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Biomedical subjects

M Hargreaves

Publications and source records attributed to M Hargreaves.

At least 73 records · Page 4Linked to original sources
PubMed1994

Muscle metabolism during exercise and heat stress in trained men: effect of acclimation.

Exercise metabolism was examined in 13 endurance athletes who exercised on three occasions for 40 min at 70% of maximal O2 uptake in an environmental chamber at either 20 degrees C and 20% relative humidity (RTT) or 40 degrees C and 20% relative humidity before (PRE ACC) or after (POST ACC) 7 days of acclimation. Exercise in the heat resulted in a lower (P < 0.05) mean O2 uptake (0.13 l/min) and higher (P < 0.01) heart rate and respiratory exchange ratio. Acclimation resulted in a lower (P < 0.01) mean heart rate and respiratory exchange ratio. Postexercise rectal temperature, muscle temperature, muscle and blood lactate, and blood glucose were higher (P < 0.01) in the PRE ACC than in the RTT trial, but all were reduced (P < 0.01) in the POST ACC compared with the PRE ACC trial. Muscle glycogenolysis and percentage of type I muscle fibers showing glycogen depletion were greater (P < 0.05) in the PRE ACC than in the RTT trial. Muscle glycogenolysis was unaffected by acclimation during exercise in the heat, although the percentage of depleted type I fibers was higher (P < 0.05) in the unacclimated state. Plasma epinephrine was higher (P < 0.01) during exercise in the heat in the unacclimated individual relative to RTT but was lower (P < 0.01) in the POST ACC than in the PRE ACC trial. The greater reliance on carbohydrate as a fuel source during exercise in the heat appears to be partially reduced after acclimation. These alterations are consistent with the observed changes in plasma epinephrine concentrations.

Acclimatization↗
PubMed1994

Effect of carbohydrate ingestion on glucose kinetics during exercise.

Six well-trained men (peak pulmonary oxygen uptake = 5.03 +/- 0.11 l/min) were studied during 2 h of exercise at 69 +/- 1% peak pulmonary oxygen uptake to examine the effect of carbohydrate (CHO) ingestion on glucose kinetics. Subjects ingested 250 ml of either a 10% glucose solution containing 6-[3H]glucose (CHO) or a sweet placebo every 15 min during exercise. Glucose kinetics were assessed by 6,6-[2H]glucose infusion corrected for gut-derived glucose in CHO. Plasma glucose was higher (P < 0.05) in CHO from 20 min. Total glucose appearance was higher in CHO due to glucose delivery from the gut (68 +/- 7 g), since hepatic glucose production was reduced by 51% (29 +/- 5 vs. 59 +/- 5 g). Glucose uptake was higher in CHO (96 +/- 7 vs. 60 +/- 6 g) with the ingested glucose supplying 67 +/- 4 g and, with the assumption that it was fully oxidized, accounted for 14 +/- 1% of total energy expenditure. In conclusion, CHO ingestion during prolonged exercise results in suppression of hepatic glucose production and increased glucose uptake. These effects appear to be mediated mainly by increased plasma glucose and insulin levels.

Administration, Oral↗
PubMed1994

Skeletal muscle GLUT-4 and glucose uptake during exercise in humans.

The present study examined the relationship between total skeletal muscle GLUT-4 protein level and glucose uptake during exercise. Eight active non-endurance-trained men cycled at 72 +/- 1% peak pulmonary oxygen consumption for 40 min, with rates of glucose appearance and disappearance (Rd) determined by utilizing a primed continuous infusion of [3-3H]glucose commencing 2 h before exercise. Muscle glycogen content and utilization, citrate synthase activity, and total GLUT-4 protein were measured on muscle biopsy samples obtained from the vastus lateralis. A direct relationship existed between preexercise muscle glycogen content and glycogen utilization during exercise (r = 0.76, P < 0.05). Citrate synthase activity and glucose Rd at the end of exercise averaged 21.9 +/- 3.0 mumol.min-1.g-1 and 27.3 +/- 2.5 mumol.kg-1.min-1, respectively. There was a direct correlation between citrate synthase activity and GLUT-4 protein (r = 0.78, P < 0.05); however, at the end of exercise, glucose Rd was inversely related to both GLUT-4 (r = -0.89, P < 0.01) and citrate synthase activity (r = -0.72, P < 0.05). Plasma insulin, which decreased during exercise, was not related to glucose Rd. In conclusion, glucose uptake during 40 min of exercise at 72% peak pulmonary oxygen consumption was inversely related to the total muscle GLUT-4 protein level. This suggests that factors other than the total GLUT-4 protein level are important in the regulation of glucose uptake during exercise.

Adult↗
PubMed1994

Effect of heat stress on muscle energy metabolism during exercise.

To examine the effect of heat stress on muscle energy metabolism during submaximal exercise, 12 endurance-trained men cycled on two occasions for approximately 40 min at 70% maximal O2 uptake in an environmental chamber at either 20 degrees C and 20% relative humidity (T20) or 40 degrees C and 20% relative humidity (T40). Trials were conducted > or = 1 wk apart in random order. No difference in mean O2 uptake was observed when exercise in T40 was compared with that in T20. In contrast, exercise in T40 resulted in a higher mean heart rate (P < 0.01) and respiratory exchange ratio (P < 0.05) compared with that in T20. Postexercise rectal and muscle temperatures were also higher (P < 0.01) in T40 than in T20. Lower (P < 0.01) postexercise creatine phosphate and higher creatine (P < 0.01) and ammonia (P < 0.05) were observed in muscle after exercise in T40 compared with T20. In addition, an increased (P < 0.01) muscle glycogenolysis and higher (P < 0.01) postexercise muscle lactate accumulation were observed during exercise in T40 compared with T20. In contrast, no differences were observed in postexercise concentrations of total adenine nucleotide pool (ATP+ADP+AMP), ATP/ADP ratio, or inosine 5'-monophosphate (IMP) when T40 was compared with T20. These results indicate that the rate of ATP utilization may be increased during exercise in the heat but that this increased energy demand is predominantly met by an increase in anaerobic glycolysis and creatine phosphate hydrolysis, preventing a reduction in total adenine nucleotide pool.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Carbohydrate and lipid requirements of soccer.

Soccer involves high-intensity, intermittent exercise and places a heavy demand on the body's liver and muscle glycogen reserves. Thus, carbohydrate depletion may contribute, in part, to fatigue and reduced performance during a soccer match. In addition, fatty acids derived from both adipose tissue and intramuscular triglyceride stores will be utilized. Soccer players engaged in strenuous competition and training should be encouraged to consume a diet that is relatively high in carbohydrate (at least 55% of total energy intake). Furthermore, the inclusion of carbohydrate in beverages consumed during and after a soccer match is likely to enhance performance and facilitate the recovery of liver and muscle glycogen reserves.

Dietary Carbohydrates↗
PubMed1994

Influence of sodium on glucose bioavailability during exercise.

To examine the influence of beverage sodium content on glucose bioavailability during exercise, six trained men were studied during 30 min of cycle ergometer exercise at 65% VO2max. Immediately prior to exercise, subjects ingested 400 ml of a 10% glucose solution containing 100 microCi of D-(3-3H]-glucose, with a sodium concentration of either 0, 25, or 50 mmol.l-1. Trials were conducted in the morning after an overnight fast and in randomized order at least 1 wk apart. Blood samples were obtained from a forearm vein before and during exercise and subsequently analyzed for plasma glucose and 3H-glucose activity and blood lactate. No differences in oxygen uptake, heart rate, or blood lactate were observed between trials. Resting plasma glucose levels were not different between trials. The increases in plasma glucose and the plasma accumulation of 3H-glucose were similar in the three trials. These results indicate that alterations in beverage sodium content, from 0-50 mmol.l-1, have no effect on glucose bioavailability, as measured by increases in plasma glucose and 3H-glucose activity during moderate intensity exercise.

Adult↗
PubMed1993

Heat stress increases ammonia accumulation during exercise in humans.

Seven men were studied during 40 min of exercise at 70% VO2 peak, in an environmental chamber maintained at either 20 or 40 degrees C, to examine the effect of heat stress on ammonia metabolism during exercise. Heart rate and rectal and muscle temperatures were higher during exercise in the heat, while no differences were observed in pulmonary oxygen uptake or respiratory exchange ratio. Plasma ammonia levels and muscle ammonia accumulation were higher during exercise at 40 degrees C compared with 20 degrees C. Such metabolic alterations may be associated with reduced performance during exercise in the heat.

Adenosine Diphosphate↗
PubMed1993

Effect of bradykinin on respiratory rate in anaesthetized rabbits; role of rapidly adapting receptors.

1. This study was performed in anaesthetized, spontaneously breathing rabbits: (a) to determine the effect of bradykinin administered into the right atrium on the respiratory rate, and (b) to elucidate the potential role of rapidly adapting receptors (RARs) in mediating this effect. The role of RARs was established by graded cooling of the cervical vagi. The respiratory rate was measured from an intrapleural pressure tracing. 2. Dose-response curves relating right atrial injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) to the respiratory rate were established in the control state (i.e. vagi at 37 degrees C). The respiratory rate increased significantly (P < 0.01, ANOVA) from a control value of 51.3 +/- 6.8 breaths/min by 12 +/- 3, 25 +/- 5, 43 +/- 7 and 58 +/- 11% respectively. At doses of 1.0 and 1.5 micrograms/kg I.V., the increase in rate was preceded by apnoea. 3. The dose-response curves were repeated with bolus injections of bradykinin (0.25, 0.5, 1.0 and 1.5 micrograms/kg) after cooling the cervical vagi to 8-9 degrees C. The increase in respiratory rate was attenuated significantly (P < 0.01 ANOVA). The rate increased from a control value of 27.2 +/- 2.1 breaths/min by 5 +/- 2, 6 +/- 2, 16 +/- 5 and 21 +/- 8% respectively. With vagi cooled, apnoea was increased in duration and occurred at lower doses. On rewarming vagi, the original responses were reestablished. 4. When the study was repeated after bilateral vagotomy, apnoea was abolished but there was a small residual increase in rate. This increase was similar to that seen after cooling the vagi (P > 0.05). 5. RAR (n = 5) activity was recorded from the cervical vagus. Right atrial injections of bradykinin (0.25-1.0 micrograms/kg) stimulated RARs. On cooling the vagi to 8-9 degrees C caudal to the recording site, the increase in activity was blocked. 6. These data support the proposition that bradykinin increases the respiratory rate in rabbits and that this response is, in part, a reflex mediated by RARs. In addition, bradykinin has other secondary effects on respiration: an aponea which is mediated by non-myelinated vagal afferents and a small stimulatory effect on respiration which persists after bilateral vagotomy.

Adaptation, Physiological↗
PubMed1993

Sprint training increases human skeletal muscle Na(+)-K(+)-ATPase concentration and improves K+ regulation.

This study investigated the effects of sprint training on muscle Na(+)-K(+)-adenosinetriphosphatase (ATPase) concentration, plasma [K+] regulation, muscle performance, and fatigue during severe intermittent exercise. Six untrained male subjects underwent intensive cycle-sprint training for 7 wk. Muscle biopsies were taken at rest from the vastus lateralis muscle before and after 7 wk of training and were assayed for Na(+)-K(+)-ATPase concentration using vanadate-facilitated [3H]ouabain binding to intact samples. Before and after the training period, subjects performed four maximal 30-s exercise bouts (EB) on a cycle ergometer, each separated by a 4-min recovery. Arterialized venous blood samples were drawn immediately before and after each sprint bout and were analyzed for plasma [K+]. The work output was significantly elevated (11%) across all four EBs after training. The muscle [3H]ouabain binding site concentration was significantly increased (16%) from 333 +/- 19 to 387 +/- 15 (SE) pmol/g wet wt after training but was unchanged in muscle obtained from three control subjects. Plasma [K+] rose by 1-2 mmol/l with each EB and declined rapidly by the end of each recovery period. The increases in plasma [K+] resulting from each EB were significantly lower (19%) after training. The ratios of rise in plasma [K+] relative to work output during each EB were also significantly lower (27%) after training. The increased muscle [3H]ouabain binding site concentration and the reduced ratio of rise in [K+] relative to work output with exercise are both consistent with improved plasma and skeletal muscle K+ regulation after sprint training.

Adolescent↗
PubMed1993

Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings.

The effect of the glycemic index (GI) of postexercise carbohydrate intake on muscle glycogen storage was investigated. Five well-trained cyclists undertook an exercise trial to deplete muscle glycogen (2 h at 75% of maximal O2 uptake followed by four 30-s sprints) on two occasions, 1 wk apart. For 24 h after each trial, subjects rested and consumed a diet composed exclusively of high-carbohydrate foods, with one trial providing foods with a high GI (HI GI) and the other providing foods with a low GI (LO GI). Total carbohydrate intake over the 24 h was 10 g/kg of body mass, evenly distributed between meals eaten 0, 4, 8, and 21 h postexercise. Blood samples were drawn before exercise, immediately after exercise, immediately before each meal, and 30, 60, and 90 min post-prandially. Muscle biopsies were taken from the vastus lateralis immediately after exercise and after 24 h. When the effects of the immediate postexercise meal were excluded, the totals of the incremental glucose and insulin areas after each meal were greater (P < or = 0.05) for the HI GI meals than for the LO GI meals. The increase in muscle glycogen content after 24 h of recovery was greater (P = 0.02) with the HI GI diet (106 +/- 11.7 mmol/kg wet wt) than with the LO GI diet (71.5 +/- 6.5 mmol/kg). The results suggest that the most rapid increase in muscle glycogen content during the first 24 h of recovery is achieved by consuming foods with a high GI.

Adolescent↗
PubMed1993

Carbohydrate ingestion during exercise: liquid vs solid feedings.

Six trained men were studied during 2 h of exercise at 65% VO2max to examine the influence of the physical form of carbohydrate supplementation on blood glucose and insulin responses. Three exercise tests were performed in randomized order, at least 1 wk apart. Subjects ingested 25 g of carbohydrate or 500 ml of a sweet placebo (CON) at 0, 30, 60, and 90 min of exercise. Carbohydrate was ingested as 500 ml of a 5% rice-based liquid (L-CHO) or as a 31-g food bar (S-CHO). No differences in oxygen uptake, respiratory exchange ratio, or heart rate were observed between trials. Blood glucose levels were higher (P < 0.05) throughout exercise when carbohydrate was ingested, compared with CON. No differences in blood glucose during exercise were observed between L-CHO and S-CHO. Plasma insulin levels were higher (P < 0.05) after 120 min of exercise when carbohydrate was ingested. The results of this study indicate that carbohydrate supplements with differing physical form (liquid vs solid) but equal carbohydrate content produce similar blood glucose and insulin responses during exercise.

Adult↗
PubMed1992

Responses of airway rapidly adapting receptors to bradykinin before and after administration of enalapril in rabbits.

1. The present study was performed in anaesthetized, artificially ventilated, open-chested rabbits to examine whether (a) the rapidly adapting receptors of the airways were stimulated by exogenously administered bradykinin, and (b) if this sensitivity could be enhanced by the angiotensin-converting-enzyme inhibitor, enalapril. 2. Rapidly adapting receptor activity (n = 8) was recorded from the cervical vagus. Bradykinin was injected intravenously (0.25-1.0 microgram/kg) and a dose-response curve relating receptor activity to bradykinin was elicited. In the control state, the threshold dose of bradykinin required for stimulation of rapidly adapting receptors was 0.53 +/- 0.11 microgram/kg. Five minutes after the administration of enalapril maleate (2 mg intravenously), the dose-response curve was shifted to the left significantly (P < 0.01). 3. In seven other rapidly adapting receptors, enalapril (2 mg) increased the resting activity significantly (P < 0.05) over a period of 60 min. This increase was significantly different from the spontaneous variation in neural activity of rapidly adapting receptors (n = 7) recorded over a period of 60 min. 4. Bradykinin either alone (0.25-1.0 microgram/kg) or in the presence of enalapril did not stimulate the slowly adapting receptors (n = 5) of the airways. 5. These results show that (a) exogenous bradykinin stimulates the rapidly adapting receptors, (b) the sensitivity of rapidly adapting receptors to bradykinin is enhanced by enalapril and (c) enalapril increases the resting activity of rapidly adapting receptors. It is suggested that the cough reported after the administration of enalapril may be due to stimulation of rapidly adapting receptors of the airways.

Action Potentials↗
PubMed1992

Effect of arm-cranking on leg blood flow and noradrenaline spillover during leg exercise in man.

Controversy exists whether recruitment of a large muscle mass in dynamic exercise may outstrip the pumping capacity of the heart and require neurogenic vasoconstriction in exercising muscle to prevent a fall in arterial blood pressure. To elucidate this question, seven healthy young men cycled for 70 minutes at a work load of 55-60% VO2max. At 30 to 50 minutes, arm cranking was added and total work load increased to (mean +/- SE) 82 +/- 4% of VO2max. During leg exercise, leg blood flow average 6.15 +/- .511 minutes-1, mean arterial blood pressure 137 +/- 4 mmHg and leg conductance 42.3 +/- 2.2 ml minutes-1 mmHg-1. When arm cranking was added to leg cycling, leg blood flow did not change significantly, mean arterial blood pressure increased transiently to 147 +/- 5 mmHg and leg vascular conductance decreased transiently to 33.5 +/- 3.1 ml minutes-1 mmHg-1. Furthermore, arm cranking doubled leg noradrenaline spillover. When arm cranking was discontinued and leg cycling continued, leg blood flow was unchanged but mean arterial blood pressure decreased to values significantly below those measured in the first leg exercise period. Furthermore, leg vascular conductance increased transiently, and noradrenaline spillover decreased towards values measured during the first leg exercise period. It is concluded that addition of arm cranking to leg cycling increases leg noradrenaline spillover and decreases leg vascular conductance but leg blood flow remains unchanged because of a simultaneous increase in mean arterial blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Muscle glycogen and glucose uptake during exercise in humans.

Six men were studied during 40 min of cycling exercise to examine the relationship between leg glucose uptake and muscle glycogen concentration. Exercise resulted in significant increases in leg glucose uptake, while muscle glycogen and arterial blood glucose concentrations declined. Arterial plasma insulin levels did not change significantly. There was a significant inverse relationship between muscle glycogen concentration and glucose uptake during exercise which suggests a possible regulatory influence of muscle glycogen on glucose uptake in the early stages of exercise in humans.

Adult↗
PubMed1992

Potassium and ventilation during incremental exercise in trained and untrained men.

The present investigation was undertaken to examine the relationship between plasma potassium (K+) and ventilation (VE) during incremental exercise. Blood lactate (La-) was also measured, and its relationship with VE was similarly examined. Eight endurance-trained triathletes (ET) and eight active but untrained men (UT) performed an incremental cycling test to volitional fatigue. Maximal oxygen uptake (VO2max) and oxygen uptake (VO2) at lactate threshold (LT) were higher (P < 0.05) in ET (VO2max 4.60 +/- 0.10 l/min, LT 2.77 +/- 0.85 l/min) than in UT (VO2max 3.79 +/- 0.11 l/min, LT 1.94 +/- 0.60 l/min). There were significant (P < 0.05) correlations between VE and K+ (UT 0.87, ET 0.77) and between VE and La- (UT 0.88, ET 0.85). In ET compared with UT, VE was lower (P < 0.05) at 330 W, K+ was lower at 300 and 330 W, and La- was lower at all work loads > 90 W. These results suggest that K+ may make an important contribution to the regulation of ventilation during incremental exercise and that endurance training attenuates the K+ response to that exercise.

Adult↗
PubMed1992

Carbohydrate nutrition and fatigue.

Carbohydrates are important substrates for contracting muscle during prolonged, strenuous exercise, and fatigue is often associated with muscle glycogen depletion and/or hypoglycaemia. Thus, the goals of carbohydrate nutritional strategies before, during and after exercise are to optimise the availability of muscle and liver glycogen and blood glucose, with a view to maintaining carbohydrate availability and oxidation during exercise. During heavy training, the carbohydrate requirements of athletes may be as high as 8 to 10 g/kg bodyweight or 60 to 70% of total energy intake. Ingestion of a diet high in carbohydrate should be encouraged in order to maintain carbohydrate reserves and the ability to train intensely. Ingestion of a high carbohydrate meal 3 to 4 hours prior to exercise ensures adequate carbohydrate availability and enhances exercise performance. Although hyperinsulinaemia associated with carbohydrate ingestion in the hour prior to exercise may result in some metabolic alterations during exercise, it may not necessarily impair exercise performance and may, in some cases, enhance performance. Carbohydrate ingestion during prolonged, strenuous exercise, where performance is often limited by carbohydrate availability, delays fatigue. This is due to maintenance of blood glucose levels and a high rate of carbohydrate oxidation, rather than a slowing of muscle glycogen utilisation, although liver glycogen reserves may be spared. During recovery from exercise, muscle glycogen resynthesis is critically dependent upon the ingestion of carbohydrate. Factors influencing the rate of muscle glycogen resynthesis include the timing, amount and type of carbohydrate ingested and muscle damage. Adequate carbohydrate availability before, during and after exercise will maintain carbohydrate oxidation during exercise and is associated with enhanced exercise performance.

Blood Glucose↗