Glucose-fatty Acid Cycle Research Articles

Impairment of Glucose Tolerance in Normal Adults Following a Lowered Carbohydrate Intake.

Some normal people are falsely classified as having impaired glucose tolerance (IGT) if they are given an oral glucose tolerance test (OGTT) when their last meal contained very few carbohydrates. In this study, the duration of carbohydrate restriction was extended to one and three days and the relationship between the carbohydrate restriction and the glucose tolerance after an OGTT was examined. Two different groups of normal subjects were placed on high-carbohydrate (80% carbohydrates) and low-carbohydrate (10%) diets before an OGTT; one group for one day and the other for 3 days. None of the subjects showed impairment of glucose tolerance when placed on the high-carbohydrate regimens. In contrast, 3 of 12 subjects and 2 of 8 subjects placed on the low-carbohydrate diets for 1 and 3 days, respectively, were classified as having IGT. The impairment of glucose tolerance was invariably accompanied by an increase in the fasting plasma free fatty acid level. The longer the period of carbohydrate restriction, the severer was the glucose tolerance impairment. However, the number of subjects who were classified as having IGT did not depend on the duration of carbohydrate restriction. The impairment of glucose tolerance after carbohydrate restriction may be associated with the Randle effect, which is the activation of the glucose-free fatty acid cycle.

The antihyperglycaemic effect of metformin: therapeutic and cellular mechanisms.

Metformin is regarded as an antihyperglycaemic agent because it lowers blood glucose concentrations in type 2 (non-insulin-dependent) diabetes without causing overt hypoglycaemia. Its clinical efficacy requires the presence of insulin and involves several therapeutic effects. Of these effects, some are mediated via increased insulin action, and some are not directly insulin dependent. Metformin acts on the liver to suppress gluconeogenesis mainly by potentiating the effect of insulin, reducing hepatic extraction of certain substrates (e.g. lactate) and opposing the effects of glucagon. In addition, metformin can reduce the overall rate of glycogenolysis and decrease the activity of hepatic glucose-6-phosphatase. Insulin-stimulated glucose uptake into skeletal muscle is enhanced by metformin. This has been attributed in part to increased movement of insulin-sensitive glucose transporters into the cell membrane. Metformin also appears to increase the functional properties of insulin- and glucose-sensitive transporters. The increased cellular uptake of glucose is associated with increased glycogen synthase activity and glycogen storage. Other effects involved in the blood glucose-lowering effect of metformin include an insulin-independent suppression of fatty acid oxidation and a reduction in hypertriglyceridaemia. These effects reduce the energy supply for gluconeogenesis and serve to balance the glucose-fatty acid (Randle) cycle. Increased glucose turnover, particularly in the splanchnic bed, may also contribute to the blood glucose-lowering capability of metformin. Metformin improves insulin sensitivity by increasing insulin-mediated insulin receptor tyrosine kinase activity, which activates post-receptor insulin signalling pathways. Some other effects of metformin may result from changes in membrane fluidity in hyperglycaemic states. Metformin therefore improves hepatic and peripheral sensitivity to insulin, with both direct and indirect effects on liver and muscle. It also exerts effects that are independent of insulin but cannot substitute for this hormone. These effects collectively reduce insulin resistance and glucotoxicity in type 2 diabetes.

Malonyl-CoA, fuel sensing, and insulin resistance.

Malonyl-CoA is an allosteric inhibitor of carnitine palmitoyltransferase (CPT) I, the enzyme that controls the transfer of long-chain fatty acyl (LCFA)-CoAs into the mitochondria where they are oxidized. In rat skeletal muscle, the formation of malonyl-CoA is regulated acutely (in minutes) by changes in the activity of the beta-isoform of acetyl-CoA carboxylase (ACCbeta). This can occur by at least two mechanisms: one involving cytosolic citrate, an allosteric activator of ACCbeta and a precursor of its substrate cytosolic acetyl-CoA, and the other involving changes in ACCbeta phosphorylation. Increases in cytosolic citrate leading to an increase in the concentration of malonyl-CoA occur when muscle is presented with insulin and glucose, or when it is made inactive by denervation, in keeping with a diminished need for fatty acid oxidation in these situations. Conversely, during exercise, when the need of the muscle cell for fatty acid oxidation is increased, decreases in the ATP/AMP and/or creatine phosphate-to-creatine ratios activate an isoform of an AMP-activated protein kinase (AMPK), which phosphorylates ACCbeta and inhibits both its basal activity and activation by citrate. The central role of cytosolic citrate links this malonyl-CoA regulatory mechanism to the glucose-fatty acid cycle concept of Randle et al. (P. J. Randle, P. B. Garland. C. N. Hales, and E. A. Newsholme. Lancet 1: 785-789, 1963) and to a mechanism by which glucose might autoregulate its own use. A similar citrate-mediated malonyl-CoA regulatory mechanism appears to exist in other tissues, including the pancreatic beta-cell, the heart, and probably the central nervous system. It is our hypothesis that by altering the cytosolic concentrations of LCFA-CoA and diacylglycerol, and secondarily the activity of one or more protein kinase C isoforms, changes in malonyl-CoA provide a link between fuel metabolism and signal transduction in these cells. It is also our hypothesis that dysregulation of the malonyl-CoA regulatory mechanism, if it leads to sustained increases in the concentrations of malonyl-CoA and cytosolic LCFA-CoA, could play a key role in the pathogenesis of insulin resistance in muscle. That it may contribute to abnormalities associated with the insulin resistance syndrome in other tissues and the development of obesity has also been suggested. Studies are clearly needed to test these hypotheses and to explore the notion that exercise and some pharmacological agents that increase insulin sensitivity act via effects on malonyl-CoA and/or cytosolic LCFA-CoA.

Mediation of glucoregulation at rest and during exercise by the glucose-fatty acid cycle: in vivo and in vitro studies.

Himsworth (1934) demonstrated that increased fat consumption leads to decreased glucose tolerance due to decreased insulin sensitivity. Randle and colleagues (1964) named this interplay between fat and carbohydrate metabolism the glucose-fatty acid cycle (GFAC) and proposed a series of feedback mechanisms by which elevated levels of free fatty acids (FFAs) impair glucose uptake and oxidation in rat heart and diaphragm muscle. Numerous investigators have extended these studies to clarify the existence of GFAC and provide insight into the mechanisms and conditions under which it occurs. This paper reviews the literature and highlights other indirect means by which FFAs affect carbohydrate metabolism. Numerous in vitro studies are reviewed, emphasizing the importance of FFA concentration, carbon length, and degree of saturation. This article addresses evidence that the interplay between fat and carbohydrate metabolism is not a function of FFA concentration but a result of the impact that FFA levels have on insulin.

Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years.

Competition for respiration between substrates in animal tissues has been known for at least 80 years. The most important interaction, quantitatively is between glucose and fatty acids. The starting point in 1963 for the so called Glucose Fatty Acid Cycle was the realisation that the metabolic relationship between glucose and fatty acids is reciprocal and not dependent. Glucose provision promotes glucose oxidation and glucose and lipid storage, and inhibits fatty acid oxidation. Provision of free fatty acids promotes fatty acid oxidation and storage, inhibits glucose oxidation and may promote glucose storage if glycogen reserves are incomplete. This review is concerned predominantly with evidence in man in vivo. In the authors opinion the evidence for inhibitory effects of fatty acids on whole body glucose utilization ad oxidation (predominantly muscles) is decisive and enzyme mechanisms mediating these effects are well established. There is also much evidence that fatty acid oxidation inhibits glucose oxidation and stimulates glucose formation in liver and again enzyme mechanism are known. A permissive role for fatty acids in the insulin secretory response of islet beta-cells has now been firmly established and can be visualised as a mechanism to protect continuing provision of respiratory substrate. Longer term exposure of islet beta-cells to fatty acids impairs the insulin secretory response to glucose and mechanisms are known. There is compelling evidence that fatty acid oxidation may impair glucose oxidation in uncontrolled Type 1 and Type 2 diabetes, but no convincing evidence that fatty acids have a role in diminished glucose storage (glycogen deposition) in Type 2 diabetes. The inhibition of glucose storage which may follow prolonged elevation of plasma FFA in man and experimental animals is associated with glycogen repletion whereas the inhibition of glucose storage in Type 2 diabetes is associated with glycogen depletion. The precise role of fatty acids in disturbed carbohydrate metabolism in Type 2 diabetes is an area where future progress is confidently predicted.

Fuel metabolism during exercise: the role of the glucose-fatty acid cycle in mediating carbohydrate and fat metabolism.

not available

Malonyl CoA, long chain fatty acyl CoA and insulin resistance in skeletal muscle.

Malonyl CoA is an inhibitor of carnitine palmitoyl transferase 1 (CPT1), the enzyme that regulates the transfer of long chain fatty acyl CoA into mitochondria. By virtue of this effect, it is thought to play a key role in regulating fatty acid oxidation. Thus, when the supply of glucose to muscle is increased, malonyl CoA levels increase in keeping with a decreased need for fatty acid oxidation, and fatty acids are preferentially esterified to form diaglycerol and triglycerides. In contrast, during exercise, when the need for fatty acid oxidation is increased, malonyl CoA levels fall. Changes in glucose supply regulate malonyl CoA by modulating the concentration of cytosolic citrate, an allosteric activator of acetyl CoA carboxylase (ACC), the rate-limiting enzyme for malonyl CoA formation and a precursor of its substrate cytosolic acetyl CoA. Conversely, exercise lowers the concentration of malonyl CoA, by activating an AMP-activated protein kinase, which phosphorylates and inhibits ACC. A number of reports have linked sustained increases in the concentration of malonyl CoA in muscle to insulin resistance. In this paper, we review these reports, as well as the notion that changes in malonyl CoA contribute to the increases in long chain fatty acyl CoA, (LCFA CoA), diacylglycerol and triglyceride content and changes in protein kinase C activity and distribution observed in insulin-resistant muscle. We also review the implications of the malonyl CoA/LCFA CoA hypothesis to two other proposed mechanisms for insulin resistance, the glucose-fatty acid cycle and the hexosamine theory.

The Role of Glucose in the Regulation of Substrate Interaction During Exercise

Glucose and fatty acids are the main energy sources for oxidative metabolism in endurance exercise. Although a reciprocal relationship exists between glucose and fatty acid contribution to energy production for a given metabolic rate, the controlling mechanism remains debatable. Randle et al.'s (1963) glucose-fatty acid cycle hypothesis provides a potential mechanism for regulating substrate interaction during exercise. The cornerstone of this hypothesis is that the rate of lipolysis, and therefore fatty acid availability, controls how glucose and fatty acids contribute to energy production. Increasing fatty acid availability attenuates carbohydrate oxidation during exercise, mainly via sparing intramuscular glycogen. However, there is little evidence for a direct inhibitory effect of fatty acids on glucose oxidation. We found that glucose directly determines the rate of fat oxidation by controlling fatty acid transport into the mitochondria. We propose that the intracellular availability of glucose, rather than fatty acids, regulates substrate interaction during exercise.

The body mass cycle of the migratory garden warbler ( Sylvia borin) is associated with changes of basal plasma metabolite levels

Garden warblers show pronounced seasonal changes in body mass in relation to migration. In order to reveal if fattening and defattening is associated with changes in plasma levels of metabolic key factors, body mass, food intake, molt, visible fat, and plasma metabolite, and electrolyte levels were measured under constant indoor conditions for the non-reproductive period (September to May). During the phase of high body mass and high fat loads, the plasma levels of glucose, triglycerides, cholesterol and free fatty acids were higher, sodium concentration and glucose tolerance lower than in the phase of low body mass and molt. Urea, uric acid, β-hydroxybutyrate and potassium levels showed no or only small variations throughout the year with no significant relationship to the body mass cycle. In preparation for migration a metabolic change in preferring fat as substrate for energy metabolism is assumed. This perhaps results in constraints as predicted by the glucose fatty-acid cycle, which appear to be similar to those in mammalian obesity.

Glucose utilization rate and pancreatic hormone response to oral glucose loads are influenced by the migratory condition and fasting in the garden warbler (Sylvia borin).

Substrate utilization and regulatory mechanisms of metabolism were studied in migratory garden warblers by measuring plasma levels of glucose, free fatty acids (FFAs), beta-hydroxybutyrate, insulin and glucagon in response to oral glucose loads. Three different physiological states were examined: (a) the autumnal migratory period on a high and (b) on a fasted low body mass level, and (c) the postmigratory period with low body mass. Glucose tolerance was better in the postmigratory lean than fat condition. However, total food deprivation of 5-7 days with fat birds reaching their lean body mass further reduced the glucose utilization rate. Initial levels of FFAs were highest in the starved, intermediate in the fat and lowest in the lean condition. Changes in plasma FFAs during glucose tolerance tests were opposite to those of the glucose levels. Ten minutes after the glucose load plasma glucagon levels decreased and insulin increased. These effects were larger in the fat than in the postmigratory lean condition. There were no differences between sexes. It appears that during premigratory and migratory periods glucose utilization may be inhibited by a more favorable oxidation of fatty acids as would be predicted by the glucose fatty acid cycle. However, the inhibition of glucose utilization seems to be counterregulated by stronger responses of insulin and glucagon. These results may be important also in the consideration of food selection during premigratory periods and refueling abilities of birds crossing ecological barriers.

Fat metabolism during exercise: a review--part II: regulation of metabolism and the effects of training.

This part discusses the complex regulation of fat metabolism. Catecholamines as a stimulator of lipolysis and insulin as a suppressor play very important roles in the regulation of fat oxidation. The interaction of carbohydrate and fat metabolism has been extensively studied in the past decennia but the understanding of this multifactorial regulation is complex and still incompletely understood. In 1963, Randle et al. proposed the glucose-fatty acid cycle as a possible mechanism, and more recently, regulation through malonyl-CoA has been put forward as a possible way to explain shifts in carbohydrate and fat metabolism at rest and during exercise. The exercise intensity affects fat oxidation mainly by increasing lipolysis and fatty acid availability during exercise of low to moderate intensity. At high exercise intensities, both a reduction in fatty acid availability (decreased RaFa) and intramuscular factors reduce fat oxidation. These intramuscular factors are largely unknown. The increased mitochondrial density after training and increased oxidative enzymes may partly explain the increased fatty acid oxidation during exercise as observed after training. However, also supply of fatty acids to the mitochondria may be important. The available evidence suggests that the additional fatty acids oxidized after training are primarily derived from intramuscular triacylglycerols and not from adipose tissue derived fatty acids or circulating triacylglycerols.

Regulation of skeletal muscle carbohydrate oxidation during steady-state contraction.

Pyruvate dehydrogenase complex (PDC) activation status has been described as being central in the regulation of tissue substrate oxidation as outlined by the glucose fatty-acid cycle. In the present study we examined the effects of reduced lipolysis, with use of nicotinate, and increased PDC activation, with use of dichloroacetate (DCA), on substrate utilization during 20 min of submaximal steady-state contraction (approximately 80% of maximal O2 uptake) in canine gracilis skeletal muscle. At rest, PDC activation was unchanged by nicotinate but was approximately 2.5-fold higher in the DCA group than in the control group (P < 0.05). During contraction, PDC activation status increased to 3.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the control group, remained at 4.5 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the DCA group, but only increased to 2.2 mmol acetyl-CoA.min-1.kg-1 at 37 degrees C in the nicotinate group (P < 0.05). However, the estimated amount of carbohydrate oxidized during the 20-min contraction was similar across groups and did not follow the degree of PDC activation (81.2 +/- 22.9, 95.9 +/- 11.7, and 89.3 +/- 18.9 mmol glucosyl units/kg dry muscle for control, nicotinate, and DCA, respectively). Thus it would appear that, during steady-state contraction, PDC activation status does not determine the rate of carbohydrate oxidation in skeletal muscle.

Role of non-esterified fatty acids in the pathogenesis of type 2 diabetes mellitus.

Over 30 years after the original description of Randle's cycle, the mechanisms by which elevated circulating non-esterified fatty acids might be causally related to glucose intolerance in Type 2 diabetes mellitus remain uncertain. This review examines the evidence that elevated plasma NEFA can inhibit glucose-stimulated insulin secretion by pancreatic beta-cells and impair glucose- and insulin-stimulated glucose disposal by peripheral tissues. Experimental evidence beginning to examine the mechanisms of these phenomena is discussed.

Metabolic interactions between glucose and fatty acids in humans

In vivo energy production results largely from the oxidative metabolism of either glucose or fatty acids. Under diverse physiologic and nutritional conditions, the oxidation of either glucose or fatty acids may predominate. The nature of the control of the availability and oxidation of each substrate has been studied extensively for > or = 30 y. The most popular and enduring hypothesis was proposed by Randle et al in 1963 and is termed the glucose-fatty acid cycle. This proposal places great significance on the regulation of lipolysis as a factor controlling substrate metabolism. Our work has led to an opposite perspective, which could be called the glucose-fatty acid cycle reversed. According to our hypothesis, the rate of glycolysis, determined by the intracellular availability of glucose-6-phosphate, is the predominant factor determining the rate of glucose oxidation. Whereas the rate of lipolysis may have some effect on the availability of glucose, both via a fatty acid-mediated inhibition of plasma glucose uptake and also by supplying glycerol for gluconeogenesis, there is little evidence for a direct inhibitory effect of fatty acid oxidation on the intracellular oxidation of glucose. In contrast, increased glucose oxidation limits oxidation of long-chain fatty acids directly by inhibiting their transport into the mitochondria. Consequently, whereas there is a close coupling between glucose availability and oxidation, fatty acids are generally available in greater quantities than are required for oxidation. We propose that fatty acid oxidation is largely controlled at the site of oxidation, which is in turn determined by the availability of glucose, rather than by its availability via lipolysis.

Effect of lipid oxidation on glucose utilization in humans

The mechanisms responsible for the competition between glucose and fatty acids as oxidative fuels are not yet completely understood in humans. Maintenance of plasma fatty acid concentrations by means of lipid and heparin infusion during hyperinsulinemic, euglycemic clamps in humans first suppresses glucose oxidation and only later decreases nonoxidative glucose disposal; muscle pyruvate dehydrogenase activity is inhibited when plasma fatty acid concentrations are maintained during hyperinsulinemic, euglycemic clamps. The maintenance of plasma fatty acid concentrations impairs insulin-stimulated muscle glucose uptake only if fatty acid uptake by skeletal muscles is increased. The role of hyperglycemia in glucose-fatty acid competition has recently been emphasized. Fatty acid utilization by muscle is impaired in patients with type 2 diabetes mellitus during fasting hyperglycemia: both lipid oxidation and uptake of plasma fatty acids by skeletal muscle are impaired during postabsorptive conditions. Hyperglycemia indirectly activates pyruvate dehydrogenase, the rate-limiting enzyme for glucose oxidation. The ability of increased glucose availability to stimulate glucose oxidation and reduce lipid oxidation by skeletal muscle can be considered a corollary of the Randle glucose-fatty acid cycle. It can be concluded that within a reasonable range of carbohydrate-to-fat ratios, the addition of fat to a meal does not decrease postprandial carbohydrate oxidation. Furthermore, high-fat meals do not promote fat oxidation leading to fat storage in adipose tissue.

Corrigendum

CORRIGENDUMCorrigendumPublished Online:01 Mar 1998https://doi.org/10.1152/ajpendo.1998.274.3.Ea1Original articleMoreSectionsPDF (18 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmail Volume 273, September 1997 Volume 36, September 1997Pages E650–E654: C. Meyer, V. Nadkarni, M. Stumvoll, and J. Gerich. “Human kidney free fatty acid and glucose uptake: evidence for a renal glucose-fatty acid cycle.” Page E650: the primed-continuous infusion rates of [6-3H]glucose were given erroneously as “20–30 μCi, 0.2–0.3 μCi/min.” The correct rates should have read “∼50 μCi, ∼0.5 μCi/min.”This article has no references to display. Previous Back to Top FiguresReferencesRelatedInformationRelated articlesHuman kidney free fatty acid and glucose uptake: evidence for a renal glucose-fatty acid cycle. 01 Sep 1997American Journal of Physiology-Endocrinology and Metabolism More from this issue > Volume 274Issue 3March 1998Pages Ea1-Ea1 Copyright & PermissionsCopyright © 1998 the American Physiological Societyhttps://doi.org/10.1152/ajpendo.1998.274.3.Ea1History Published online 1 March 1998 Published in print 1 March 1998 PDF download Metrics Downloaded 87 times

Regulation of fat/carbohydrate interaction in human skeletal muscle during exercise.

There has been continued interest in the regulation of fat and carbohydrate utilization in muscle tissue, as they are the main substrates for energy production during exercise in well fed humans. Many investigations have demonstrated that increasing fat availability increases fat oxidation and decreases carbohydrate use in the whole body and skeletal muscle. However, little work has been performed in skeletal muscle to identify the mechanisms, and specific biochemical signals which mediate these shifts in fuel selection. The classic work in this area, using contracting heart and resting diaphragm muscle, led to a theory that could explain reciprocal changes in fat and carbohydrate use from a biochemical perspective (glucose-fatty acid cycle). Using this information, we embarked on a number of studies demonstrating that the biochemical regulation of fat/carbohydrate interaction in human skeletal muscle during exercise is different than exists in the continually active heart and diaphragm muscles. By increasing the availability of free fatty acids to the working muscles, it was demonstrated that carbohydrate downregulation during moderate and intense aerobic exercise occurred mainly at glycogen phosphorylase, the enzyme that regulates the degradation of muscle glycogen. There was also coordinate downregulation of pyruvate dehydrogenase activity at low and moderate exercise intensities. We have also investigated the roles that carnitine palmitoyl-transferase I and its regulator malonyl-CoA play in governing the transport of long-chain fatty acids into the mitochondria for oxidation, and therefore the impact on carbohydrate use in human skeletal muscle. The regulation of Carnitine Palmitoyltransferase I (CPTI) activity appears to involve more than changes in the concentration of malonyl-CoA. Several additional mechanisms may exist that interact with or override the effects of malonyl-CoA during exercise, although this work is just beginning. The present work has identified several regulatory sites in the pathways of carbohydrate and fat metabolism that help explain the regulation of fat/carbohydrate interaction in human skeletal muscle during exercise.

Free fatty acids (FFA), a link between obesity and insulin resistance.

Evidence, gained from human studies, is reviewed showing that elevation of plasma FFA levels produce peripheral and probably also hepatic insulin resistance in obese healthy and diabetic subjects. First, plasma FFA levels are elevated in most obese subjects. Second, physiological elevations of plasma FFA inhibit acutely as well as chronically insulin stimulated glucose uptake in a dose dependent fashion. Responsible for this inhibition is a FFA induced defect in insulin stimulated glucose transport and/or phosphorylation which develops after 3-4 hours of raising plasma FFA and a second defect, consisting of inhibition of glycogen synthase, the rate limiting enzyme of glycogen synthesis, which develops after 4-6 hours. FFA induced inhibition of fatty acid oxidation (Randle effect) does not affect insulin stimulated glucose uptake or glycogen synthesis and thus does not cause insulin resistance. Elevated plasma FFA levels also modestly increase insulin suppressed endogenous glucose production (EGP) although this effect has not been found by all investigators. The reasons why it has been difficult to demonstrate unequivocal effects of FFA on EGP include 1) the fact that FFA promote insulin secretion which counteracts its effect on EGP (FFA increase, while insulin decreases EGP); 2) the recognition that FFA induced increase in gluconeogenesis may be compensated by intrahepatic downregulation of EGP (i.e., by a decrease in glycogenolysis). The FFA induced insulin resistance is physiologically important during starvation by preserving carbohydrate for oxidation in the central nervous system and during pregnancy, where the well recognized accelerated starvation pattern provides carbohydrate for the growing fetus. In obesity, however, there is no need to spare carbohydrate and the FFA induced insulin resistance may result in type 2 diabetes and other cardiovascular risk factors.

Mechanisms of liver and muscle insulin resistance induced by chronic high-fat feeding.

To elucidate cellular mechanisms of insulin resistance induced by excess dietary fat, we studied conscious chronically high-fat-fed (HFF) and control chow diet-fed rats during euglycemic-hyperinsulinemic (560 pmol/l plasma insulin) clamps. Compared with chow diet feeding, fat feeding significantly impaired insulin action (reduced whole body glucose disposal rate, reduced skeletal muscle glucose metabolism, and decreased insulin suppressibility of hepatic glucose production [HGP]). In HFF rats, hyperinsulinemia significantly suppressed circulating free fatty acids but not the intracellular availability of fatty acid in skeletal muscle (long chain fatty acyl-CoA esters remained at 230% above control levels). In HFF animals, acute blockade of beta-oxidation using etomoxir increased insulin-stimulated muscle glucose uptake, via a selective increase in the component directed to glycolysis, but did not reverse the defect in net glycogen synthesis or glycogen synthase. In clamp HFF animals, etomoxir did not significantly alter the reduced ability of insulin to suppress HGP, but induced substantial depletion of hepatic glycogen content. This implied that gluconeogenesis was reduced by inhibition of hepatic fatty acid oxidation and that an alternative mechanism was involved in the elevated HGP in HFF rats. Evidence was then obtained suggesting that this involves a reduction in hepatic glucokinase (GK) activity and an inability of insulin to acutely lower glucose-6-phosphatase (G-6-Pase) activity. Overall, a 76% increase in the activity ratio G-6-Pase/GK was observed, which would favor net hepatic glucose release and elevated HGP in HFF rats. Thus in the insulin-resistant HFF rat 1) acute hyperinsulinemia fails to quench elevated muscle and liver lipid availability, 2) elevated lipid oxidation opposes insulin stimulation of muscle glucose oxidation (perhaps via the glucose-fatty acid cycle) and suppression of hepatic gluconeogenesis, and 3) mechanisms of impaired insulin-stimulated glucose storage and HGP suppressibility are not dependent on concomitant lipid oxidation; in the case of HGP we provide evidence for pivotal involvement of G-6-Pase and GK in the regulation of HGP by insulin, independent of the glucose source.

Glycolysis inhibition by palmitate in renal cells cultured in a two-chamber system.

A major shortcoming of renal proximal tubular cells (RPTC) in culture is the gradual modification of their energy metabolism from the oxidative type to the glycolytic type. To test the possible reduction of glycolysis by naturally occurring long-chain fatty acids, RPTC were cultured in a two-chamber system, with albumin-bound palmitate (0.4 mM) added to the basolateral chamber after confluency. Twenty-four hours of contact with palmitate decreased glycolysis by 38% provided that carnitine was present; lactate production was decreased by 38%, and the decrease in glycolysis resulted from a similar decrease of basolateral and apical net uptake of glucose. In contrast to the previously described effect of the nonphysiological oxidative substrate heptanoate, palmitate promoted a long-term decrease in lactate production and sustained excellent cellular growth. After 4 days of contact, decreased glycolysis was maintained even in the absence of carnitine and resulted from a decrease of basolateral uptake only, suggestive of long-term regulation different from the earlier effects. Thus, although cultured RPTC lost their oxidative phenotype, they exhibited a type of regulation (Randle effect) that is found in the oxidative-type but not in the glycolytic-type tissues, therefore unmasking a regulative capacity barely detectable in fresh RPTC. Low PO2 (50 mmHg in the apical chamber) could be a major cause of elevated glycolysis and could hinder the effects of palmitate.