Regulation of haemoglobin concentration at high altitude

Lowlanders sojourning for more than 1 day at high altitude (HA) experience a reduction in plasma volume (PV) that increases haemoglobin concentration and thus restores arterial oxygen content. If the sojourn extends over weeks, an expansion of total red cell volume (RCV) occurs and contributes to the haemoconcentration. While the reduction in PV was classically attributed to an increased diuretic fluid loss, recent studies support fluid redistribution, rather than loss, as the underlying mechanism. The fluid redistribution is presumably driven by a disappearance of proteins from the circulation and the resulting reduction in oncotic pressure exerted by the plasma, although the fate of the disappearing proteins remains unclear. The RCV expansion is the result of an accelerated erythropoietic activity secondary to enhanced renal erythropoietin release, but a contribution of other mechanisms cannot be excluded. After return from HA, intravascular volumes return to normal values and the normalisation of RCV might involve selective destruction of newly formed erythrocytes, although this explanation has been strongly challenged by recent studies. In contrast to acclimatised lowlanders, native highlanders originating from the Tibetan and the Ethiopian plateaus present with a normal or only mildly elevated haemoglobin concentration. Genetic adaptations blunting the erythropoietic response to HA exposure have been proposed as an explanation for the absence of more pronounced haemoconcentration in these populations, but new evidence also supports a contribution of a larger than expected PV. The functional significance of the relatively low haemoglobin concentration in Tibetan and Ethiopian highlanders is incompletely understood and warrants further investigation.

Transcerebral exchange kinetics of large neutral amino acids during acute inspiratory hypoxia in humans

Hypoxaemia is present in many critically ill patients, and may contribute to encephalopathy. Changes in the passage of large neutral amino acids (LNAAs) across the blood-brain barrier (BBB) with an increased cerebral influx of aromatic amino acids into the brain may concurrently be present and also contribute to encephalopathy, but it has not been established whether hypoxaemia per se may trigger such changes. We measured cerebral blood flow (CBF) in 11 healthy men using the Kety-Schmidt technique and obtained paired arterial and jugular-venous blood samples for the determination of LNAAs by high performance liquid chromatography at baseline and after 9 hours of poikilocapnic normobaric hypoxia (12% O₂). Transcerebral net exchange was determined by the Fick principle, and transport of LNAAs across the BBB was determined mathematically. Hypoxia increased both the systemic and corresponding cerebral delivery of the aromatic amino acid phenylalanine, and the branched-chain amino acids leucine and isoleucine. Despite this, the transcerebral net exchange values and mathematically derived brain extracellular concentrations for all LNAAs were unaffected. In conclusion, the observed changes in circulating LNAAs triggered by hypoxaemia do not affect the transcerebral exchange kinetics of LNAAs to such an extent that their brain extracellular concentrations are affected.

Maximizing Cellular Adaptation to Endurance Exercise in Skeletal Muscle

The application of molecular techniques to exercise biology has provided novel insight into the complexity and breadth of intracellular signaling networks involved in response to endurance-based exercise. Here we discuss several strategies that have high uptake by athletes and, on mechanistic grounds, have the potential to promote cellular adaptation to endurance training in skeletal muscle. Such approaches are based on the underlying premise that imposing a greater metabolic load and provoking extreme perturbations in cellular homeostasis will augment acute exercise responses that, when repeated over months and years, will amplify training adaptation.

No evidence for direct effects of recombinant human erythropoietin on cerebral blood flow and metabolism in healthy humans

Erythropoietin (EPO) is expressed in human brain tissue, but its exact role is unknown. EPO may improve the efficiency of oxidative metabolism and has neuroprotective properties against hypoxic injuries in animal models. We aimed to investigate the effect of recombinant human EPO (rHuEPO) administration on healthy cerebral metabolism in humans during normoxia and during metabolic stress by inhalation of 10% O₂ hypoxic air. Twenty-four healthy men participated in a two-arm double-blind placebo-controlled trial. rHuEPO was administered as a low dose (5,000 IU) over 4 wk ( n = 12) or as a high dose (500 IU·kg body wt^-1·day^-1) for three consecutive days ( n = 12). Global cerebral blood flow (CBF) and metabolic rate of glucose (CMR_glc) were measured with positron emission tomography. CBF, metabolic rate of oxygen ([Formula: see text]), and cerebral lactate concentration were measured by magnetic resonance imaging and spectroscopy. Low-dose treatment increased hemoglobin and was associated with a near-significant decrease in CBF during baseline normoxia. High-dose treatment caused no change in CBF. Neither treatment had an effect on normoxia CMR_glc, [Formula: see text], or lactate concentration or an effect on the cerebral metabolic response to inhalation of hypoxic air. In conclusion, the study found no evidence for a direct effect of rHuEPO on cerebral metabolism. NEW & NOTEWORTHY We demonstrate with magnetic resonance imaging and positron emission tomography that administration of erythropoietin does not have a substantial direct effect on healthy human resting cerebral blood flow or effect on cerebral glucose and oxygen metabolism. Also, administration of erythropoietin did not have a direct effect on the metabolic response to acute hypoxic stress in healthy humans, and a suggested neuroprotective effect from erythropoietin is therefore likely not a direct effect of erythropoietin on cerebral metabolism.

Cutaneous exposure to hypoxia does not affect skin perfusion in humans

AIM

Experiments have indicated that skin perfusion in mice is sensitive to reductions in environmental O₂ availability. Specifically, a reduction in skin-surface PO₂ attenuates transcutaneous O₂ diffusion, and hence epidermal O₂ supply. In response, epidermal HIF-1α expression increases and facilitates initial cutaneous vasoconstriction and subsequent nitric oxide-dependent vasodilation. Here, we investigated whether the same mechanism exists in humans.

METHODS

In a first experiment, eight males rested twice for 8 h in a hypobaric chamber. Once, barometric pressure was reduced by 50%, while systemic oxygenation was preserved by O₂ -enriched (42%) breathing gas (Hypoxia_Skin ), and once barometric pressure and inspired O₂ fraction were normal (Control₁ ). In a second experiment, nine males rested for 8 h with both forearms wrapped in plastic bags. O₂ was expelled from one bag by nitrogen flushing (Anoxia_Skin ), whereas the other bag was flushed with air (Control₂ ). In both experiments, skin blood flux was assessed by laser Doppler on the dorsal forearm, and HIF-1α expression was determined by immunohistochemical staining in forearm skin biopsies.

RESULTS

Skin blood flux during Hypoxia_Skin and Anoxia_Skin remained similar to the corresponding Control trial (P = 0.67 and P = 0.81). Immunohistochemically stained epidermal HIF-1α was detected on 8.2 ± 6.1 and 5.3 ± 5.7% of the analysed area during Hypoxia_Skin and Control₁ (P = 0.30) and on 2.3 ± 1.8 and 2.4 ± 1.8% during Anoxia_Skin and Control₂ (P = 0.90) respectively.

CONCLUSION

Reductions in skin-surface PO₂ do not affect skin perfusion in humans. The unchanged epidermal HIF-1α expression suggests that epidermal O₂ homoeostasis was not disturbed by Hypoxia_Skin /Anoxia_Skin , potentially due to compensatory increases in arterial O₂ extraction.

Prolyl-4-hydroxylase 2 and 3 coregulate murine erythropoietin in brain pericytes

A classic response to systemic hypoxia is the increased production of red blood cells due to hypoxia-inducible factor (HIF)-mediated induction of erythropoietin (EPO). EPO is a glycoprotein hormone that is essential for normal erythropoiesis and is predominantly synthesized by peritubular renal interstitial fibroblast-like cells, which express cellular markers characteristic of neuronal cells and pericytes. To investigate whether the ability to synthesize EPO is a general functional feature of pericytes, we used conditional gene targeting to examine the von Hippel-Lindau/prolyl-4-hydroxylase domain (PHD)/HIF axis in cell-expressing neural glial antigen 2, a known molecular marker of pericytes in multiple organs. We found that pericytes in the brain synthesized EPO in mice with genetic HIF activation and were capable of responding to systemic hypoxia with the induction of Epo. Using high-resolution multiplex in situ hybridization, we determined that brain pericytes represent an important cellular source of Epo in the hypoxic brain (up to 70% of all Epo-expressing cells). We furthermore determined that Epo transcription in brain pericytes was HIF-2 dependent and cocontrolled by PHD2 and PHD3, oxygen- and 2-oxoglutarate-dependent prolyl-4-hydroxylases that regulate HIF activity. In summary, our studies provide experimental evidence that pericytes in the brain have the ability to function as oxygen sensors and respond to hypoxia with EPO synthesis. Our findings furthermore suggest that the ability to synthesize EPO may represent a functional feature of pericytes in the brain and kidney.

Performance Enhancement: What Are the Physiological Limits?

Our objective is to highlight some key physiological determinants of endurance exercise performance and to discuss how these can be further improved. V̇o2max remains remarkably stable throughout an athletic career. By contrast, exercise economy, lactate threshold, and critical power may be improved in world-class athletes by specific exercise training regimes and/or with more years of training.

Red cell volume expansion at altitude: a meta-analysis and Monte Carlo simulation

INTRODUCTION

Altitude acclimatization is associated with a rapid increase in hematocrit. The time course and the contribution of the red cell volume expansion are not clear. The purpose of the present meta-analysis was to explore how much altitude exposure is required to induce polycythemia in healthy lowlanders.

METHODS

A systematic review was performed of 66 published articles (including 447 volunteers) identified through literature search. We performed a mixed-model random-effects meta-analysis and a Monte Carlo simulation on the extracted data.

RESULTS

The following results were obtained in this study: 1) the red cell volume expansion for a given duration of exposure is dependent on altitude (P < 0.0001), that is, that the increase in red cell volume was accelerated at higher altitudes; and 2) the extent of the erythropoietic response depends on the initial red cell volume (P < 0.0001). It seems that exposure time must exceed 2 wk at an altitude of more than 4000 m to exert a statistically significant effect. At lower altitudes, longer exposure times are needed with altitudes lower than 3000 m not yielding an increase within 4 wk.

CONCLUSIONS

Red cell volume response to hypoxia is generally slow, although it accelerates with increasing altitude. This, in combination with a dependency on initial red cell volume, suggests that, for example, athletes may need to spend more time at altitude to see an effect on red cell volume than commonly recommended.

A multi-center study on the regenerative effects of erythropoietin in burn and scalding injuries: study protocol for a randomized controlled trial

Background

Although it was initially assumed that erythropoietin (EPO) was a hormone that only affected erythropoiesis, it has now been proposed that EPO plays an additional key role in the regulation of acute and chronic tissue damage.

Via the inhibition of inflammatory reactions and of apoptosis, stem cell recruitment, advancement of angiogenesis and growth factor release, EPO enhances healing and thus restitutio ad integrum after trauma. Human skin contains EPO receptors and is able to synthesize EPO. We therefore hypothesize that EPO is able to optimize wound healing in thermally injured patients.

Methods/Design

This is a large, prospective, randomized, double-blind, multi-center study, funded by the German Federal Ministry of Education and Research, and fully approved by the designated ethics committee. The trial, which is to investigate the effects of EPO in severely burned patients, is in its recruitment phase and is being carried out in 13 German burn care centers. A total of 150 patients are to be enrolled to receive study medication every other day for 21 days (EPO 150 IU/kg body weight or placebo). A follow-up of one year is planned. The primary endpoint of this study is the time until complete re-epithelialization of a defined skin graft donor site is reached. Furthermore, clinical parameters such as wound healing, scar formation (using the Vancouver scar scale), laboratory values, quality of life (SF-36), angiogenic effects, and gene- and protein-expression patterns are to be determined. The results will be carefully evaluated for gender differences.

Discussion

We are seeking new insights into the mechanisms of wound healing in thermally injured patients and more detailed information about the role EPO plays, specifically in these complex interactions. We additionally expect that the biomimetic effects of EPO will be useful in the treatment of acute thermal dermal injuries.

Trial registration

EudraCT Number: 2006-002886-38, Protocol Number: 0506, ISRCT Number: http://controlled-trials.com/ISRCTN95777824/ISRCTN95777824.

Human skin hypoxia modulates cerebrovascular and autonomic functions

Because the skin is an oxygen sensor in amphibians and mice, we thought to confirm this function also in humans. The human upright posture, however, introduces additional functional demands for the maintenance of oxygen homeostasis in which cerebral blood flow and autonomic nervous system (ANS) function may also be involved. We examined nine males and three females. While subjects were breathing ambient air, at sea level, we changed gases in a plastic body-bag during two conditions of the experiment such as to induce skin hypoxia (with pure nitrogen) or skin normoxia (with air). The subjects performed a test of hypoxic ventilatory drive during each condition of the experiment. We found no differences in the hypoxic ventilatory drive tests. However, ANS function and cerebral blood flow velocities were modulated by skin hypoxia and the effect was significantly greater on the left than right middle cerebral arteries. We conclude that skin hypoxia modulates ANS function and cerebral blood flow velocities and this might impact life styles and tolerance to ambient hypoxia at altitude. Thus the skin in normal humans, in addition to its numerous other functions, is also an oxygen sensor.