Effects of beta-adrenergic receptor stimulation and blockade on substrate metabolism during submaximal exercise

A bout of aerobic exercise in the heat increases carbohydrate use but does not enhance the disposal of an oral glucose load, in healthy active individuals

We investigated if a bout of exercise in a hot environment (HEAT) would reduce the postprandial hyperglycemia induced by glucose ingestion. The hypothesis was that HEAT stimulating carbohydrate oxidation and glycogen use would increase the disposal of an ingested glucose load [i.e., oral glucose tolerance test (OGTT); 75 g of glucose]. Separated by at least 1 wk, nine young healthy individuals underwent three trials after an overnight fast in a randomized order. Two trials included 50 min of pedaling at 58 ± 5% V̇o2max either in a thermoneutral (21 ± 1°C; NEUTRAL) or in a hot environment (33 ± 1°C; HEAT) eliciting similar energy expenditure (503 ± 101 kcal). These two trials were compared with a no-exercise trial (NO EXER). Twenty minutes after exercise (or rest), subjects underwent an OGTT, while carbohydrate oxidation (CHOxid, using indirect calorimetry) plasma blood glucose, insulin concentrations (i.e., [glucose], [insulin]), and double tracer glucose kinetics ([U-13C] glucose ingestion and [6,6-2H2] glucose infusion) were monitored for 120 min. At rest, [glucose], [insulin], and rates of appearance/disappearance of glucose in plasma (glucose Ra/Rd) were similar among trials. During exercise, heart rate, tympanic temperature, [glucose], glycogen oxidation, and total CHOxid were higher during HEAT than NEUTRAL (i.e., 149 ± 35 vs. 124 ± 31 µmol·kg-1·min-1, P = 0.010). However, during the following OGTT, glucose Rd was similar in HEAT and NEUTRAL trials (i.e., 25.1 ± 3.6 vs. 25.2 ± 5.3 µmol·kg-1·min-1, P = 0.981). Insulin sensitivity (i.e., ISIndexMATSUDA) only improved in NEUTRAL compared with NO EXER (10.1 ± 4.6 vs. 8.8 ± 3.7 au; P = 0.044). In summary, stimulating carbohydrate use with exercise in a hot environment does not improve postprandial plasma glucose disposal or insulin sensitivity in a subsequent OGTT.NEW & NOTEWORTHY Exercise in the heat increases estimated muscle glycogen use. Reduced muscle glycogen after exercise in the heat could increase insulin-mediated glucose uptake during a subsequent oral glucose tolerance test (OGTT). However, plasma glucose kinetics are not improved during the OGTT in response to a bout of exercise in the heat, and insulin sensitivity worsens. Heat stress activates glucose counterregulatory hormones whose actions may linger during the OGTT, preventing increased glucose uptake.

Insulin and cancer: a tangled web

For a century, since the pioneering work of Otto Warburg, the interwoven relationship between metabolism and cancer has been appreciated. More recently, with obesity rates rising in the U.S. and worldwide, epidemiologic evidence has supported a link between obesity and cancer. A substantial body of work seeks to mechanistically unpack the association between obesity, altered metabolism, and cancer. Without question, these relationships are multifactorial and cannot be distilled to a single obesity- and metabolism-altering hormone, substrate, or factor. However, it is important to understand the hormone-specific associations between metabolism and cancer. Here, we review the links between obesity, metabolic dysregulation, insulin, and cancer, with an emphasis on current investigational metabolic adjuncts to standard-of-care cancer treatment.

Exercising with low muscle glycogen content increases fat oxidation and decreases endogenous, but not exogenous carbohydrate oxidation

BACKGROUND

Initiating aerobic exercise with low muscle glycogen content promotes greater fat and less endogenous carbohydrate oxidation during exercise. However, the extent exogenous carbohydrate oxidation increases when exercise is initiated with low muscle glycogen is unclear.

PURPOSE

Determine the effects of muscle glycogen content at the onset of exercise on whole-body and muscle substrate metabolism.

METHODS

Using a randomized, crossover design, 12 men (mean ± SD, age: 21 ± 4 y; body mass: 83 ± 11 kg; VO_2peak: 44 ± 3 mL/kg/min) completed 2 cycle ergometry glycogen depletion trials separated by 7-d, followed by a 24-h refeeding to elicit low (LOW; 1.5 g/kg carbohydrate, 3.0 g/kg fat) or adequate (AD; 6.0 g/kg carbohydrate, 1.0 g/kg fat) glycogen stores. Participants then performed 80 min of steady-state cycle ergometry (64 ± 3% VO_2peak) while consuming a carbohydrate drink (95 g glucose +51 g fructose; 1.8 g/min). Substrate oxidation (g/min) was determined by indirect calorimetry and ¹³C. Muscle glycogen (mmol/kg dry weight), pyruvate dehydrogenase (PDH) activity, and gene expression were assessed in muscle.

RESULTS

Initiating steady-state exercise with LOW (217 ± 103) or AD (396 ± 70; P < 0.05) muscle glycogen did not alter exogenous carbohydrate oxidation (LOW: 0.84 ± 0.14, AD: 0.87 ± 0.16; P > 0.05) during exercise. Endogenous carbohydrate oxidation was lower and fat oxidation was higher in LOW (0.75 ± 0.29 and 0.55 ± 0.10) than AD (1.17 ± 0.29 and 0.38 ± 0.13; all P < 0.05). Before and after exercise PDH activity was lower (P < 0.05) and transcriptional regulation of fat metabolism (FAT, FABP, CPT1a, HADHA) was higher (P < 0.05) in LOW than AD.

CONCLUSION

Initiating exercise with low muscle glycogen does not impair exogenous carbohydrate oxidative capacity, rather, to compensate for lower endogenous carbohydrate oxidation acute adaptations lead to increased whole-body and skeletal muscle fat oxidation.

Understanding the factors that effect maximal fat oxidation

Lipids as a fuel source for energy supply during submaximal exercise originate from subcutaneous adipose tissue derived fatty acids (FA), intramuscular triacylglycerides (IMTG), cholesterol and dietary fat. These sources of fat contribute to fatty acid oxidation (FAox) in various ways. The regulation and utilization of FAs in a maximal capacity occur primarily at exercise intensities between 45 and 65% VO_2max, is known as maximal fat oxidation (MFO), and is measured in g/min. Fatty acid oxidation occurs during submaximal exercise intensities, but is also complimentary to carbohydrate oxidation (CHOox). Due to limitations within FA transport across the cell and mitochondrial membranes, FAox is limited at higher exercise intensities. The point at which FAox reaches maximum and begins to decline is referred to as the crossover point. Exercise intensities that exceed the crossover point (~65% VO_2max) utilize CHO as the predominant fuel source for energy supply. Training status, exercise intensity, exercise duration, sex differences, and nutrition have all been shown to affect cellular expression responsible for FAox rate. Each stimulus affects the process of FAox differently, resulting in specific adaptions that influence endurance exercise performance. Endurance training, specifically long duration (>2 h) facilitate adaptations that alter both the origin of FAs and FAox rate. Additionally, the influence of sex and nutrition on FAox are discussed. Finally, the role of FAox in the improvement of performance during endurance training is discussed.

Fat mass localization alters fuel oxidation during exercise in normal weight women

PURPOSE

Abdominal and lower body fat mass tissues exhibit particular metabolic profiles at rest and during exercise. However, data are missing in normal weight women during exercise. The purpose of this study was to investigate the effect of low (LA/LB) and high (HA/LB) abdominal to lower body (A/LB) fat mass ratio on metabolic and hormonal responses during exercise in premenopausal normal weight women.

METHODS

After preliminary testing (V˙O2max and body composition assessment), substrate oxidation (RER, lipid, and carbohydrate oxidation rates), metabolic response (glycerol, free fatty acids, and glucose), and hormonal response (insulin, growth hormone, atrial natriuretic peptide, adrenaline, and noradrenaline) were determined during exercise (45 min at 65% of V˙O2max) in 21 premenopausal normal weight women (10 HA/LB women vs 11 LA/LB women).

RESULTS

Waist circumference was significantly higher in HA/LB women compared with LA/LB women (P < 0.01). No difference in other anthropometric characteristics, V˙O2max, and resting blood values was observed between the two groups. LA/LB subjects exhibited greater lipid oxidation rates compared with HA/LB women during exercise (P < 0.01). This occurred with lower plasma insulin (P < 0.05) and glucose (P < 0.05) concentrations and higher plasma free fatty acids (P < 0.05), glycerol (P < 0.05), growth hormone (P < 0.05), and atrial natriuretic peptide levels (P < 0.01) during exercise in the LA/LB group compared with the HA/LB group.

CONCLUSIONS

The present study demonstrated that LA/LB women exhibited an increase in whole-body lipid mobilization and use during exercise compared with HA/LB counterparts. This greater reliance on lipid as fuel metabolism during exercise could be explained by substrate availability and metabolic and hormonal responses. It appeared that LA/LB women exhibited greater metabolic flexibility during an exercise bout of 45 min at 65% of V˙O2max on cycle ergometer.

Antiischemic effects of metoprolol and the risk of carbohydrate metabolism disturbances in angina patients

Aim. To study the association between antiischemic effects (AIE) of metoprolol (MP), glucose tolerance, and insulin sensitivity in patients with stable angina (SA). Material and methods. The study included 28 male patients, aged 46-68 years, with stable effort angina, Functional Class II-III, and positive exercise stress test (EST). The time of the ST segment depression by ≥1 mm defined the threshold exercise stress time. MP in a selected dose was administered twice a day, for one month. Its hemodynamic effects were assessed by the dynamics of heart rate (HR), blood pressure (BP), and double product (DP). Glucose tolerance test (GTT) was performed at baseline (before MP administration) and after one month of MP treatment. Tissue insulin sensitivity and insulin resistance (IR) were assessed by ISI0.120 and HOMA-IR parameters, respectively. Results. AIE was registered in 57% of the patients, while 43% failed to demonstrate it. Both groups did not differ by the extent of MP impact on the levels of HR, BP, and DP. The presence or absence of AIE was linked to selected parameters of glucose metabolism. In patients with AIE, the pre-treatment levels of glucose and insulin 2 hours after glucose load were higher (p=0,028 and 0,043, respectively) and ISI1,120 values lower than in patients without AIE (p=0,023). Among participants with AIE, impaired glucose tolerance (IGT) was observed in 4 at baseline and in 8 one month later; among patients without AIE, IGT was not registered. Conclusion. For the first time, the presence of AIE during MP therapy of SA patients was linked to the decreased insulin sensitivity of peripheral tissues (ISI0.120). Paired EST with a single MP dose at baseline provides an opportunity to identify the patients with a higher risk of metabolic disturbances during the longer-term MP treatment.

Epigallocatechin-3-gallate increases maximal oxygen uptake in adult humans

Epigallocatechin-3-gallate (EGCG), a component of green tea, increases endurance performance in animals and promotes fat oxidation during cycle ergometer exercise in adult humans.

PURPOSE

We have investigated the hypothesis that short-term consumption of EGCG delays the onset of the ventilatory threshold (VT) and increases maximal oxygen uptake (VO2max).

METHODS

In this randomized, repeated-measures, double-blind study, 19 healthy adults (11 males and 8 females, age = 26 ± 2 yr (mean ± SE)) received seven placebo or seven EGCG (135-mg) pills. Forty-eight hours before data collection, participants began consuming three pills per day; the last pill was taken 2 h before exercise testing. VT and VO2max were determined from breath-by-breath indirect calorimetry data collected during continuous incremental stationary cycle ergometer exercise (20-35 W·min(-1)), from rest until volitional fatigue. Each condition/exercise test was separated by a minimum of 14 d.

RESULTS

Compared with placebo, short-term EGCG consumption increased VO2max (3.123 ± 0.187 vs 3.259 ± 0.196 L·min(-1), P = 0.04). Maximal work rate (301 ± 15 vs 301 ± 16 W, P = 0.98), maximal RER (1.21 ± 0.01 vs 1.22 ± 0.02, P = 0.27), and maximal HR were unaffected (180 ± 3 vs 180 ± 3 beats·min(-1), P = 0.87). In a subset of subjects (n = 11), maximal cardiac output (determined via open-circuit acetylene breathing) was also unaffected by EGCG (29.6 ± 2.2 vs 30.2 ± 1.4 L·min(-1), P = 0.70). Contrary to our hypothesis, EGCG decreased VO2 at VT (1.57 ± 0.11 vs 1.48 ± 0.10 L·min(-1)), but this change was not significant (P = 0.06).

CONCLUSIONS

Short-term consumption of EGCG increased VO2max without affecting maximal cardiac output, suggesting that EGCG may increase arterial-venous oxygen difference.

Aerobic fitness determines whole-body fat oxidation rate during exercise in the heat

The purpose of this study was to determine whole-body fat oxidation in endurance-trained (TR) and untrained (UNTR) subjects exercising at different intensities in the heat. On 3 occasions, 10 TR cyclists and 10 UNTR healthy subjects (peak oxygen uptake = 60 ± 6 vs. 44 ± 3 mL·kg-1·min-1; p < 0.05) exercised at 40%, 60%, and 80% peak oxygen uptake in a hot, dry environment (36 °C; 25% relative humidity). To complete the same amount of work in all 3 trials, exercise duration varied (107 ± 4, 63 ± 1, and 45 ± 0 min for 40%, 60%, and 80% peak oxygen uptake, respectively). Substrate oxidation was calculated using indirect calorimetry. Blood samples were collected at the end of exercise to determine plasma epinephrine ([EPI]plasma) and norepinephrine ([NEPI]plasma) concentrations. The maximal rate of fat oxidation was achieved at 60% peak oxygen uptake for the TR group (0.41 ± 0.01 g·min-1) and at 40% peak oxygen uptake for the UNTR group (0.28 ± 0.01 g·min-1). TR subjects oxidized absolutely (g·min-1) and relatively (% of total energy expenditure) more fat than UNTR subjects at 60% and 80% peak oxygen uptake (p < 0.05). At these exercise intensities, TR subjects also had higher [NEPI]plasma concentrations than UNTR subjects (p < 0.05). In the heat, whole-body peak fat oxidation occurs at higher relative exercise intensities in TR than in UNTR subjects (60% vs. 40% peak oxygen uptake). Moreover, TR subjects oxidize more fat than UNTR subjects when exercising at moderate to high intensities (>60% peak oxygen uptake).

Acute β-adrenergic stimulation does not alter mitochondrial protein synthesis or markers of mitochondrial biogenesis in adult men

Exercise-induced expression of peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α) is dramatically inhibited in mice pretreated with a β-adrenergic receptor (β-AR) antagonist, suggesting that β-ARs play an important role in the regulation of skeletal muscle PGC-1α expression, and potentially, mitochondrial biogenesis. Accordingly, we hypothesized that acute β-AR stimulation would induce transcriptional pathways involved in skeletal muscle mitochondrial biogenesis in humans. Whole body protein turnover (WBPT), myofibrillar protein synthesis (MyPS), skeletal muscle mitochondrial protein synthesis (MiPS), and mitochondrial biogenic signaling were determined in samples of vastus lateralis obtained on two separate occasions in 10 young adult males following 1 h of continuous intravenous administration of saline (CON) or a nonspecific β-AR agonist [isoproterenol (ISO): 12 ng·kg fat free mass−1·min−1], combined with coinfusion of [1,2]13C-leucine. β-AR stimulation induced appreciable increases in heart rate and systolic blood pressure (both P < 0.001) but did not affect mitochondrial biogenic signaling (no change in PGC-1α, TFAM, NRF-1, NRF-2, COX, or NADHox expression via RT-PCR; P > 0.05). Additionally, MiPS [CON: 0.099 ± 0.028, ISO: 0.074 ± 0.046 (mean ± SD); P > 0.05] and MyPS (CON: 0.059 ± 0.008, ISO: 0.055 ± 0.009; P > 0.05), as well as measures of WBPT were unaffected. On the basis of this investigation, we conclude that acute intravenous β-AR stimulation does not increase mitochondrial protein synthesis or biogenesis signals in skeletal muscle.

Glucose metabolism and catecholamines

Until now, catecholamines were the drugs of choice to treat hypotension during shock states. Catecholamines, however, also have marked metabolic effects, particularly on glucose metabolism, and the degree of this metabolic response is directly related to the beta2-adrenoceptor activity of the individual compound used. Under physiologic conditions, infusing catecholamine is associated with enhanced rates of aerobic glycolysis (resulting in adenosine triphosphate production), glucose release (both from glycogenolysis and gluconeogenesis), and inhibition of insulin-mediated glycogenesis. Consequently, hyperglycemia and hyperlactatemia are the hallmarks of this metabolic response. Under pathophysiologic conditions, the metabolic effects of catecholamines are less predictable because of changes in receptor affinity and density and in drug kinetics and the metabolic capacity of the major gluconeogenic organs, both resulting from the disease per se and the ongoing treatment. It is also well-established that shock states are characterized by a hypermetabolic condition with insulin resistance and increased oxygen demands, which coincide with both compromised tissue microcirculatory perfusion and mitochondrial dysfunction. This, in turn, causes impaired glucose utilization and may lead to inadequate glucose supply and, ultimately, metabolic failure. Based on the landmark studies on intensive insulin use, a crucial role is currently attributed to glucose homeostasis. This article reviews the effects of the various catecholamines on glucose utilization, both under physiologic conditions, as well as during shock states. Because, to date (to our knowledge), no patient data are available, results from relevant animal experiments are discussed. In addition, potential strategies are outlined to influence the catecholamine-induced effects on glucose homeostasis.

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O2 uptake (V̇o2 max)] were measured in eight well-trained male subjects [V̇o2 max = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in 13C was ingested: plasma glucose oxidation was computed from 13C/12C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% V̇o2 max, respectively) and 13CO2 production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% V̇o2 max [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% V̇o2 max [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% V̇o2 max. However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.

Lipid mobilization of long-distance migrant birds in vivo: the high lipolytic rate of ruff sandpipers is not stimulated during shivering

For long migrations, birds must rely on high flux capacities at all steps of lipid metabolism, from the mobilization of adipose reserves to fatty acid oxidation in flight muscle mitochondria. Substrate kinetics and indirect calorimetry were used to investigate key parameters of lipid metabolism in a highly aerobic shorebird: the ruff sandpiper Philomachus pugnax. In this study, we have quantified the effects of cold exposure because such measurements are presently impossible during flight. Lipolytic rate was monitored by continuous infusion of 2-[3H]-glycerol and lipid oxidation by respirometry. Plasma lipid concentrations (non-esterified fatty acids, neutral lipids and phospholipids) and their fatty acid composition were also measured to assess whether cold exposure causes selective metabolism of specific lipids. Results show that shivering leads to a 47% increase in metabolic rate (44.4±3.8 ml O2kg–1min–1 to 65.2±8.1 ml O2kg–1 min–1), almost solely by stimulating lipid oxidation (33.3± 3.3 ml O2 kg–1min–1 to 48.2±6.8 ml O2kg–1 min–1) because carbohydrate oxidation remains close to 11.5± 0.5 ml O2 kg–1min–1. Sandpipers support an unusually high lipolytic rate of 55–60 μmol glycerol kg–1 min–1. Its stimulation above thermoneutral rates is unnecessary during shivering when the birds are still able to re-esterify 50% of released fatty acids. No changes in plasma lipid composition were observed, suggesting that cold exposure does not lead to selective metabolism of particular fatty acids. This study provides the first measurements of lipolytic rate in migrant birds and shows that their capacity for lipid mobilization reaches the highest values measured to date in vertebrates. Extending the limits of conventional lipid metabolism has clearly been necessary to achieve long-distance migrations.

Beta-adrenergic and atrial natriuretic peptide interactions on human cardiovascular and metabolic regulation

CONTEXT

Atrial natriuretic peptide (ANP) has well-known cardiovascular effects and modifies lipid and carbohydrate metabolism in humans.

OBJECTIVE

The objective of the study was to determine the metabolic and cardiovascular interaction of beta-adrenergic receptors and ANP.

DESIGN

This was a crossover study, conducted 2004-2005.

SETTING

The study was conducted at an academic clinical research center.

PATIENTS

PATIENTS included 10 healthy young male subjects (body mass index 24 +/- 1 kg/m2).

INTERVENTION

We infused iv incremental ANP doses (6.25, 12.5, and 25 ng/kg.min) with and without propranolol (0.20 mg/kg in divided doses followed by 0.033 mg/kg.h infusion). Metabolism was monitored through venous blood sampling, im, and sc microdialysis and indirect calorimetry. Cardiovascular changes were monitored by continuous electrocardiogram and beat-by-beat blood pressure recordings.

MAIN OUTCOME MEASURES

Venous nonesterified fatty acid, glycerol, glucose, and insulin; and microdialysate glucose, glycerol, lactate, and pyruvate were measured.

RESULTS

ANP increased heart rate dose dependently. beta-Adrenergic receptor blockade abolished the response. ANP elicited a dose-dependent increase in serum nonesterified fatty acid and glycerol concentrations. The response was not suppressed with propranolol. Venous glucose and insulin concentrations increased with ANP, both without or with propranolol. ANP induced lipid mobilization in sc adipose tissue. In skeletal muscle, microdialysate lactate increased, whereas the lactate to pyruvate ratio decreased, both with and without propranolol. Higher ANP doses increased lipid oxidation, whereas energy expenditure remained unchanged. Propranolol tended to attenuate the increase in lipid oxidation.

CONCLUSIONS

Selected cardiovascular ANP effects are at least partly mediated by beta-adrenergic receptor stimulation. ANP-induced changes in lipid mobilization and glycolysis are mediated by another mechanism, presumably stimulation of natriuretic peptide receptors, whereas substrate oxidation might be modulated through adrenergic mechanisms.

Oral glucose ingestion attenuates exercise-induced activation of 5'-AMP-activated protein kinase in human skeletal muscle

5'-AMP-activated protein kinase (AMPK) has been suggested to be a 'metabolic master switch' regulating various aspects of muscle glucose and fat metabolism. In isolated rat skeletal muscle, glucose suppresses the activity of AMPK and in human muscle glycogen loading decreases exercise-induced AMPK activation. We hypothesized that oral glucose ingestion during exercise would attenuate muscle AMPK activation. Nine male subjects performed two bouts of one-legged knee-extensor exercise at 60% of maximal workload. The subjects were randomly assigned to either consume a glucose containing drink or a placebo drink during the two trials. Muscle biopsies were taken from the vastus lateralis before and after 2 h of exercise. Plasma glucose was higher (6.0 +/- 0.2 vs. 4.9 +/- 0.1 mmol L-1, P < 0.001), whereas glycerol (44.8 +/- 7.8 vs. 165.7 +/- 22.3 micromol L-1), and free fatty acid (169.3 +/- 9.5 vs. 1161 +/- 144.9 micromol L-1) concentrations were lower during the glucose compared to the placebo trial (both P < 0.001). Calculated fat oxidation was lower during the glucose trial (0.17 +/- 0.02 vs. 0.25 +/- 0.03 g min-1, P < 0.001). Activation of alpha2-AMPK was attenuated in the glucose trial compared to the placebo trial (0.24 +/- 0.07 vs. 0.46 +/- 0.14 pmol mg-1 min-1, P = 0.03), whereas the alpha1-AMPK activity was not different between trials or affected by exercise. AMPK and the downstream target of AMPK, acetyl-CoA carboxylase-beta, were phosphorylated as a response to exercise, but neither was significantly different between the two trials. We conclude that oral glucose ingestion attenuates the exercise-induced activation of alpha2-AMPK, bringing further support for a fuel-sensing role of AMPK in skeletal muscle.

Effects of eccentric exercise on cycling efficiency

The aim of this study was to find out whether the efficiency of concentric muscle contraction is impaired by eccentric squatting exercise. The study involved 25 male physical education students in two experiments. In the first experiment 14 subjects undertook cycling exercise at 65% VO(2)max until exhaustion on two occasions. During the experimental condition their cycling was interrupted every 10 min so they could perform eccentric squatting exercise, whereas in the control condition they rested seated on the bike during the interruptions. Eccentric squatting consisted of 10 series of 25 reps with a load equivalent to 150% of the subject's body mass on the shoulders. During the first experiment gross efficiency decreased (mean +/- SE) from 17.1 +/- 0.3 to 16.0 +/- 0.4%, and from 17.2 +/- 0.3 to 16.5 +/- 0.4%, between the 2nd and 9th cycling bouts of the experimental and control conditions, respectively (both p < 0.05). The reduction in cycling efficiency was similar in both conditions (p = 0.10). Blood lactate concentration [La] was higher during the experimental than in the control condition (p < 0.05), but substrate oxidation was similar. MVC was decreased similarly (25-28%) in both conditions. The 11 subjects participating in the second experiment undertook 25 reps of eccentric squatting exercise only, each with a load equivalent to 95% of his maximal voluntary contraction (MVC), repeated every 3 min until exhaustion. One hour after the end of the eccentric squatting exercise series cycling, VO(2) and gross cycling efficiency were comparable to the values observed before the eccentric exercise. Both experimental protocols with eccentric exercise elicited similar muscle soreness 2 days later; however, at this time cycling efficiency was similar to that observed prior to eccentric exercise. The interposition of cycling exercise between the eccentric exercise bouts accelerated the recovery of MVC. We conclude that eccentric exercise does not alter or has only a marginal effect on gross cycling efficiency even in presence of marked muscle soreness.

Manipulation of dietary carbohydrate and muscle glycogen affects glucose uptake during exercise when fat oxidation is impaired by beta-adrenergic blockade

We have recently reported that, during moderate intensity exercise, low muscle glycogen concentration and utilization caused by a high-fat diet is associated with a marked increase in fat oxidation with no effect on plasma glucose uptake (R(d) glucose). It is our hypothesis that this increase in fat oxidation compensates for low muscle glycogen, thus preventing an increase in R(d) glucose. Therefore, the purpose of this study was to determine whether low muscle glycogen availability increases R(d) glucose under conditions of impaired fat oxidation. Six cyclists exercised at 50% peak O(2) consumption (Vo(2 peak)) for 1 h after 2 days on either a high-fat (HF, 60% fat, 24% carbohydrate) or control (CON, 22% fat, 65% carbohydrate) diet to manipulate muscle glycogen to low and normal levels, respectively. Two hours before the start of exercise, subjects ingested 80 mg of propanolol (betaB), a nonselective beta-adrenergic receptor blocker, to impair fat oxidation during exercise. HF significantly decreased calculated muscle glycogen oxidation (P < 0.05), and this decrease was partly compensated for by an increase in fat oxidation (P < 0.05), accompanied by an increase in whole body lipolysis (P < 0.05), despite the presence of betaB. Although HF increased fat oxidation, plasma glucose appearance rate, R(d) glucose, and glucose clearance rate were also significantly increased by 13, 15, and 26%, respectively (all P < 0.05). In conclusion, when lipolysis and fat oxidation are impaired, in this case by betaB, fat oxidation cannot completely compensate for a reduction in muscle glycogen utilization, and consequently plasma glucose turnover increases. These findings suggest that there is a hierarchy of substrate compensation for reduced muscle glycogen availability after a high-fat, low-carbohydrate diet, with fat being the primary and plasma glucose the secondary compensatory substrate. This apparent hierarchy likely serves to protect against hypoglycemia when endogenous glucose availability is low.

Excess body fat in men decreases plasma fatty acid availability and oxidation during endurance exercise

The effect of relative body fat mass on exercise-induced stimulation of lipolysis and fatty acid oxidation was evaluated in 15 untrained men (5 lean, 5 overweight, and 5 obese with body mass indexes of 21 +/- 1, 27 +/- 1, and 34 +/- 1 kg/m2, respectively, and %body fat ranging from 12 to 32%). Palmitate and glycerol kinetics and substrate oxidation were assessed during 90 min of cycling at 50% peak aerobic capacity (VO2 peak) by use of stable isotope-labeled tracer infusion and indirect calorimetry. An inverse relationship was found between %body fat and exercise-induced increase in glycerol appearance rate relative to fat mass (r2 = 0.74; P < 0.01). The increase in total fatty acid uptake during exercise [(micromol/kg fat-free mass) x 90 min] was approximately 50% smaller in obese (181 +/- 70; P < 0.05) and approximately 35% smaller in overweight (230 +/- 71; P < 0.05) than in lean (354 +/- 34) men. The percentage of total fatty acid oxidation derived from systemic plasma fatty acids decreased with increasing body fat, from 49 +/- 3% in lean to 39 +/- 4% in obese men (P < 0.05); conversely, the percentage of nonsystemic fatty acids, presumably derived from intramuscular and possibly plasma triglycerides, increased with increasing body fat (P < 0.05). We conclude that the lipolytic response to exercise decreases with increasing adiposity. The blunted increase in lipolytic rate in overweight and obese men compared with lean men limits the availability of plasma fatty acids as a fuel during exercise. However, the rate of total fat oxidation was similar in all groups because of a compensatory increase in the oxidation of nonsystemic fatty acids.

High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise

This study determined the role of intramuscular triglyceride (IMTG) and adipose lipolysis in the elevated fat oxidation during exercise caused by a high-fat diet. In four separate trials, six endurance-trained cyclists exercised at 50% peak O2 consumption for 1 h after a two-day control diet (22% fat, CON) or an isocaloric high-fat diet (60% fat, HF) with or without the ingestion of acipimox, an adipose lipolysis inhibitor, before exercise. During exercise, HF elevated fat oxidation by 72% and whole body lipolysis [i.e., the appearance rate of glycerol in plasma (Ra glycerol)] by 79% compared with CON (P < 0.05), and this was associated with a 36% increase (P < 0.05) in preexercise IMTG concentration. Although acipimox lowered plasma free fatty acid (FFA) availability, HF still increased fat oxidation and Ra glycerol to the same magnitude above control as the increase caused by HF without acipimox (i.e., both increased fat oxidation 13-14 micromol.kg(-1).min(-1)). In conclusion, the marked increase in fat oxidation after a HF diet is associated with elevated IMTG concentration and whole body lipolysis and does not require increased adipose tissue lipolysis and plasma FFA concentration during exercise. This suggests that altered substrate storage in skeletal muscle is responsible for increased fat oxidation during exercise after 2 days of an HF diet.

Gender differences in the regulation of amino acid metabolism

Exercising men, compared with women, have a greater increase in leucine oxidation but not lysine rate of appearance. The cause for this sexual dimorphism is unknown; however, an inhibition of beta-adrenoreceptor activity has previously been shown to mediate amino acid metabolism (Lamont LS, McCullough AJ, and Kalhan SC. Am J Physiol Endocrinol Metab 268: E910-E916, 1995; Lamont LS, Patel DG, and Kalhan SC. J Appl Physiol 67: 221-225, 1989). This study was a gender comparison of leucine and lysine kinetics during a beta-adrenoreceptor blockade (beta1,beta2-blockade) and a placebo control by using a double-blind crossover protocol. Subjects exercised at 50% of their trial-specific maximal O2 consumption (1 h) after 7 days of dietary control. During exercise with beta-blockade, men had an increased nonprotein respiratory exchange ratio (P < 0.001), whereas women had an increased circulation of free fatty acids (P < 0.001). The genders also displayed distinct differences in exercise amino acid kinetics. The men, but not the women, increased leucine oxidation (P < 0.005) and lysine rate of appearance (P < 0.009) when exercising during beta-adrenergic blockade. This study indicates that during beta-blockade, exercising men increase their need for amino acids (and carbohydrate) to fuel energy needs, whereas women increase their mobilization of fat, thereby requiring less alternative fuels such as carbohydrate and amino acids. Gender-specific fuel preferences during exercise are regulated by beta-adrenergic-receptor activity. Substrate availability during exercise appears to modulate the amino acid oxidation differences between genders.

Systemic adverse effects of beta-adrenergic blockers: an evidence-based assessment

PURPOSE

To present an evidence-based review of the systemic adverse effects of beta-adrenergic blockers and recommend safety guidelines for use of ophthalmic beta-adrenergic blockers.

DESIGN

Literature review of published articles in peer-reviewed journals and medical texts.

METHODS

Pre-MEDLINE and MEDLINE search of relevant English language articles from 1966 to the present. Cardiovascular, pulmonary, endocrine/metabolic, central nervous system, sexual, exercise, and neuromuscular effects of systemic or ophthalmic beta-adrenergic blockers were reviewed.

RESULTS

Systemic beta-adrenergic antagonists unequivocally reduce mortality in patients with mild, moderate, and even severe congestive heart failure. Development of symptomatic bradycardia on systemic or ophthalmic beta-adrenergic blockers alone likely indicates underlying cardiac conduction disturbances. Beta 2-adrenergic blockade, regardless of route of administration, may exacerbate or trigger bronchospasm in patients with asthma or pulmonary disease associated with hyper-reactive airways. This review identifies no scientific studies supporting the development of worsening claudication, depression, hypoglycemic unawareness, or prolonged hypoglycemia in non-insulin-dependent diabetes, sexual dysfunction, or impaired neuromuscular transmission with systemic or ophthalmic beta-adrenergic blockers.

CONCLUSIONS

Many commonly presumed adverse beta-adrenergic blocker effects observed via systemic or ocular administration are not supported by published randomized clinical trials. Wide acceptance of such traditionally purported side effects has been largely due to propagation of isolated case reports and short series as well as personal communication felt to reflect expert opinion. Many more patients may be eligible to use these drugs. Obtaining a careful medical history and checking pulse rate and rhythm in the office should identify the vast majority of patients with potential cardiopulmonary contraindications.

Propranolol prevents epinephrine from limiting insulin-stimulated muscle glucose uptake during contraction

Beta-blockade results in rapid glucose clearance and premature fatigue during exercise. To investigate the cause of this increased glucose clearance, we studied the acute effects of propranolol on insulin-stimulated muscle glucose uptake during contraction in the presence of epinephrine with an isolated rat muscle preparation. Glucose uptake increased in both fast- (epitrochlearis) and slow-twitch (soleus) muscle during insulin or contraction stimulation. In the presence of 24 nM epinephrine, glucose uptake during contraction was completely suppressed when insulin was present. This suppression of glucose uptake by epinephrine was accompanied by a decrease in insulin receptor substrate (IRS)-1-phosphatidylinositol 3 (PI3)-kinase activity. Propranolol had no direct effect on insulin-stimulated glucose uptake during contraction. However, epinephrine was ineffective in attenuating insulin-stimulated glucose uptake during contraction in the presence of propranolol. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake during contraction occurred in conjunction with its inability to completely suppress IRS-1-PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake during contraction is severely diminished in muscle exposed to propranolol. Thus the increase in glucose clearance and premature fatigue associated with beta-blockade could result from the inability of epinephrine to attenuate insulin-stimulated muscle glucose uptake.

Effect of gender on lipid kinetics during endurance exercise of moderate intensity in untrained subjects

We evaluated lipid metabolism during 90 min of moderate-intensity (50% VO(2) peak) cycle ergometer exercise in five men and five women who were matched on adiposity (24 +/- 2 and 25 +/- 1% body fat, respectively) and aerobic fitness (VO(2) peak: 49 +/- 2 and 47 +/- 1 ml x kg fat-free mass(-1) x min(-1), respectively). Substrate oxidation and lipid kinetics were measured by using indirect calorimetry and [(13)C]palmitate and [(2)H(5)]glycerol tracer infusion. The total increase in glycerol and free fatty acid (FFA) rate of appearance (R(a)) in plasma during exercise (area under the curve above baseline) was approximately 65% greater in women than in men (glycerol R(a): 317 +/- 40 and 195 +/- 33 micromol/kg, respectively; FFA R(a): 652 +/- 46 and 453 +/- 70 micromol/kg, respectively; both P < 0.05). Total fatty acid oxidation was similar in men and women, but the relative contribution of plasma FFA to total fatty acid oxidation was higher in women (76 +/- 5%) than in men (46 +/- 5%; P < 0.05). We conclude that lipolysis of adipose tissue triglycerides during moderate-intensity exercise is greater in women than in men, who are matched on adiposity and fitness. The increase in plasma fatty acid availability leads to a greater rate of plasma FFA tissue uptake and oxidation in women than in men. However, total fat oxidation is the same in both groups because of a reciprocal decrease in the oxidation rate of fatty acids derived from nonplasma sources, presumably intramuscular and possibly plasma triglycerides, in women.