Effect of hypoxia on pig heart short-chain acylcarnitines

The right ventricles of pig heart were perfused with hypoxic blood and the left ventricles were perfused with normally ventilated arterial blood. Free carnitine and short-chain acylcarnitines in hypoxic ventricles were lower than in perfused controls, and much lower than in non-perfused heart. Acetylcarnitine levels decreased and the branched-chain acylcarnitines and propionylcarnitine were elevated in the hypoxic perfused ventricles. These data indicate that both hypoxia and anaesthesia caused loss of carnitine and short-chain acylcarnitines from the heart and hypoxia also changed the distribution of short-chain acylcarnitines in the heart.     

Thermospray liquid chromatography/mass spectrometry for the analysis of l-carnitine and its short-chain acyl derivatives

A method utilizing thermospray high-performance liquid chromatography/mass spectrometry for the separation and direct analysis of carnitine, acetylcarnitine, and propionylcarnitine is described. On-column analysis of mixtures of the acylcarnitines with their corresponding stable, isotope-labeled analogs at nanomolar concentrations has indicated that isotope dilution assays can be applied towards the analysis of carnitine and short-chain acylcarnitines present in biological samples.     

Carnitine and acylcarnitine profiles in dried blood spots of patients with acute myocardial infarction

Earlier studies have suggested an important role of carnitine pathway in cardiovascular pathology. However, the redistribution of carnitine and acylcarnitine pools, as a result of altered carnitine metabolism, is not clearly known in patients with acute myocardial infarction (AMI). We compared the carnitine and acylcarnitine profiles of 65 AMI patients, including 26 ST-elevated myocardial infarction (STEMI) and 39 non-ST-elevated myocardial infarction (NSTEMI), 28 patients with chest pain and 154 normal controls. The levels of carnitine and acylcarnitines in the blood spots were determined using LC-MS/MS. Total and free carnitine levels were significantly higher in all the patient groups in the following order: STEMI > NSTEMI > chest pain. The levels of short- and medium-chain acylcarnitines were significantly higher in patient groups. Among the long-chain acylcarnitines, C14:2 and C16:1 levels were significantly increased in STEMI and NSTEMI. The ratio of free carnitine to short-chain or medium-chain acylcarnitines was significantly decreased in STEMI, NSTEMI and chest pain patients however a significant increase was observed in the ratio of carnitine to long-chain acylcarnitines in all the patient groups as compared to normal controls. In conclusion, alterations in carnitine and acylcarnitine levels in the blood of AMI patients indicate the possibility of impaired carnitine homeostasis in ischemic myocardium. The clinical implications of these findings for the risk screening or diagnosis and prognosis of AMI require additional follow-up studies on large number of patients. We also suggest that a dual-marker strategy using carnitine (longer plasma half-life) in combination with troponin (shorter plasma half-life) could be a more promising biomarker strategy in risk stratification of patients.     

Ester composition of carnitine in the perfusate of liver and in the plasma of donor rats

When the carnitine pool of fed rats was labelled with tritium, in non-recirculating perfusate of their liver 44% of acid-soluble 3H activity was identified as free carnitine and 47% as short-chain acylcarnitine. Of the latter component acetylcarnitine accounted for 30% and propionylcarnitine for 10% of total acid-soluble. In plasma the contribution of short-chain acylcarnitines to total carnitine in fed, fasted and diabetic rats was 15.6%, 43.1% and 48.0%, respectively. Recirculating perfusion of livers from the same animals revealed that livers from fed rats released short-chain acylcarnitines as much as 56.2% of total and this proportion did not increase further in the other two groups. At the same time, ketone bodies in the perfusate increased gradually in the fed, fasted and diabetic group, paralleling the plasma ketone levels. Although liver supplies the organism with carnitine the increment of plasma short-chain acylcarnitines seen in ketosis is not a result of some extra output by the liver.     

Separation of acylcarnitines from biological samples using high-performance liquid chromatography

A reverse-phase high-performance liquid chromatography technique to separate carnitine and acylcarnitines from a biological matrix is described. The method utilizes a step gradient to provide baseline resolution of acylcarnitines (individually or by class) for subsequent quantification using a sensitive radioenzymatic assay. The method requires minimal sample preparation and prevents any contamination among groups of acylcarnitines. This technique has been applied to liver tissues of rats obtained under a variety of conditions. These studies demonstrate the validity and utility of the HPLC method while confirming the applicability of the perchloric acid fractionation of acylcarnitines by functional class. The present HPLC method permits resolution of long-chain acylcarnitines in the presence of large excess concentrations of carnitine and short-chain acylcarnitines (coelution of unesterified carnitine with long-chain acylcarnitines less than or equal to 0.05%). Thus, the method will be of use in the study of acylcarnitines in biological systems over a broad spectrum of metabolic conditions.     

Carnitine metabolism during exercise in patients with peripheral vascular disease

The distribution between carnitine and the acyl derivatives of carnitine reflects changes in the metabolic state of a variety of tissues. Patients with peripheral vascular disease (PVD) develop skeletal muscle ischemia with exertion. This impairment in oxidative metabolism during exercise may result in the generation of acylcarnitines. To test this hypothesis, 11 patients with PVD and 7 age-matched control subjects were evaluated with graded treadmill exercise. Subjects with PVD walked to maximal claudication pain at a peak O2 consumption (VO2) of 19.9 +/- 1.3 ml X kg-1 X min-1 (mean +/- SE). Control subjects were taken to a near-maximal work load at a VO2 of 31.3 +/- 1.0 ml X kg-1 X min-1. In patients with PVD, the plasma concentration of total acid-soluble, long-chain acylcarnitine and total carnitine was increased at peak exercise compared with resting values. Four minutes postexercise, the plasma short-chain acylcarnitine concentration was also increased. In control subjects taken to the higher work load, only the long-chain acylcarnitine concentration was increased at peak exercise. In patients with PVD, plasma short-chain acylcarnitine concentration at rest was negatively correlated with subsequent maximal walking time (r = -0.51, P less than 0.05). In conclusion, acylcarnitines increased in patients with PVD who walked to maximal claudication pain, whereas control subjects did not show equivalent changes even when taken to a higher work load. The relationship between short-chain acylcarnitine concentration at rest and subsequent exercise performance suggests that repeated episodes of ischemia may cause chronic accumulation of short-chain acylcarnitine in plasma in proportion to the severity of disease.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myocardial carnitine and carnitine palmitoyltransferase deficiencies in patients with severe heart failure

We studied myocardial tissue from 25 cardiac transplant recipients, who had end-stage congestive heart failure (CHF), and from 21 control donor hearts. Concentrations of total carnitine (TC), free carnitine (FC), short-chain acylcarnitines, long-chain acylcarnitines (LCAC) as well as carnitine palmitoyltransferase (CPT) activities were measured in myocardial tissue homogenates and referred to the concentration of non-collagen protein. Compared to controls, the concentrations of TC and FC as well as total CPT activities were significantly lower in patients. LCAC levels and the LCAC to FC ratio values were significantly greater in patients than in controls. While the malonyl-CoA sensitive fraction of CPT, which represents CPT I activity, was similar in patients and controls, the residual CPT activity after inhibition by malonyl-CoA, representing CPT II activity, was significantly reduced in patients compared to controls. Moreover, the activity of CPT in the presence of Triton X-100, which also represents the activity of CPT II, was significantly lower in patients than in controls. Malonyl-CoA concentrations required for half-maximal inhibition of CPT activity were significantly greater in patients than in controls. There was a linear relationship between ejection fraction (EF) values and concentrations of TC, FC, or total CPT activities. Values for LCAC and the LCAC to FC ratio were inversely related to EF values. We conclude that failing heart shows decreased total CPT and CPT II activities and carnitine deficiency that may be related to ventricle function.     

Interorgan cooperativity in carnitine metabolism in the trained state

This study was designed to evaluate the effects of chronic exercise training on carnitine acetyl- and palmitoyltransferase activity and the distribution of carnitine forms and concentrations in various organs and tissues of female rats. Sprague-Dawley rats were swim trained 6 days/wk and progressed to 75-min swims twice daily (with 3% of their total body weight attached to the medial portion of the tail) at the end of 5 wk of training. Sedentary (S, n = 12) and trained (T, n = 13) animals were killed by decapitation, and the livers, kidneys, hearts, and several skeletal muscle types were removed and immediately frozen in liquid N2 and/or extracted for enzyme activity assays. Blood was collected and plasma was stored frozen. Samples were assayed for free, acid-soluble, and acid-insoluble carnitine. Free carnitine increased significantly (P less than 0.03) in T hearts. Free carnitine remained unchanged in liver, but short-chain acylcarnitines increased significantly (P less than 0.001). There was a significant (P less than 0.001) reduction in long-chain acylcarnitines in kidney in the trained rats, and plasma short-chain acylcarnitine levels also decreased (P less than 0.001). Several significant changes in carnitine distribution also occurred in the superficial and deep portions of the vastus lateralis and in the mixed gastrocnemius muscles. There was a significant reduction in carnitine acetyltransferase activity with training in both the soleus (P less than 0.02) and superficial gastrocnemius (P less than 0.002) muscles. The deep portion of the gastrocnemius muscle contained significantly higher activity than either the superficial portion or the soleus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Uptake, metabolism and release of carnitine and acylcarnitines in the perfused rat liver

The uptake and release of carnitine and isovalerylcarnitine have been studied in the perfused rat liver. Labelled carnitine accumulates in rat livers perfused with 50 or 500 microM [3H]carnitine. When alpha-ketoisocaproate (5 mM) is added to the perfusate after 30 min of perfusion, the net uptake of carnitine in the liver stops, and there is even a decrease in liver radioactivity. The decrease in liver carnitine can be attributed to an enhanced formation and efflux to the perfusate of short-chain acylcarnitines. Thin-layer chromatography of liver and perfusate extracts showed that efflux rates for branched-chain acylcarnitines (isovalerylcarnitine) formed are at least 2.5-fold the efflux rate for carnitine. Acetylcarnitine is released about twice as fast as carnitine from the liver. Perfusion with 50 microM [3H]isovalerylcarnitine showed that the influx rate of isovalerylcarnitine exceeds that of carnitine 1.5-fold. Since the efflux rate is still higher, a net loss of carnitine from the liver to the perfusate will result when branched-chain acylcarnitines are formed in the perfused liver. The addition of 500 microM unlabelled carnitine to the perfusate does not influence the release of labelled carnitine or acylcarnitines from the liver, showing that uptake and release are independent processes. Isovalerylcarnitine accumulates faster than carnitine does, also in the perfused rat heart. A mechanism for the development of secondary carnitine deficiencies associated with organic acidemia is proposed.     

Skeletal muscle carnitine metabolism in patients with unilateral peripheral arterial disease.

Patients with peripheral arterial disease (PAD) have abnormalities of carnitine metabolism that may contribute to their functional impairment. To test the hypothesis that muscle acylcarnitine generation (intermediates in oxidative metabolism) in patients with PAD provides a marker of the muscle dysfunction, 10 patients with unilateral PAD and 6 age-matched control subjects were studied at rest, and the patients were studied during exercise. At rest, biopsies of the gastrocnemius muscle in the patients' nonsymptomatic leg revealed a normal carnitine pool and lactate content compared with control subjects. In contrast, the patients' diseased leg had higher contents of lactate and long-chain acylcarnitines than controls. The muscle short-chain acylcarnitine content in the patients' diseased leg at rest was inversely correlated with peak exercise performance (r = -0.75, P less than 0.05). With graded treadmill exercise, only patients who exceeded their individual lactate threshold had an increase in muscle short-chain acylcarnitine content in the nonsymptomatic leg, which was identical to the muscle carnitine response in normal subjects. In the patients' diseased leg, muscle short-chain acylcarnitine content increased with exercise from 440 +/- 130 to 900 +/- 200 (SE) nmol/g (P less than 0.05). In contrast to the nonsymptomatic leg, there was no increase in muscle lactate content in the diseased leg with exercise, and the change in muscle carnitine metabolism was correlated with exercise duration (r = 0.82, P less than 0.01) and not with the lactate threshold. We conclude that energy metabolism in ischemic muscle of patients with PAD is characterized by the accumulation of acylcarnitines.(ABSTRACT TRUNCATED AT 250 WORDS)     

Substrate specificity of human carnitine acetyltransferase: Implications for fatty acid and branched-chain amino acid metabolism

Carnitine acyltransferases catalyze the reversible conversion of acyl-CoAs into acylcarnitine esters. This family includes the mitochondrial enzymes carnitine palmitoyltransferase 2 (CPT2) and carnitine acetyltransferase (CrAT). CPT2 is part of the carnitine shuttle that is necessary to import fatty acids into mitochondria and catalyzes the conversion of acylcarnitines into acyl-CoAs. In addition, when mitochondrial fatty acid β-oxidation is impaired, CPT2 is able to catalyze the reverse reaction and converts accumulating long- and medium-chain acyl-CoAs into acylcarnitines for export from the matrix to the cytosol. However, CPT2 is inactive with short-chain acyl-CoAs and intermediates of the branched-chain amino acid oxidation pathway (BCAAO). In order to explore the origin of short-chain and branched-chain acylcarnitines that may accumulate in various organic acidemias, we performed substrate specificity studies using purified recombinant human CrAT. Various saturated, unsaturated and branched-chain acyl-CoA esters were tested and the synthesized acylcarnitines were quantified by ESI-MS/MS. We show that CrAT converts short- and medium-chain acyl-CoAs (C2 to C10-CoA), whereas no activity was observed with long-chain species. Trans-2-enoyl-CoA intermediates were found to be poor substrates for this enzyme. Furthermore, CrAT turned out to be active towards some but not all the BCAAO intermediates tested and no activity was found with dicarboxylic acyl-CoA esters. This suggests the existence of another enzyme able to handle the acyl-CoAs that are not substrates for CrAT and CPT2, but for which the corresponding acylcarnitines are well recognized as diagnostic markers in inborn errors of metabolism.     

Relationship between acid-soluble carnitine and coenzyme A pools in vivo

The relationship between the acid-soluble carnitine and coenzyme A pools was studied in fed and 24-h-starved rats after carnitine administration. Carnitine given by intravenous injection at a dose of 60μmol/100g body wt. was integrated into the animal's endogenous carnitine pool. Large amounts of acylcarnitines appeared in the plasma and liver within 5min of carnitine injection. Differences in acid-soluble acylcarnitine concentrations were observed between fed and starved rats after injection and reflected the acylcarnitine/carnitine relationship seen in the endogenous carnitine pool of the two metabolic states. Thus, a larger acylcarnitine production was seen in starved animals and indicated a greater source of accessible acyl-CoA molecules. In addition to changes in the amount of acylcarnitines present, the specific acyl groups present also varied between groups of animals. Acetylcarnitine made up 37 and 53% of liver acid-soluble acylcarnitines in uninjected fed and starved animals respectively. At 5min after carnitine injection hepatic acid-soluble acylcarnitines were 41 and 73% in the form of acetylcarnitine in fed and starved rats respectively. Despite these large changes in carnitine and acylcarnitines, no changes were observed in plasma non-esterified fatty acid or β-hydroxybutyrate concentrations in either fed or starved rats. Additionally, measurement of acetyl-CoA, coenzyme A, total acid-soluble CoA and acid-insoluble CoA demonstrated that the hepatic CoA pool was resistant to carnitine-induced changes. This lack of change in the hepatic CoA pool or ketone-body production while acyl groups are shunted from acyl-CoA molecules to acylcarnitines suggests a low flux through the carnitine pool compared with the CoA pool. These results support the concept that the carnitine/acid-soluble acylcarnitine pool reflects changes in, rather than inducing changes in, the hepatic CoA/acyl-CoA pool.     

Changes in haematological parameters associated with capture and captivity of the marine teleost, Pleuronectes platessa L

After capture by trawling, the blood parameters of plaice (Pleuronectes platessa L.) are perturbed for up to 5 days post-capture. Whole blood values recovered from an initial stress-induced haemoconcentration within 12 hr. There is a marked hyperglycaemia following capture: blood glucose concentration increased four-fold to 87.92 +/- 10.41 mg/100 ml (N = 6) after 12 hr and remained elevated for 3-4 days before returning to normal values. Monovalent blood electrolytes (Na+, K+, Cl-) significantly increased during the initial stages post-capture (4-10 hr) but then recovered. The divalent cations (Ca2+, Mg2+) similarly increased but for a longer period (24-72 hr). Liver and muscle glycogen concentrations were very variable during the recovery period. All blood parameters achieved stable values within 5 days of capture. This study provides comprehensive haematological data on post-trawl recovery and tank-acclimation in plaice, for up to 28 days following capture.     

Carnitine is associated with fatty acid metabolism in plants

The finding of acylcarnitines alongside free carnitine in Arabidopsis thaliana and other plant species, using tandem mass spectrometry coupled to liquid chromatography shows a link between carnitine and plant fatty acid metabolism. Moreover the occurrence of both medium- and long-chain acylcarnitines suggests that carnitine is connected to diverse fatty acid metabolic pathways in plant tissues. The carnitine and acylcarnitine contents in plant tissues are respectively a hundred and a thousand times lower than in animal tissues, and acylcarnitines represent less than 2% of the total carnitine pool whereas this percentage reaches 30% in animal tissues. These results suggest that carnitine plays a lesser role in lipid metabolism in plants than it does in animals.     

Quantitation of the efflux of acylcarnitines from rat heart, brain, and liver mitochondria

The efflux of individual short-chain and medium-chain acylcarnitines from rat liver, heart, and brain mitochondria metabolizing several substrates has been measured. The acylcarnitine efflux profiles depend on the substrate, the source of mitochondria, and the incubation conditions. The largest amount of any acylcarnitine effluxing per mg of protein was acetylcarnitine produced by heart mitochondria from pyruvate. This efflux of acetylcarnitine from heart mitochondria is almost 5 times greater with 1 mM than 0.2 mM carnitine. Apparently the acetyl-CoA generated from pyruvate by pyruvate dehydrogenase is very accessible to carnitine acetyltransferase. Very little acetylcarnitine effluxes from heart mitochondria when octanoate is the substrate except in the presence of malonate. Acetylcarnitine production from some substrates peaks and then declines, indicating uptake and utilization. The unequivocal demonstration that considerable amounts of propionylcarnitine or isobutyrylcarnitine efflux from heart mitochondria metabolizing alpha-ketoisovalerate and alpha-keto-beta-methylvalerate provides evidence for a role (via removal of non-metabolizable propionyl-CoA or slowly metabolizable acyl-CoAs) for carnitine in tissues which have limited capacity to metabolize propionyl-CoA. These results also show propionyl-CoA must be formed during the metabolism of alpha-ketoisovalerate and that extra-mitochondrial free carnitine rapidly interacts with matrix short-chain aliphatic acyl-CoA generated from alpha-keto acids of branched-chain amino acids and pyruvate in the presence and absence of malate.     

Association of plasma acylcarnitines with insulin sensitivity, insulin secretion,and prediabetes in a biracial cohort

The ability to predict prediabetes, which affects ∼90 million adults in the US and ∼400 million adults worldwide, would be valuable to public health. Acylcarnitines, fatty acid metabolites, have been associated with type 2 diabetes risk in cross-sectional studies of mostly Caucasian subjects, but prospective studies on their link to prediabetes in diverse populations are lacking. Here, we determined the association of plasma acylcarnitines with incident prediabetes in African Americans and European Americans enrolled in a prospective study. We analyzed 45 acylcarnitines in baseline plasma samples from 70 adults (35 African-American, 35 European-American) with incident prediabetes (progressors) and 70 matched controls (non-progressors) during 5.5-year (mean 2.6 years) follow-up in the Pathobiology of Prediabetes in a Biracial Cohort (POP-ABC) study. Incident prediabetes (impaired fasting glucose/impaired glucose tolerance) was confirmed with OGTT. We measured acylcarnitines using tandem mass spectrometry, insulin sensitivity by hyperinsulinemic euglycemic clamp, and insulin secretion using intravenous glucose tolerance test. The results showed that progressors and non-progressors during POP-ABC study follow-up were concordant for 36 acylcarnitines and discordant for nine others. In logistic regression models, beta-hydroxy butyryl carnitine (C4-OH), 3-hydroxy-isovaleryl carnitine/malonyl carnitine (C5-OH/C3-DC), and octenoyl carnitine (C8:1) were the only significant predictors of incident prediabetes. The combined cut-off plasma levels of <0.03 micromol/L for C4-OH, <0.03 micromol/L for C5-OH/C3-DC, and >0.25 micromol/L for C8:1 acylcarnitines predicted incident prediabetes with 81.9% sensitivity and 65.2% specificity. Thus, circulating levels of one medium-chain and two short-chain acylcarnitines may be sensitive biomarkers for the risk of incident prediabetes among initially normoglycemic individuals with parental history of type 2 diabetes.     

A fixation procedure suitable for autoradiography of endogenous long-chain acyl carnitine

A tissue processing procedure was evaluated for fixation of endogenous long-chain acyl carnitine (LCA) to facilitate autoradiographic subcellular localization of this amphiphile. Suspensions of neonatal rat myocytes labeled with exogenous 14C-palmitoyl carnitine retained 85.2% of the radiolabel after tissue processing. Autoradiography demonstrated no significant translocation of radiolabeled LCA from myocytes to unlabeled sheep erythrocytes mixed in equal proportions and processed together. To evaluate endogenous LCA fixation, cultured myocytes were incubated for 3 days with 3H-carnitine. Radioactivity was distributed in LCA, short-chain acyl carnitine, and free carnitine pools in proportion to the physiological concentrations of the metabolites traced. Before tissue processing, LCA contained 4.5% of total radioactivity. After tissue processing, labeled water-soluble components were lost and 88% of the retained radioactivity was in the LCA pool. The enrichment of endogenous LCA radioactivity was attributable to the selective extraction of endogenous short-chain and free carnitine. Nearly 75% of endogenous LCA was preserved. In contrast, 99.5% of both endogenous short-chain and free carnitine were extracted. Thus, endogenous LCA can be selectively preserved, permitting quantitative subcellular localization of this amphiphile with ultrastructural autoradiography.     

Plasma carnitine in women. Effects of the menstrual cycle and of oral contraceptives

The plasma concentrations of carnitine were determined in a group of 35 women and 35 men admitted to a clinic, and in another group of 18 women during their menstrual cycle. The values found for the women (45.1 +/- 2.6 nmol/ml of free carnitine and 59.1 +/- 2.8 nmol/ml of total carnitine) were not significantly different from the values obtained in men (respectively 42.4 +/- 1.7 and 55.5 +/- 1.9 nmol/ml). No direct relationship between the free or total carnitine concentrations and the concentrations of circulating lipids could be demonstrated. During the menstrual cycle the plasma concentrations of free and total carnitine remained unchanged. Intake of oral contraceptives caused an elevation in blood triacylglycerols and decreases in the levels of luteinizing hormone, follicle-stimulating hormone, and free and total carnitine.     

Carnitine metabolism in livers and hearts of 48h-starved rats: the contribution of fat and carbohydrate to acylcarnitine formation

The work investigated the effects of administration of 2-tetradecylglycidate (TDG), an inhibitor of mitochondrial long-chain fatty acid oxidation, alone or in combination with glucose, on concentrations of free and acylated carnitine in livers and hearts of 48 h-starved rats. The only significant effect of TDG in the heart was to decrease [short-chain acylcarnitine]. This demonstrates that in heart, fat oxidation is linked to the formation of short-chain acylcarnitine. Cardiac [short-chain acylcarnitine] was not significantly decreased by TDG if the rats were also administered glucose, suggesting that acyl CoA derived from glucose may be used for short-chain acylcarnitine formation in TDG-treated rats. TDG significantly decreased in [free carnitine]. No changes in [short-chain acylcarnitine] were observed. This indicates that formation of short-chain acylcarnitine in liver is not determined by the rates of fat oxidation. It was calculated that at least 63% of the acyl-groups esterified to carnitine were generated by intramitochondrial beta-oxidation. The effects of glucose and TDG on hepatic concentrations of free and long-chain acylcarnitine were additive, suggesting that extramitochondrial fat oxidation can contribute to acylcarnitine formation in liver.     

Carnitine metabolism during prolonged exercise and recovery in humans

Lennon et al. (J. Appl. Physiol. 55: 489-495, 1983) have recently reported a large loss of muscle total carnitine (TC) after 40 min of moderate exercise. These authors have also suggested that elevations in plasma esterified carnitine (EC) were due to the release of these carnitine esters from muscle during exercise. After 10 male subjects underwent 90 min of cycle egometry we found no alteration in muscle TC from preexercise values. Plasma EC progressively increased above resting values during exercise and remained elevated above rest at 0.75 and 1.5 h into recovery. Elevations of plasma EC were largely due to a decrement in free carnitine (FC) in both conditions. Immediately postexercise the urinary fractional reabsorbsion of EC and FC were similar to that at rest. These results suggest that a net loss of TC from exercising muscle does not occur. As in other conditions marked by falling insulin concentrations, elevations in plasma EC could result from an exchange of carnitine with the hepatic carnitine pool.