Hematological Adaptations to Prolonged Heat Acclimation in Endurance-Trained Males.

Heat acclimation is associated with plasma volume (PV) expansion that occurs within the first week of exposure. However, prolonged effects on hemoglobin mass (Hbmass) are unclear as intervention periods in previous studies have not allowed sufficient time for erythropoiesis to manifest. Therefore, Hbmass, intravascular volumes, and blood volume (BV)-regulating hormones were assessed with 5½ weeks of exercise-heat acclimation (HEAT) or matched training in cold conditions (CON) in 21 male cyclists [(mean ± SD) age: 38 ± 9 years, body weight: 80.4 ± 7.9 kg, VO2peak: 59.1 ± 5.2 ml/min/kg]. HEAT (n = 12) consisted of 1 h cycling at 60% VO2peak in 40°C for 5 days/week in addition to regular training, whereas CON (n = 9) trained exclusively in cold conditions (<15°C). Before and after the intervention, Hbmass and intravascular volumes were assessed by carbon monoxide rebreathing, while reticulocyte count and BV-regulating hormones were measured before, after 2 weeks and post intervention. Total training volume during the intervention was similar (p = 0.282) between HEAT (509 ± 173 min/week) and CON (576 ± 143 min/week). PV increased (p = 0.004) in both groups, by 303 ± 345 ml in HEAT and 188 ± 286 ml in CON. There was also a main effect of time (p = 0.038) for Hbmass with +34 ± 36 g in HEAT and +2 ± 33 g in CON and a tendency toward a higher increase in Hbmass in HEAT compared to CON (time × group interaction: p = 0.061). The Hbmass changes were weakly correlated to alterations in PV (r = 0.493, p = 0.023). Reticulocyte count and BV-regulating hormones remained unchanged for both groups. In conclusion, Hbmass was slightly increased following prolonged training in the heat and although the mechanistic link remains to be revealed, the increase could represent a compensatory response in erythropoiesis secondary to PV expansion.

Changes in metabolism but not myocellular signaling by training with CHO-restriction in endurance athletes.

Carbohydrate (CHO) restricted training has been shown to increase the acute training response, whereas less is known about the acute effects after repeated CHO restricted training. On two occasions, the acute responses to CHO restriction were examined in endurance athletes. Study 1 examined cellular signaling and metabolic responses after seven training-days including CHO manipulation (n = 16). The protocol consisted of 1 h high-intensity cycling, followed by 7 h recovery, and 2 h of moderate-intensity exercise (120SS). Athletes were randomly assigned to low (LCHO: 80 g) or high (HCHO: 415 g) CHO during recovery and the 120SS. Study 2 examined unaccustomed exposure to the same training protocol (n = 12). In Study 1, muscle biopsies were obtained at rest and 1 h after 120SS, and blood samples drawn during the 120SS. In Study 2, substrate oxidation and plasma glucagon were determined. In Study 1, plasma insulin and proinsulin C-peptide were higher during the 120SS in HCHO compared to LCHO (insulin: 0 min: +37%; 60 min: +135%; 120 min: +357%, P = 0.05; proinsulin C-peptide: 0 min: +32%; 60 min: +52%; 120 min: +79%, P = 0.02), whereas plasma cholesterol was higher in LCHO (+15-17%, P = 0.03). Myocellular signaling did not differ between groups. p-AMPK and p-ACC were increased after 120SS (+35%, P = 0.03; +59%, P = 0.0004, respectively), with no alterations in p-p38, p-53, or p-CREB. In Study 2, glucagon and fat oxidation were higher in LCHO compared to HCHO during the 120SS (+26-40%, P = 0.03; +44-76%, P = 0.01 respectively). In conclusion, the clear respiratory and hematological effects of CHO restricted training were not translated into superior myocellular signaling after accustomization to CHO restriction.

Determinants of maximal whole-body fat oxidation in elite cross-country skiers: Role of skeletal muscle mitochondria.

Elite endurance athletes possess a high capacity for whole-body maximal fat oxidation (MFO). The aim was to investigate the determinants of a high MFO in endurance athletes. The hypotheses were that augmented MFO in endurance athletes is related to concomitantly increments of skeletal muscle mitochondrial volume density (MitoVD ) and mitochondrial fatty acid oxidation (FAOp ), that is, quantitative mitochondrial adaptations as well as intrinsic FAOp per mitochondria, that is, qualitative adaptations. Eight competitive male cross-country skiers and eight untrained controls were compared in the study. A graded exercise test was performed to determine MFO, the intensity where MFO occurs (FatMax ), and V ˙ O 2 Max . Skeletal muscle biopsies were obtained to determine MitoVD (electron microscopy), FAOp , and OXPHOSp (high-resolution respirometry). The following were higher (P < 0.05) in endurance athletes compared to controls: MFO (mean [95% confidence intervals]) (0.60 g/min [0.50-0.70] vs 0.32 [0.24-0.39]), FatMax (46% V ˙ O 2 Max [44-47] vs 35 [34-37]), V ˙ O 2 Max (71 mL/min/kg [69-72] vs 48 [47-49]), MitoVD (7.8% [7.2-8.5] vs 6.0 [5.3-6.8]), FAOp (34 pmol/s/mg muscle ww [27-40] vs 21 [17-25]), and OXPHOSp (108 pmol/s/mg muscle ww [104-112] vs 69 [68-71]). Intrinsic FAOp (4.0 pmol/s/mg muscle w.w/MitoVD [2.7-5.3] vs 3.3 [2.7-3.9]) and OXPHOSp (14 pmol/s/mg muscle ww/MitoVD [13-15] vs 11 [10-13]) were, however, similar in the endurance athletes and untrained controls. MFO and MitoVD correlated (r2  = 0.504, P < 0.05) in the endurance athletes. A strong correlation between MitoVD and MFO suggests that expansion of MitoVD might be rate-limiting for MFO in the endurance athletes. In contrast, intrinsic mitochondrial changes were not associated with augmented MFO.

Increased capillary density in skeletal muscle is not associated with impaired insulin sensitivity induced by bed rest in healthy young men.

Physical inactivity alters glucose homeostasis in skeletal muscle, potentially developing into overt metabolic disease. The present study sought to investigate the role of skeletal muscle capillarization in glucose tolerance and insulin sensitivity (IS) using a classic human model of physical inactivity. Thirteen healthy males (age = 23 ± 2 years) underwent 4 days of full-time supervised and diet-controlled bed rest. Oral glucose tolerance test, indices of IS (quantitative insulin sensitivity check index (QUICKI), Matsuda index), as well as skeletal muscle biopsies with measurement of fiber type distribution, fiber cross-sectional area (FCSA), capillary-to-fiber ratio (C/F ratio), and capillary density (CD) were assessed prior to and after bed rest. Body weight and composition were unaltered by bed rest. Fasting glucose/insulin ratio (G0/I0 ratio) (-25%, P = 0.016), QUICKI (-7%, P = 0.023), and Matsuda index (-24%, P = 0.003) diminished with bed rest. Skeletal muscle FCSA decreased (-737.4 ± 763.2 µm-2 (-12%), P = 0.005) while C/F ratio was preserved, resulting in augmented CD (+93.9 ± 91.5 capillaries·mm-2 (+37%), P = 0.003) with bed rest. No association was detected between changes in skeletal muscle variables and metabolic outcomes. Independently of bed rest-induced effects, a positive linear relationship was detected between C/F ratio and G0/I0 ratio (ß = 17.09, P = 0.021). In conclusion, impaired glucose homeostasis with bed rest is not prevented nor associated with enhanced skeletal muscle capillarization in healthy individuals.

Sexual dimorphism of substrate utilization: Differences in skeletal muscle mitochondrial volume density and function.

NEW FINDINGS: What is the central question of this study? Females rely to a greater extent than males on fat oxidation during exercise. Whether any difference in skeletal muscle mitochondrial phenotype and oxidative capacity contributes to this sexual dimorphism remains incompletely explored. What is the main finding and its importance? Female prioritization of fat during exercise occurs in parallel to augmented mitochondrial volume density and intrinsic fatty acid and lactate oxidation in skeletal muscle fibres compared with males, independently of aerobic exercise capacity. The enlarged metabolic machinery in skeletal muscle of females is associated with lower body size and leg mass. ABSTRACT: Fat oxidation during exercise is greater in females than in males. We sought to determine whether sex differences in substrate metabolism are paralleled by distinct skeletal muscle mitochondrial volume density and oxidative capacity. Whole-body substrate (fat and carbohydrate) utilization during submaximal treadmill running was assessed, and skeletal muscle biopsies were taken to determine mitochondrial volume density and function in healthy young females (n = 12) and males (n = 12) matched by aerobic exercise capacity and exercise performance. Females presented a lower respiratory exchange ratio (0.87 ± 0.04 versus 0.91 ± 0.04, P = 0.023) and whole-body carbohydrate oxidation (27.8 ± 8.3 versus 35.8 ± 6.5 mg kg-1  min-1 , P = 0.027), whereas fat oxidation was higher (8.7 ± 2.8 versus 5.9 ± 2.6 mg kg-1  min-1 , P = 0.034) during submaximal exercise compared with males. In skeletal muscle biopsies, females demonstrated augmented mitochondrial volume density (7.51 ± 1.77 versus 5.90 ± 1.72%, P = 0.035) and oxidative capacity for fatty acid [36.6 ± 12.8 versus 24.5 ± 7.3 pmol O2  s-1  (mg wet weight)-1 , P = 0.009] and lactate [71.1 ± 24.4 versus 53.2 ± 14.6 pmol O2  s-1  (mg wet weight)-1 , P = 0.040]. No sex differences in respiratory exchange ratio, whole-body fat oxidation and skeletal muscle variables were detected when adjusted for anthropometric variables including body mass or leg mass, which were lower in females. In conclusion, female prioritization of fat over carbohydrate oxidation during exercise is underpinned by augmented body size-related mitochondrial volume density, fatty acid and lactate oxidative capacity in skeletal muscle fibres.

Reliability and validity of non-invasive determined haemoglobin mass and blood volumes.

INTRODUCTION: The carbon monoxide (CO) rebreathing method used for the determination of haemoglobin mass (Hbmass ) is associated with blood sample analysis (in this study: Radiometer ABL800). As an alternative hereto the aim of the present study was to evaluate the use of a portable and non-invasive CO pulse oximeter (Rad-57). METHOD: With simultaneous determination of CO in the circulation by ABL800 (%HbCO) and Rad-57 (SpCO), Hbmass and blood volume (BV) were determined in duplicates in 24 volunteers. Percentage of typical errors (%TE) within methods and linear correlations between the two procedures were computed. RESULTS: Hbmass (Rad-57 = 798 ± 230 g; ABL800 = 781 ± 192 g) and BV (Rad-57 = 5700 ± 1373 ml; ABL800 = 5581 ± 1096 ml) were similar between methods. However, the %TE for Hbmass was higher (P<0·001) for Rad-57 (5·84 ± 5·29%) than for ABL800 (1·35 ± 1·13%). Similarly, the %TE for BV was higher (P<0·001) for Rad-57 (6·06 ± 5·76%) than for ABL800 (1·48 ± 1·25%). Lower (P<0·05) correlation coefficients between the methods were found when Hbmass  > 905 g and BV > 6193 ml. CONCLUSION: Assessment of SpCO by Rad-57 resulted in considerably less precise determinations of Hbmass and BV, especially for high values. Thus, non-invasive assessment of Hbmass and BV cannot be recommended for scientific purposes, but may nonetheless be useful in clinical settings.

Combined effects of physical inactivity and acute hyperglycemia on arterial distensibility.

Arterial distensibility, an independent predictor of cardiovascular events, is transiently increased with acute hyperglycemia (AHG) in healthy individuals. Whether this response interacts with physical inactivity remains unknown. We examined the effects of short-term bed rest (BR) on the response of carotid artery distensibility (CD) to AHG, and the influence of underlying changes in insulin resistance and blood volume. CD was assessed with ultrasonography before as well as 30 and 120 minutes following ingestion of 75 g of glucose prior to and after 3 days of BR in 15 healthy male volunteers. Plasma insulin/glucose concentrations and blood volumes were concomitantly determined. On day 4 of BR, blood volume was re-established to pre-BR levels by albumin infusion and CD and insulin/glucose concentrations were determined as in the previous experimental days. Basal CD was not affected by BR. AHG increased CD before and after BR but reached a higher peak increment after BR (12% vs 60% at 30 min OGTT, p=0.028). BR also increased the plasma insulin concentration during AHG ( p=0.007). In regression analyses, plasma insulin and glucose concentrations were positively correlated to CD, particularly after BR ( r=0.31, p<0.05). Restoration of the BR-induced loss (5%) in blood volume did not affect the response of CD to AHG. In conclusion, short-term physical inactivity strongly accentuates the initial increase in CD in response to AHG in healthy individuals. This effect is associated with concomitant increases in circulating insulin concentration attributable to early insulin resistance.

Erythropoiesis with endurance training: dynamics and mechanisms.

The purpose of the present study was to characterize the progression of red blood cell volume (RBCV) expansion and potential volumetric and endocrine regulators of erythropoiesis during endurance training (ET). Nine healthy, untrained volunteers (age = 27 ± 4 yr) underwent supervised ET consisting of 3-4 × 60 min cycle ergometry sessions per week for 8 wk. Plasma volume (PV), RBCV, and overnight fasting hematological markers were determined before and at weeks 2, 4, and 8 of ET. In addition, plasma erythropoietin (EPO), cortisol, copeptin, and proatrial natriuretic peptide concentrations were measured during a 3-h morning period at baseline and postexercise at weeks 1 and 8 PV increased from baseline (2,405 ± 335 ml) at weeks 2, 4, and 8 (+374 ± 194, +505 ± 156, and +341 ± 160 ml, respectively, P < 0.001). Increases in RBCV from baseline (1,737 ± 442 ml) were manifested at week 4 (+109 ± 114 ml, P = 0.030) and week 8 (+205 ± 109 ml, P = 0.001). Overnight fasting plasma EPO concentration increased from baseline (11.3 ± 4.8 mIU/ml) at week 2 (+2.5 ± 2.8 mIU·ml-1, P = 0.027) and returned to baseline concentration at weeks 4 and 8 Higher 3-h-postexercise EPO concentration was observed at week 1 (11.6 mIU/ml) compared with week 8 (8.4 ± 3.9 mIU/ml, P = 0.009) and baseline (9.0 ± 4.2 mIU/ml, P = 0.019). Linear relationships between EPO concentration and hematocrit (ß = -56.2, P < 0.001) and cortisol (ß = 0.037, P < 0.001) were detected throughout the ET intervention. In conclusion, ET leads to mild, transient increases in circulating EPO concentration, concurring with early PV expansion and lowered hematocrit, preceding gradual RBCV enhancement.

Detection of blood volumes and haemoglobin mass by means of CO re-breathing and indocyanine green and sodium fluorescein injections.

The main aim of the present study was to quantify the magnitude of differences introduced when estimating a given blood volume compartment (e.g. plasma volume) through the direct determination of another compartment (e.g. red cell volume) by multiplication of venous haematocrit and/or haemoglobin concentration. However, since whole body haematocrit is higher than venous haematocrit such an approach might comprise certain errors. To test this experimentally, four different methods for detecting blood volumes and haemoglobin mass (Hbmass) were compared, namely the carbon monoxide (CO) re-breathing (for Hbmass), the indocyanine green (ICG; for plasma volume [PV]) and the sodium fluorescein (SoF; for red blood cell volume [RBCV]) methods. No difference between ICG and CO re-breathing derived PV could be established when a whole body/venous haematocrit correction factor of 0.91 was applied (p = 0.11, r = 0.43, mean difference -340 ± 612 mL). In contrast, when comparing RBCV derived by the CO re-breathing and the SoF method, the SoF method revealed lower RBCV values as compared to the CO re-breathing method (p < 0.05, r = 0.95, mean difference -728 ± 184 mL). However, compared to the ICG and the SoF methods, the typical error (%TE) and hence reliability of the CO re-breathing method was lower for all measured parameters. Therefore, estimating blood volume compartments by the direct assessment of another compartment can be considered a suitable approach. The CO re-breathing method proved accurate in determining the induced phlebotomy and is at the same time judged easier to perform than any of the other methods.

Recombinant erythropoietin in humans has a prolonged effect on circulating erythropoietin isoform distribution.

The membrane-assisted isoform immunoassay (MAIIA) quantitates erythropoietin (EPO) isoforms as percentages of migrated isoforms (PMI). We evaluated the effect of recombinant human EPO (rhEPO) on the distribution of EPO isoforms in plasma in a randomized, placebo-controlled, double-blinded, cross-over study. 16 healthy subjects received either low-dose Epoetin beta (5000 IU on days 1, 3, 5, 7, 9, 11 and 13); high-dose Epoetin beta (30.000 IU on days 1, 2 and 3 and placebo on days 5, 7, 9, 11 and 13); or placebo on all days. PMI on days 4, 11 and 25 was determined by interaction of N-acetyl glucosamine with the glycosylation dependent desorption of EPO isoforms. At day 25, plasma-EPO in both rhEPO groups had returned to values not different from the placebo group. PMI with placebo, reflecting the endogenous EPO isoforms, averaged 82.5 (10.3) % (mean (SD)). High-dose Epoetin beta decreased PMI on days 4 and 11 to 31.0 (4.2)% (p<0.00001) and 45.2 (7.3)% (p<0.00001). Low-dose Epoetin beta decreased PMI on days 4 and 11 to 46.0 (12.8)% (p<0.00001) and 46.1 (10.4)% (p<0.00001). In both rhEPO groups, PMI on day 25 was still decreased (high-dose Epoetin beta: 72.9 (19.4)% (p=0.029); low-dose Epoetin beta: 73.1 (17.8)% (p=0.039)). In conclusion, Epoetin beta leaves a footprint in the plasma-EPO isoform pattern. MAIIA can detect changes in EPO isoform distribution up til at least three weeks after administration of Epoetin beta even though the total EPO concentration has returned to normal.