We used muscle contraction, which increases fatty acid oxidation, as a model to determine whether fatty acid transport is acutely regulated by fatty acid translocase (FAT/CD36). Palmitate uptake by giant vesicles, obtained from skeletal muscle, was increased by muscle contraction. Kinetic studies indicated that muscle contraction increased V max , but K m remained unaltered. Sulfo-N-succinimidyl oleate, a specific inhibitor of FAT/ CD36, fully blocked the contraction-induced increase in palmitate uptake. In giant vesicles from contracting muscles, plasma membrane FAT/CD36 was also increased in parallel with the increase in long chain fatty acid uptake. Further studies showed that like GLUT-4, FAT/CD36 is located in both the plasma membrane and intracellularly (endosomally). With muscle contraction, FAT/CD36 at the surface of the muscle was increased, while concomitantly, FAT/CD36 in the intracellular pool was reduced. Similar responses were observed for GLUT-4. We conclude that fatty acid uptake is subject to short term regulation by muscle contraction and involves the translocation of FAT/CD36 from intracellular stores to the sarcolemma, analogous to the regulation of glucose uptake by GLUT-4.

Giant vesicles were used to study the rates of uptake of long-chain fatty acids by heart, skeletal muscle, and adipose tissue of obese and lean Zucker rats. With obesity there was an increase in vesicular fatty acid uptake of 1.8-fold in heart, muscle and adipose tissue. In some tissues only fatty acid translocase (FAT) mRNA (heart, ؉37%; adipose, ؉80%) and fatty acid-binding protein (FABPpm) mRNA (heart, ؉148%; adipose, ؉196%) were increased. At the protein level FABPpm expression was not changed in any tissues except muscle (؉14%), and FAT/CD36 protein content was altered slightly in adipose tissue (؉26%). In marked contrast, the plasma membrane FAT/CD36 protein was increased in heart (؉60%), muscle (؉80%), and adipose tissue (؉50%). The plasma membrane FABPpm was altered only in heart (؉50%) and adipose tissues (؉70%). Thus, in obesity, alterations in fatty acid transport in metabolically important tissues are not associated with changes in fatty acid transporter mRNAs or altered fatty acid transport protein expression but with their increased abundance at the plasma membrane. We speculate that in obesity fatty acid transporters are relocated from an intracellular pool to the plasma membrane in heart, muscle, and adipose tissues.Fatty acids (FA) 1 are important substrates for most mammalian tissues. Based on their hydrophobic structure it has been postulated that FA are sequestered by cells through passive diffusion across the plasma membrane (cf. Ref. 1). However, other evidence has shown that FA also traverse the plasma membrane via a protein-mediated mechanism (cf. Refs. 2 and 3). Indeed, this latter system is quantitatively more important than passive diffusion, as FA uptake can be reduced markedly by inhibitors of protein-mediated membrane transport (4 -6) and by a reactive ester of oleate (4). Thus, a number of groups began to search for FA transport proteins.Several putative fatty acid transport proteins have been identified that promote the cellular uptake of FA. These are a 43-kDa plasma membrane fatty acid-binding protein (FABPpm) (7), identical to mitochondrial aspartate aminotransferase (7-9), and an 88-kDa heavily glycosylated fatty acid translocase (FAT/CD36), the rat homologue of human CD36 (10). In addition, to these membrane-associated proteins, a soluble cytoplasmic fatty acid-binding protein (FABPc) is also important for cellular FA uptake, because in FABPc null mice there is a marked decrease of FA influx into cardiac myocytes (11). FATP1, another putative fatty acid transport protein (12, 13), correlates inversely with fatty acid transport in muscle and heart (4), and this protein appears to be a very long-chain acyl-CoA synthetase (14, 15). These observations suggest that FATP1 is unlikely to be involved directly in fatty acid translocation across the plasma membrane.To examine the regulation of transmembrane fatty acid transport, we have characterized giant vesicles (4, 16, 17), which can be prepared from metabolically important tissue such as heart (4) and skeletal muscle (16, 17) as...

-It is well known that muscle contraction and insulin can independently translocate GLUT-4 from an intracellular depot to the plasma membrane. Recently, we have shown that the fatty acid transporter FAT/CD36 is translocated from an intracellular depot to the plasma membrane by muscle contraction (Ͻ30 min) (Bonen et al. J Biol Chem 275: 14501-14508, 2000). In the present study, we examined whether insulin also induced the translocation of FAT/CD36 in rat skeletal muscle. In studies in perfused rat hindlimb muscles, we observed that insulin increased fatty acid uptake by ϩ51%. Insulin increased the rate of palmitate incorporation into triacylglycerols, diacylglycerols, and phospholipids (P Ͻ 0.05) while reducing muscle palmitate oxidation (P Ͻ 0.05). Perfusing rat hindlimb muscles with insulin increased plasma membrane FAT/CD36 by ϩ48% (P Ͻ 0.05), whereas concomitantly the intracellular FAT/CD36 depot was reduced by 68% (P Ͻ 0.05). These insulin-induced effects on FAT/CD36 translocation were inhibited by the phosphatidylinositol 3-kinase inhibitor LY-294002. Thus these studies have shown for the first time that insulin can induce the translocation of FAT/CD36 from an intracellular depot to the plasma membrane.This reveals a previously unknown level of regulation of fatty acid transport by insulin and may well have important consequences in furthering our understanding of the relation between fatty acid metabolism and insulin resistance.

We have examined the effects of streptozotocin (STZ)-induced diabetes (moderate and severe) on fatty acid transport and fatty acid transporter (FAT/CD36) and plasma membrane-bound fatty acid binding protein (FABPpm) expression, at the mRNA and protein level, as well as their plasmalemmal localization. These studies have shown that, with STZ-induced diabetes, 1) fatty acid transport across the plasma membrane is increased in heart, skeletal muscle, and adipose tissue and is reduced in liver; 2) changes in fatty acid transport are generally not associated with changes in fatty acid transporter mRNAs, except in the heart; 3) increases in fatty acid transport in heart and skeletal muscle occurred with concomitant increases in plasma membrane FAT/CD36, whereas in contrast, the increase and decrease in fatty acid transport in adipose tissue and liver, respectively, were accompanied by concomitant increments and reductions in plasma membrane FABPpm; and finally, 4) the increases in plasma membrane transporters (FAT/CD36 in heart and skeletal muscle; FABPpm in adipose tissue) were attributable to their increased expression, whereas in liver, the reduced plasma membrane FABPpm appeared to be due to its relocation within the cell in the face of slightly increased expression. Taken together, STZ-induced changes in fatty acid uptake demonstrate a complex and tissue-specific pattern, involving different fatty acid transporters in different tissues, in combination with different underlying mechanisms to alter their surface abundance.

. Different mechanisms can alter fatty acid transport when muscle contractile activity is chronically altered. Am J Physiol Endocrinol Metab 286: E1042-E1049, 2004; 10.1152/ajpendo.00531.2003.-We examined whether skeletal muscle transport rates of long-chain fatty acids (LCFAs) were altered when muscle activity was eliminated (denervation) or increased (chronic stimulation). After 7 days of chronically stimulating the hindlimb muscles of female Sprague-Dawley rats, the LCFA transporter proteins fatty acid translocase (FAT)/CD36 (ϩ43%) and plasma membrane-associated fatty acid-binding protein (FABPpm; ϩ30%) were increased (P Ͻ 0.05), which resulted in the increased plasmalemmal content of these proteins (FAT/CD36, ϩ42%; FABPpm ϩ13%, P Ͻ 0.05) and a concomitant increase in the LCFA transport rate into giant sarcolemmal vesicles (ϩ44%, P Ͻ 0.05). Although the total muscle contents of FAT/CD36 and FABPpm were not altered (P Ͼ 0.05) after 7 days of denervation, the LCFA transport rate was markedly decreased (Ϫ39%). This was associated with reductions in plasmalemmal FAT/CD36 (Ϫ24%) and FABPpm (Ϫ28%; P Ͻ 0.05). These data suggest that these LCFA transporters were resequestered to their intracellular depot(s) within the muscle. Combining the results from these experiments indicated that changes in rates of LCFA transport were correlated with concomitant changes in plasmalemmal FAT/CD36 and FABPpm, but not necessarily with their total muscle content. Thus chronic alterations in muscle activity can alter the rates of LCFA transport via different mechanisms, either 1) by increasing the total muscle content of FAT/CD36 and FABPpm, resulting in a concomitant increase at the sarcolemma, or 2) by reducing the plasma membrane content of these proteins in the absence of any changes in their total muscle content. giant vesicles; tibialis anterior; gastrocnemius; denervation; chronic stimulation SKELETAL MUSCLES depend on long-chain fatty acids (LCFAs) to maintain ATP production during contractile activity. When muscle activity is increased, progressively more LCFAs are taken up into the muscle (16). It is now recognized that LCFAs cross the sarcolemma via simple diffusion and a proteinmediated system (3,4,35,47). Several proteins have been identified as LCFA transporters. Fatty acid translocase (FAT/ CD36) and plasma membrane-associated fatty acid-binding protein (FABPpm) have been shown to facilitate LCFA transport into heart and skeletal muscle (3, 24, 38, 47), whereas fatty acid transport protein 1 (FATP1) appears to be involved in the transport of LCFAs and acylating very long-chain fatty acids (15,45,53,58).Skeletal muscle metabolism is remarkably capable of adapting to changes in muscle activity pattern (7, 44a). Chronic changes in muscle activity can also alter the expression of several substrate transporters, and hence substrate transport. For example, increasing muscle activity by chronic stimulation enhances glucose and lactate transport and their respective transporters GLUT4 (28,46,56) and the monocarboxylate transp...