Erythropoiesis in women during 11 days at 4,300 m is not affected by menstrual cycle phase

Heterogeneity of Hematological Response to Hypoxia and Short-Term or Medium-Term Bed Rest

Hematological changes are commonly observed following prolonged exposure to hypoxia and bed rest. Typically, such responses have been reported as means and standard deviations, however, investigation into the responses of individuals is insufficient. Therefore, the present study retrospectively assessed individual variation in the hematological responses to severe inactivity (bed rest) and hypoxia. The data were derived from three-bed rest projects: two 10-d (LunHab project: 8 males; FemHab project: 12 females), and one 21-d (PlanHab project: 11 males). Each project comprised a normoxic bed rest (NBR; P_IO₂=133mmHg) and hypoxic bed rest (HBR; P_IO₂=91mmHg) intervention, where the subjects were confined in the Planica facility (Rateče, Slovenia). During the HBR intervention, subjects were exposed to normobaric hypoxia equivalent to an altitude of 4,000m. NBR and HBR interventions were conducted in a random order and separated by a washout period. Blood was drawn prior to (Pre), during, and post bed rest (R1, R2, R4) to analyze the individual variation in the responses of red blood cells (RBC), erythropoietin (EPO), and reticulocytes (Rct) to bed rest and hypoxia. No significant differences were found in the mean ∆(Pre-Post) values of EPO across projects (LunHab, FemHab, and PlanHab; p>0.05), however, female EPO responses to NBR (Range - 17.39, IQR – 12.97 mIU^.ml⁻¹) and HBR (Range – 49.00, IQR – 10.91 mIU^.ml⁻¹) were larger than males (LunHab NBR Range – 4.60, IQR – 2.03; HBR Range – 7.10, IQR – 2.78; PlanHab NBR Range – 7.23, IQR – 1.37; HBR Range – 9.72, IQR – 4.91 mIU^.ml⁻¹). Bed rest duration had no impact on the heterogeneity of EPO, Rct, and RBC responses (10-d v 21-d). The resultant hematological changes that occur during NBR and HBR are not proportional to the acute EPO response. The following cascade of hematological responses to NBR and HBR suggests that the source of variability in the present data is due to mechanisms related to hypoxia as opposed to inactivity alone. Studies investigating hematological changes should structure their study design to explore these mechanistic responses and elucidate the discord between the EPO response and hematological cascade to fully assess heterogeneity.

Short exposure to intermittent hypoxia increases erythropoietin levels in healthy individuals

Few minutes of hypoxic exposure stabilizes hypoxia-inducible factor-1α, resulting in erythropoietin (EPO) gene transcription and production. The objective of this study was to identify the shortest intermittent hypoxia protocol necessary to increase serum EPO levels in healthy individuals. In a first experiment, spontaneous EPO changes under normoxia (NORM) and the EPO response to five 4-min cycles of intermittent hypoxia (IH5) were determined in six individuals. In a second experiment, the EPO response to eight 4-min cycles of intermittent hypoxia (IH8) and 120 min of continuous hypoxia (CONT) was determined in six individuals. All hypoxic protocols were performed at a targeted arterial oxygen saturation of 80%. There was no significant change in EPO levels in response to normoxia or in response to five cycles of intermittent hypoxia (NORM: 9.5 ± 1.8 to 10.5 ± 1.8, IH5: 11.4 ± 2.3 to 13.4 ± 2.1 mU/mL, main effect for time P = 0.35). There was an increase in EPO levels in response to eight cycles of intermittent hypoxia and 120 min of continuous hypoxia, with peak levels observed 4.5 h after the onset of hypoxia (IH8: 11.2 ± 2.0 to 16.7 ± 2.2, CONT: 11.1 ± 3.8 to 19.4 ± 3.8 mU/mL, main effect for time P < 0.01). Eight cycles of intermittent hypoxia increased EPO levels to a similar extent as 120 min of continuous hypoxia (main effect for condition P = 0.36). Eight 4-min cycles of intermittent hypoxia represent the shortest protocol to increase serum EPO levels in healthy individuals.NEW & NOTEWORTHY The objective of this study was to identify the shortest intermittent hypoxia protocol necessary to increase serum erythropoietin levels in healthy individuals. Eight 4-min bouts of intermittent hypoxia, representing a hypoxic duration of 32 min at an arterial oxygen saturation of 80%, significantly increased erythropoietin levels in healthy individuals. These findings suggest that a short session of intermittent hypoxia has the potential to increase oxygen-carrying capacity.

Ventilatory Response to Hypoxia and Tolerance to High Altitude in Women: Influence of Menstrual Cycle, Oral Contraception, and Menopause

Introduction: Tolerance to high altitude in women might be influenced by hormonal status since female hormones modulate ventilation. Methods: Our objectives were (i) to explore in 1060 women, the influence of the phase of menstrual cycle, oral contraception, and menopause with or without hormonal treatment, on hypoxic ventilatory response at exercise (HVRe) and hypoxic cardiac response at exercise (HCRe) measured during a routine hypoxia exercise test, before an exposure to high altitude; (ii) to determine in 260 women exposed to high altitude, the influence of menopause and oral contraceptive and other drug use, on the prevalence of severe acute mountain sickness (sAMS). Four groups were defined: premenopausal with or without oral contraception and postmenopausal with or without hormonal treatment. Results: In premenopausal women without contraception, HVRe was higher in the early luteal/midluteal phase than in the early follicular phase (0.89 ± 0.37 vs. 0.75 ± 0.27 mL/[min · kg], p = 0.03). HVRe was similar in postmenopausal versus premenopausal women. HCRe was lower in postmenopausal women (p < 0.001), due to aging. HVRe decreased from second to fourth decade of age and increased from fourth to eighth decade, while HCRe consistently decreased with aging. Oral contraception or hormonal treatment had no effect on responses to hypoxia. The prevalence of sAMS was similar in all groups. Severe high-altitude illness score was higher and HVRe lower in women with sAMS. Conclusion: (i) physiological responses to hypoxic exercise depend on the ovarian cycle phase and menopause status, (ii) oral contraception and hormonal treatment have no influence on the tolerance to high altitude, and (iii) independent of hormonal status, aging modulates physiological responses to hypoxia.

Physiological effects of high-altitude trekking on gonadal, thyroid hormones and macrophage migration inhibitory factor (MIF) responses in young lowlander women

Altitude hypoxia is often associated with impairment of human reproduction. In this study, hormones and macrophage migration inhibitory factor (MIF, a proinflammatory cytokine with key roles in human reproduction) were determined in seven regularly menstruating, lowlander native women living at sea level participating in 14 days of trekking at moderate and high altitude. Blood and saliva samples were collected from each subject at high altitude (5050 m a.s.l. [above sea level]), and at sea level before and after the expedition. Testosterone level was lowered by high altitude and was restored after the end of the expedition, while progesterone decreased significantly in all participants at the end of the expedition, although most of the participants were in the luteal phase. The salivary concentration of MIF decreased greatly at altitude, but its levels were completely restored after the return to sea level. Our findings showed high sensitivity and rapid changes in the determined parameters in response to the high‐altitude hypoxic environment, particularly MIF.

Diurnal changes of arterial oxygen saturation and erythropoietin concentration in male and female highlanders

In Caucasians and Native Americans living at altitude, hemoglobin mass is increased in spite of erythropoietin concentrations ([Epo]) not markedly differing from sea level values. We hypothesized that a nocturnal decrease of arterial oxygen saturation (SaO₂) causes a temporary rise of [Epo] not detected by morning measurements. SaO₂ (continuous, finger oximeter) and [Epo] (ELISA, every 4 h) were determined in young highlanders (altitude 2600 m) during 24 h of usual daily activity. In Series I (six male, nine female students), SaO₂ fell during the night with the nadir occurring between 01:00 and 03:00; daily means (range 92.4–95.2%) were higher in females (+1.7%, P < 0.01). [Epo] showed opposite changes with zenith occurring at 04:00 without a sex difference. Mean daily values (22.9 ± 10.7SD U/L) were higher than values obtained at 08:00 (17.2 ± 9.5 U/L, P < 0.05). In Series II (seven females), only SaO₂ was measured. During follicular and luteal phases, SaO₂ variation was similar to Series I, but the rhythm was disturbed during menstruation. While daily [Epo] variations at sea level are not homogeneous, there is a diurnal variation at altitude following changes in SaO₂. Larger hypoventilation‐dependent decreases of alveolar PO₂ decreases during the night probably cause a stronger reduction of SaO₂ in highlanders compared to lowlanders. This variation might be enlarged by a diurnal fluctuation of Hb concentration. In spite of a lower [Hb], the higher SaO₂ in women compared to men led to a similar arterial oxygen content, likely explaining the absence of differences in [Epo] between sexes.

Increased ventilation in female erythropoietin-deficient mouse line is not progesterone and estrous stage-dependent

Previous studies suggest that chronic erythropoietin (Epo) deficiency in male mice does not alter normoxic/hypoxic ventilation. As effects of Epo are sex specific and as progesterone could be a respiratory stimulant, we evaluated the impact of Epo deficiency and its possible interaction with progesterone in ventilatory control in female mice during estrous cycle phases. Compared to wild type (WT) animals, Epo-TAg^h female mice exhibited higher ventilation in hypoxia. However, when data were separated into luteal and follicular phases of the estrous cycle, basal ventilation and hypoxic ventilation were not different in both mice strains. As progesterone is known to be a potent respiratory stimulant, additional experiments were performed to elucidate its role. Interestingly, after mifepristone treatment, HVR was not modified in WT and Epo-TAg^h mice, showing that the ventilatory stimulation observed in females was not directly mediated by progesterone. We conclude that Epo-TAg^h female mice show no estrous stage-dependent increase of ventilatory control and progesterone independent response to hypoxia.

AltitudeOmics: rapid hemoglobin mass alterations with early acclimatization to and de-acclimatization from 5260 m in healthy humans

It is classically thought that increases in hemoglobin mass (Hbmass) take several weeks to develop upon ascent to high altitude and are lost gradually following descent. However, the early time course of these erythropoietic adaptations has not been thoroughly investigated and data are lacking at elevations greater than 5000 m, where the hypoxic stimulus is dramatically increased. As part of the AltitudeOmics project, we examined Hbmass in healthy men and women at sea level (SL) and 5260 m following 1, 7, and 16 days of high altitude exposure (ALT1/ALT7/ALT16). Subjects were also studied upon return to 5260 m following descent to 1525 m for either 7 or 21 days. Compared to SL, absolute Hbmass was not different at ALT1 but increased by 3.7±5.8% (mean ± SD; n = 20; p<0.01) at ALT7 and 7.6±6.6% (n = 21; p<0.001) at ALT16. Following descent to 1525 m, Hbmass was reduced compared to ALT16 (−6.0±3.7%; n = 20; p = 0.001) and not different compared to SL, with no difference in the loss in Hbmass between groups that descended for 7 (−6.3±3.0%; n = 13) versus 21 days (−5.7±5.0; n = 7). The loss in Hbmass following 7 days at 1525 m was correlated with an increase in serum ferritin (r = −0.64; n = 13; p<0.05), suggesting increased red blood cell destruction. Our novel findings demonstrate that Hbmass increases within 7 days of ascent to 5260 m but that the altitude-induced Hbmass adaptation is lost within 7 days of descent to 1525 m. The rapid time course of these adaptations contrasts with the classical dogma, suggesting the need to further examine mechanisms responsible for Hbmass adaptations in response to severe hypoxia.

Testosterone action on erythropoiesis does not require its aromatization to estrogen: Insights from the testosterone and estrogen treatment of two aromatase-deficient men

Androgens act on erythropoiesis, but the relative role of testosterone (T) and estradiol (E(2)) on erythropoietic parameters in men is a poorly investigated issue. In order to evaluate separately the effects on erythropoiesis of high-dose T administration alone and of physiological dose of E(2) administration alone two adult men with aromatase deficiency were assessed before and during each treatment. Blood cell count, hemoglobin (Hb), hematocrit (Hct), erythrocyte mean cell volume (MCV), erythrocyte mean corpuscular hemoglobin (MCH), erythrocyte mean corpuscular hemoglobin concentration (MCHC), serum ferritin, iron and total iron-binding capacity (TIBC), serum erythropoietin, serum total testosterone and estradiol were evaluated. Hb, Hct and red cell count rose during testosterone treatment, consistently with the increase in circulating testosterone, but failed to increase during estradiol treatment. A decrease in Hb, Hct and red cell count was recorded in one of the two subjects during estradiol treatment, with a concomitant decrease in serum testosterone. Circulating T alone is capable of and sufficient to influence erythropoiesis, especially at supraphysiological dosage, while circulating E(2) have not the same effect on erythropoietic parameters, suggesting the hypothesis that the erythropoietic changes induced by androgens are not mediated via its aromatization to estrogens.

Jack Reeves and his science

John T. (Jack) Reeves' science is reviewed across the 37 years of his research career at the University of Colorado Health Sciences Center, a period which occupied approximately half his remarkable life. His contributions centered on understanding the inter-relatedness as well as the underlying mechanisms controlling the various components of the O(2) transport system. We review here his studies on exercise performance; these encompassed about half his scientific output with the other half being devoted to the study of hypoxic pulmonary hypertension. Early studies concerned cardiac output, showing how it was a balance between O(2) uptake and O(2) extraction, and that cardiac output during exercise at high altitude was reduced, most likely because of decreased plasma volume and left ventricular filling. Jack's many studies addressed virtually every aspect of the O(2) transport system -- adding significantly to our understanding of the syndromes of altitude illness, the mechanisms by which ventilatory sensitivity to hypoxia and hypercapnia influenced ventilatory acclimatization, and the contributions of the various limbs of the autonomic nervous system on systemic blood pressure, vascular resistance and substrate utilization. His scientific career ended abruptly in 2004 when struck by a car while biking to work, but his legacy remains in his more than 385+ research articles or chapters, the 40+ fellows he trained, and the countless number of younger (and older) scientists for whom he served as a role model for learning how to scrutinize their data and present their findings in clear and sometimes bold prose. An integral man, he is sorely missed.

Biomedical Research on Health and Performance of Military Women: Accomplishments of the Defense Women's Health Research Program (DWHRP)

In 1994, Congress provided dollar 40 M for biomedical research on issues of importance for military women. This supported 104 intramural and 30 extramural studies and launched an era of research to narrow the knowledge gap on protection and enhancement of health and performance of military women. Projects addressed issues specific to female physiology (e.g., gynecological health in the field, maternal malaria), problems with higher prevalence for women (e.g., marginal iron deficiency, stress fracture), and issues of drug and materiel safety that had only been extrapolated from studies of men (e.g., chemical agent prophylaxis, fatigue countermeasures). Several important assumptions about female physiology and occupational risks were found to be astoundingly wrong. Hormonal changes through the menstrual cycle were less important to acute health risks and performance than predicted, exercise did not increase risk for amenorrhea and consequent bone mineral loss, and women tolerated G-forces and could be as safe as men in the cockpit if their equipment was designed for normal size and strength ranges. Data on personal readiness issues, such as body fat, physical fitness, nutrition, and postpartum return to duty, allowed reconsideration of standards that were gender appropriate and not simply disconnected adjustments to existing male standards. Other discoveries directly benefited men as well as women, including development of medical surveillance databases, identification of task strength demands jeopardizing safety and performance, and greater understanding of the effects of psychosocial stress on health and performance. This surge of research has translated into advances for the welfare of service women and the readiness of the entire force; relevant gender issues are now routine considerations for researchers and equipment developers, and some key remaining research gaps of special importance to military women continue to be investigated.

Errors of measurement for blood volume parameters: a meta-analysis

The volume of red blood cells (V(RBC)) is used routinely in the diagnostic workup of polycythemia, in assessing the efficacy of erythropoietin administration, and to study factors affecting oxygen transport. However, errors of various methods of measurement of V(RBC) and related parameters are not well characterized. We meta-analyzed 346 estimates of error of measurement of V(RBC) for techniques based on Evans blue (V(RBC,Evans)), 51chromium-labeled red blood cells (V(RBC,51Cr)), and carbon monoxide (CO) rebreathing (V(RBC,CO)), as well as hemoglobin mass with the carbon-monoxide method (M(Hb,CO)), in athletes and active and inactive subjects undergoing various experimental and control treatments lasting minutes to months. Subject characteristics and experimental treatments had little effect on error of measurement, but measures with the smallest error showed some increase in error with increasing time between trials. Adjusted to 1 day between trials and expressed as coefficients of variation, mean errors for M(Hb,CO) (2.2%; 90% confidence interval 1.4-3.5%) and V(RBC,51Cr) (2.8%; 2.4-3.2%) were much less than those for V(RBC,Evans) (6.7%; 4.9-9.4%) and V(RBC,CO) (6.7%; 3.4-14%). Most of the error of V(RBC,Evans) was due to error in measurement of volume of plasma via Evans blue dye (6.0%; 4.5-7.8%), which is the basis of V(RBC,Evans). Most of the error in V(RBC,CO) was due to estimates from laboratories with a relatively large error in M(Hb,CO), the basis of V(RBC,CO). V(RBC,51Cr) and M(Hb,CO) are the best measures for research on blood-related changes in oxygen transport. With care, V(RBC,Evans) is suitable for clinical applications of blood-volume measurement.