Intramuscular fatty acid metabolism in contracting and non-contracting human skeletal muscle

The present study was undertaken to investigate the fate of blood-borne non-esterified fatty acids (NEFA) entering contracting and non-contracting knee extensor muscles of healthy young individuals. [U-(13)C]-palmitate was infused into a forearm vein during 5 h of one-legged knee extensor exercise at 40 % of maximal work capacity and the NEFA kinetics, oxidation and rate of incorporation into intramuscular triacylglycerol (mTAG) were determined for the exercising and the non-exercising legs. During 4 h of one-legged knee extensor exercise, mTAG content decreased by 30 % (P < 0.05) in the contracting muscle, whereas it was unchanged in the non-contracting muscle. The uptake of plasma NEFA, as well as the proportion directed towards oxidation, was higher in the exercising compared to the non-exercising leg, whereas the rate of palmitate incorporation into mTAG was fourfold lower (0.70 +/- 0.14 vs. 0.17 +/- 0.04 micromol (g dry wt)(-1) h(-1); P < 0.05), resulting in fractional synthesis rates of 1.0 +/- 0.2 and 3.8 +/- 0.9 % h(-1) (P < 0.01) for the contracting and non-contracting muscle, respectively. These findings demonstrate that mTAG in human skeletal muscle is continuously synthesised and degraded and that the metabolic fate of plasma NEFA entering the muscle is influenced by muscle contraction, so that a higher proportion is directed towards oxidation at the expense of storage in mTAG.

Fat digestion modulates gastrointestinal sensations induced by gastric distention and duodenal lipid in humans

BACKGROUND AND AIMS

It is unclear whether fat digestion is required for the induction of gastrointestinal sensations and whether different fats have different effects. We investigated the effect of fat digestion and of medium-chain triglycerides (MCTs; C < 12) and long-chain triglycerides (LCTs; C > 16) on gastrointestinal sensations.

METHODS

In a double-blind study, 15 healthy subjects were studied on 5 occasions during which LCT or MCT emulsions (2 kcal/min), with or without 120 mg tetrahydrolipstatin (THL, lipase inhibitor), or sucrose polyester (SPE, nondigestible fat) were infused intraduodenally in randomized order. After 30 minutes, the proximal stomach was distended in 1 mm Hg steps/min. Intensity of gastrointestinal sensations (on a 0-10 visual analog scale), plasma cholecystokinin (CCK) levels, and gastric volumes were assessed throughout.

RESULTS

LCT and MCT increased gastric volume at baseline pressure compared with SPE, and LCT more than MCT. THL entirely abolished this effect (volumes [mL]: LCT, 213 +/- 19; LCT-THL, 39 +/- 3; MCT, 155 +/- 12; MCT-THL, 43 +/- 5; SPE, 44 +/- 5). Only LCT increased plasma CCK levels (pmol/L per 30 minutes: LCT, 21 +/- 2; LCT-THL, 9 +/- 1; MCT, 9 +/- 1; MCT-THL, 11 +/- 1; SPE, 9 +/- 1). During distentions, intragastric volumes were greater during infusion of LCT and MCT than during the respective THL conditions or SPE, but plasma CCK levels did not change. The intensity of sensations increased (hunger decreased) more with LCT than with MCT. During infusion of THL or SPE, the effects were smaller than during LCT or MCT.

CONCLUSIONS

Fat digestion is required for the modulation of gastrointestinal sensations during gastric distention. The effects of fat depend on the fatty acid chain length and are not entirely explained by release of CCK.

Preferential channeling of energy fuels toward fat rather than muscle during high free fatty acid availability in rats

The preferential channeling of different fuels to fat and changes in the transcription profile of adipose tissue and skeletal muscle are poorly understood processes involved in the pathogenesis of obesity and insulin resistance. Carbohydrate and lipid metabolism may play relevant roles in this context. Freely moving lean Zucker rats received 3- and 24-h infusions of Intralipid (Pharmacia and Upjohn, Milan, Italy) plus heparin, or saline plus heparin, to evaluate how an increase in free fatty acids (nonesterified fatty acid [NEFA]) modulates fat tissue and skeletal muscle gene expression and thus influences fuel partitioning. Glucose uptake was determined in various tissues at the end of the infusion period by means of the 2-deoxy-[1-3H]-D-glucose technique after a euglycemic-hyperinsulinemic clamp: high NEFA levels markedly decreased insulin-mediated glucose uptake in red fiber-type muscles but enhanced glucose utilization in visceral fat. Using reverse transcriptase-polymerase chain reaction and Northern blotting analyses, the mRNA expression of fatty acid translocase (FAT)/CD36, GLUT4, tumor necrosis factor (TNF)-alpha, peroxisome proliferator-activated receptor (PPAR)-gamma, leptin, uncoupling protein (UCP)-2, and UCP-3 was investigated in different fat depots and skeletal muscles before and after the study infusions. GLUT4 mRNA levels significantly decreased (by approximately 25%) in red fiber-type muscle (soleus) and increased (by approximately 45%) in visceral adipose tissue. Furthermore, there were marked increases in FAT/CD36, TNF-alpha, PPAR-gamma, leptin, UCP2, and UCP3 mRNA levels in the visceral fat and muscle of the treated animals in comparison with those measured in the saline-treated animals. These data suggest that the in vivo gene expression of FAT/CD36, GLUT4, TNF-alpha, PPAR-gamma, leptin, UCP2, and UCP3 in visceral fat and red fiber-type muscle are differently regulated by circulating lipids and that selective insulin resistance seems to favor, at least in part, a prevention of fat accumulation in tissues not primarily destined for fat storage, thus contributing to increased adiposity and the development of a prediabetic syndrome.

Kinetics of intramuscular triglyceride fatty acids in exercising humans

A pulse ([(14)C]palmitate)-chase ([(3)H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at approximately 45% peak O(2) consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; (3)H(2)O production, breath (14)CO(2) excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise.

RESULTS

during exercise, 8.2+/-1.2 and 8.4+/-0.7 micromol x kg(-1) x min(-1) of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P = 0.6) from lipid oxidation by indirect calorimetry (15.4 +/-1.6 micromol x kg(-1) x min(-1)); the isotopic and indirect calorimetry values were correlated (r = 0.79, P<0.005). During exercise, imTG turnover rate was 0.32+/-0.07%/min (6.0+/-2.0 micromol of imTG x kg wet muscle(-1) x min(-1)) and plasma FFA were incorporated into imTG at a rate of 0.7+/-0.1 micromol x kg wet muscle(-1) x min(-1). The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.

Partitioning of pyruvate between oxidation and anaplerosis in swine hearts

The goal of this study was to measure flux through pyruvate carboxylation and decarboxylation in the heart in vivo. These rates were measured in the anterior wall of normal anesthetized swine hearts by infusing [U-(13)C(3)]lactate and/or [U-(13)C(3)] pyruvate into the left anterior descending (LAD) coronary artery. After 1 h, the tissue was freeze-clamped and analyzed by gas chromatography-mass spectrometry for the mass isotopomer distribution of citrate and its oxaloacetate moiety. LAD blood pyruvate and lactate enrichments and concentrations were constant after 15 min of infusion. Under near-normal physiological concentrations of lactate and pyruvate, pyruvate carboxylation and decarboxylation accounted for 4.7 +/- 0.3 and 41.5 +/- 2.0% of citrate formation, respectively. Similar relative fluxes were found when arterial pyruvate was raised from 0.2 to 1.1 mM. Addition of 1 mM octanoate to 1 mM pyruvate inhibited pyruvate decarboxylation by 93% without affecting carboxylation. The absence of M1 and M2 pyruvate demonstrated net irreversible pyruvate carboxylation. Under our experimental conditions we found that pyruvate carboxylation in the in vivo heart accounts for at least 3-6% of the citric acid cycle flux despite considerable variation in the flux through pyruvate decarboxylation.

Fatty acids for myocardial imaging

Radioiodinated free fatty acids are tracers that can be used to assess both myocardial perfusion and metabolism. There have been several fatty acids and structurally modified fatty acids studied since Evans' initial report of radiolabeled I-123 oleic acid in 1965. The radiolabeling of a phenyl group added to the long chain fatty acids in the omega-terminal position opposite the carboxyl terminal group prevents nonspecific deiodination and the rapid release of free iodine as the tracer undergoes beta-oxidation. The additional inclusion of a methyl or dimethyl group to the chain slows oxidation resulting in prolonged myocardial retention. The longer retention of the radiolabel permits longer image acquisitions more compatible with single photon emission computed tomography (SPECT) imaging, especially with single-detector imaging systems. Several protocols have been implemented using these compounds, particularly 15-(para-iodophenyl)-3-R,S-methyl pentadecanoic BMIPP, to detect abnormal fatty acid metabolism in ischemic heart disease as well as in nonischemic and hypertrophic cardiomyopathies. Successful management of patients with ischemic cardiomyopathies depends on the accurate identification of hibernating myocardium. The studies covered in this review suggest that both IPPA and BMIPP, especially when combined with markers of myocardial perfusion, may be excellent tracers of viable and potentially functional myocardium. Future studies with larger numbers of patients are needed to confirm the results of these studies and to compare their efficacy with that of other available imaging modalities. Cost and distribution issues will have to be resolved for these metabolic tracers to compete in the commercial marketplace. Otherwise they will likely be available only on a limited basis for research use. As progress is made with these issues and with the development of newer imaging systems, the use of radioiodinated and fluorinated fatty acids is likely to be increasingly attractive.

Reduced nitric oxide production and altered myocardial metabolism during the decompensation of pacing-induced heart failure in the conscious dog

The aim of the present study was to determine whether cardiac nitric oxide (NO) production changes during the progression of pacing-induced heart failure and whether this occurs in association with alterations in myocardial metabolism. Dogs (n=8) were instrumented and the heart paced until left ventricular end-diastolic pressure reached 25 mm Hg and clinical signs of severe failure were evident. Every week, hemodynamic measurements were recorded and blood samples were withdrawn from the aorta and the coronary sinus for measurement of NO metabolites, O2 content, free fatty acids (FFAs), and lactate and glucose concentrations. Cardiac production of NO metabolites or consumption of O2 or utilization of substrates was calculated as coronary sinus-arterial difference times coronary flow. In end-stage failure, occurring at 29+/-1.6 days, left ventricular end-diastolic pressure was 25+/-1 mm Hg, left ventricular systolic pressure was 92+/-3 mm Hg, mean arterial pressure was 75+/-2.5 mm Hg, and dP/dtmax was 1219+/-73 mm Hg/s (all P<0.05). These changes in hemodynamics were associated with a fall of cardiac NO metabolite production from 0.37+/-0.16 to -0.28+/-0.13 nmol/beat (P<0.05). O2 consumption and lactate uptake did not change significantly from control, while FFA uptake decreased from 0.16+/-0.03 to 0.05+/-0.01 microEq/beat and glucose uptake increased from -2.3+/-7.0 to 41+/-10 microgram/beat (P<0.05). The cardiac respiratory quotient also increased significantly by 28%. In 14 normal dogs the same measurements were performed at control and 1 hour after we injected 30 mg/kg of nitro-L-arginine, a competitive inhibitor of NO synthase .O2 consumption increased from 0.05+/-0.002 mL/beat at control to 0.071+/-0.003 mL/beat after nitro-L-arginine, while FFA uptake decreased from 0.1+/-0.01 to 0.06+/-0.01 microEq/beat, lactate uptake increased from 0.15+/-0.04 to 0.31+/-0.03 micromol/beat, glucose uptake increased from 8.2+/-5.0 to 35.4+/-9.5 microgram/beat, and RQ increased by 23% (all P<0.05). Our results indicate that basal cardiac production of NO falls below normal levels during cardiac decompensation and that there are shifts in substrate utilization. This switch in myocardial substrate utilization also occurs after acute pharmacological blockade of NO production in normal dogs.

Gustatory mechanisms for the detection of fat

Recent evidence suggests that free fatty acids may be one of the important stimuli used by taste receptor cells for the detection of fat. Consistent with this interpretation, the proteins necessary for the release and transport of lipophilic fatty acids are found in the oral cavity, and taste cells have recently been shown to contain fatty-acid-sensitive ion channels and transport molecules for the uptake of fatty acids.

Measurement of free fatty acid kinetics during non-equilibrium tracer conditions in man: implications for the estimation of the rate of appearance of free fatty acids

BACKGROUND

This study aimed to document the applicability and variability of free fatty acid (FFA) kinetic parameters during non-equilibrium and equilibrium tracer conditions in man.

METHODS

FFA kinetic parameters were assessed after an overnight fast in six healthy non-obese and three obese subjects as well as in three patients with non-insulin-dependent diabetes mellitus (NIDDM) by infusion of [14C]-palmitate of 60 min (study A) and 10 min duration (study B).

RESULTS

The kinetic parameters estimated from the upstroke and downstroke of the plasma FFA specific activity curve (non-equilibrium) were not statistically different within studies A and B. Furthermore, there were no significant differences in any of the FFA kinetic parameters between studies A and B. The averaged plasma levels of FFA obtained during the up- and downstroke from studies A and B were higher in obese subjects and NIDDM patients than in non-obese subjects (P < 0.01). The averaged total rate of appearance (TRa) of FFA was higher in obese subjects than in non-obese subjects (P < 0.02). The TRa and metabolic clearance rate (MCR), estimated from non-equilibrium conditions, were about 25% higher than the apparent values obtained from steady-state measurement in all subjects combined (P < 0.01), suggesting considerable recirculation of label from hydrolysis of labelled esterified fatty acids. Indeed, in three non-obese subjects, the radiolabel in esterified fatty acids was approximately 50% of labelled FFA at 60 min of label infusion. The coefficients of variation of the kinetic parameters were consistently larger in study A than in study B.

CONCLUSION

FFA kinetic parameters can be estimated with sufficient precision using non-equilibrium data from short-term labelled palmitate infusion. Short-term label infusion has the advantage that label recirculation is prevented and exposure to radiation is limited.

Tissue uptake and interconversion of plasma unesterified 14C linoleic acid in the guinea pig

Part of the arachidonic acid (20:4, n - 6) pools in the gastrointestinal tract and blood forming tissues may be formed by local interconversion of linoleic acid (18:2, n - 6) taken up as a free fatty acid from blood. This study examined the rate of uptake and interconversion of unesterified 14C-18:2 by different tissues in young guinea pigs. The clearance rate of 14C-18:2 was fast, and the initial half-life was 6.3 s. The retention of 14C in tissue lipids was 1.6-1.8% g-1 in the liver, 0.4% g-1 in stomach, 0.7% g-1 in small intestine, 0.2% g-1 in colon, 0.4% g-1 in bone marrow and 0.7% g-1 in spleen. Autoradiographic localization of 3H-18:2 under light microscope demonstrated that most of the 3H radioactivity of the gastrointestinal tract was in the mucosa, in both villus and crypt cells. In bone marrow smears, a high density of silver grains was found in megakaryocytes. The percent of 14C in delta6 desaturase products was higher in gastrointestinal tract, heart, lung, bone marrow and spleen than in liver. The ratio of 14C-20:3/14C-20:4 formation in most tissues was high, and a notable finding being a lower rate of 20:4 formation from plasma free 18:2 in the liver, (170 pmol min-1) than in the gastrointestinal tract (428 pmol min-1) and bone marrow (1203 pmol min-1). The local interconversion of 18:2 into delta6 desaturase products is thus an important source of 20:4 in these organs in guinea pigs.

Stimulation of intestinal apolipoprotein A-IV by lipid is independent of capsaicin-sensitive afferent signals

We tested the hypothesis that stimulation of synthesis and secretion of intestinal apolipoprotein A-IV (apo A-IV) by intestinally infused lipid is mediated by capsaicin-sensitive afferent signals. Vehicle or capsaicin (125 mg/kg) was systemically administered to rats; then the effects of intestinal infusion of lipid emulsions on lymph lipid and apo A-IV transport were determined in rats equipped with duodenal infusion cannulas and mesenteric lymph fistulas. Capsaicin treatment did not significantly affect lymph outputs of triglyceride, phospholipid, and apo A-IV during duodenal infusion of triglyceride emulsion. In separate studies the effect of capsaicin treatment on ileal lipid-elicited stimulation of intestinal mucosal apo A-IV synthesis was also examined. Ileal lipid infusion increased apo A-IV synthesis in distal ileum, proximal jejunum, and jejunal Thirty-Vella fistulas; this finding was unaffected by capsaicin pretreatment. However, capsaicin treatment significantly attenuated the inhibitory effects of duodenal acid and fat on gastric emptying. These results do not support a role for capsaicin-sensitive, sensory afferent nerves in the stimulation of intestinal apo A-IV by dietary lipid.

An evaluation of transovarian uptake of metabolites using arterio-venous difference methods in dairy cattle

Arterio-venous (A-V) difference techniques were used in cattle to examine ovarian energy metabolism, cholesterol uptake and steroid hormone outputs. Catheters were inserted into the ovarian vein and facial artery, and Transonic flow transducers were placed around the ovarian A-V plexus. Further, in some cows, the effects of a challenge with GnRH were examined. Glucose uptake and lactate output were significant in most individual cows. Nonesterified fatty acids (NEFA) uptake were not significant in any cow in dioestrus. Ovarian uptake of beta-Hydroxy-butyrate (3-OHB) was significant in 4 cows in dioestrus. Cholesterol uptake was significant in only 1 cow. Oxygen uptake was significant in all cows at all stages of the oestrous cycle. All cows had significant output of progesterone and oestradiol-17 beta. These data show that the bovine ovary utilises significant amounts of glucose, and Respiratory quotient (RQ) estimates demonstrated that glucose was the primary fuel used by the ovary. The significant output of lactate suggested that anaerobic pathways were mainly used for glucose oxidation. The observed uptakes of 3-OHB indicated that the ovary utilises 3-OHB as a source of energy. Cholesterol uptake was not a rate-limiting factor for steroid hormone production in the ovary. Despite the high metabolic rate in the luteal ovary, the small difference in PO2 between arterial and ovarian venous blood indicated that the ovary consumes only a small proportion of available oxygen. GnRH had no significant effect on the uptake of metabolites and energy metabolism, but it increased OBF and the output of progesterone and oestradiol-17 beta. The use of A-V methods to determine the metabolic needs of the ovary is useful in understanding the means by which nutrition can influence fertility.

Short-term studies of ovarian metabolism in the ewe

The ovarian uptake of metabolites in anaesthetised ewes was determined. In both studies, catheters were inserted into the ovarian vein and femoral artery, and Transonic flow transducers were placed around the ovarian arterio-venous plexus. Arterio-venous differences in glucose, lactate, free fatty acids (FFA), 3-hydroxybutyrate (3-OHB), acetate, cholesterol and progesterone and oestradiol-17 beta levels were determined every 10 min over a 3.5 h period. In study one, glucose uptake was significant in three sheep, and one sheep only had a significant uptake of FFA. Ovarian 3-OHB uptake was significant in two sheep. significant uptake of acetate or cholesterol was identified in one sheep. Progesterone secretion was significant in three sheep and two sheep had significant progesterone uptake. In study 2, glucose uptake was significant in four sheep and lactate release was significant in the same sheep. There was uptake of FFA and 3-OHB, cholesterol, and acetate in each of three different sheep. Oestradiol-17 beta output was significant for sheep in oestrus and prooestrus. While the effects of gonadotrophin-releasing hormone (GnRH) treatment were confounded by time spent under anaesthesia, exogenous GnRH appeared to have no significant effect on the uptake of most metabolites and steroid hormone outputs. The metabolic requirements for energy and precursors for progesterone was small. Glucose was the major source of energy for the ovary and appears to be metabolised through anaerobic pathways, as indicated by significant lactate output.

Metabolic effects of liver transplantation in cirrhotic patients

To assess whether liver transplantation (LTx) can correct the metabolic alterations of chronic liver disease, 14 patients (LTx-5) were studied 5+/-1 mo after LTx, 9 patients (LTx-13) 13+/-1 mo after LTx, and 10 patients (LTx-26) 26+/-2 months after LTx. Subjects with chronic uveitis (CU) and healthy volunteers (CON) were also studied. Basal plasma leucine and branched-chain amino acids were reduced in LTx-5, LTx-13, and LTx-26 when compared with CU and CON (P < 0.01). The basal free fatty acids (FFA) were reduced in LTx-26 with respect to CON (P < 0.01). To assess protein metabolism, LTx-5, LTx-13, and LTx-26 were studied with the [1-14C]leucine turnover combined with a 40-mU/m2 per min insulin clamp. To relate changes in FFA metabolism to glucose metabolism, eight LTx-26 were studied with the [1-14C]palmitate and [3-3H]glucose turnovers combined with a two-step (8 and 40 mU/m2 per min) euglycemic insulin clamp. In the postabsorptive state, LTx-5 had lower endogenous leucine flux (ELF) (P < 0.005), lower leucine oxidation (LO) (P < 0.004), and lower non-oxidative leucine disposal (NOLD) (P < 0.03) with respect to CON (primary pool model). At 2 yr (LTx-26) both ELF (P < 0.001 vs. LTx-5) and NOLD (P < 0.01 vs. LTx-5) were normalized, but not LO (P < 0.001 vs. CON) (primary and reciprocal pool models). Suppression of ELF by insulin (delta-reduction) was impaired in LTx-5 and LTx-13 when compared with CU and CON (P < 0.01), but normalized in LTx-26 (P < 0.004 vs. LTx-5 and P = 0.3 vs. CON). The basal FFA turnover rate was decreased in LTx-26 (P < 0.01) and CU (P < 0.02) vs. CON. LTx-26 showed a lower FFA oxidation rate than CON (P < 0.02). Tissue glucose disposal was impaired in LTx-5 (P < 0.005) and LTx-13 (P < 0.03), but not in LTx-26 when compared to CON. LTx-26 had normal basal and insulin-modulated endogenous glucose production. In conclusion, LTx have impaired insulin-stimulated glucose, FFA, and protein metabolism 5 mo after surgery. Follow-up at 26 mo results in (a) normalization of insulin-dependent glucose metabolism, most likely related to the reduction of prednisone dose, and, (b) maintenance of some alterations in leucine and FFA metabolism, probably related to the functional denervation of the graft and to the immunosuppressive treatment.

Biological evaluation of 5-methyl-branched-chain omega-[18F]fluorofatty acid: a potential myocardial imaging tracer for positron emission tomography

5-Methyl-17-[18F]fluoroheptadecanoic acid (5-MFHA) has been proposed as a new myocardial imaging tracer for positron emission tomography (PET), containing methyl-branching at the odd-numbered position, except the 3-position. To compare the site of methyl-branching of fatty acids on the contribution to myocardial imaging, the biological evaluation of 5-MFHA-using accumulation studies, metabolic studies, and PET studies of the heart muscle-was investigated. In the comparative biodistribution studies for 16-[18F]fluoropalmitic acid (FPA), 3-methyl-17-[18F]fluoroheptadecanoic acid (3-MFHA) and 5-MFHA, the initial myocardial uptake of 5-MFHA (2.64 [%dose/g tissue]) was relatively high between those of FPA and 3-MFHA (3.45 and 1.58, respectively), and the washout from myocardium of 5-MFHA was midway between those of FPA and 3-MFHA. In the lipid analysis studies, 5-MFHA was mainly metabolized to triglycerides in the myocardium, and its metabolic pattern was similar to that of straight-chain fatty acids (FPA). In the PET studies using 5-MFHA in canines, good myocardial images were obtained for up to 30 min after injection.

Effect of high FFA on glycogenolysis in oxidative rat hindlimb muscles during twitch stimulation

The effect of elevated free fatty acids (FFA) on carbohydrate (CHO) utilization in the oxidative muscles of the isolated hindlimb was determined using twitch contraction paradigms evoking a wide range of O2 uptakes and glycogenolysis. The hindlimb was perfused with either 0 or 1.8 mM FFA for 10 min at rest and then subjected to 20 min of stimulation at 0.4, 0.7, 1, 2, 3, or 4 Hz. Soleus (Sol), plantaris (Pl), and red gastrocnemius (RG) were sampled after rest perfusion or stimulation. FFA had little effect on glycogenolysis during stimulation, although glycogen sparing occurred with one of the lesser intensity protocols in each muscle (Sol, 0.4 Hz; RG, 0.7 Hz; Pl, 1 Hz). Muscle citrate and acetyl-CoA were elevated in Sol during several stimulation protocols with high FFA, but this effect was inconsistent in Pl and RG. The sparing of glycogen, when it did occur, was generally unrelated to increases in either citrate or acetyl-CoA content. Furthermore, protocols in which citrate or acetyl-CoA were elevated in the presence of elevated FFA did not demonstrate glycogen sparing. Hindlimb lactate efflux at rest was reduced with FFA but unaffected during stimulation. Glucose uptake was unaffected by FFA at rest and during stimulation protocols, except 3 Hz. The present study does not support the classically proposed roles of citrate and acetyl-CoA in the FFA-induced downregulation of CHO utilization in electrically stimulated rat skeletal muscle.

The effect of beta-adrenergic blockade on non-esterified fatty acid uptake of exercising skeletal muscle during arm cranking

beta-Adrenoceptor blocking agents impair endurance exercise performance in healthy subjects and in patients with hypertension. A possible explanation for the reduced exercise tolerance is a diminished availability of plasma non-esterified fatty acids (NEFA) for energy production during exercise. This study investigated the effect of beta-adrenoceptor blockade on NEFA uptake of exercising skeletal muscle at elevated blood NEFA concentrations. In 11 healthy volunteers a triacylglycerol emulsion was infused at increasing rate for 1 hour before and 1 hour during one-armed cranking exercise at 60% Wpeak with and without prior administration of the beta 1+2-adrenoceptor blocking agent propranolol (80 mg per os). Arteriovenous concentration differences of NEFA across the active forearm were measured and forearm blood flow was estimated using venous occlusion plethysmography. Heart rate and blood flow were significantly lower after propranolol (p < 0.05). Propranolol did not affect arterial NEFA concentration, arteriovenous NEFA difference or NEFA flux significantly. Net NEFA uptake increased with increasing arterial NEFA concentration or inflow in a similar manner with and without prior propranolol administration. Therefore, the results do not support the hypothesis that beta-adrenergic activity plays a role in the regulation of active skeletal muscle NEFA uptake under the conditions studied.

The hepatocellular uptake of free fatty acids is selectively preserved during starvation

BACKGROUND/AIMS

The liver loses protein during fasting. This study sought to determine if hepatic protein loss during fasting selectively preserves functions important to survival such as uptake of fatty acids, which are major energy substrates in that condition.

METHODS

Initial [3H]oleate uptake and efflux rates in hepatocytes from starved (for 48 hours) and fed male rats were measured in media containing 250 mumol/L albumin at oleate/albumin ratios of 0.2:1-2:1. Uptake rates of sulfobromophthalein, taurocholate, and glucose were also determined.

RESULTS

Initial oleate uptake rate was saturable with respect to unbound oleate concentration. Maximum initial velocity expressed per cell number did not differ between fasted and fed animals, but measured cell volume and estimated surface area were decreased in starved vs. fed hepatocytes (921 +/- 21 vs. 1623 +/- 58 microns2, respectively; P < 0.001). Consequently, when expressed per surface area, maximum initial velocity was greater in starved cells (17 +/- 3 vs. 10 +/- 2 [pmol.min-1.micron2] x 10(-7); P < 0.02). Expressed similarly, oleate efflux was also greater from starved hepatocytes and was inhibited by an antibody to plasma membrane fatty acid binding protein (FABPpm). FABPpm concentration per unit area of plasma membrane also increased in starved hepatocytes (P < 0.05). By contrast, uptake rates of sulfobromophthalein, taurocholate, and glucose by starved hepatocytes were decreased when expressed per cell number and unchanged per unit area.

CONCLUSIONS

During fasting, the hepatocellular uptake mechanism for oleate is selectively preserved compared with those for sulfobromophthalein, taurocholate, or glucose.

[Use of free fatty acids in animal nutrition]

The description of the processes of absorption of free fatty acids in monogastric animals and in ruminants is the first part of the review. The feeding problems of free fatty acids are discussed in the second part. In pig diets is a level up 6% free fatty acids in the dry matter of the ration not disadvantageous. In the feeding of ruminants additionally free fatty acids reduce the fermentation processes in the rumen. Only protected free fatty acids (Ca-fatty acids) are favourable in feeding systems for high yielding cows.

Regulatory factors in the development of fatty infiltration of the liver during gram-negative sepsis

To further understand the development of fatty liver during gram-negative sepsis, we measured fatty acid uptake in addition to esterification and secretion of lipids by freshly isolated hepatocytes from fasted and fed control and Escherichia coli-treated rats. Rats were made septic by intravenous (IV) injection of 8 x 10(7) live E coli colonies per 100 g body weight. For the fasted groups, food was removed after E coli injection. Fed rats received a nutritionally adequate diet intragastrically for 5 days before and 24 hours after inducing sepsis. Twenty-four hours after E coli injection, the esterification of newly synthesized fatty acids, as measured by 3H2O incorporation, and the esterification of exogenous fatty acids, measured from 14C-palmitate incorporation, into triglyceride (TG), total cholesterol, and total phospholipid phosphorus were significantly greater in hepatocytes from fasted septic rats compared with their control rats. In fed septic rats, esterification of 14C-palmitate into TG was fourfold greater than in the fed control rats. The increased rates of esterification in hepatocytes from fasted and fed septic rats were not accompanied by an increase in the labeled TG in the medium. This inability to secrete the additional TG that the hepatocytes produce resulted in a higher concentration of cellular TG in fasted and fed septic rats than in their controls. The enzymes glycerol-3-phosphate acyltransferase (GPAT) and phosphatidate phosphohydrolase (PPH) do not appear to be factors contributing to the increased TG synthesis, since the increase in enzyme activity was not accompanied by a similar increase in TG synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of canine donor heart preservation temperature on posttransplant left ventricular function and myocardial metabolism

The generally accepted method of preserving donor heart integrity during transfer is to arrest it with cold cardioplegic solution, then store it in a plastic bag immersed in an iced electrolyte solution. Temperatures between 0 degree C and 4 degrees C are maintained by this method until the heart is transplanted. Although profound hyperthermia best inhibits metabolic processes, it may damage the myocardium. Higher myocardial temperatures may be more advantageous and may result in better preservation. The efficacy of this hypothesis has been investigated in a canine model. The hearts of 40 dogs were isolated, arrested with cold cardioplegia, removed from the animal, and stored at different temperature ranges from 0-3 degree C to 12-15 degrees C for 4 hr. After this time period, the hearts were transplanted into a recipient animal in the cervical heterotopic position. The degree and speed of myocardial functional recovery were monitored by measuring end-systolic elastance generated from pressure-diameter loops using sonomicrometry techniques. Myocardial metabolism was studied simultaneously by monitoring coronary flow, O2, glucose, lactate, pyruvate, and free fatty acid uptakes. The results were compared with those from a control group of hearts transplanted immediately after their removal. Our results indicate that donor heart function was significantly depressed 30 min after heterotopic transplantation, but returned to "control" levels after 2 hr when stored between 0 degrees C and 6 degrees C. Myocardial function remained significantly depressed throughout the 2-hr recovery period in hearts stored at higher (6-15 degrees C) temperatures. Hearts stored at all temperatures continued to extract glucose, lactate, and free fatty acids, but produced significantly higher levels of pyruvate at higher storage temperatures, which may be related to the favored use of free fatty acids. In conclusion, donor hearts stored at colder temperatures for 4 hr regain complete left ventricular function faster than hearts stored at higher temperatures. Our experiments support the presently applied methods of donor heart preservation for 4 hr.

Cytosolic acetylation of sulfamethazine decreases hepatic release of ketone bodies in vivo in fasting rats

The effect of sulfamethazine (SMZ) on ketogenesis was studied in this report. SMZ, a sulfonamide metabolized by cytosolic acetylation in liver, was intraperitoneally injected (2 mmol/kg) to ketamine-anesthetized, overnight-fasted rats. Ketogenesis was measured by the Fick principle from the transhepatic (A-V) gradients of acetoacetate (AcAc) and D-beta-hydroxybutyrate (beta-OHB). Prior to SMZ injection, A-V gradient of ketone bodies (KB) (AcAc + beta-OHB) was -1.38 mM, indicating release from the liver. After SMZ injection, the release of KB decreased rapidly, maintaining at a level approximately 50% less than controls throughout the 2-h experimental period. Plasma concentrations of AcAc and beta-OHB also decreased. In contrast, plasma concentrations and trans-hepatic gradients of free fatty acids (FFA) were not significantly affected. Our results thus indicate that SMZ acetylation in liver mobilizes acetyl CoA from mitochondria. Decreased hepatic ketogenesis limits the availability of KB and may thus affect energy metabolism in the extrahepatic tissues. The incomplete inhibition on ketogenesis may indicate compartmentation of acetyl CoA in liver mitochondria.

Postprandial free fatty acid kinetics are abnormal in upper body obesity

Excess free fatty acid release (rate of appearance) is seen in overnight postabsorptive, upper body obese women and, if present postprandially, could contribute to glucose intolerance. These studies examine the antilipolytic effect of a mixed meal in upper body obese, lower body obese, and nonobese women and the contribution of meal triglyceride fatty acids to circulating free fatty acids. Eight upper body obese, 8 lower body obese, and 8 nonobese age-matched, premenopausal women were studied. Systemic oleate Ra ([3H]oleate) was measured before and after an evening meal that contained triolein labeled with [14C]triolein as the only source of fat. Premeal oleate Ra was greater in both upper body obese and lower body obese women than nonobese women. The nadir of total oleate Ra occurred 90-240 min postprandially and was less (P < 0.01) in nonobese and lower body obese women (63 +/- 10 and 87 +/- 17 mumol/min) than in upper body obese women (140 +/- 21 mumol/min). Meal oleate Ra contributed substantially to total oleate Ra. The nadir for endogenous oleate Ra in nonobese and lower body obese women was less (P < 0.01) than that observed in upper body obese women. We conclude that the antilipolytic effect of a mixed meal is reduced in upper body obese women and that meal triglyceride fatty acids contribute significantly to postprandial free fatty acid flux.

Effect of valproate, sodium 2-propyl-4-pentenoate and sodium 2-propyl-2-pentenoate on renal substrate uptake and ammoniagenesis in the rat

Experiments were carried out in the intact functioning rat kidney to study the effect of valproate (VPA), a widely used antiepileptic drug and an hyperammonemic agent, but usually without clinical relevance, and of two of its metabolites, sodium 2-propyl-4-pentenoate (4-en-VPA) and sodium 2-propyl-2-pentenoate (2-en-VPA), on the renal production of ammonia and on the renal uptake of glutamine, glutamate and of inhibitors of renal ammoniagenesis; mainly lactate, fatty acids, ketone bodies and alpha-ketoglutarate. Administration of VPA and 4-en-VPA stimulated the uptake of glutamine and glutamate and the production of ammonia by the rat kidney, resulting in an increase in the renal venous release of ammonia and in a hyperammonemia. By contrast, no hyperammonemia was observed after the administration of 2-en-VPA which stimulated renal ammoniagenesis to a lesser extent than VPA and 4-en-VPA, resulting in no stimulation of the renal venous release of ammonia. The three compounds tested caused, in a qualitatively different but, in terms of substrate carbons, in a quantitatively similar manner, a significant diminution of the renal uptake of fatty acids, ketone bodies and alpha-ketoglutarate. These results suggest that, in the rat kidney, VPA, 4-en-VPA and 2-en-VPA stimulate the production of ammonia at least in part by reducing the renal uptake and metabolism of ammoniagenesis inhibitors; the more potent stimulation of renal ammoniagenesis caused by VPA and 4-en-VPA also suggest that these compounds exert their stimulatory effect by an additional mechanism.

Disappearance of palmitic acid from plasma of fetal and newborn sheep

These studies were undertaken to measure the kinetic constants that characterize the disappearance of a representative free fatty acid (FFA) from the plasma of fetal and newborn sheep. A bolus of albumin-complexed [14C]palmitic acid was infused intravenously, and during the next 8 min, 24 arterial samples were collected to characterize the disappearance curve. Palmitic acid disappearance from plasma was well described by a double-exponential model. When birth was simulated in utero, kinetic values were not changed by cooling. However, after intrauterine ventilation with O2, the volume of distribution of the FFA increased 29%, its plasma clearance rate decreased 26%, and its apparent half-life in the plasma lengthened from 0.8 to 1.2 min (all P < 0.01, n = 8). After umbilical cord occlusion, plasma clearance rate decreased a further 19% and half-life lengthened to 1.4 min. About 60% of the increase in FFA concentration during simulated birth is explained by increased release from adipose stores, and 40% is explained by decreased clearance. Further experiments tested the influence of FFA concentrations themselves. After infusion of unlabeled FFA, clearance of the tracer decreased 23% (P < 0.05, n = 5), a result consistent with a saturable membrane transporter of FFAs.