Influence of alpha-adrenergic blockade on the catecholamine response to exercise at 4,300 meters

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.

Acute hypoxia reduces exogenous glucose oxidation, glucose turnover, and metabolic clearance rate during steady-state aerobic exercise

BACKGROUND

Exogenous carbohydrate oxidation is lower during steady-state aerobic exercise in native lowlanders sojourning at high altitude (HA) compared to sea level (SL). However, the underlying mechanism contributing to reduction in exogenous carbohydrate oxidation during steady-state aerobic exercise performed at HA has not been explored.

OBJECTIVE

To determine if alterations in glucose rate of appearance (R_a), disappearance (R_d) and metabolic clearance rate (MCR) at HA provide a mechanism for explaining the observation of lower exogenous carbohydrate oxidation compared to during metabolically-matched, steady-state exercise at SL.

METHODS

Using a randomized, crossover design, native lowlanders (n = 8 males, mean ± SD, age: 23 ± 2 yr, body mass: 87 ± 10 kg, and VO_2peak: SL 4.3 ± 0.2 L/min and HA 2.9 ± 0.2 L/min) consumed 145 g (1.8 g/min) of glucose while performing 80-min of metabolically-matched (SL: 1.66 ± 0.14 V̇O₂ L/min 329 ± 28 kcal, HA: 1.59 ± 0.10 V̇O₂ L/min, 320 ± 19 kcal) treadmill exercise in SL (757 mmHg) and HA (460 mmHg) conditions after a 5-h exposure. Substrate oxidation rates (g/min) and glucose turnover (mg/kg/min) during exercise were determined using indirect calorimetry and dual tracer technique (¹³C-glucose oral ingestion and [6,6-²H₂]-glucose primed, continuous infusion).

RESULTS

Total carbohydrate oxidation was higher (P < 0.05) at HA (2.15 ± 0.32) compared to SL (1.39 ± 0.14). Exogenous glucose oxidation rate was lower (P < 0.05) at HA (0.35 ± 0.07) than SL (0.44 ± 0.05). Muscle glycogen oxidation was higher at HA (1.67 ± 0.26) compared to SL (0.83 ± 0.13). Total glucose R_a was lower (P < 0.05) at HA (12.3 ± 1.5) compared to SL (13.8 ± 2.0). Exogenous glucose R_a was lower (P < 0.05) at HA (8.9 ± 1.3) compared to SL (10.9 ± 2.2). Glucose R_d was lower (P < 0.05) at HA (12.7 ± 1.7) compared to SL (14.3 ± 2.0). MCR was lower (P < 0.05) at HA (9.0 ± 1.8) compared to SL (12.1 ± 2.3). Circulating glucose and insulin concentrations were higher in response carbohydrate intake during exercise at HA compared to SL.

CONCLUSION

Novel results from this investigation suggest that reductions in exogenous carbohydrate oxidation at HA may be multifactorial; however, the apparent insensitivity of peripheral tissue to glucose uptake may be a primary determinate.

Effect of acute hypoxia on regional cerebral blood flow: effect of sympathetic nerve activity

We examined 1) whether global cerebral blood flow (CBF) would increase across a 6-h bout of normobaric poikilocapnic hypoxia and be mediated by a larger increase in blood flow in the vertebral artery (VA) than in the internal carotid artery (ICA); and 2) whether additional increases in global CBF would be evident following an α1-adrenergic blockade via further dilation of the ICA and VA. In 11 young normotensive individuals, ultrasound measures of ICA and VA flow were obtained in normoxia (baseline) and following 60, 210, and 330 min of hypoxia (FiO2 = 0.11). Ninety minutes prior to final assessment, participants received an α1-adrenoreceptor blocker (prazosin, 1 mg/20 kg body mass) or placebo. Compared with baseline, following 60, 220, and 330 min of hypoxia, global CBF [(ICAFlow + VAFlow) ∗ 2] increased by 160 ± 52 ml/min (+28%; P = 0.05), 134 ± 23 ml/min (+23%; P = 0.02), and 113 ± 51 (+19%; P = 0.27), respectively. Compared with baseline, ICAFlow increased by 23% following 60 min of hypoxia (P = 0.06), after which it progressively declined. The percentage increase in VA flow was consistently larger than ICA flow during hypoxia by ∼20% (P = 0.002). Compared with baseline, ICA and VA diameters increased during hypoxia by ∼9% and ∼12%, respectively (P ≤ 0.05), and were correlated with reductions in SaO2. Flow and diameters were unaltered following α1 blockade (P ≥ 0.10). In conclusion, elevations in global CBF during acute hypoxia are partly mediated via greater increases in VA flow compared with ICA flow; this regional difference was unaltered following α1 blockade, indicating that a heightened sympathetic nerve activity with hypoxia does not constrain further dilation of larger extracranial blood vessels.

Acute exposure to altitude

Acute exposure to altitude principally encompasses aviation and space activities. These environments can be associated with very acute changes in pressure, oxygenation and temperature due to rates and magnitude of ascent that are not experienced in more chronic exposure such as mountaineering. The four key physiological challenges during acute exposure to altitude are: hypoxia (and hyperventilation), gas volume changes, decompression sickness and cold. The brief nature of aviation exposure to altitude provides little opportunity for acclimatisation, leading to markedly different effects when an individual is exposed to the same altitude acutely compared with an acclimatised individual climbing an 8000m (26 347ft) peak. Challenges such as hypobaric decompression sickness are not considered a hazard for chronic altitude exposure but are routine considerations for those flying to high altitude. Protective systems are essential for aircrew and passengers to survive and function during acute exposure to altitude.

Urinary adrenalin and cortisol secretion patterns of social voles in response to adrenergic blockade under basal conditions

The effect of alpha(1)- and beta-adrenergic blockade on daily rhythms of urinary adrenalin (ADR) and cortisol (CORT) under basal conditions were evaluated. Voles acclimated to a 12:12 h light/dark cycle at 26+/-2 degrees C received a single dose of either propranolol (PROP; 4.5 mg/kg) or prazosin (PRAZ; 1 mg/kg) 1 h before lights off. Urine samples were collected for 24 h at 4 h intervals. PROP evokes a significant increase in mean urinary ADR; although CORT was unaffected by PROP, PRAZ administration significantly decreased both urinary ADR and CORT during the scotophase as compared with control voles. Cosinor analysis indicated a significant 24 h rhythm in urinary ADR, but not in CORT secretion. ADR mesor and amplitude were increased and acrophase was significantly delayed by 5 h in PROP-treated voles; PRAZ elicited opposite effects. Unexpectedly, these changes in the 24 h ADR rhythm persisted 4-weeks after PROP-treatment. The 24 h rhythm components of urinary CORT were marginally altered 4-weeks post-PROP, but only the acrophase showed a significant change. Collectively, the results indicate that sympathetic activity has a redundant compensatory mechanism defending against physiological changes induced by beta-blockade. The simultaneous decrease in adrenal hormones induced by PRAZ suggests that alpha(1)-adrenoceptors may contribute to the mechanism of integrated stress responses.

During hypoxic exercise some vasoconstriction is needed to match O2 delivery with O2 demand at the microcirculatory level

To test the hypothesis that the increased sympathetic tonus elicited by chronic hypoxia is needed to match O(2) delivery with O(2) demand at the microvascular level eight male subjects were investigated at 4559 m altitude during maximal exercise with and without infusion of ATP (80 mug (kg body mass)(-1) min(-1)) into the right femoral artery. Compared to sea level peak leg vascular conductance was reduced by 39% at altitude. However, the infusion of ATP at altitude did not alter femoral vein blood flow (7.6 +/- 1.0 versus 7.9 +/- 1.0 l min(-1)) and femoral arterial oxygen delivery (1.2 +/- 0.2 versus 1.3 +/- 0.2 l min(-1); control and ATP, respectively). Despite the fact that with ATP mean arterial blood pressure decreased (106.9 +/- 14.2 versus 83.3 +/- 16.0 mmHg, P < 0.05), peak cardiac output remained unchanged. Arterial oxygen extraction fraction was reduced from 85.9 +/- 5.3 to 72.0 +/- 10.2% (P < 0.05), and the corresponding venous O(2) content was increased from 25.5 +/- 10.0 to 46.3 +/- 18.5 ml l(-1) (control and ATP, respectively, P < 0.05). With ATP, leg arterial-venous O(2) difference was decreased (P < 0.05) from 139.3 +/- 9.0 to 116.9 +/- 8.4(-1) and leg .VO(2max) was 20% lower compared to the control trial (1.1 +/- 0.2 versus 0.9 +/- 0.1 l min(-1)) (P = 0.069). In summary, at altitude, some degree of vasoconstriction is needed to match O(2) delivery with O(2) demand. Peak cardiac output at altitude is not limited by excessive mean arterial pressure. Exercising leg .VO(2peak) is not limited by restricted vasodilatation in the altitude-acclimatized human.

Catecholamine analysis with microcolumn LC-peroxyoxalate chemiluminescence reaction detection

BACKGROUND

Plasma catecholamines (CAs) are widely used as an index of sympathetic nervous system activity. In addition, CAs are known to be metabolized by catechol-O-methyltransferase (COMT) to produce their 3-O-methyl metabolites. We previously established a sensitive determination method of CAs and their 3-O-methyl metabolites using HPLC-peroxyoxalate chemiluminescence (POCL) reaction detection system. In this study, a microcolumn (100 x 1.0 mm I.D.) was used for separation to obtain higher sensitivity and shorter analysis time.

METHODS

The system included automated precolumn ion-exchange extraction of amines, followed by separation on an ODS column, coulometric oxidation, fluorescence derivatization with ethylenediamine, and finally POCL reaction detection.

RESULTS

The detection limits for CAs and their 3-O-methyl metabolites were 0.3-2.0 fmol. The analysis time was about 35 min, about half that of previously reported results. The method developed was used in monitoring changes in CAs and 3-O-methyl metabolite concentrations in human plasma during exercise.

CONCLUSION

The simultaneous determination method for concentrations of CAs and their 3-O-methyl metabolites in human plasma was developed using micro LC-peroxyoxalate chemiluminescence detection. We were successful in quantitating the changes in plasma CAs and their 3-O-methyl metabolites during exercise.

Cytokine Responses at High Altitude

PURPOSE

This study tested the hypothesis that antioxidant supplementation would attenuate plasma cytokine (IL-6, tumor necrosis factor (TNF)-alpha), and C-reactive protein (CRP) concentrations at rest and in response to exercise at 4300-m elevation.

METHODS

A total of 17 recreationally trained men were matched and assigned to an antioxidant (N = 9) or placebo (N = 8) group in a double-blinded fashion. At sea level (SL), energy expenditure was controlled and subjects were weight stable. Then, 3 wk before and throughout high altitude (HA), an antioxidant supplement (10,000 IU beta-carotene, 200 IU alpha-tocopherol acetate, 250 mg ascorbic acid, 50 microg selenium, 15 mg zinc) or placebo was given twice daily. At HA, energy expenditure increased approximately 750 kcal.d(-1) and energy intake decreased approximately 550 kcal.d, resulting in a caloric deficit of approximately 1200-1500 kcal.d(-1). At SL and HA day 1 (HA1) and day HA13, subjects exercised at 55% of VO2peak until they expended approximately 1500 kcal. Blood samples were taken at rest, end of exercise, and 2, 4, and 20 h after exercise.

RESULTS

No differences were seen between groups in plasma IL-6, CRP, or TNF-alpha at rest or in response to exercise. For both groups, plasma IL-6 concentration was significantly higher at the end of exercise, 2, 4, and 20 h after exercise at HA1 compared with SL and HA13. Plasma CRP concentration was significantly elevated 20 h postexercise for both groups on HA1 compared to SL and HA13. TNF-alpha did not differ at rest or in response to exercise.

CONCLUSION

Plasma IL-6 and CRP concentrations were elevated following exercise at high altitude on day 1, and antioxidant supplementation did not attenuate the rise in plasma IL-6 and CRP concentrations associated with hypoxia, exercise, and caloric deficit.

Living high-training low altitude training: effects on mucosal immunity

Secretory immunoglobulin A (sIgA) is the major immunoglobulin of the mucosal immune system. Whereas the suppressive effect of heavy training on mucosal immunity is well documented, little is known regarding the influence of hypoxia exposure on sIgA during altitude training. This investigation examined the impact of an 18-day Living high-training low (LHTL) training camp on sIgA levels in 11 (six females and five males) elite cross-country skiers. Subjects from the control group (n=5) trained and lived at 1,200 m of altitude, whereas, subjects from the LHTL group (n=6) trained at 1,200 m, but lived at a simulated altitude of 2,500, 3,000 and 3,500 m (3x6-day, 11 h day(-1)) in hypoxic rooms. Saliva samples were collected before, after each 6-day phases and 2 weeks thereafter (POST). Salivary sIgA, protein and cortisol were measured. There was a downward trend in sIgA concentrations over the study, which reached significance in LHTL (P<0.01), but not in control (P=0.08). Salivary IgA concentrations were still lower baseline at POST (P<0.05). Protein concentration increased in LHTL (P<0.05) and was negatively correlated with sIgA concentration after the 3,000 and 3,500 m-phase and at POST (P<0.05 all). Cortisol concentrations were unchanged over the study and no relationship was found between cortisol and sIgA. In summary, data were strongly suggestive of a cumulative negative effect of physical exercise and hypoxia on sIgA levels during LHTL training. Two weeks of active recovery did not allow for proper sIgA recovery. The mechanism underlying this depression of sIgA could be mediated by neural factors.

Intravenous catecholamine administration affects mouse intestinal lymphocyte number and apoptosis

The purposes of this study were to determine plasma and intestinal epinephrine (E) and norepinephrine (NE) concentrations in mice after exercise stress and, the effect of intravenous injection of E and NE (at concentrations during exercise) on viability of intestinal lymphocytes (IL). Exhaustive exercise significantly elevated plasma E and NE, and intestinal E, compared with sedentary animals. Twenty-four hours after intravenous NE administration, IL counts were higher (p<0.001) and % apoptotic IL were lower (p<0.001) than saline conditions. E resulted in fewer apoptotic IL at 24 h compared to saline controls. E and NE differentially influence IL numbers at 24 h after injection although both result in fewer % apoptotic IL relative to mice given saline only.

β-adrenergic receptor blockade during exercise decreases intestinal lymphocyte apoptosis but not cell loss in mice

Catecholamines induce apoptosis in various lymphoid populations. This process can occur with both α- and β-adrenoreceptors. Heavy exercise increases plasma catecholamine concentrations, and is also a cause of lymphocyte apoptosis, a possible explanation for postexercise lymphocytopenia. The purpose of this study was to examine the effects of adrenoreceptor antagonism on exercise-induced decreases and apoptosis of intestinal lymphocytes. Mice received an intraperitoneal injection of phentolamine (a nonselective α-blocker), nadolol (a nonselective β-blocker), or saline (vehicle) prior to an exhaustive bout of exercise. Total intestinal lymphocyte numbers, percent and number of CD3+ lymphocytes, and cell viability were assessed. Neither α- nor β-antagonism prevented exercise-induced cell loss in the intestine; however, pretreatment with nadolol significantly reduced the number of apoptotic and necrotic cells. Phentolamine administration appeared to increase the incidence of cell death among intestinal lymphocytes. Both drugs decreased the percentage of CD3+ intestinal lymphocytes. Our study suggests that catecholamines are not responsible for postexercise lymphocytopenia, but β-adrenoceptor blockade may confer protection against exercise-induced apoptosis of intestinal lymphocytes.Key words: catecholamines, exhaustive exercise, apoptosis, intestinal lymphocytes, rodents.