Metabolism of Free Fatty Acids and Ketone Bodies in Skeletal Muscle

Metabolic effects of beta-adrenoceptor blockade on skeletal muscle at rest and during exercise

beta-Adrenoceptor antagonists influence the metabolic responses in man at rest and during exercise. Impaired working capacity and muscular fatigue have been reported in patients on beta-blockers and this could be due to an altered substrate supply to the muscles. The results from several studies show that the main effect of beta-blockade on metabolism is decreased lipolysis, with less fat available to the muscles. This results in an increased carbohydrate demand to maintain an unchanged aerobic metabolism, and liver and muscle glycogen stores are more rapidly depleted. beta-blockade also results in decreased lactate release from the muscles, probably due to a membrane effect and/or changed perfusion. It is concluded that beta-blockade a) decreases fat metabolism in the muscle, which secondarily increases the use of carbohydrates during exercise, resulting in earlier hypoglycaemia and/or depletion of muscle glycogen with reduction of the working capacity, b) impairs lactate transport from the muscle but does not cause lactate accumulation within the muscle which could be responsible for muscular fatigue.

beta-Adrenoceptor blockade and exercise: effects on endurance and physical training

beta-adrenoceptor antagonists influence almost all haemodynamic and metabolic actions in the body. High levels of sympathetic stimulation accompany aerobic exercise and it is known that beta-blockade results in a decreased working capacity. Furthermore it has also been questioned whether beta-blockade inhibits the normal response to physical training. Although adrenergic mechanisms are involved in muscle and liver glycogen breakdown, beta-blockade does not seem to reduce glycogen utilisation during exercise. Both selective and non-selective beta-blockade inhibit lipolysis and result in less free fatty acids being available for muscle utilisation. Surgical and chemical sympathectomy in animals has been shown to inhibit the responses to physical training but results are now available showing that beta-adrenergic blockade does not prevent the effect of physical conditioning in patients treated with propranolol. It is concluded that beta-blockade during prolonged exercise a) does not reduce oxygen uptake by the working muscles b) decreases fat metabolism, which secondarily increases the use of carbohydrates, resulting in earlier hypoglycaemia and/or depletion of muscle glycogen with reduction in working capacity c) does not inhibit central and peripheral adaptation to physical conditioning.

High-intensity exercise and muscle glycogen availability in humans

This study investigated the effects of muscle glycogen availability on performance and selected physiological and metabolic responses during high-intensity intermittent exercise. Seven male subjects completed a regimen of exercise and dietary intake (48 h) to either lower and keep low (LOW-CHO) or lower and then increase (HIGH-CHO) muscle glycogen stores, on two separate occasions at least a week apart. On each occasion the subjects completed a short-term (<10 min) and prolonged (>30 min) intermittent exercise (IEX) protocol, 24 h apart, which consisted of 6-s bouts of high-intensity exercise performed at 30-s intervals on a cycle ergometer. Glycogen concentration (mean +/- SEM) in m. vastus lateralis before both IEx(short) and IEx(long) was significantly lower following LOW-CHO [180 (14), 181 (17) mmol kg (dw)(-1)] compared with HIGH-CHO [397 (35), 540 (25) mmol kg (dw)(-1)]. In both IEx(short) and IEx(long), significantly less work was performed following LOW-CHO compared with HIGH-CHO. In IEx(long), the number of exercise bouts that could be completed at a pre-determined target exercise intensity increased by 265% from 111 (14) following LOW-CHO to 294 (29) following HIGH-CHO (P < 0.05). At the point of fatigue in IEx(long), glycogen concentration was significantly lower with the LOW-CHO compared with HIGH-CHO [58 (25) vs. 181 (46) mmol kg (dw)(-1), respectively]. The plasma concentrations of adrenaline and nor-adrenaline (in IEx(short) and IEx(long)), and FFAand glycerol (in IEx(long)), increased several-fold above resting values with both experimental conditions. Oxygen uptake during the exercise periods in IEx(long), approached 70% of Vo2max. These results suggest that muscle glycogen availability can affect performance during both short-term and more prolonged high-intensity intermittent exercise and that with repeated exercise periods as short as 6 s, there can be a relatively high aerobic contribution.

Utilization of lipids during exercise in human subjects: metabolic and dietary constraints

During endurance exercise, skeletal muscle relies mainly on both carbohydrate (CHO) and fat oxidation to cover energy needs. Numerous scientific studies have shown that increasing the exercise intensity leads to a progressive utilization of CHO. The latter will induce a state of glycogen depletion which is generally recognized as being a limiting factor for the continuation of strenuous exercise. Different dietary interventions have been proposed to overcome this limitation. A high-CHO diet during periods of intense training and competition, as well as CHO intake during exercise, are known to maintain a high rate of CHO oxidation and to delay fatigue. However, it has been recognized also that enhancing fatty acid (FA) oxidation during exercise induces a reduced rate of glycogen degradation, resulting in an improved endurance capacity. This is most strikingly observed as a result of frequent endurance exercise which improves a number of factors known to govern the FA flux and the oxidative capacity of skeletal muscle. Such factors are: (1) blood flow and capillarization; (2) lipolysis of triacylglycerol (TAG) in adipose tissue and circulating TAG and transport of FA from blood plasma to the sarcoplasm; (3) availability and rate of hydrolysis of intramuscular TAG; (4) activation of the FA and transport across the mitochondrial membrane; (5) the activity of enzymes in the oxidative pathway; (6) hormonal adaptations, i.e. sensitivity to catecholamines and insulin. The observation that the plasma FA concentration is an important factor in determining the rate of FA oxidation, and that some dietary factors may influence the rate of FA supply to muscle as well as to the mitochondria, has led to a number of dietary interventions with the ultimate goal to enhance FA oxidation and endurance performance. It appears that experimental data are not equivocal that dietary interventions, such as a high-fat diet, medium-chain TAG-fat emulsions and caffeine intake during exercise, as well as L-carnitine supplementation, do significantly enhance FA oxidation during exercise. So far, only regular endurance exercise can be classified as successful in achieving adaptations which enhance FA mobilization and oxidation.

Whole-body, peripheral and intestinal endogenous acetate turnover in dogs using stable isotopes

Acetate metabolism supplies about 10% of energy requirements in food-deprived nonruminant animals. This study used a stable isotope dilution method to investigate the fate of acetate in 24-h food-deprived dogs free of colonic fermentation. Three dogs received intravenous bolus injections of 40 or 70 micromol/kg of [1-13C] acetate, and carotid blood was then sampled during a 15-min period to estimate the acetate distribution volume. Ten dogs received intravenous [1-13C] acetate infusions of 1.05 +/- 0.02 or 2.10 +/- 0. 10 micromol/(kg.min) for 120 or 200 min after a prime of 200 or 70 micromol/kg, respectively. Cephalic venous and carotid arterial blood were sampled for all dogs, and portal blood for five. Acetate distribution volume was 0.27 +/- 0.16 L/kg (mean +/- SEM). The concentrations of acetate in arterial (144 +/- 17 micromol/L), venous (155 +/- 20 micromol/L) and portal plasma (131 +/- 16 micromol/L) were not significantly different during infusion, whereas isotopic enrichments [mole percent excess (MPE): labeled acetate/all acetate molecules] in portal (1.2 +/- 0.2 MPE) and venous plasma (1.7 +/- 0.3 and 2.6 +/- 0.7 MPE) were lower than in arterial plasma for both infusion rates (4.9 +/- 0.6 and 7.6 +/- 0.8 MPE, respectively, P < 0.005). Whole-body acetate turnover was 24.4 +/- 2.4 micromol/(kg.min). Fractional acetate extractions for forelimb and intestine were 62 +/- 7 and 72 +/- 6%, respectively, and the production for each organ was 0.3 and 1.1 micromol/(kg.min) respectively, similar to that of utilization (P > 0.05). It is concluded that the forelimb and intestine produce and utilize acetate as an energy source in 24-h food-deprived dogs free of colonic fermentation.

Fuel selection, muscle fibre

The fuel selection of muscle fibres at rest is dependent on substrate availability. Increased lipid availability results in an increase citrate concentration with inhibition of glycolysis. Fat utilization also increases the concentration ratio acetyl-CoA:CoASH, with inhibition of PDH transformation to the active form. The result is an inhibition of carbohydrate utilization in conformity with the classical glucose-fatty acid style. During exercise fuel selection is dependent on the intensity of exercise, the recruitment pattern of fibre type and the availability of fuels. During exercise at maximum intensity the main fuels are PCr and muscle glycogen, the highest energy release occurring with type II fibres. At exercise intensities between 70 and 100% VO2max carbohydrate is the main fuel after the intake of normal mixed or carbohydrate-rich diets. No inhibition of PDHa formation was observed by increased concentration ratio acetyl-CoA:CoASH during the exercise, but the activation and transport of fatty-acyl groups from NEFA may be inhibited by a decrease in the concentration of CoASH. This mechanism may limit the contribution of fat to metabolism during exercise at intensities above 60% VO2max, after an intake of carbohydrate-rich diets. After carbohydrate starvation or an infusion of a fat emulsion, there was a substantial increase in the utilization of fat which, after the infusion, was concomitant with a high PDHa and a high lactate production. This is thought to be due to a decrease in glycolysis and in the catalytic activity of PDHa, especially in type I fibres, while lactate production continues in type II fibres. When exercise intensities fall below 60% VO2max, fat becomes the dominant fuel during prolonged exercise. At the same time the recruitment pattern is shifted toward type I fibres which have the lowest activation threshold and the highest oxidative capacity.

Glycogen and triglyceride utilization in relation to muscle metabolic characteristics in men performing heavy-resistance exercise

Nine bodybuilders performed heavy-resistance exercise activating the quadriceps femoris muscle. Intermittent 30-s exhaustive exercise bouts comprising 6-12 repetitions were interspersed with 60-s periods for 30 min. Venous blood samples were taken repeatedly during and after exercise for analyses of plasma free fatty acid (FFA) and glycerol concentration. Muscle biopsies were obtained from the vastus lateralis muscle before and after exercise and assayed for glycogen, glycerol-3-phosphate, lactate and triglyceride (TG) content. The activities of citrate synthase (CS), lactate dehydrogenase, hexokinase (HK), myokinase, creatine kinase and 3-hydroxyacyl-CoA dehydrogenase (HAD), were analysed. Histochemical staining procedures were used to assess fibre type composition, fibre area and capillary density. TG content before and after exercise averaged (SD) 23.9 (13.3) and 16.7 (6.4) mmol kg-1 dry wt. The basal triglyceride content varied sixfold among individuals and the higher the levels the greater was the change during exercise. The glycogen content decreased (P less than 0.001) from 690 (82) to 495 (95) mmol kg-1 dry wt. and lactate and glycerol-3-phosphate increased (P less than 0.001) to 79.5 (5.5) and 14.5 (7.3) mmol kg-1 dry wt., respectively, after exercise. The HK and HAD/CS content respectively correlated with glycogen or TG content at rest and with changes in these metabolites during exercise. FFA and glycerol concentrations increased slightly (P less than 0.001) during exercise. Lipolysis may, therefore, provide energy during heavy-resistance exercise of relatively short duration. Also, storage and utilization of intramuscular substrates appear to be influenced by the metabolic profile of muscle.

Individual free fatty acids and lactate uptake in the human heart during severe sepsis

Coronary haemodynamics and myocardial metabolism of nonesterified fatty acids (NEFA) and lactate were studied in 11 patients with severe sepsis, and compared to 10 control subjects. Coronary sinus blood flow was evaluated by thermodilution. Arterial and coronary sinus blood samples were collected for the measurement of lactate and total and individual NEFA concentrations both in septic and control patients. There was an increase in lactate and total NEFA arterial concentrations with a marked increase in palmitic and linolenic acids. The uptake of the main NEFA (C14:0 to C18:2) was significantly decreased. In the control group, individual NEFA uptake was proportional to their arterial concentrations. This relationship was not observed in patients with sepsis: there was no preferential extraction of any particular NEFA. Furthermore, in patients with sepsis, myocardial oxygen consumption did not correlate with NEFA, but only with lactate uptake. Alterations in NEFA uptake were found to be constant during severe sepsis and are consistent with major disturbances in myocardial metabolism.

The effect of dietary carbohydrate intake on the metabolic response to prolonged walking on consecutive days

Six healthy subjects walked 37 km per day for four consecutive days on two occasions one month apart; on one walk, subjects consumed a high carbohydrate (CHO) diet (85 +/- 1% CHO, Mean +/- SE) and on the other walk an isocaloric low CHO diet (2 +/- 0% CHO) was consumed. Subjects were fasted each day until after the completion of the walk. Blood samples were obtained at rest prior to exercise and after completion of each of three laps of 12.3 km. Exercise intensity corresponded to approximately 17% of VO2max. The first day of each walk demonstrated that the pattern of substrate mobilisation in response to this type of exercise is highly reproducible, there being no difference in any of the parameters measured between the two walks. Circulating glucose, lactate, insulin and triglyceride levels remained essentially unchanged; alanine fell progressively and glycerol, free fatty acid (FFA) and 3-hydroxybutyrate (BHB) rose progressively. After the first day there was a general tendency for the blood glucose concentration to decline as exercise progressed; by the end of the walk on Day 2, blood glucose was lower on the low CHO diet than on the high CHO diet. On Day 4 plasma insulin was higher (p less than 0.05) on the high CHO diet than on the low CHO diet and declined progressively on both diets. Blood lactate and alanine concentrations were generally higher at rest on the high CHO diet, but fell so that no differences existed by the end of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolism of ketone bodies in pregnant sheep

A combination of isotope-dilution and arteriovenous-difference techniques was used to determine the significance of ketones to energy homoeostasis in fasted pregnant ewes. 2. There was incomplete interconversion of D(-) 3-hydroxybutyrate (3HB) and acetoacetate (AcAc) and therefore neither entry rate nor oxidation of total ketone bodies could be estimated by assuming circulating ketone bodies represent a single metabolic compartment. Total ketone body metabolism was satisfactorily summarized using a three-compartment model. In fasted pregnant ewes the mean entry rate of total ketones was 1 mmol/h per kg body-weight and of the ketones entering the circulating 87% were promptly oxidized to carbon dioxide accounting for 30% of the total CO2 production. 3. Ketone bodies are readily utilized by hind-limb skeletal muscle such that if completely oxidized, 18 +/- 4 and 48 +/- 3% of the oxygen utilized could be accounted for in fed and fasted pregnant ewes respectively. For both 3HB and AcAc there was a hyperbolic relationship between utilization and arterial concentration. The apparent Michaelis constant (Km) values were 0.55 and 1.42 mM respectively and the maximum velocity (Vmax) 2.9 and 5.6 mmol/h per kg muscle. The arterial concentration of AcAc is always below the Km value and this limits the utilization rate. THe D(-) 3HB concentration however, may surpass that required for maximum utilization and ketoacidosis may be a consequence of this. 4. A two-compartment model was used to analyse ketone body metabolism by hind-limb skeletal muscle. The results suggested substantial interconversion and production of AcAc and 3HB. 5. The pregnant uterus utilized 3HB which if completely oxidized accounted for 12 +/- 2 (fed) and 25 +/- 4 (fasted) % of its O2 consumption. At least 64% of the net 3HB utilized was oxidized. AcAc was not utilized in significant quantitites.

Effect of diet on the utilization of blood-borne and intramuscular substrates during exercise in man

20 subjects were studied at rest and during a 25 min submaximal exercise (65% of VO2 max) on two occasions, the first preceded by a fat rich diet and the second by a carbohydrate rich diet. Oxygen uptake, respiratory exchange ratio (R) and arterial-femoral venous differences for glucose, lactate, beta-hydroxybutyrate and FFA (based on the fractional extraction of 3H-palmitate) were measured at rest and during exercise. Changes in intramuscular glycogen, triglyceride and lactate concentrations were determined in muscle biopsies taken before and immediately after exercise form m. quadriceps femoris. R was lower after the fat than after the carbohydrate diet and simultaneously the FFA extraction by the exercising leg was higher. The muscle triglycerides did not changes significantly during exercise after either diet. The glucose extraction was insignificantly greater after the fat diet. The glycogen reduction was numerically smaller after the fat diet, but the difference was uncertain and difficult to evaluate due to a large variation after the carbohydrate diet. However, muscle lactate accumulation and release by the exercising leg was smaller after the fat diet, indicating a slower rate of muscle glycogenolysis. It is concluded that a fat rich diet increases the relative contribution of fat to the oxidative metabolism, that this increase, to a great extent, is covered by plasma FFA and that the concomitant decrease in carbohydrate utilization concerns muscle glycogen rather than blood glucose.

Metabolic responses to bicycle exercise after several days of physical work and energy deficiency

The responses of plasma free fatty acids (FFA), free glycerol, beta-hydroxybutyrate (BUT) and glucose to bicycle exercise (corresponding to 50% of the maximal VO2) were investigated in 11 cadets of the Norwegian Military Academy, before a combat course (control experiment) and on the third and fifth day of such a course which involved nearly continuous intense military activities and pronounced energy deficiency. Pre-exercise levels of FFA, glycerol and BUT were greatly elevated on days 3 and 5 as compared with pre-exercise levels before the course. The increases in plasma FFA, glucose and BUT in response to bicycle exercise were much more pronounced on days 3 and 5 than before the course. The increases observed during (and before) the course were approximately in mmol/l: FFA 1.0 (0.3), glycerol 0.3 (0.08), BUT 0.6 (0.0). The day 5 responses were lower than those on day 3. The plasma glucose concentration appeared to decrease slightly after exercise on days 3 and 5. The results demonstrate that several days of strenuous work and pronounced energy deficiency do appreciably increase acute metabolic responses to exercise, and indicate that there is a very high FFA-utilization under such conditions.

Free fatty acid and ketone body metabolism during exercise in diabetes

In diabetic patients the metabolic response to physical exercise is to a large extent determined by the degree of insulin deficiency at the onset of exercise. Thus, in well-controlled insulin-treated patients with mild hyperglycaemia and no or minimal ketonemia the utilization of FFA, blood glucose and glycogen by working muscle is similar to that of healthy subjects, and exercise is accompanied by a fall in blood glucose levels. In contrast, patients in poor metabolic control with marked hyperglycaemia, elevated FFA levels and hyperketonemia may respond to exercise with a further rise in FFA, ketone levels and blood glucose, reflecting augmented rates of adipose tissue lipolysis and hepatic ketogenesis. The accelerated rate of ketogenesis seen during exercise continues unabated in diabetics but not in healthy subjects during the post-exercise recovery period, thereby contributing to the development of post-exercise ketosis. These considerations underscore the importance of adequate insulin administration in connection with exercise in diabetic patients.

Energy Metabolism in Diabetic Distance Runners

The authors discuss chronic effects of serious endurance training on the management of diabetes and recommend ways of adjusting insulin administration to accommodate training schedules.

Altered hormonal response to short-term bicycle exercise in young men after prolonged physical strain, caloric deficit, and sleep deprivation

The hormonal response to a standardized bicycle exercise test was studied in 11 male cadets exposed to a course of 107 h of continuous activity with less than 2 h sleep. The subjects expended only about 8,600-11,000 kcal/24 h whereas their daily food intake contained only about 1,500 kcal. The exercise test was performed once 12 days before the course (control experiment) and on day 3 and day 5 during the course, always between 0700-0900 h. A two to six fold increase was seen in the resting levels of noradrenaline, adrenaline, dopamine, and growth hormone during the course whereas a decrease was observed for thyroxine, triiodothyronine, and prolactin. Cortisol increased on day 3 and then decreased to precourse levels on day 5. The response to the exercise test during the course for all catecholamines was a further increase above and proportional to the raisted resting levels. Growth hormone increased by about 6-8 microgram/l both before and during the course. During the exercise test, cortisol decreased before the course whereas it increased during the course. All plasma levels of cortisol were higher on day 3 than on day 5 and in the control experiment. The post-exercise insulin increase was reduced during the course corresponding to a reduction in blood glucose levels. Prolactin decreased during and after exercise in the control experiment, whereas on day 5 the opposite response was seen. No changes in the disappearance rate of different hormones were observed during the course. The present investigation has demonstrated that prolonged strain severely affects the resting plasma levels of different hormones as well as the endocrine response to a short-term physical exercise.

Acetate production and substrate availability in the dog

Arterial plasma acetate, glucose and NEFA concentrations were measured during loading with glucose/insulin and fat emulsion/heparin, and during epinephrine and norepinephrine stimulation in dogs. The expected alterations in glucose and NEFA concentrations were found. Acetate concentrations were not influenced, and remained constant in all experiments. Since acetate removal from the body pool is mainly concentration dependent, it may be concluded that acetate production was constant under the conditions described. "Overflow disposal" of AcSCoA to yield free acetate in cases of high rates of AcSoA-production does not occur as long as the Krebs cycle is intact.

Plasma acetate concentrations during canine haemorrhagic shock

Acetate, pyruvate, lactate and NEFA concentrations, as well as acid-base-parameters were followed during bleeding, stable hypotension and re-infusion in five dogs. Mean arterial blood pressures were kept at 30 mmHg during the shock phase. An increase in acetate concentrations (P less than 0.01) was found in arterial as well as in venous plasma samples. The maximal mean acetate concentration was 0.19 mmol/l (during reinfusion) as compared to 0.06 mmol/l prior to bleeding. There was no difference between arterial and inferior caval venous concentrations. A definite correlation (r = 0.81, P less than 0.02) was found between blood pyruvate and plasma acetate concentrations. There was no correlation between plasma glucose or NEFA and acetate concentrations or between blood excess lactate and plasma acetate. The plasma acetate accumulation was negligible compared to the concomitant lactate accumulation (1:60), and did not contribute to the metabolic acidosis of shock. The correlation between acetate and pyruvate concentrations may indicate that pyruvate is the main substrate of acetate production in hypovolemic shock.

The effect of different diets and of insulin on the hormonal response to prolonged exercise

The importance of carbohydrate availability during exercise for metabolism and plasma hormone levels was studied. Seven healthy men ran on a treadmill at 70% of individual maximal oxygen uptake having eaten a diet low (F) or high (CH) in carbohydrate through 4 days. At exhaustion the subjects were encouraged to continue to run while glucose infusion increased plasma glucose to preexercise levels. Forearm venous blood, biopsies from vastus muscle and expiratory gas were analyzed. Time to exhaustion was longer in CH- (106 +/- 5 min (S.E.)) than in F-expts. (64 +/- 6). During exercise, overall carbohydrate combustion rate, muscular glycogen depletion and glucose and lactate concentrations, carbohydrate metabolites in plasma, and estimated rate of hepatic glucose production were higher, fat metabolites lower, and the decrease in plasma glucose slower in CH- than in F-expts. Plasma norepinephrine increased and insulin decreased similarly in CH- and F-expts., whereas the increase in glucagon, epinephrine, growth hormone and cortisol was enhanced in F-expts. Glucose infusion eliminated hypoglycemic symptoms but did not substantially increase performance time. During the infusion epinephrine decreased markedly and glucagon even to preexercise levels. Infusion of insulin (to 436% of preexercise concentration) in addition to glucose in F-expts. did not change the plasma levels of the other hormones more than infusion of glucose only but reduced fat metabolites in plasma. At exhaustion muscular glycogen depletion was slow, and the glucose gradient between plasma and sarcoplasma as well as the muscular glucose 6-phosphate concentration had decreased.

CONCLUSIONS

The preceding diet modifies the energy depots, the state of which (as regards size, receptors and enzymes) is of prime importance for metabolism during prolonged exercise. Plentiful carbohydrate stores favor both glucose oxidation and lactate production. During exercise norepinephrine increases and insulin decreases independent of plasma glucose changes whereas receptors sensitive to glucose privation but not to acute changes in insulin levels enhance the exercise-induced secretion of glucagon, epinephrine, growth hormone and cortisol. Abolition of cerebral hypoglycemia does not inevitably increase performance time, because elimination of the hypoglycemia may not abolish muscular energy lack.

The effects of partial chronic denervation on forearm metabolism

Effects of chronic denervation upon in vivo forearm metabolism were studied in six patients and six controls. The diagnosis was amyotrophic lateral sclerosis in four patients, the neuronal form of Charcot-Marie Tooth disease in one patient, and an unclassified chronic disease of the lower motor neurons in one patient. In all cases the forearm muscles showed clinical weakness and electrical evidence of denervation, while muscle biopsy from a proximal muscle of the upper limb showed typical denervation atrophy. At rest there was increased oxygen utilization and lactate output as well as a tendency for increased uptake of glucose and long chain fatty acids from arterial blood per 100 ml of forearm tissue. During exercise the abnormally high lactate output increased further. An increased arterial lactate concentration was present during rest and exercise. Oxidation of fatty acids was not impaired. It is suggested that these abnormalities are consistent with an augmented utilization of blood borne fuels at rest by denervated muscles. A concurrent regional ischemia of muscles during rest and exercise, possibly due to defective autoregulation of skeletal muscle blood flow, may explain the abnormally high lactate generation.

Effect of glucose on plasma glucagon and free fatty acids during prolonged exercise

The effects of glucose ingestion on the changes in blood glucose, FFA, insulin and glucagon levels induced by a prolonged exercise at about 50% of maximal oxygen uptake were investigated. Healthy volunteers were submitted to the following procedures: 1. a control test at rest consisting of the ingestion of 100 g glucose, 2. an exercise test without, or 3. with ingestion of 100 g of glucose. Exercise without glucose induced a progressive decrease in blood glucose and plasma insulin; plasma glucagon rose significantly from the 60th min onward (+45 pg/ml), the maximal increase being recorded during the 4th h of exercise (+135 pg/ml); plasma FFA rose significantly from the 60th min onward and reached their maximal values during the 4th h of exercise (2177 +/- 144 muEq/l, m +/- SE). Exercise with glucose ingestion blunted almost completely the normal insulin response to glucose. Under these conditions, exercise did not increase plasma glucagon before the 210th min; similarly, the exercise-induced increase in plasma FFA was markedly delayed and reduced by about 60%. It is suggested that glucose availability reduces exercise-induced glucagon secretion and, possibly consequently, FFA mobilization.

Effect of heparin on the substrate utilization during prolonged exercise

Leg and splanchnic exchange of free fatty acids (FFA) and glucose was determined during 90 min bicycle exercise after heparin administration in six healthy male volunteers, and the results compared to those obtained previously under identical exercise conditions without the administration of heparin. Heparin elicited a marked elevation of plasma FFA at rest but the exercise values were not higher than during exercise without prior heparin administration. The FFA uptake in the exercising legs was, however, augmented after heparin due to an increase in the fractional leg uptake. Leg glucose uptake and splanchnic glucose production during exercise were not affected by heparin administration. It is concluded that inhibition of glucose uptake by FFA is of minor importance in the regulation of muscle substrate utilization during exercise in man.

[In vivo incorporation of labeled palmitic and oleic acids into skeletal muscle lipids of normal and thyroidectomized rats during swimming]

The purpose of this work was to study the incorporation of [1-14C] palmitic and and [9,10(-3)H] oleic acids, after intravenous administration in the lipids of rat hind leg muscles. The animals were fasting or fed, at rest or swimming during 10 min before test, euthyroid or thyroidectomised. All these animals have been taking the same daily swimming training, during 15 days before the injection of labelled molecules. They were killed 10 min (+/-1)later. The lipidic muscle composition, the incorporation rate of labelled fatty acids in these lipids and the radioactivity distribution among the different lipids in the various cases have been determined. Moreover the plasmatic non-esterified acid radioactivity has been measured. These various values are affected by nutritional, hormonal state, and by physical activity of the animals. Particularly, it seems that supplementary energy spent during swimming test will be covered by the oxidation of different nutriments, according to the nutritional and hormonal state of animals.