Effect of various agents and conditions on palmitate oxidation by homogenates of rat liver and rat and human skeletal muscle

The effect of various inhibitors of fatty acid transport and of respiratory chain on palmitate oxidation was investigated in homogenates and mitochondria of rat muscle and homogenates of rat liver and human muscle. Inhibition of fatty acid transport by carnitine omission, malonyl-CoA, tetradecylglycidic acid and mersalyl decreased oxidation more with muscle than with rat liver. Antimycin and KCN decreased markedly palmitate oxidation and caused a larger accumulation of peroxisomal oxidation products. Inhibition of mitochondrial long-chain fatty acid transport decreased accumulation of peroxisomal products in comparison to the control. The effect of malonyl-CoA was dependent on the nutritional state, the pH and the palmitate-albumin ratio with liver homogenates, and only on the latter parameter with muscle homogenates. Effects observed were comparable for rat and human muscle homogenates.     

Peroxisomal fatty acid oxidation in rat and human tissues. Effect of nutritional state, clofibrate treatment and postnatal development in the rat

Oxidation of palmitate 14C-labeled in different positions was assayed in the absence and presence of antimycin and rotenone in homogenates of various rat and human tissues to determine total and peroxisomal oxidation and acetyl group production. Total and antimycin-insensitive palmitate oxidation rates were higher in rat heart, liver and quadriceps muscle than in the corresponding human tissues. The proportion of antimycin-insensitive oxidation of [1-14C]palmitate was 17-35% in tissues of starved rats and in human muscles and fibroblasts, but peroxisomal production of acetyl groups amounted only to 5-11% of that by mitochondria. The mean number of peroxisomal beta-oxidation cycles was 1.5-2.5 per palmitate molecule. The nutritional state markedly influenced the total oxidation rate and the antimycin-insensitive proportion in rat liver. Clofibrate feeding increased total and antimycin-insensitive oxidation rates in liver, heart and kidney, but not in quadriceps muscle. Total oxidation capacity was maximal in rat liver at weaning, and in rat heart at an age of 70 days. Antimycin-insensitive oxidation rates increased in rat liver and heart at postnatal development up to weaning. A marked proportion of lignocerate oxidation was antimycin-sensitive in rat tissues.     

Identification of fatty acid translocase on human skeletal muscle mitochondrial membranes: essential role in fatty acid oxidation

Fatty acid translocase (FAT/CD36) is a transport protein with a high affinity for long-chain fatty acids (LCFA). It was recently identified on rat skeletal muscle mitochondrial membranes and found to be required for palmitate uptake and oxidation. Our aim was to identify the presence and elucidate the role of FAT/CD36 on human skeletal muscle mitochondrial membranes. We demonstrate that FAT/CD36 is present in highly purified human skeletal mitochondria. Blocking of human muscle mitochondrial FAT/CD36 with the specific inhibitor sulfo-N-succimidyl-oleate (SSO) decreased palmitate oxidation in a dose-dependent manner. At maximal SSO concentrations (200 muM) palmitate oxidation was decreased by 95% (P<0.01), suggesting an important role for FAT/CD36 in LCFA transport across the mitochondrial membranes. SSO treatment of mitochondria did not affect mitochondrial octanoate oxidation and had no effect on maximal and submaximal carnitine palmitoyltransferase I (CPT I) activity. However, SSO treatment did inhibit palmitoylcarnitine oxidation by 92% (P<0.001), suggesting that FAT/CD36 may be playing a role downstream of CPT I activity, possibly in the transfer of palmitoylcarnitine from CPT I to carnitine-acylcarnitine translocase. These data provide new insight regarding human skeletal muscle mitochondrial fatty acid (FA) transport, and suggest that FAT/CD36 could be involved in the cellular and mitochondrial adaptations resulting in improved and/or impaired states of FA oxidation.     

Incomplete palmitate oxidation in cell-free systems of rat and human muscles

The palmitate oxidation capacity was determined in whole homogenates, postnuclear fractions and mitochondrial fractions of various rat and human muscles and in rat liver, kidney, brain and lung. The oxidation rate (production of 14CO2 and 14C-labeled acid-soluble intermediates) was [1-14C]palmitate greater than [U-14C]palmitate greater than [16-14C]palmitate in all cell-free systems. Oxidation rates were highest in rat heart and liver, intermediate in kidney, diaphragm and m. quadriceps, and low in brain and lung. The capacity of human heart was much lower than that of rat heart and about twice that of human skeletal muscles. Omission of L-carnitine and addition of malonyl-CoA, KCN or antimycin A decreased the oxidation rates in whole homogenates and mitochondrial fractions. Antimycin or KCN increased and malonyl-CoA decreased the ratio of the oxidation rates with [1-14C]- and [16-14C]palmitate. The carnitine concentration had no significant effect on the ratio. 14C-labeled dodecanoic and tetradecanoic acids were identified in homogenates and mitochondrial fractions of m. quadriceps and liver of rat as acid-insoluble intermediates of [16-14C]palmitate oxidation in the presence and absence of antimycin A. Their amounts recovered can account for the differences in oxidation rates found with [1-14C]- and [16-14C]palmitate. The incomplete palmitate oxidation in cell-free systems appears to be mainly caused by an inadequate mitochondrial degradation of peroxisomal oxidation products.     

Fatty acid oxidation in human and rat heart. Comparison of cell-free and cellular systems

Oxidation rates of palmitate and activities of the mitochondrial marker enzymes cytochrome c oxidase and citrate synthase have been determined in homogenates, isolated mitochondria and slices of human and rat heart and in calcium-tolerant rat cardiac myocytes. Homogenates and mitochondria from rat heart showed a 6- and 2.5-fold higher palmitate oxidation rate than the corresponding preparations from human heart. From the palmitate oxidation rates and cytochrome c oxidase and citrate synthase activities as parameters, the mitochondrial protein contents of human and rat heart were calculated to be about 18 and 45 mg/g wet weight, respectively. Based on citrate synthase activities, the fatty acid oxidation rates were about the same in homogenates and isolated mitochondria, much lower in myocytes and lowest in slices. In the cellular systems the palmitate molecule was more completely oxidized than in homogenates or isolated mitochondria. Fatty acid oxidation rates were concentration-dependent in slices, but not with myocytes. With the cellular systems, palmitate oxidation was synergistically stimulated by the addition of carnitine, coenzyme A and ATP to the incubation medium. This stimulation could be attributed only partly to an increased oxidation in damaged cells.     

Effect of the fatty acid oxidation inhibitor 2-tetradecylglycidic acid (TDGA) on glucose and fatty acid oxidation in isolated rat soleus muscle

1. The effect of 2-tetradecylglycidic acid (TDGA), a potent, specific inhibitor of long-chain fatty acid oxidation, on fatty acid and glucose oxidation by isolated rat soleus muscle was studied. 2. TDGA inhibited [1-14C]palmitate oxidation by soleus muscle in a concentration-dependent manner. 3. TDGA inhibited the activity of soleus muscle mitochondrial carnitine palmitoyltransferase A (CPT-A). 4. Added palmitate (0.5 mM) significantly inhibited D-[U-14C]glucose oxidation and, under conditions where TDGA inhibited palmitate oxidation, the oxidation of D-[U-14C]glucose by isolated soleus muscle was significantly stimulated. 5. TDGA stimulation of glucose oxidation was reversed by octanoate, a medium-chain fatty acid whose oxidation is not inhibited by TDGA. 6. When nondiabetic rats were treated with TDGA (10 mg/kg p.o./day x 3 days), fasting plasma glucose was significantly lowered and the ability of isolated contralateral soleus muscles to oxidize palmitate was inhibited while glucose oxidation was significantly stimulated.     

An accurate and sensitive assay of [14C]octanoate oxidation and its application on tissue homogenates and fibroblasts

A procedure was developed to assay [14C]octanoate oxidation from the production of both 14CO2 and 14C-labeled acid-soluble products. Octanoic acid and its CoA and carnitine esters were eliminated from the acid-soluble products by alkaline hydrolysis of the esters and acidification and binding of the acid to Lipidex 1000. The method was evaluated with homogenates of various rat tissues and human muscles and with human fibroblasts. 14CO2 production was variable and comprised less than 3% of the total oxidation products with homogenates and 26 +/- 19% with fibroblasts. As compared to palmitate, oxidation rates of octanoate were higher in rat liver and heart homogenates, of the same magnitude in muscle homogenates, but lower in fibroblasts. The proportion of antimycin-insensitive oxidation was much lower with octanoate than with palmitate. Using the assay a case of medium-chain acyl-CoA dehydrogenase deficiency could be indicated.     

14CO2 production is no adequate measure of [14C]fatty acid oxidation

Palmitate oxidation was comparatively assayed in various cell-free and cellular systems by 14CO2 production and by the sum of 14CO2 and 14C-labeled acid-soluble products. The 14CO2 production rate was dependent on incubation time and amount of tissue in contrast to the total oxidation rate. The 14CO2 contribution to the oxidation rate of [1-14C]palmitate varied with homogenates from 1% with rat liver to 28% with rat kidney and amounted to only 2-4% with human muscles. With cellular systems the 14CO2 contribution varied between 20% in human fibroblasts and 70% in rat muscles and myocytes. Addition of cofactors increased the oxidation rate, but decreased the 14CO2 contribution. Various conditions appeared also to influence to a different extent the 14CO2 production and the total oxidation rate with rat tissue homogenates and with rat muscle mitochondria. Incorporation of radioactivity from [1-14C]palmitate into protein was not detectable in cell-free systems and only 2-3% of the sum of 14CO2 and 14C-labeled acid-soluble products in cellular systems. Assay of 14CO2 and 14C-labeled acid-soluble products is a much more accurate and sensitive estimation of fatty acid oxidation than assay of only 14CO2.     

Palmitate oxidation in suspended skeletal muscle fibers from the rat

Palmitate oxidation in rat skeletal muscle was investigated with a suspension of intact isolated cells. M. flexor digitorum brevis was dissociated by a 6 h collagenase treatment to yield single myofibers of which 76% were viable. The contributions of 14CO2 and 14C-labeled acid-soluble intermediates to total oxidation products from palmitate were evaluated. The myofiber suspension exhibited a higher total oxidation rate than the isolated whole muscle, due to improved transport of palmitate to the sarcolemma. Addition of cytoplasmic cofactors L-carnitine, CoASH and ATP did not increase the palmitate oxidation. 14CO2 amounted to about 37% of oxidation products. With [1(-14)C]- and [16(-14)C]palmitate, the oxidation rates were equal. These findings indicate that the cellular integrity was well preserved. The oxidation rates were sharply decreased in fibers with damaged sarcolemmas, and in intact fibers when rotenon and antimycin A were applied. The damaged fibers restored the production of acid-soluble intermediates in the presence of cofactors. The results indicate that suspended skeletal myofibers are an adequate in vitro system for measurements of metabolic activities in the resting muscle.     

Acute endurance exercise increases plasma membrane fatty acid transport proteins in rat and human skeletal muscle

Fatty acid transport proteins are present on the plasma membrane and are involved in the uptake of long-chain fatty acids into skeletal muscle. The present study determined whether acute endurance exercise increased the plasma membrane content of fatty acid transport proteins in rat and human skeletal muscle and whether the increase was accompanied by an increase in long-chain fatty acid transport in rat skeletal muscle. Sixteen subjects cycled for 120 min at ～60 ± 2% Vo(2) peak. Two skeletal muscle biopsies were taken at rest and again following cycling. In a parallel study, eight Sprague-Dawley rats ran for 120 min at 20 m/min, whereas eight rats acted as nonrunning controls. Giant sarcolemmal vesicles were prepared, and protein content of FAT/CD36 and FABPpm was measured in human and rat vesicles and whole muscle homogenate. Palmitate uptake was measured in the rat vesicles. In human muscle, plasma membrane FAT/CD36 and FABPpm protein contents increased 75 and 20%, respectively, following 120 min of exercise. In rat muscle, plasma membrane FAT/CD36 and FABPpm increased 20 and 30%, respectively, and correlated with a 30% increase in palmitate transport following 120 min of running. These data suggest that the translocation of FAT/CD36 and FABPpm to the plasma membrane in rat skeletal muscle is related to the increase in fatty acid transport and oxidation that occurs with endurance running. This study is also the first to demonstrate that endurance cycling induces an increase in plasma membrane FAT/CD36 and FABPpm content in human skeletal muscle, which is predicted to increase fatty acid transport.     

Regulation of AMP-activated protein kinase and acetyl-CoA carboxylase phosphorylation by palmitate in skeletal muscle cells

The purpose of this study was to investigate the effects of long-chain fatty acids (LCFAs) on AMP-activated protein kinase (AMPK) and acetyl-coenzyme A carboxylase (ACC) phosphorylation and beta-oxidation in skeletal muscle. L6 rat skeletal muscle cells were exposed to various concentrations of palmitate (1-800 microM). Subsequently, ACC and AMPK phosphorylation and fatty acid oxidation were measured. A 2-fold increase in both AMPK and ACC phosphorylation was observed in the presence of palmitate concentrations as low as 10 microM, which was also accompanied by a significant increase in fatty acid oxidation. The effect of palmitate on AMPK and ACC phosphorylation was dose-dependent, reaching maximum increases of 3.5- and 4.5-fold, respectively. Interestingly, ACC phosphorylation was coupled with AMPK activation at palmitate concentrations ranging from 10 to 100 microM; however, at concentrations >200 microM, ACC phosphorylation and fatty acid oxidation remained high even after AMPK phosphorylation was completely prevented by the use of a selective AMPK inhibitor. This indicates that LCFAs regulate ACC activity by AMPK-dependent and -independent mechanisms, based on their abundance in skeletal muscle cells. Here, we provide novel evidence that the AMPK/ACC pathway may operate as a mechanism to sense and respond to the lipid energy charge of skeletal muscle cells.     

Phosphorylation of cardiac protein kinase B is regulated by palmitate

In this study isolated perfused working rat hearts were used to investigate the role of palmitate-regulated protein kinase B (PKB) phosphorylation on glucose metabolism. Rat hearts were perfused aerobically in working mode with 11 mM glucose and either 100 microU/ml insulin or 100 microU/ml insulin and 1.2 mM palmitate. PKB activity and phosphorylation state were reduced in the presence of 1.2 mM palmitate, which correlates with a decrease in glycolysis (47%), glucose oxidation (84%), and glucose uptake (43%). In contrast to skeletal muscle, neither p38 nor ERK underwent changes in their phosphorylation states in response to insulin or insulin and palmitate. Moreover, pharmacological restoration of glucose oxidation rates in hearts perfused with 1.2 mM palmitate demonstrated no increase in PKB phosphorylation state. In cultured mouse cardiac muscle HL-1 cells, insulin markedly increased PKB phosphorylation, which was blunted by pre- and cotreatment with 1.2 mM palmitate. However, neither palmitate nor C(2)-ceramide treatment of insulin-stimulated cells was able to accelerate PKB dephosphorylation beyond that observed following the removal of insulin alone. Taken together, these experiments show the control of PKB phosphorylation by palmitate is independent of ceramide and suggest that this signaling event may be an important regulator of myocardial glucose uptake and oxidation.     

Palmitate acutely induces insulin resistance in isolated muscle from obese but not lean humans

Exposure to high fatty acids (FAs) induces whole body and skeletal muscle insulin resistance. The globular form of the adipokine, adiponectin (gAd), stimulates FA oxidation and improves insulin sensitivity; however, its ability to prevent lipid-induced insulin resistance in humans has not been tested. The purpose of this study was to determine 1) whether acute (4 h) exposure to 2 mM palmitate would impair insulin signaling and glucose transport in isolated human skeletal muscle, 2) whether muscle from obese humans is more susceptible to the effects of palmitate, and 3) whether the presence of 2 mM palmitate + 2.5 mug/ml gAd (P+gAd) could prevent the effects of palmitate. Insulin-stimulated (10 mU/ml) glucose transport was not different, relative to control, following exposure to palmitate (-10%) or P+gAd (-3%) in lean muscle. In obese muscle, the absolute increase in glucose transport from basal to insulin-stimulated conditions was significantly decreased following palmitate (-55%) and P+gAd (-36%) exposure (control vs. palmitate; control vs. P+gAd, P < 0.05). There was no difference in the absolute increase in glucose transport between palmitate and P+gAd, indicating that in the presence of palmitate, gAd did not improve glucose transport. The palmitate-induced reduction in insulin-stimulated glucose transport in muscle from obese individuals may have been due to reduced Ser Akt (control vs. palmitate; P+gAd, P < 0.05) and Akt substrate 160 (AS160) phosphorylation (control vs. palmitate; P+gAd, P < 0.05). FA oxidation was significantly increased in muscle of lean and obese individuals in the presence of gAd (P < 0.05), suggesting that the stimulatory effects of gAd on FA oxidation may not be sufficient to entirely prevent palmitate-induced insulin resistance in obese muscle.     

Increased uncoupling proteins and decreased efficiency in palmitate-perfused hyperthyroid rat heart

The physiological role of mitochondrial uncoupling proteins (UCPs) in heart and skeletal muscle is unknown, as is whether mitochondrial uncoupling of oxidative phosphorylation by fatty acids occurs in vivo. In this study, we found that UCP2 and UCP3 protein content, determined using Western blotting, was increased by 32 and 48%, respectively, in hyperthyroid rat heart mitochondria. Oligomycin-insensitive respiration rate, a measure of mitochondrial uncoupling, was increased in all mitochondria in the presence of palmitate: 36% in controls and 71 and 100% with 0.8 and 0.9 mM palmitate, respectively, in hyperthyroid rat heart mitochondria. In the isolated working heart, 0.4 mM palmitate significantly lowered cardiac output by 36% and cardiac efficiency by 38% in the hyperthyroid rat heart. Thus increased mitochondrial UCPs in the hyperthyroid rat heart were associated with increased uncoupling and decreased myocardial efficiency in the presence of palmitate. In conclusion, a physiological effect of UCPs on fatty acid oxidation has been found in heart at the mitochondrial and whole organ level.     

Palmitate oxidation by rat skeletal muscle mitochondria: Comparison of polarographic and radiochemical experiments

Oxidation of palmitate by rat skeletal muscle mitochondria was determined polarographically and radiochemically under state 3 conditions. Maximal oxidation rate is reached at 4 μm palmitate, palmitoyl-CoA, or palmitoyl-l-carnitine. At palmitoyl-CoA concentrations higher than 30 μm oxidation is inhibited. At limiting substrate concentrations as used in polarographic experiments palmitate is totally degraded to CO₂. At higher concentrations the palmitate molecule is only partially degraded, due to the accumulation of intermediates. Citric acid cycle intermediates, especially 2-oxoglutarate, accumulate during oxidation of palmitate in the presence of malate. It is suggested that this accumulation is stimulated by dicarboxylate exchange. The rate of formation of ¹⁴CO₂ and ¹⁴C-labeled perchloric acid-soluble products is higher from [1-¹⁴C]palmitate than that from [U-¹⁴C]palmitate. This difference, which is enhanced by higher carnitine concentrations indicates incomplete oxidation during the β-oxidation in state 3. The simultaneous determination of ¹⁴CO₂ production and ¹⁴C-labeled perchloric acid-soluble products appears to be a more accurate and sensitive method for measuring ¹⁴C-fatty acid oxidation than that of ¹⁴CO₂ production alone.     

Fatty acid and glucose oxidation by cultured rat heart cells

Monolayer cultures of fetal rat myocardial cells can be utilized to examine substrate preferences and interactions. The specific activity of glucose oxidation by myocardial cell cultures was high in sparse cultures but decreased with increased cell density. In contrast, palmitate oxidation was independent of initial cell density. Palmitate inhibited glucose oxidation by 50% in rat heart cultures. Glucose had only a slight sparing effect on palmitate oxidation. This suggests that fetal and newborn rat myocardial cells in culture preferentially oxidize palmitate similar to adult heart. The sparing effect of palmitate on glucose oxidation is accounted for by inhibition of the glycolytic-aerobic pathway and not by inhibition of the pentose phosphate pathway. Data on oxidation of ¹⁴C-pyruvate specifically labelled suggest that palmitate or a product of its oxidation such as acetyl-CoA may be acting directly to inhibit the pyruvate dehydrogenase complex. Palmitate oxidation per mg of cell protein was constant from 15 days gestational age to 2 days postnatal age. The observed differences between cultured cells and the intact heart may relate to decreased aerobic metabolism in monolayer cell culture and suggest that the increase in fatty acid oxidation observed in vivo is controlled by the oxygen environment of the cell. These studies show that heart cells in monolayer culture can be utilized to obtain metabolic information similar to an adult organ perfusion model.     

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside-induced AMP-activated protein kinase phosphorylation inhibits basal and insulin-stimulated glucose uptake, lipid synthesis, and fatty acid oxidation in isolated rat adipocytes

The objective of this study was to investigate the effects of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR)-induced AMP-activated protein kinase (AMPK) activation on basal and insulin-stimulated glucose and fatty acid metabolism in isolated rat adipocytes. AICAR-induced AMPK activation profoundly inhibited basal and insulin-stimulated glucose uptake, lipogenesis, glucose oxidation, and lactate production in fat cells. We also describe the novel findings that AICAR-induced AMPK phosphorylation significantly reduced palmitate (32%) and oleate uptake (41%), which was followed by a 50% reduction in palmitate oxidation despite a marked increase in AMPK and acetyl-CoA carboxylase phosphorylation. Compound C, a selective inhibitor of AMPK, not only completely prevented the inhibitory effect of AICAR on palmitate oxidation but actually caused a 2.2-fold increase in this variable. Compound C also significantly increased palmitate oxidation in the presence of inhibitory concentrations of malonyl-CoA and etomoxir indicating an increase in CPT1 activity. In contrast to skeletal muscle in which AMPK stimulates fatty acid oxidation to provide ATP as a fuel, we propose that AMPK activation inhibits lipogenesis and fatty acid oxidation in adipocytes. Inhibition of lipogenesis would conserve ATP under conditions of cellular stress, although suppression of intra-adipocyte oxidation would spare fatty acids for exportation to other tissues where their utilization is crucial for energy production. Additionally, the stimulatory effect of compound C on long chain fatty acid oxidation provides a novel pharmacological approach to promote energy dissipation in adipocytes, which may be of therapeutic importance for obesity and type II diabetes.     

Training-induced elevation in FABP(PM) is associated with increased palmitate use in contracting muscle.

To evaluate the effects of endurance training in rats on fatty acid metabolism, we measured the uptake and oxidation of palmitate in isolated rat hindquarters as well as the content of fatty acid-binding proteins in the plasma membranes (FABP(PM)) of red and white muscles from 16 trained (T) and 18 untrained (UT) rats. Hindquarters were perfused with 6 mM glucose, 1,800 microM palmitate, and [1-(14)C]palmitate at rest and during electrical stimulation (ES) for 25 min. FABP(PM) content was 43-226% higher in red than in white muscles and was increased by 55% in red muscles after training. A positive correlation was found to exist between succinate dehydrogenase activity and FABP(PM) content in muscle. Palmitate uptake increased by 64-73% from rest to ES in both T and UT and was 48-57% higher in T than UT both at rest (39.8 +/- 3.5 vs. 26.9 +/- 4. 4 nmol. min(-1). g(-1), T and UT, respectively) and during ES (69.0 +/- 6.1 vs. 43.9 +/- 4.4 nmol. min(-1). g(-1), T and UT, respectively). While the rats were resting, palmitate oxidation was not affected by training; palmitate oxidation during ES was higher in T than UT rats (14.8 +/- 1.3 vs. 9.3 +/- 1.9 nmol. min(-1). g(-1), T and UT, respectively). In conclusion, endurance training increases 1) plasma free fatty acid (FFA) uptake in resting and contracting perfused muscle, 2) plasma FFA oxidation in contracting perfused muscle, and 3) FABP(PM) content in red muscles. These results suggest that an increased number of these putative plasma membrane fatty acid transporters may be available in the trained muscle and may be implicated in the regulation of plasma FFA metabolism in skeletal muscle.     

Palmitate oxidation rate and action on glycogen synthase in myoblasts from insulin-resistant subjects

Elevated plasma lipid and nonesterified fatty acid concentrations reduce insulin-mediated glucose disposal in skeletal muscle. Cultured myoblasts from 21 subjects were studied for rates of palmitate oxidation and the effect of palmitate on glycogen synthase activity at the end of an 18-h incubation in serum- and glucose-free media. Oxidation rates of 40 microM palmitate in cultured myoblasts correlated with the fasting glucose (r = 0.71, P = 0.001), log fasting insulin (r = 0.52, P = 0.03), and insulin-mediated glucose storage rate (r = -0.50, P = 0.04) of the muscle donors. Myoblast glycogen synthase activity can be regulated by 240 microM palmitate, but the changes are associated with the basal respiratory quotient and not with the insulin resistance of the muscle donor. These results indicate that myoblasts producing elevated palmitate oxidation rates in vitro can be used to identify skeletal muscle abnormalities which are primary contributors to insulin resistance in vivo. Effects of 240 microM palmitate on myoblast glycogen synthase activity appear to be mechanistically different from the relationship between myoblast palmitate oxidation rates and insulin resistance of the muscle donor.     

The effect of three hypolipidemic drugs on catalase activity and peroxisomal and mitochondrial palmitate oxidation in rat cardiac and skeletal muscle

Catalase activity and peroxisomal and mitochondrial palmitate oxidation have been investigated in cardiac and skeletal muscle from rats fed clofibrate, ciprofibrate or nafenopin in an unrefined diet for different periods of time. Nafenopin was also added to either a high carbohydrate (70% of kilocalories from glucose) or high fat (70% of kilocalories from lard) diet and fed to rats for either 1 or 3 weeks. Catalase activity was elevated in all muscles from rats fed the hypolipidemic drugs. The response of catalase activity in muscle to clofibrate was dose-dependent. The response time of catalase activity was different in individual muscles. Peroxisomal palmitate oxidation was elevated in the heart and soleus muscle from rats fed nafenopin in either the high-carbohydrate or the high-fat diet. There was no change in peroxisomal palmitate oxidation in psoas or extensor digitorum longus muscle from rats fed the drugs. Mitochondrial palmitate oxidation was only slightly increased by nafenopin in the heart and soleus muscles after 3 weeks of nafenopin feeding. The results suggest that the cardiac muscle, like the liver, responds to hypolipidemic drug treatment with an increase in peroxisomal fat oxidation. The skeletal muscle response is less specific and that tissue may not contribute to the hypolipidemic effect of the drugs. The findings also suggest that these drugs do not induce peroxisome proliferation in skeletal muscle.