). A metabolic characterization of young and adult rats was obtained by measuring serum insulin, leptin, nonesterified fatty acid (NEFA), and glucose levels.
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RESEARCH DESIGN AND METHODS |
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On the day they were killed and without any previous food deprivation, the rats were anesthetized by an injection of chloral hydrate (40 mg/100 g body wt i.p.) and blood was collected from the inferior cava vein. Immediately after blood collection, hind leg muscles were rapidly removed and used for the preparation of SS and IMF mitochondria (18), while the rat carcasses were used for determination of body composition (18).
Measurements of mitochondrial respiration parameters, thermodynamic coupling, and efficiency.
Oxygen consumption was measured polarographically with a Clark-type electrode (Yellow Springs Instruments, Yellow Springs, OH) at 30°C in a medium containing 30 mmol/l KCl, 6 mmol/l MgCl2, 75 mmol/l sucrose, 1 mmol/l EDTA, 20 mmol/l KH2PO4, pH 7.0, and 0.1% (wt/vol) fatty acid-free BSA. Substrates used were 10 mmol/l succinate plus 3.75 µmol/l rotenone, 10 mmol/l glutamate plus 2.5 mmol/l malate, or 40 µmol/l palmitoylCoA plus 2 mmol/l carnitine plus 2.5 mmol/l malate. Measurements were performed in the absence (state 4) and presence (state 3) of 0.6 mmol/l ADP. Respiratory control ratio (RCR) was calculated as the ratio between states 3 and 4.
The degree of thermodynamic coupling (q) was obtained by applying equation 11 by Cairns et al. (23).
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where Jo represents oxygen consumption. (Jo)sh and (Jo)unc were measured as above using succinate plus rotenone in the presence of 2 µg/ml oligomycin or 1 µmol/l FCCP [carbonyl cyanide 4-(trifluoro-methoxy)phenylhydrazone], respectively. The optimal thermodynamic efficiency of oxidative phosphorylation was calculated by using equation 13 by Cairns et al. (23).
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The adenine nucleotide translocase (ANT) content of SS and IMF mitochondria was determined by titrating state 3 respiration with increasing concentrations of carboxyatractyloside (24) in a medium containing 30 mmol/l KCl, 6 mmol/l MgCl2, 75 mmol/l sucrose, 1 mmol/l EDTA, 20 mmol/l KH2PO4, 0.1% (wt/vol) fatty acid-free BSA, 10 mmol/l succinate, and 3.75 µmol/l rotenone (pH 7.0). Cytochrome oxidase (COX) specific activity was measured polarographically with a Clark-type electrode (Yellow Springs Instruments) at 30°C in a medium containing 30 µmol/l cytochrome c, 4 µmol/l rotenone, 0.5 mmol/l dinitrophenol, 10 mmol/l Na-malonate, and 75 mmol/l Hepes (pH 7.4) (25).
Measurements of basal proton leak kinetics.
Oxygen consumption was measured polarographically with a Clark-type electrode (Yellow Springs Instruments) maintained at 30°C in a medium containing 30 mmol/l LiCl, 6 mmol/l MgCl2, 75 mmol/l sucrose, 1 mmol/l EDTA, 20 mmol/l Tris-PO4, pH 7.0, and 0.1% (wt/vol) fatty acid-free BSA. Titration of state 4 respiration was carried out by sequential additions of up to 5 mmol/l malonate in the presence of 10 mmol/l succinate, 3.75 µmol/l rotenone, 2 µg/ml oligomycin, 83.3 nmol/mg safranin O, and 80 ng/ml nigericin. Mitochondrial membrane potential recordings were performed in parallel with safranin O using a JASCO dual-wavelength spectrophotometer (511533 nm) (26). The absorbance readings were transformed into mV membrane potential using the Nernst equation: = 61 mV x log ([K+]in/[K+]out). Calibration curves made for each preparation were obtained from traces in which the extramitochondrial K+ level ([K+]out) was altered in the 0.1- to 20-mmol/l range. The change in absorbance caused by the addition of 3 µmol/l valinomycin was plotted against [K+]out. Then, [K+]in was estimated by extrapolation of the line to the zero uptake point.
Measurement of palmitate-induced proton leak kinetics.
Mitochondrial membrane potential and oxygen consumption were measured as above in the presence of 10 mmol/l succinate, 3.75 µmol/l rotenone, 2 µg/ml oligomycin, 83.3 nmol/mg safranin O, and palmitate at a concentration of 45 µmol/l for SS mitochondria or 75 µmol/l for IMF mitochondria. Due to the presence of 0.1% BSA in the incubation medium, the above concentrations of palmitate correspond to 17 (for SS mitochondria) and 62 (for IMF mitochondria) nmol/l free (not bound) fatty acid calculated using the equation of Richieri et al. (27). Palmitate-induced proton leak was estimated from titration of respiration by sequential additions of up to 600 µmol/l malonate for IMF and 1 mmol/l for SS mitochondria.
Measurement of uncoupling protein 3 (UCP3) protein content.
Mitochondrial protein content of UCP3 was estimated by Western blot analysis, as previously reported (28), by using UCP3 polyclonal antibodies (Chemicon).
Circulating substrate and hormone serum concentrations.
Serum insulin and leptin concentrations were measured using radioimmunoassay kits in a single assay to remove interassay variations (Linco Research, St. Charles, MO). Serum glucose and NEFA concentrations were measured by colorimetric enzymatic method using commercial kits (Pokler Italia, Genova, Italy, and Roche Diagnostics, Mannheim, Germany, respectively). Insulin resistance was assessed by homeostasis model assessment (HOMA) index ([glucose {mg/dl} x insulin {mU/l}]/405) (29).
Statistical analysis.
Data are summarized using means ± SE for eight rats. Statistical analyses were performed using a two-tailed unpaired Students t test or nonlinear regression curve fit. All analyses were performed using GraphPad Prism (version 4; GraphPad Software, San Diego, CA).
ADP, succinate, rotenone, glutamate, malate, carnitine, palmitoylCoA, ascorbate, dinitrophenol, TMPD (N, N, N', N'-tetramethyl-p-phenylenediamine), nigericin, nagarse, oligomycin, and safranin O were purchased from Sigma Chemical (St. Louis, MO). Carboxyatractyloside was purchased from Calbiochem (San Diego, CA). All other reagents were of the highest purity commercially available.
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RESULTS |
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DISCUSSION |
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The generalized increase in state 3 oxidative capacity found in SS mitochondria from 180-day-old rats suggests an increase in one of the steps below complex II. The most feasible target of the above increase should be at the level of cytochromes and/or ATP synthetase, since COX activity was not significantly altered with increasing age, although a tendency to increase was detected. On the other hand, only a stimulation of complex I activity was evident in IMF mitochondria. The determination of ANT content in the two mitochondrial populations allows us to exclude that the above increase in respiratory rates is due to ANT, which was found unchanged or decreased in SS and IMF mitochondria, respectively. The divergent effect of increasing age on respiratory rates and ANT content is not contradictory, since ANT exerts a low control on mitochondrial respiration in skeletal muscle (30). A possible explanation for the different effect of increasing age on SS and IMF mitochondria could be that these two mitochondrial populations have different cellular localization and subserve different metabolic roles in the cell. In fact, IMF mitochondria mainly meet ATP requirements of the contractile elements, while SS mitochondria supply ATP for cytoplasmic reactions.
Previous results have shown that old rats display a decrease in skeletal muscle mitochondrial activities (3133), and when SS and IMF mitochondria have been studied separately in the heart, the above decrease was found to be restricted to IMF organelles (22). In addition, we have previously found that an aging-induced decline in hepatic mitochondrial activity of complex II is already evident in 180-day-old rats (7), indicating that the liver is affected early by the aging process. Our present results show that skeletal muscle behaves very differently from liver and that, at 180 days of age, oxidative capacity has not yet reached its maximum. In agreement, it has been found that complex II-IV activity of skeletal muscle mitochondria peaks in middle life (1 year) and then declines with increasing age (32).
As for mitochondrial efficiency, both mitochondrial populations display an increase in the degree of thermodynamic coupling and optimal thermodynamic efficiency as well as a decreased fatty acid-dependent proton leak. For IMF mitochondria, decreased fatty acid-dependent proton leak is in agreement with the decreased ANT content of this mitochondrial population, while in SS mitochondria, the reduced sensitivity to palmitate is probably due to the other carrier systems involved, since their ANT content was not affected. Our results also underline the dissociation between regulation of mitochondrial fatty acid-dependent proton leak and UCP3 levels. In fact, the increased UCP3 protein content of SS and IMF mitochondria from 180-day-old rats is in contrast with the lower fatty acid-dependent leak but is consistent with the increased serum NEFA levels, which have been shown to be closely linked to changes in UCP3 protein expression (34).
A number of recent human studies have focused the attention on the mitochondrial compartment as a cellular site involved in the pathogenesis of insulin resistance. In fact, mitochondrial dysfunction has been found in old men (9) and insulin-resistant young subjects (35). In addition, a significant direct relationship between skeletal muscle oxidative capacity and insulin sensitivity has been found (10). An alternative way to reduce mitochondrial oxidation of energy substrates is to increase the efficiency of oxidative phosphorylation. In fact, a higher efficiency implies that less substrate needs to be burned to obtain ATP. Since NEFA serum levels are significantly higher in adult rats, a deleterious consequence of reduced substrate burning could be intracellular triglyceride accumulation and lipotoxicity. The above consequence is of relevance because one of the main causes leading to insulin resistance in skeletal muscle is intramuscular triglygeride deposition (17,36). In addition, skeletal muscle is the primary site of insulin action and is thus inherently linked to the development of whole-body insulin resistance (17).
The increased mitochondrial efficiency and resulting lower thermogenesis in 180-day-old rats could be due to a reduced sensitivity of skeletal muscle cell to leptin. In fact, it has been shown that leptin directly stimulates thermogenesis in skeletal muscle (37) and that 180-day-old rats display age-induced leptin resistance (6). Therefore, leptin resistance could be regarded as one of the primary events leading to insulin resistance.
In conclusion, our present results show that although at 180 days of age no clear biochemical impairment of mitochondrial machinery can be detected, an increased efficiency in the production of ATP takes place. The impact of this modification on cellular metabolism could contribute to the onset of skeletal muscle insulin resistance.
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ACKNOWLEDGMENTS |
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We thank Dr. Emilia De Santis for her skilful management of the animal house.
Address correspondence and reprint requests to Susanna Iossa, Dipartimento di Fisiologia Generale ed Ambientale, Via Mezzocannone 8, I-80134, Napoli, Italy. E-mail: susiossa{at}unina.it
Received for publication May 21, 2004 and accepted in revised form July 23, 2004
ANT, adenine nucleotide translocase; COX, cytochrome oxidase; HOMA, homeostasis model assessment; IMF, intermyofibrillar; NEFA, nonesterified fatty acid; RCR, respiratory control ratio; SS, subsarcolemmal; UCP3, uncoupling protein 3
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References |
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