Liver glucagon receptors (GluR): effect of exercise and fasting on binding characteristics, GluR-mRNA, and GluR protein content in rats.

The purpose of the study was to investigate the effects of acute exercise and fasting on glucagon receptor (GluR) binding characteristics, GluR-mRNA, and protein content in rat liver. Liver homogenates were prepared and plasma membranes were purified by aqueous 2-phase affinity partitioning in rats fed at rest (control) and after 180 min of swimming exercise and 24 h of fasting (7 rats/group). Saturation curve of plasma membranes incubated with [125I]-glucagon showed significant higher GluR density following exercise and fasting than in the control group (8.19±0.29 and 8.01±0.65 vs. 3.09±0.12 pmol/mg of proteins, respectively). When compared to control rats, GluR Kd was also higher following exercise and fasting (0.46±0.05 and 0.56±0.13 vs. 0.33±0.05 nM, respectively; significantly different for fasting only). Expression of GluR-mRNA and protein content were both significantly higher (~100% and ~90%, respectively) following the 24-h fast than in the control rats, but not following exercise. These results, in line with the literature showing an increased sensitivity of the liver to glucagon following exercise and fasting, indicate that an increased density of GluR on plasma membranes can be obtained by 2 complementary mechanisms: externalization of pre-existing GluR from intracellular pools operative in response to the prolonged exercise, and de novo synthesis of GluR operative only in response to fasting. The reduction in plasma insulin concentration and/or depletion of liver glycogen stores, which results from both prolonged exercise and fasting, could be involved in the control of these mechanisms.

An off-normal fibre-like texture in thin films on single-crystal substrates.

In the context of materials science, texture describes the statistical distribution of grain orientations. It is an important characteristic of the microstructure of polycrystalline films, determining various electrical, magnetic and mechanical properties. Three types of texture component are usually distinguished in thin films: random texture, when grains have no preferred orientation; fibre texture, for which one crystallographic axis of the film is parallel to the substrate normal, while there is a rotational degree of freedom around the fibre axis; and epitaxial alignment (or in-plane texture) on single-crystal substrates, where an in-plane alignment fixes all three axes of the grain with respect to the substrate. Here we report a fourth type of texture--which we call axiotaxy--identified from complex but symmetrical patterns of lines on diffraction pole figures for thin films formed by solid-state reactions. The texture is characterized by the alignment of planes in the film and substrate that share the same d-spacing. This preferred alignment of planes across the interface manifests itself as a fibre texture lying off-normal to the sample surface, with the fibre axis perpendicular to certain planes in the substrate. This texture forms because it results in an interface, which is periodic in one dimension, preserved independently of interfacial curvature. This new type of preferred orientation may be the dominant type of texture for a wide class of materials and crystal structures.

Roles for alpha(2)p24 and COPI in endoplasmic reticulum cargo exit site formation.

A two-step reconstitution system for the generation of ER cargo exit sites from starting ER-derived low density microsomes (LDMs; 1.17 g/cc) is described. The first step is mediated by the hydrolysis of Mg(2+)ATP and Mg(2+)GTP, leading to the formation of a transitional ER (tER) with the soluble cargo albumin, transferrin, and the ER-to-Golgi recycling membrane proteins alpha(2)p24 and p58 (ERGIC-53, ER-Golgi intermediate compartment protein) enriched therein. Upon further incubation (step two) with cytosol and mixed nucleotides, interconnecting smooth ER tubules within tER transforms into vesicular tubular clusters (VTCs). The cytosolic domain of alpha(2)p24 and cytosolic COPI coatomer affect VTC formation. This is deduced from the effect of antibodies to the COOH-terminal tail of alpha(2)p24, but not of antibodies to the COOH-terminal tail of calnexin on this reconstitution, as well as the demonstrated recruitment of COPI coatomer to VTCs, its augmentation by GTPgammaS, inhibition by Brefeldin A (BFA), or depletion of beta-COP from cytosol. Therefore, the p24 family member, alpha(2)p24, and its cytosolic coat ligand, COPI coatomer, play a role in the de novo formation of VTCs and the generation of ER cargo exit sites.

RGS-PX1, a GAP for GalphaS and sorting nexin in vesicular trafficking.

Heterotrimeric GTP-binding proteins (G proteins) control cellular functions by transducing signals from the outside to the inside of cells. Regulator of G protein signaling (RGS) proteins are key modulators of the amplitude and duration of G protein-mediated signaling through their ability to serve as guanosine triphosphatase-activating proteins (GAPs). We have identified RGS-PX1, a Galpha(s)-specific GAP. The RGS domain of RGS-PX1 specifically interacted with Galpha(s), accelerated its GTP hydrolysis, and attenuated Galpha(s)-mediated signaling. RGS-PX1 also contains a Phox (PX) domain that resembles those in sorting nexin (SNX) proteins. Expression of RGS-PX1 delayed lysosomal degradation of the EGF receptor. Because of its bifunctional role as both a GAP and a SNX, RGS-PX1 may link heterotrimeric G protein signaling and vesicular trafficking.

Effect of a 20-day ski trek on fuel selection during prolonged exercise at low workload with ingestion of 13C-glucose.

Fuel selection was measured in five subjects (36.0 +/- 10.5 years old; 87.3 +/- 12.5 kg; mean +/- SD) during a 120-min tethered walking with ski poles (1.12 l O(2) min(-1)) with ingestion of (13)C-glucose (1.5 g kg(-1)), before and after a 20-day 415-km ski trek [physical activity level (PAL) approximately 3], using respiratory calorimetry, urea excretion, and (13)C/(12)C in expired CO(2) and in plasma glucose. Before the ski trek, protein oxidation contributed 9.7 +/- 1.6% to the energy yield (%En) while fat and carbohydrate (CHO) oxidation provided 73.5 +/- 5.5 and 16.7 +/- 6.5%En. Plasma glucose was the main source of CHO (52.9 +/- 9.5%En) with similar contributions from exogenous glucose (27.2 +/- 3.1%En), glucose from the liver (25.6 +/- 8.3%En) and muscle glycogen (20.9 +/- 4.0%En). Endogenous CHO contributed 46.6 +/- 3.9%En. Following the ski trek %En from protein, fat, CHO, exogenous glucose and endogenous CHO were not significantly modified (10.1 +/- 1.3, 15.8 +/- 6.7, 74.1 +/- 6.5, 28.7 +/- 3.0 and 45.5 +/- 7.5%En, respectively) but the %En from plasma glucose and glucose from the liver (41.1 +/- 3.6 and 12.4 +/- 4.0%En) were reduced, while that from muscle glycogen increased (33.0 +/- 4.5%En). These results show that in subjects in the fed state with glucose ingestion during exercise, CHO is the main substrate oxidized, with major contributions from both exogenous and endogenous CHO. Following a ~3-week period of prolonged low intensity exercise, the %En from protein, fat, CHO, exogenous glucose and endogenous CHO were not modified. However, the %En from glucose released from the liver was reduced (possibly due to an increased insulin sensitivity of the liver) while that from muscle glycogen was increased.

Substrate source utilization during moderate intensity exercise with glucose ingestion in Type 1 diabetic patients.

Substrate oxidation and the respective contributions of exogenous glucose, glucose released from the liver, and muscle glycogen oxidation were measured by indirect respiratory calorimetry combined with tracer technique in eight control subjects and eight diabetic patients (5 men and 3 women in both groups) of similar age, height, body mass, and maximal oxygen uptake, over a 60-min exercise period on cycle ergometer at 50.8% (SD 4.0) maximal oxygen uptake [131.0 W (SD 38.2)]. The subjects and patients ingested a breakfast (containing approximately 80 g of carbohydrates) 3 h before and 30 g of glucose (labeled with 13C) 15 min before the beginning of exercise. The diabetic patients also received their usual insulin dose [Humalog = 9.1 U (SD 0.9); Humulin N = 13.9 U (SD 4.4)] immediately before the breakfast. Over the last 30 min of exercise, the oxidation of carbohydrate [1.32 g/min (SD 0.48) and 1.42 g/min (SD 0.63)] and fat [0.33 g/min (SD 0.10) and 0.30 g/min (SD 0.10)] and their contribution to the energy yield were not significantly different in the control subjects and diabetic patients. Exogenous glucose oxidation was also not significantly different in the control subjects and diabetic patients [6.3 g/30 min (SD 1.3) and 5.2 g/30 min (SD 1.6), respectively]. In contrast, the oxidation of plasma glucose and oxidation of glucose released from the liver were significantly lower in the diabetic patients than in control subjects [14.5 g/30 min (SD 4.3) and 9.3 g/30 min (SD 2.8) vs. 27.9 g/30 min (SD 13.3) and 21.6 g/30 min (SD 12.8), respectively], whereas that of muscle glycogen was significantly higher [28.1 g/30 min (SD 15.5) vs. 11.6 g/30 min (SD 8.1)]. These data indicate that, compared with control subjects, in diabetic patients fed glucose before exercise, substrate oxidation and exogenous glucose oxidation overall are similar but plasma glucose oxidation is lower; this is associated with a compensatory higher utilization of muscle glycogen.

Role of p97 and syntaxin 5 in the assembly of transitional endoplasmic reticulum.

Transitional endoplasmic reticulum (tER) consists of confluent rough and smooth endoplasmic reticulum (ER) domains. In a cell-free incubation system, low-density microsomes (1.17 g cc(-1)) isolated from rat liver homogenates reconstitute tER by Mg(2+)GTP- and Mg(2+)ATP-hydrolysis-dependent membrane fusion. The ATPases associated with different cellular activities protein p97 has been identified as the relevant ATPase. The ATP depletion by hexokinase or treatment with either N-ethylmaleimide or anti-p97 prevented assembly of the smooth ER domain of tER. High-salt washing of low-density microsomes inhibited assembly of the smooth ER domain of tER, whereas the readdition of purified p97 with associated p47 promoted reconstitution. The t-SNARE syntaxin 5 was observed within the smooth ER domain of tER, and antisyntaxin 5 abrogated formation of this same membrane compartment. Thus, p97 and syntaxin 5 regulate assembly of the smooth ER domain of tER and hence one of the earliest membrane differentiated components of the secretory pathway.

Is early and late post-meal exercise so different in type 1 diabetic lispro users?

To compare blood glucose (BG) responses during a 60 min moderate intensity exercise session performed in early or late postprandial periods. Nine generally well-controlled (HbA(1c): 7.3+/-0.1%) type 1 diabetic patients performed, at least one week apart, two exercise sessions, 60 (early exercise) and 180 min (late exercise) after a standardized breakfast. All subjects were using Humulin N (N) and Humalog (Lispro, LI) insulin. During exercise, the overall decrease in BG was 4.8+/-0.6 mmol/l and 3.6+/-0.8 mmol/l in early and late exercise, respectively (P=0.051). To prevent hypoglycemia, a dextrose infusion was initiated when BG reached 5 mmol/l. The quantity of dextrose infused was 6.2+/-3.0 g and 10.5+/-3.2g in early and late exercise, respectively (NS). The time free of dextrose infusion during exercise was 41.2+/-7.8 min and 31.7+/-7.5 min in early and late exercise, respectively (NS). In N-LI users, overall drop in BG during exercise tends to be greater in the early postprandial period. However, early and late exercise present similar quantity of dextrose infused and time free of dextrose infusion. Consequently, the similar risk of exercise-induced hypoglycemia suggests similar precautions in either exercise times.

Physical activity barriers in diabetes: development and validation of a new scale.

To develop and validate a questionnaire measuring perceived Barriers to Physical Activity in Diabetes (type 1) or BAPAD1. Initially, an open-ended questionnaire was filled by 36 patients. The modal accessible beliefs obtained on this pilot study were analysed and a scale composed of 12 items (BAPAD1) was developed and validated. Seventy-four type 1 diabetic patients filled the BAPAD1 scale. Cronbach alpha coefficient was 0.85 and the correlation between the test-retest scores was 0.84, both indicating adequate reliability of the barriers scale. Each item of BAPAD1 scale displayed very good item characteristic curve except for item 12, which was withdrawn. The test reliability curve indicated that the BAPAD1 scale is informative (value>or=0.82) at all levels of perceived barriers toward physical activity. Moreover, among diabetic-related items, the risk of hypoglycemia showed a particularly good item characteristic curve. In summary, the BAPAD1 scale presents excellent psychometric proprieties and among diabetic-related items, the risk of hypoglycemia should be considered as a significant target to overcome in order to increase physical activity. This new validated tool should be useful in identifying the most salient barriers toward the practice of physical activity and thus, permit more focused intervention in order to overcome those barriers.

Accumulation of polyunsaturated free fatty acids coincident with the fusion of rough endoplasmic reticulum membranes.

The accumulation of polyunsaturated free fatty acids (PUFAs) was observed coincident with GTP-dependent fusion of liver rough microsomes. Whereas 0.5 mM NADPH led to a parallel reduction (greater than 50%) in membrane fusion and PUFA accumulation, indomethacin (50 microM) either had little effect or slightly augmented both processes. CTP was observed to stimulate accumulation of PUFAs and diacylglycerol (DAG). Therefore PUFAs may be relevant for GTP-dependent membrane fusion and together with DAG may play a role in fusion stimulated in the presence of CTP.

Dbp45A encodes a Drosophila DEAD box protein with similarity to a putative yeast helicase involved in ribosome assembly.

Proteins of the DEAD family of putative ATP-dependent RNA helicases have been implicated in translation initiation, ribosome assembly, and RNA processing in a variety of organisms from Escherichia coli to man. Among these proteins are eIF-4A, an essential component of the cap-binding complex, numerous yeast proteins required for pre-mRNA splicing, and proteins from yeast and E. coli necessary for ribosome assembly. We report the isolation of a new DEAD gene from Drosophila, Dbp45A, which is most abundantly expressed in 6-12 h embryos and adults. The predicted amino acid sequence of the Dbp45A product contains all eight highly conserved DEAD family motifs, and most closely resembles the Saccharomyces cerevisiae DRS1p among known DEAD box proteins. DRS1p has been implicated in ribosomal RNA processing.

Modulation of GTP-dependent fusion by linoleic and arachidonic acid in derivatives of rough endoplasmic reticulum from rat liver.

The effect of modulation of the content of unsaturated free fatty acids on GTP-dependent fusion of stripped rough microsomes from rat liver was determined. Cytidine monophosphate, CDP and CTP were all observed to be able to stimulate free fatty acid accumulation and coincident membrane fusion. GTP was required for membrane fusion in the presence of cytidine nucleotide but was not required for free fatty acid accumulation. In the presence of GTP and cytidine nucleotide, the addition of ATP and CoA led to the synthesis of triacyglycerol and marked inhibition of both free fatty acid accumulation and membrane fusion. Delipidated bovine serum albumin also inhibited both free fatty acid accumulation and membrane fusion. Analysis by gas chromatography indicated that linoleic acid and arachidonic acid were the most actively fluctuating of the accumulated free fatty acids. Comparison by quantitation indicated a high correlation between GTP-dependent membrane fusion and changes in amount of unesterified linoleic acid and arachidonic acid. The results suggest that polyunsaturated free fatty acids may be required for GTP-dependent membrane fusion.

The Drosophila cystoblast differentiation factor, benign gonial cell neoplasm, is related to DExH-box proteins and interacts genetically with bag-of-marbles.

Selection of asymmetric cell fates can involve both intrinsic and extrinsic factors. Previously we have identified the bag-of-marbles (bam) gene as an intrinsic factor for cystoblast fate in Drosophila germline cells and shown that it requires active product from the benign gonial cell neoplasm (bgcn) gene. Here we present the cloning and characterization of bgcn. The predicted Bgcn protein is related to the DExH-box family of RNA-dependent helicases but lacks critical residues for ATPase and helicase functions. Expression of the bgcn gene is extremely limited in ovaries but, significantly, bgcn mRNA is expressed in a very limited number of germline cells, including the stem cells. Also, mutations in bgcn dominantly enhance a bam mutant phenotype, further corroborating the interdependence of these two genes' functions. On the basis of known functions of DExH-box proteins, we propose that Bgcn and Bam may be involved in regulating translational events that are necessary for activation of the cystoblast differentiation program.

Regulation of µ and δ opioid receptor functions: involvement of cyclin-dependent kinase 5.

BACKGROUND AND PURPOSE: Phosphorylation of δ opioid receptors (DOP receptors) by cyclin-dependent kinase 5 (CDK5) was shown to regulate the trafficking of this receptor. Therefore, we aimed to determine the role of CDK5 in regulating DOP receptors in rats treated with morphine or with complete Freund's adjuvant (CFA). As µ (MOP) and DOP receptors are known to be co-regulated, we also sought to determine if CDK5-mediated regulation of DOP receptors also affects MOP receptor functions. EXPERIMENTAL APPROACH: The role of CDK5 in regulating opioid receptors in CFA- and morphine-treated rats was studied using roscovitine as a CDK inhibitor and a cell-penetrant peptide mimicking the second intracellular loop of DOP receptors (C11-DOPri2). Opioid receptor functions were assessed in vivo in a series of behavioural experiments and correlated by measuring ERK1/2 activity in dorsal root ganglia homogenates. KEY RESULTS: Chronic roscovitine treatment reduced the antinociceptive and antihyperalgesic effects of deltorphin II (Dlt II) in morphine- and CFA-treated rats respectively. Repeated administrations of C11-DOPri2 also robustly decreased Dlt II-induced analgesia. Interestingly, DAMGO-induced analgesia was significantly increased by roscovitine and C11-DOPri2. Concomitantly, in roscovitine-treated rats the Dlt II-induced ERK1/2 activation was decreased, whereas the DAMGO-induced ERK1/2 activation was increased. An acute roscovitine treatment had no effect on Dlt II- or DAMGO-induced analgesia. CONCLUSIONS AND IMPLICATIONS: Together, our results demonstrate that CDK5 is a key player in the regulation of DOP receptors in morphine- and CFA-treated rats and that the regulation of DOP receptors by CDK5 is sufficient to modulate MOP receptor functions through an indirect process.

Operative management of "hockey groin syndrome": 12 years of experience in National Hockey League players.

BACKGROUND: At the elite level of hockey, groin injuries can threaten a player's career. The aim of this review is to describe the clinical presentation and evaluate our operative approach to "hockey groin syndrome" in National Hockey League (NHL) players. METHODS: Between November 1989 and June 2000, 22 NHL players with debilitating groin pain underwent operative exploration. A repair, including ablation of the ilioinguinal nerve and reinforcement of the external oblique aponeurosis with a Goretex (W.L. Gore & Associates, Inc, Flagstaff, Ariz) mesh, was performed. Medical records were reviewed, and the players or their trainers were contacted by telephone after a mean follow-up period of 31.2 months to assess function, symptoms, and overall satisfaction. RESULTS: All patients had tearing of the external oblique aponeurosis, with branches of the ilioinguinal nerve emerging from the torn areas. At follow-up, 18 players (82%) had no pain, whereas 4 (18%) reported mild, intermittent pain. All 22 patients returned to playing hockey, with 19 (85%) able to continue their careers in the NHL. CONCLUSIONS: The "hockey groin syndrome," marked by tearing of the external oblique aponeurosis and entrapment of the ilioinguinal nerve, is a cause of groin pain in professional hockey players. Ilioinguinal nerve ablation and reinforcement of the external oblique aponeurosis successfully treats this incapacitating entity.

Breath [13CO2] recovery from an oral glucose load during exercise: comparison between [U-13C] and [1,2-13C]glucose.

The purpose of the present experiment was to compare 13CO2 recovery at the mouth, and the corresponding exogenous glucose oxidation computed, during a 100-min exercise at 63 +/- 3% maximal O2 uptake with ingestion of glucose (1.75 g/kg) in six active male subjects, by use of [U-13C] and [1,2-13C]glucose. We hypothesized that 13C recovery and exogenous glucose oxidation could be lower with [1,2-13C] than [U-13C]glucose because both tracers provide [13C]acetate, with possible loss of 13C in the tricarboxylic acid (TCA) cycle, but decarboxylation of pyruvate from [U-13C]glucose also provides 13CO2, which is entirely recovered at the mouth during exercise. The recovery of 13C (25.8 +/- 2.3 and 27.4 +/- 1.2% over the exercise period) and the amounts of exogenous glucose oxidized computed were not significantly different with [1,2-13C] and [U-13C]glucose (28.9 +/- 2.6 and 30.7 +/- 1.3 g, between minutes 40 and 100), suggesting that no significant loss of 13C occurred in the TCA cycle. This stems from the fact that, during exercise, the rate of exogenous glucose oxidation is probably much larger than the flux of the metabolic pathways fueled from TCA cycle intermediates. It is thus unlikely that a significant portion of the 13C entering the TCA cycle could be diverted to these pathways. From a methodological standpoint, this result indicates that when a large amount of [13C]glucose is ingested and oxidized during exercise, 13CO2 production at the mouth accurately reflects the rate of glucose entry in the TCA cycle and that no correction factor is needed to compute the oxidative flux of exogenous glucose.

Respective oxidation of 13C-labeled lactate and glucose ingested simultaneously during exercise.

The purpose of this experiment was to measure, by using 13C labeling, the oxidation rate of exogenous lactate (25 g, as Na+, K+, Ca2+, and Mg2+ salts) and glucose (75 g) ingested simultaneously (in 1,000 ml of water) during prolonged exercise (120 min, 65 +/- 3% maximum oxygen uptake in 6 male subjects). The percentage of exogenous glucose and lactate oxidized were similar (48 +/-3 vs. 45 +/- 5%, respectively). However, because of the small amount of oral lactate that could be tolerated without gastrointestinal discomfort, the amount of exogenous lactate oxidized was much smaller than that of exogenous glucose (11.1 +/- 0.5 vs. 36.3 +/- 1.3 g, respectively) and contributed to only 2.6 +/- 0.4% of the energy yield (vs. 8.4 +/- 1.9% for exogenous glucose). The cumulative amount of exogenous glucose and lactate oxidized was similar to that observed when 100 g of [13C]glucose were ingested (47.3 +/- 1.8 vs. 50.9 +/- 1.2 g, respectively). When [13C]glucose was ingested, changes in the plasma glucose 13C/12C ratio indicated that between 39 and 61% of plasma glucose derived from exogenous glucose. On the other hand, the plasma glucose 13C/12C ratio remained unchanged when [13C]lactate was ingested, suggesting no prior conversion into glucose before oxidation.

Oral [13C]glucose oxidation during prolonged exercise after high- and low-carbohydrate diets.

The effect of a diet either high or low in carbohydrates (CHO) on exogenous 13C-labeled glucose oxidation (200 g) during exercise (ergocycle: 120 min at 64.0 +/- 0.5% maximal oxygen uptake) was studied in six subjects. Between 40 and 80 min, exogenous glucose oxidation was significantly higher after the diet low in CHO (0.63 +/- 0.05 vs. 0.52 +/- 0.04 g/min), but this difference disappeared between 80 and 120 min (0.71 +/- 0.03 vs. 0.69 +/- 0.04 g/min). The oxidation rate of plasma glucose, computed from the volume of 13CO2 produced the 13C-to-12C ratio in plasma glucose at 80 min, and of glucose released from the liver, computed from the difference between plasma glucose and exogenous glucose oxidation, was higher after the diet low in CHO (1.68 +/- 0.26 vs. 1.41 +/- 0.17 and 1.02 +/- 0.20 vs. 0.81 +/- 0.14 g/min, respectively). In contrast the oxidation rate of glucose plus lactate from muscle glycogen (computed from the difference between total CHO oxidation and plasma glucose oxidation) was lower (0.31 +/- 0.35 vs. 1.59 +/- 0.20 g/min). After a diet low in CHO, the oxidation of exogenous glucose and of glucose released from the liver is increased and partly compensates for the reduction in muscle glycogen availability and oxidation.

Oxidation of [(13)C]glycerol ingested along with glucose during prolonged exercise.

The respective oxidation of glycerol and glucose (0.36 g/kg each) ingested simultaneously immediately before exercise (120 min at 68 +/- 2% maximal oxygen uptake) was measured in six subjects using (13)C labeling. Indirect respiratory calorimetry corrected for protein and glycerol oxidation was used to evaluate the effect of glucose + glycerol ingestion on the oxidation of glucose and fat. Over the last 80 min of exercise, 10.0 +/- 0.8 g of exogenous glycerol were oxidized (43% of the load), while exogenous glucose oxidation was 21% higher (12.1 +/- 0.7 g or 52% of the load). However, because the energy potential of glycerol is 18% higher than that of glucose (4.57 vs. 3.87 kcal/g), the contribution of both exogenous substrates to the energy yield was similar (4.0-4.1%). Total glucose and fat oxidation were similar in the placebo (144.4 +/- 13.0 and 60.5 +/- 4.2 g, respectively) and the glucose + glycerol (135.2 +/- 12.0 and 59.4 +/- 6.5 g, respectively) trials, whereas endogenous glucose oxidation was significantly lower than in the placebo trial (123.7 +/- 11.7 vs. 144.4 +/- 13.0 g). These results indicate that exogenous glycerol can be oxidized during prolonged exercise, presumably following conversion into glucose in the liver, although direct oxidation in peripheral tissues cannot be ruled out.

Differential metabolic fate of the carbon skeleton and amino-N of [13C]alanine and [15N]alanine ingested during prolonged exercise.

The decarboxylation/oxidation and the deamination of 13C- and [15N]alanine ingested (1 g/kg or 73.7 +/- 2 g) during prolonged exercise at low workload (180 min at 53 +/- 2% maximal O2 uptake) was measured in six healthy male subjects from V13CO2 at the mouth and [15N]urea excretion in urine and sweat. Over the exercise period, 50.6 +/- 3.5 g of exogenous alanine were oxidized (68.7 +/- 4.5% of the load), providing 10.0 +/- 0.6% of the energy yield vs. 4.8 +/- 0.4, 47.6 +/- 4.3, and 37.4 +/- 4.7% for endogenous proteins, glucose, and lipids, respectively. Alanine could have been oxidized after conversion into glucose in the liver and/or directly in peripheral tissues. In contrast, only 13.0 +/- 3.2 mmol of [(15)N]urea were excreted in urine and sweat (10.6 +/- 0.4 and 2.4 +/- 0.5 mmol, respectively), corresponding to the deamination of 2.3 +/- 0.3 g of exogenous alanine (3.1 +/- 0.4% of the load). These results confirm that the metabolic fate of the carbon skeleton and the amino-N moiety of exogenous alanine ingested during prolonged exercise at low workload are markedly different. The large positive nitrogen balance (8.5 +/- 0.3 g) suggests that in this situation protein synthesis could be increased when a large amount of a single amino acid is ingested.