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.

Altered muscle fatty acid (FA) metabolism may contribute to the presence of muscle insulin resistance in the genetically obese Zucker rat. To determine whether FA uptake and disposal are altered in insulin-resistant muscle, we measured palmitate uptake, oxidation, and incorporation into di-and triglycerides in isolated rat hindquarters, as well as muscle plasma membrane fatty acid-binding protein (FABP PM ) content of lean (n ‫؍‬ 16, fa/؉) and obese (n ‫؍‬ 15, fa/fa) Zucker rats (12 weeks of age). Hindquarters were perfused with 7 mmol/l glucose, 1,000 mol/l albumin-bound palmitate, and albumin-bound [1-14 C]palmitate at rest (no insulin). Glucose uptake was 42% lower in the obese than in the lean rats and indicated the presence of muscle insulin resistance. Fractional and total rates of palmitate uptake were 42 and 74% higher in the obese than in the lean rats and were associated with higher muscle FABP PM content (r 2 ‫؍‬ 0.69, P < 0.05). The percentage of palmitate oxidized was not significantly different between groups. FA disposal to storage was altered according to fiber type. When compared with lean rats, the rate of triglyceride synthesis in red muscle was 158% higher in obese rats, and the rate of palmitate incorporation into diglycerides in white muscle was 93% higher in obese rats. Pre-and postperfusion muscle triglyceride levels were higher in both red and white muscles of the obese rats. These results show that increased FA uptake and altered FA disposal to storage may contribute to the development of muscle insulin resistance in obese Zucker rats.

. AMPK activation is not critical in the regulation of muscle FA uptake and oxidation during low-intensity muscle contraction. Am J Physiol Endocrinol Metab 288: E592-E598, 2005. First published November 16, 2004; doi:10.1152/ajpendo.00301.2004.-To determine the role of AMP-activated protein kinase (AMPK) activation on the regulation of fatty acid (FA) uptake and oxidation, we perfused rat hindquarters with 6 mM glucose, 10 U/ml insulin, 550 M palmitate, and [14 C]palmitate during rest (R) or electrical stimulation (ES), inducing low-intensity (0.1 Hz) muscle contraction either with or without 2 mM 5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside (AICAR). AICAR treatment significantly increased glucose and FA uptake during R (P Ͻ 0.05) but had no effect on either variable during ES (P Ͼ 0.05). AICAR treatment significantly increased total FA oxidation (P Ͻ 0.05) during both R (0.38 Ϯ 0.11 vs., which was paralleled in both conditions by a significant increase and significant decrease in AMPK and acetyl-CoA carboxylase (ACC) activity, respectively (P Ͻ 0.05). Low-intensity muscle contraction increased glucose uptake, FA uptake, and total FA oxidation (P Ͻ 0.05) despite no change in AMPK (950.5 Ϯ 35.9 vs. 1,067.7 Ϯ 58.8 nmol ⅐ min Ϫ1 ⅐ g Ϫ1 ) or ACC (51.2 Ϯ 6.7 vs. 55.7 Ϯ 2.0 nmol ⅐ min Ϫ1 ⅐ g Ϫ1 ) activity from R to ES (P Ͼ 0.05). When contraction and AICAR treatment were combined, the AICAR-induced increase in AMPK activity (34%) did not account for the synergistic increase in FA oxidation (175%) observed under similar conditions. These results suggest that while AMPK-dependent mechanisms may regulate FA uptake and FA oxidation at rest, AMPK-independent mechanisms predominate during low-intensity muscle contraction. electrical stimulation; perfused hindquarter; acetyl-coenzyme A carboxylase; malonyl-coenzyme A; cellular signaling; 5-aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside; fatty acids IT HAS BEEN SHOWN REPEATEDLY in isolated muscle and whole body studies that exercise and muscle contraction increase fatty acid (FA) utilization (4,5,25). In perfused muscle as well as in giant sarcolemmal vesicles isolated from muscles, it has been shown that the rate of FA uptake is higher during acute muscle contraction induced by electrical stimulation than during rest, and that this increase in FA uptake is independent of FA delivery (6,25). In perfused and isolated rat muscle and in human muscle, it has also been shown that the rate of FA oxidation is higher during acute muscle contraction induced by electrical stimulation and knee extensor exercise, respectively, than during rest (9,25,26). However, the signaling pathway(s) regulating this contraction-induced increase in FA uptake and oxidation in muscle is not well defined.

Previous studies have demonstrated enhanced insulin sensitivity in calorie-restricted [CR, fed 60% ad libitum (AL) one time daily] compared with AL-fed rats. To evaluate the effects of reduced food intake, independent of temporal differences in consumption, we studied AL (unlimited food access)-fed and CR (fed one time daily) rats along with groups temporally matched for feeding [fed 3 meals (M) daily]: MAL and MCR, eating 100 and 60% of AL intake, respectively. Insulin-stimulated glucose transport by isolated muscle was increased in MCR and CR vs. AL and MAL; there was no significant difference for MCR vs. CR or MAL vs. AL. Intramuscular triglyceride concentration, which is inversely related to insulin sensitivity in some conditions, did not differ among groups. Muscle concentration of UDP-N-acetylhexosamines [end products of the hexosamine biosynthetic pathway (HBP)] was lower in MCR vs. MAL despite unaltered glutamine-fructose-6-phosphate aminotransferase activity (rate-limiting enzyme for HBP). These results indicate that the CR-induced increase in insulin-stimulated glucose transport in muscle is attributable to an altered amount, not timing, of food intake and is independent of lower triglyceride concentration. They further suggest that enhanced insulin action might involve changes in HBP.

) group. Conversely, values of fractional uptake and percent oxidation of octanoate were not significantly different between groups (P Ͼ 0.05). MalonylCoA levels were inversely correlated with LCFA oxidation (P Ͻ 0.05). These results demonstrate that high carbohydrate availability in muscle is associated with a decrease in LCFA oxidation that is not due to a parallel decrease in LCFA uptake; rather, the decrease in LCFA oxidation could be due to malonylCoA inhibition of mitochondrial LCFA transport. long-chain fatty acid; medium-chain fatty acid; malonyl-coenzyme A; carbohydrate; free fatty acid metabolism; free fatty acid oxidation; free fatty acid uptake CARBOHYDRATE AVAILABILITY is known as a factor that can alter the relative contribution of fuel sources to total oxidative metabolism. In whole body human studies, high intracellular carbohydrate availability resulting from the presence of hyperglycemia and hyperinsulinemia has been shown to be associated with a decrease in long-chain fatty acid (LCFA) oxidation, even when the insulin-induced fall in LCFA availability is prevented by a concomitant infusion of lipids (8,29). These results suggest that intramuscular factors must play a critical role in the regulation of substrate utilization under conditions of altered carbohydrate availability.