Increasing maximal heart rate increases maximal O2 uptake in rats acclimatized to simulated altitude

Maximal exercise heart rate (HRmax) is reduced after acclimatization to hypobaric hypoxia. The low HRmax contributes to reduce maximal cardiac output (Qmax) and may limit maximal O2 uptake (VO2max). The objective of these experiments was to test the hypothesis that the reduction in Qmax after acclimatization to hypoxia, due, in part, to the low HRmax, limits VO2max. If this hypothesis is correct, an increase in Qmax would result in a proportionate increase in VO2max. Rats acclimatized to hypobaric hypoxia [inspired PO2 (PIO2) = 69.8 +/- 3 Torr for 3 wk] exercised on a treadmill in hypoxic (PIO2 = 71.7 +/- 1.1 Torr) or normoxic conditions (PIO2 = 142.1 +/- 1.1 Torr). Each rat ran twice: in one bout the rat was allowed to reach its spontaneous HRmax, which was 505 +/- 7 and 501 +/- 5 beats/min in hypoxic and normoxic exercise, respectively; in the other exercise bout, HRmax was increased by 20% to the preacclimatization value of 600 beats/min by atrial pacing. This resulted in an approximately 10% increase in Qmax, since the increase in HRmax was offset by a 10% decrease in stroke volume, probably due to shortening of diastolic filling time. The increase in Qmax was accompanied by a proportionate increase in maximal rate of convective O2 delivery (Qmax x arterial O2 content), maximal work rate, and VO2max in hypoxic and normoxic exercise. The data show that increasing HRmax to preacclimatization levels increases VO2max, supporting the hypothesis that the low HRmax tends to limit VO2max after acclimatization to hypoxia.

Hypoxemia increases serum interleukin-6 in humans

Serum concentrations of interleukin (IL) 1 beta, IL-1 receptor antagonist (IL-1ra), IL-6, tumor necrosis factor (TNF) alpha, and C-reactive protein (CRP) were determined in ten healthy men at sea level and during four days of altitude hypoxia (4350m above sea level). The mean (SD) arterial blood oxygen saturations were 78.6 (7.3)%, 82.4 (4.9)%, and 83.4 (5.3)% in the first, second, and third days at altitude, respectively. A symptom score of acute mountain sickness (AMS) revealed that the subjects had mostly light symptoms of AMS. Mean serum IL-6 increased from 1.36 (1.04) pg x ml(-1) at sea level to 3.10 (1.65), 4.71 (2.81), and 3,54 (2.17) pg x ml(-1) during the first three days at altitude, and to 9.96 (8.90) pg x ml(-1) on the fourth day at altitude (ANOVA p = 0.002). No changes occurred in serum concentrations of IL-1 beta, IL-1ra, TNF alpha, or CRP. The serum IL-6 were related to SaO2, (r = -0.45, p = 0.003), but not to heart rates or AMS scores. In conclusion, human serum concentrations of IL-6 increased during altitude hypoxia whereas the other proinflammatory cytokines remained unchanged. The major role of IL-6 during altitude hypoxia seem not to be mediation of inflammation, instead, the role of IL-6 could be to stimulate the erythropoiesis at altitude.

Oxygen sensors in the organism: examples of regulation under altitude hypoxia in mammals

Oxygen sensing is a determinant function of mammals, especially humans, to maintain their activity under acute or chronic exposure to hypoxia. True O2 sensors (chemoreceptors, erythropoietin secreting cells) are involved in regulation loops, which aim to restore O2 availability to the cells. Pseudo O2 sensors are cells activated by the lack of oxygen but not clearly involved in regulation processes. Potassium channels in the carotid bodies have been suspected to be O2 sensitive and could mediate the chemosensitivity to hypoxia. Na,K-ATPase related ion transport in alveolar pneumocytes could be sensitive to O2 availability and regulate the flux of water and sodium in the alveolar space. Signal transduction in G-protein-dependent receptor systems is modified in hypoxia, such as in cardiac beta-receptors and adenosinergic and muscarinic receptors. Recent studies have provided some evidence to the possible role of hypoxia inducible factors (HIF-1) in the regulation of protein synthesis at the transcriptional level. Similarities between O2-sensing mechanisms in erythropoiesis and in the synthesis of vascular endothelial growth factor were recently evidenced. Both genes are upregulated in hypoxia. However, the precise structure (heme-linked enzyme?) of all these O2-sensitive sites is not known, either in the erythropoietic system or in the chemoreceptor function. An adequate balance between hypoxia-induced upregulation and downregulation processes is necessary for optimal survival in a hypoxic environment.

Effects of altitude hypoxia on middle latency auditory evoked potentials in humans

BACKGROUND

This paper presents an investigation of auditory evoked responses in humans subjected to high altitude hypoxic conditions.

METHODS

Middle latency (MLAEPs) as well as short latency (BAEPs) evoked potentials were recorded in 10 healthy subjects, first at sea level (N), then 24 h (H1) and 72 h (H3) after their arrival at an altitude of 4350 m. At the same time, arterial blood parameters (PaO2, PaCO2 and pH) were measured and the clinical status of the subjects was assessed.

RESULTS

In altitude conditions, the amplitude of BAEP peak V decreased (-17%). The MLAEP waves showed variations in the shape of their latest waveforms. Their amplitudes, however, were not affected. The Pa-Nb interpeak latency significantly decreased (-2.2 ms) between N and H1, and remained stable during the stay at high altitude.

CONCLUSION

A correlation was found between the relative decrease of PaCO2 and the shortening of Nb wave latency, suggesting that the variations in MLAEPs could be preferentially related to the ventilatory response of the subjects in hypoxic conditions. However, no correlation was found between the clinical status of the subjects (Acute Mountain Sickness score) and the parameters of the waves.

Recovery processes after repeated supramaximal exercise at the altitude of 4,350 m

We tested the hypothesis that prolonged exposure to high altitude would impair the restoration of muscle power during repeated sprints. Seven subjects performed two 20-s Wingate tests (WT1 and WT2) separated by 5 min of recovery, at sea level (N) and after 5-6 days at 4,350 m (H). Mean power output (MPO) and O2 deficit were measured during WT. O2 uptake (VO2) and ventilation (VE) were measured continuously. Blood velocity in the femoral artery (FBV) was recorded by Doppler ultrasound during recovery. Arterialized blood pH and concentrations of bicarbonate ([HCO3-]), venous plasma lactate ([La-]), norepinephrine ([NE]), and epinephrine ([Epi]) were measured before and after WT1 and WT2. MPO decreased between WT1 and WT2 by 6.9% in N (P < 0.05) and by 10.7% in H (P < 0.01). H did not further decrease MPO. O2 deficit decreased between WT1 and WT2 in H only (P < 0.01). Peak VO2 after WT was reduced by 30-40% in H (P < 0.01), but excess postexercise O2 consumption was not significantly lowered in H. During recovery in H compared with N, VE, exercise-induced acidosis, and [NE] were higher, [Epi] tented to be higher, [La-] was not altered, and [HCO3-] and FBV were lower. The similar [La-] accumulation was associated with a higher exercise-induced acidosis and a larger increase in [NE] in H. We concluded from this study that prolonged exposure to high altitude did not significantly impair the restoration of muscle power during repeated sprints, despite a limitation of aerobic processes during early recovery.

Hypoxia- and normoxia-induced reversibility of autonomic control in Andean guinea pig heart

We herein describe the regulation of cardiac receptors in a typical high-altitude native animal. Heart rate response to isoproterenol (HRIso) (beats.min-1.mg Iso.kg-1) and atropine, the density of beta-adrenergic (beta AR) and muscarinic (M2) receptors, and the ventricular content of norepinephrine (NE) and dopamine (DA) were studied in guinea pigs (Cavia porcellus). Animals native to Lima, Peru (150 m) were studied at sea level (SL) and after 5 wk at 4,300-m altitude (SL-HA). Animals native to Rancas [Pasco, Peru (4,300 m)] were studied at high altitude (HA) and after 5 wk at SL (HA-SL). HA animals had a lower HRIso, maximum number of beta AR binding sites (Bmax), beta AR dissociation constant (Kd), NE, and DA (P < 0.05) and a higher M2 Bmax (P < 0.001) when compared with the SL group. HA-SL showed an increase of the HRIso, beta Ar Kd, and NE (P < 0.05) and a decrease of the M2 Bmax and Kd (P < 0.0001) when compared with the HA group. The present study demonstrates the differential regulation and reversibility of the autonomic control in the guinea pig heart.

High altitude-induced albuminuria in normal man is enhanced by infusion of low-dose dopamine

Renal function and the urinary excretion rate of albumin (Ualb) at rest and during infusion of dopamine (3 micrograms kg-1 min-1) were investigated in eight normal volunteers at sea level and 48 h after a rapid, passive ascent to an altitude of 4350 m. Oxygen saturation decreased to 81% (77-85) (median with quartiles in parentheses) at high altitude. High altitude hypoxia increased Ualb from 3.2 micrograms min-1 (2.7-3.5) to 5.0 micrograms min-1 (3.3-6.6) (p < 0.05); increased mean arterial blood pressure from 80 mmHg (73-95) to 102 mmHg (96 108) (p < 0.01); decreased the effective renal plasma flow (ERPF) from 465 ml min-1 (412-503) to 410 ml min-1 (385-451) (p < 0.05), and increased the filtration fraction from 24% (22-27) to 28% (26-29) (p < 0.01). Glomerular filtration rate (GFR), and the renal clearances of lithium (CLi) and sodium (CNa) remained unchanged at high altitude. Dopamine increased ERPF, GFR, CLi, CNa, and decreased the filtration fraction in both environments. Infusion of dopamine further increased Ualb to 10.5 micrograms min-1 (5.5-64.8) (p < 0.05) at high altitude, but had no effect on Ualb at sea level. In conclusion, high altitude hypoxia per se increases the urinary excretion rate of albumin, which is further increased by the renal vasodilating drug dopamine. This effect of dopamine at high altitude may result from combined effects of the increase in renal plasma flow and a hypoxia-induced increase in the glomerular capillary permeability to albumin.

Water balance and acute mountain sickness before and after arrival at high altitude of 4,350 m

The present study is a first attempt to measure water balance and its components at altitude by using labeled water and bromide dilution and relating the results with acute mountain sickness (AMS). Water intake, total water output, and water output in urine and feces were measured over a 4-day interval before and a subsequent 4-day interval after transport to 4,350 m. Total body water and extracellular water were measured at the start and at the end of the two intervals. There was a close relationship between energy intake and water intake, and the relationship was unchanged by the altitude intervention. Subjects developing AMS reduced energy intake and water intake cor respondingly. The increase in total body water (TBW) in subjects developing AMS was accompanied by a reduction in total water loss. They did not show the increased urine output, compensating for the reduced evaporative water loss at altitude. Subjects showed a significant increase in TBW after 4 days at altitude. Subjects with AMS showed the biggest shifts in extracellular water relative to TBW. In conclusion, fluid retention in relation to AMS is independent of a change in water requirements due to altitude exposure. Subjects developing AMS were those showing a fluid shift of at least 1 liter from the intracellular to the extracellular compartment or from the extracellular to the intracellular compartment.

Diurnal variations of serum erythropoietin at sea level and altitude

This study tested the hypothesis that the diurnal variations of serum-erythropoietin concentration (serum-EPO) observed in normoxia also exist in hypoxia. The study also attempted to investigate the regulation of EPO production during sustained hypoxia. Nine subjects were investigated at sea level and during 4 days at an altitude of 4350 m. Median sea level serum-EPO concentration was 6 (range 6-13) U.l-1. Serum-EPO concentration increased after 18 and 42 h at altitude, [58 (range 39-240) and 54 (range 36-340) U.l-1, respectively], and then decreased after 64 and 88 h at altitude [34 (range 18-290) and 31 (range 17-104) U.l-1, respectively]. These changes of serum-EPO concentration were correlated to the changes in arterial blood oxygen saturation (r = -0.60, P = 0.0009), pH (r = 0.67, P = 0.003), and in-vivo venous blood oxygen half saturation tension (r = -0.68, P = 0.004) but not to the changes in 2, 3 diphosphoglycerate. After 64 h at altitude, six of the nine subjects had down-regulated their serum-EPO concentrations so that median values were three times above those at sea level. These six subjects had significant diurnal variations of serum-EPO concentration at sea level; the nadir occurred between 0800-1600 hours [6 (range 4-13) U.l-1], and peak concentrations occurred at 0400 hours [9 (range 8-14) U.l-1, P = 0.02]. After 64 h at altitude, the subjects had significant diurnal variations of serum-EPO concentration; the nadir occurred at 1600 hours [20 (range 16-26) U.l-1], and peak concentrations occurred at 0400 hours [31 (range 20-38) U.l-1, P = 0.02]. This study demonstrated diurnal variations of serum-EPO concentration in normoxia and hypoxia, with comparable time courses of median values. The results also suggested that EPO production at altitude is influenced by changes in pH and haemoglobin oxygen affinity.

Differential regulation of G protein expression in rat hearts exposed to chronic hypoxia

Chronic hypoxia impairs adrenergic responsiveness. A modulation of Gs and/or G1 protein alpha-subunits may be associated with the downregulation of the beta-adrenergic receptors previously found in chronic hypoxia. G protein gene expression and protein level and function in rat hearts exposed to a 30-day hypobaric chronic hypoxia were compared with control rat hearts. No change was observed in G alpha s mRNA levels in either right or left ventricles. In right ventricles, mRNA levels of G alpha i-2 increased by 40% (P < 0.05), but not in left ventricles. In both left and right ventricles, chronic hypoxia did not modify G alpha i-2 and G alpha s protein amounts, but significantly decreased functional activity of G alpha s. In conclusion, gene expression, protein levels of G alpha s and G alpha i-2, and activity of G alpha s do not change in parallel fashion with chronic hypoxia. In chronic hypoxic right ventricles, although the mRNA level of G alpha i-2 is increased, the protein level is unchanged. One potential mechanism of desensitization to catecholamines in chronic hypoxia appears to involve a decreased functional activity of G alpha s in spite of normal mRNA and protein levels.

Hormonal changes in normal and polycythemic high-altitude natives

Acute and chronic exposure to high-altitude (HA) hypoxia inhibits the renin-angiotensin-aldosterone system and may modify the release of atrial natriuretic peptide (ANP) in sea-level (SL) natives. In HA natives, the release of these hormones could be influenced by changes in blood volume or pulmonary arterial pressure. Twenty-four men residing in La Paz, Bolivia, at 3,600 m were separated into two groups: one normocythemic (HAN; with hematocrit < 57%; n = 13) and the other polycythemic (HAP; with hematocrit > 57%; n = 11). A control group of 9 SL residents was studied in normoxia (SLN) as well as after 4 days spent at 4,350 m (SLH). The groups were tested for plasma active renin (PAR), plasma aldosterone concentration, ANP, and potassium and norepineprine concentrations at rest and after a maximal exercise. Pulmonary arterial systolic pressure was assessed by a Doppler technique. It was observed that PAR and plasma aldosterone concentration at rest and after exercise were lower in the SLH than in the SLN group. PAR and norepineprine concentration were higher among highlanders than in the SLN group. Renin response to exercise was normal among the HAN group and slightly decreased among the HAP group, and an exercise-induced increase in aldosterone was attenuated in both HA groups. Aldosterone response to renin was maintained among the SLH group but was attenuated in the HA groups, possibly owing to a protective mechanism against salt and water retention. Resting and exercise ANP was lower in the HA groups than in the SLN group.(ABSTRACT TRUNCATED AT 250 WORDS)

[Cardiovascular effects of a calcium channel blocker in hypoxia caused by altitude]

OBJECTIVE

High altitude pulmonary oedema can be successfully treated and prevented by calcium channel blockers. Moreover, calcium entering in the cells could explain the congestive phenomena of acute mountain sickness (AMS). These findings led us to study the action of a calcium channel blocker, isradipine, in the prevention of non-complicated AMS.

METHODS

In a double blind randomized study, 20 healthy volunteers received 5 mg of isradipine (n = 6) or placebo (n = 6) for 8 days. After 5 days of treatment in normoxia, the subjects were rapidly transported to an altitude of 4350 m. The efficiency of the treatment was then estimated by the AMS symptom score, haemodynamic parameters and renal function.

RESULTS

The administration of isradipine did not significantly modify AMS symptom score nor most of other parameters measured in high altitude hypoxia. Heart rate was an average of 15 b/min lower in the isradipine group, probably because of a direct action of isradipine on the sinus node. Otherwise, the effects of hypoxia were similar in both groups and were in accordance with the literature. There was no clear explanation for the increase in cardiac output and stroke volume when the subjects moved from supine to standing position. Renal blood flow, measured by Doppler or para-aminohippuric acid clearance was not modified by hypoxia. Cerebral blood flow was elevated, due to the direct vasodilator effect of hypoxia. However this increase did not seem to be the main mechanism responsible for the congestive phenomena. On the other hand, the increase in capillary permeability (demonstrated by the increased transcapillary escape rate of albumin, and albuminuria) appeared to play a major role in the pathogenesis of AMS and high altitude cerebral oedema. Isradipine had no protective effect on these phenomena and its use should be restricted to the treatment of high altitude pulmonary oedema.

Energy balance at high altitude of 6,542 m

Weight loss due to malnutrition and possibly intestinal malabsorption is a well-known phenomenon in high-altitude climbers. Up to approximately 5,000 m, energy balance may be attained and intestinal energy digestibility remains normal. To see whether 1) energy balance may also be attained at 6,542 m and, if not, 2) whether decreased energy digestibility would play a significant role in the energy deficit, energy intake (EI), energy expenditure, body composition, and energy digestibility of 10 subjects (4 women, 6 men; 27-44 yr) were assessed during a 21-day sojourn on the summit of Mt. Sajama, Bolivia (6,542 m). EI was measured during two 3-day intervals: EI1 (days 7-9) and EI2 (days 17-19). Total fecal energy loss during EI1 was calculated from fecal energy measured by bomb calorimetry. Average daily metabolic rate (ADMR) at altitude was measured in six subjects (2 women, 4 men) using doubly labeled water over a 10-day interval (days 9-19). Basal metabolic rate was measured before and after the expedition by respiratory gas analysis. Body composition was estimated from skinfolds and body mass before and during the altitude sojourn. Subjects were in negative energy balance throughout the observation period (EI1-ADMR = -2.9 +/- 1.8 MJ/day and EI2-ADMR = -2.3 +/- 1.8 MJ/day based on a gross energy digestibility of 95%). The activity level, expressed as ADMR to basal metabolic rate, was 1.56-2.39. The loss of fat mass (3.7 +/- 1.5 kg) represented 74 +/- 15% of the loss of body mass.(ABSTRACT TRUNCATED AT 250 WORDS)

Albuminuria and overall capillary permeability of albumin in acute altitude hypoxia

The mechanism of proteinuria at high altitude is unclear. Renal function and urinary excretion rate of albumin (Ualb) at rest and during submaximal exercise and transcapillary escape rate of 125I-labeled albumin (TERalb) were investigated in 12 normal volunteers at sea level and after rapid and passive ascent to 4,350 m. The calcium antagonist isradipine (5 mg/day; n = 6) or placebo (n = 6) was administered to abolish hypoxia-induced rises in blood pressure. Lithium clearance and urinary excretion of beta 2-microglobulin were used to evaluate renal tubular function. High altitude increased Ualb from 2.8 to > 5.0 micrograms/min in both groups (P < 0.05). In the placebo group, high altitude significantly increased filtration fraction (P < 0.05), but this response was abolished by isradipine. Lithium clearance and urinary excretion of beta 2-microglobulin remained unchanged by hypoxia in both groups. Exercise did not reveal any further renal dysfunction. In both groups, high altitude increased TERalb from 4.8 to > 6.7%/h (P < 0.05). In conclusion, acute altitude hypoxia increases Ualb despite unchanged tubular function and independent of effects of isradipine on filtration fraction. The elevated TERalb suggests an overall increase in capillary permeability, including the glomerular endothelium, as the critical factor in high-altitude induced albuminuria.

Adrenergic status of humans during prolonged exposure to the altitude of 6,542 m

Plasma norepinephrine (NE) concentration increases with altitude exposure while maximal heart rate (HR) and chronotropic response to isoproterenol (IP) are blunted. Downregulation of cardiac beta-adrenergic receptors (beta-AR) has been evoked to explain this phenomenon. Chronotropic response was studied at extreme altitude in 10 subjects (4 women, 6 men; aged 35 +/- 6 yr). Observations were made in normoxia (N) and after 1 (H1) and 3 (H3) wk at 6,542 m. Acclimatization was accomplished by gradual climbing from 4,000 to 6,542 m over 10 days. Plasma NE was obtained at rest and during submaximal exercise. Successive doses of IP (0.02, 0.04, and 0.06 microgram/kg-1.min-1) were infused for 5 min each. Density and affinity of lymphocyte beta 2-AR were also measured. Increase in HR for maximal dose of IP decreased from 57 +/- 12 to 34 +/- 7 and 37 +/- 10 min-1 in H1 and H3, respectively (P < 0.001 for both). IP dose for which HR rises by 25 min-1 (I25) increased from 27 +/- 5 in N to 42 +/- 10 and 43 +/- 17 ng.kg-1.min-1 in H1 and H3, respectively (P < 0.001 for both). Arterial O2 saturation at rest was 98 +/- 2% in N, 72 +/- 6% in H1 (P < 0.001), and 79 +/- 5% in H3 (P < 0.001). The chronotropic response was neither restored nor further attenuated after 3 wk at 6,542 m. Plasma NE levels at rest and during exercise were higher at 6,542 m than values obtained in previous studies at 4,350 and 4,800 m.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of erythropoiesis in humans during prolonged exposure to the altitude of 6,542 m

Altitude hypoxia induces an increase in erythropoiesis. Some of the factors involved in the control of altitude polycythemia were studied. Ten subjects (4 women, 6 men) were exposed for 3 wk to extreme altitude (6,542 m). Blood was withdrawn in normoxia (N) and after 1 wk (H1), 2 wk (H2), or 3 wk (H3) at 6,542 m for the measurement of serum erythropoietin (EPO), blood hemoglobin (Hb), hematocrit (Hct), intraerythrocyte folate (Fol), and plasma ferritin (Fer) concentrations. Renal blood flow (RBF) and absolute proximal reabsorption rate (APR) were measured by the p-aminohippuric acid and lithium clearance, respectively, in N and H2 conditions. O2 supply to the kidneys was calculated using RBF and arterial O2 content (CaO2). After an initial sharp increase in EPO, it decreased at H2 and H3. Hct and Hb increased from N to H1 and H2 and then unexpectedly decreased from H2 to H3. Mean corpuscular Hb content (MCHC = Hb/Hct) was lower in all H than in N conditions. Increase in EPO at H1 varied from 3- to 134-fold among individuals. Women showed a smaller increase in Hct and Hb and a greater decrease in MCHC. Two women showed a large increase in EPO without increase in Hb. Fol was not modified by altitude hypoxia. Fer showed a marked decrease in H1 and H3 compared with N. Hb was positively related to Fer in hypoxia. Iron intake in food was markedly decreased during the 2-wk ascent to 6,542 m. EPO was inversely related to CaO2 and positively related to APR.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia-induced differential modulation of adenosinergic and muscarinic receptors in rat heart

To better understand the decreased chronotropic response to catecholamines in chronic hypoxia, we compared the inhibitory pathways regulating adenylate cyclase in rats exposed for 30 days to hypobaric hypoxia (380 Torr; HX) with those in control rats (CT) by the analysis of adenosinergic A1-receptors (8-cyclopentyl-1,3-[3H]dipropylxanthine) and muscarinic M2-receptors ([3H]quinuclidinyl benzilate). A1-receptor density was decreased by 46% in sarcolemmal preparations without a change in the affinity for agonist [(R)-phenylisopropyladenosine]. M2-receptor density was increased (HX: 280 +/- 16 fmol/mg, CT: 188 +/- 15 fmol/mg; n = 7; P < 0.001) without a change in dissociation constant. Displacement of [3H]quinuclidinyl benzilate by carbachol indicated significant decreases in the dissociation constants of both superhigh- (HX: 73 +/- 19 nM, CT: 182 +/- 42 nM; P < 0.001) and high-affinity binding sites (HX: 4 +/- 1 microM, CT: 12 +/- 3 microM; P < 0.001). Our data show that chronic hypoxia leads to differential modulation of cardiac receptors with a downregulation of adenosine receptors and increases in muscarinic receptor affinity and density, which may contribute to the blunted responsiveness of the heart to catecholamines.

Renal hemodynamics, tubular function, and response to low-dose dopamine during acute hypoxia in humans

Renal function was investigated in eight normal subjects before and during infusion of dopamine (3 micrograms.kg-1 x min-1) at sea level (SL) and at high altitude (HA, 4,350 m). Lithium clearance (CLi) was used as an index of proximal tubular outflow. HA significantly increased arterial pressure, heart rate, and plasma norepinephrine. Effective renal plasma flow (ERPF) decreased at HA by 10% (P < 0.05), but glomerular filtration rate (GFR), CLi, sodium clearance (CNa), and urine flow remained unchanged compared with SL. Dopamine at SL and HA increased ERPF by 47% (P < 0.001) and 30% (P < 0.01), respectively, but the increase at HA was smaller than that at SL (P < 0.05). Dopamine increased GFR only at SL. CLi and CNa increased by 29% (P < 0.001) and 108% (P < 0.001) at SL and by 23% (P < 0.01) and 108% (P < 0.001) at HA. Whereas dopamine at SL increased urine flow by 46% (P < 0.01), this response was abolished at HA, and free water clearance decreased (P < 0.05). The decreased ERPF at HA suggests a constriction of the renal arterioles secondary to increased adrenergic nervous activity. Although the effect of dopamine on ERPF was attenuated in hypoxia, dopamine-induced increases in CLi and CNa remained unaltered, suggesting that natriuresis in both environments was secondary to an increased outflow from the proximal tubules. The absence of a diuretic response to dopamine at HA seemed to be caused by an effect on distal tubular function.

Reduction of postprandial lipemia after acute exposure to high altitude hypoxia

The effects of acute exposure to high altitude hypoxia upon plasma levels of lipids and lipoproteins were studied in 6 healthy subjects observed under strict dietary control. Fasting and postprandial values, first measured at low altitude (< 300 m), were compared to values measured 7 days after transfer by helicopter to the Observatoire Vallot (4350 m) near Mont-Blanc. Plasma levels of catecholamines and thyroid hormones were measured in parallel. Under normoxic conditions, a rise in plasma levels of cholesterol, triglycerides and phospholipids was found 4 hr after the test-meal, whereas circulating norepinephrine fell. Under hypoxic conditions, postprandial effects on lipid parameters disappeared, lipemic responses to the test-meal were dramatically reduced and plasma levels of norepinephrine increased. These effects were accompanied by a fall in triglyceride-rich lipoproteins, generally associated with a slight elevation of HDL-cholesterol, especially the less dense HDL2 fraction. When all results obtained in normoxic and hypoxic conditions were analyzed together, a series of highly significant correlations were found between the magnitude of postprandial lipidemia and various lipid parameters, in particular HDL2-cholesterol which was negatively correlated. These observations strongly suggest that high altitude hypoxia, independent of interfering variables such as exercise training or cold exposure, induced a net stimulation of the lipolysis of plasma triglycerides.

Effects of acute hypoxia on renal and endocrine function at rest and during graded exercise in hydrated subjects

Renal effects of altitude hypoxia are unclear. Renal and hormonal function was investigated in eight males at rest and during graded exercise at sea level (SL) and 48 h after rapid ascent to 4,350 m (HA). HA did not change resting values of effective renal plasma flow (ERPF), glomerular filtration rate (GFR), sodium clearance (CNa), urine flow, or lithium clearance (CLi), which was used as an index of proximal tubular outflow. At rest, HA increased plasma norepinephrine concentration and decreased plasma concentrations of renin and aldosterone. Exercise decreased ERPF similarly in both environments. Normoxic exercise progressively reduced GFR, but at HA GFR only decreased during heavy exercise. This resulted in a higher filtration fraction during light and moderate hypoxic exercise. However, calculated absolute proximal reabsorption rate (GFR-CLi) at HA was higher during low-intensity exercise, and there were no significant differences between exercise-induced decreases in CLi, CNa, and urine flow at HA compared with SL. Exercise gradually increased plasma norepinephrine, but values were higher at HA during light and moderate exercise. The small changes in the renal response to low-intensity hypoxic exercise may be secondary to increased adrenosympathetic activity. However, antidiuretic and antinatriuretic effects of exercise were maintained in hypoxia and in both environments seemed to be the consequence of decreased proximal tubular outflow. The results demonstrate that renal glomerular and tubular function is well preserved in acute hypoxia despite marked hormonal changes.

Adrenergic system in high altitude residents

Heart rate (HR) response to isoproterenol (ISO) infusion (IP) is decreased in normal sea level (SL) natives exposed to high altitude (HA). Since norepinephrine plasma concentration is higher in HA hypoxia, a downregulation of beta-adrenoceptors (beta AR) was evoked. We explored this phenomenon at 3600 m in a HA normal population (HAN) and in polycythemic subjects (HAP). Results are compared to SL natives in normoxia (SLN), and during chronic hypoxia at 4800 m (SLH) (J Appl Physiol 65:1957-1961, 1988). ISO dose required to raise HR by 25 min-1 (I 25) is not different in HAN or HAP group when compared to SLN. Density of beta AR on lymphocytes was 39% and 25% lower in HAN and HAP than in SLN group, respectively. Chronotropic response to IP is similar in SL and HA subjects under their usual environmental conditions, while SL natives show a blunted response under hypoxia, probably due to a decrease in beta AR density. No adrenergic desensitization was found in highlanders. Lower beta AR density in HA groups could be an adaptive mechanism to chronic hypoxia. Polycythemia does not affect this responsiveness.

Hypoxia-induced downregulation of beta-adrenergic receptors in rat heart

To test the desensitization hypothesis of cardiac beta-adrenergic receptors (beta-AR) in chronic hypoxia, the effect of 1, 3, 7, 15, and 21 days of exposure to hypobaric hypoxia (380 Torr) was evaluated in Wistar rats. Exposure to hypoxia for 1-15 days did not induce any change in right and left ventricular beta-AR density (Bmax) determined with [125I]iodocyanopindolol or in antagonist affinity. After 21 days, Bmax decreased by 24% in the left ventricle. In contrast, no change in beta-AR was shown in the right hypertrophied ventricle. Agonist affinity in the left ventricle was not altered, as shown by the analysis of displacement curves of isoproterenol (normoxia 185 +/- 26 nM, hypoxia 170 +/- 11 nM). Moreover, there was no significant decrease in adenylate cyclase activity (pmol.mg-1.min-1) in the left ventricle. In the right ventricle, a 21-day exposure to hypoxia led to a decrease in basal and maximal activity when stimulated by isoproterenol. A decrease in tissue norepinephrine content was observed after 7 days of hypoxia. In conclusion, these data support the beta-AR downregulation hypothesis as one of the mechanisms of myocardial adaptation to high altitude occurring after 2-3 wk of exposure to hypoxia. The regulation pathways of beta-AR may differ between left nonhypertrophied and right hypertrophied ventricles. No evidence of profound abnormality of signal transduction was shown.