Respiratory responses of the air-breathing fish Hoplosternum littorale to hypoxia and hydrogen sulfide

The present study analyzes the respiratory responses of the neotropical air-breathing fish Hoplosternum littorale to graded hypoxia and increased sulfide concentrations. The oxygen uptake (VO2), critical O2 tension (PcO2), respiratory (fR) and air-breathing (fRA) frequencies in response to graded hypoxia were determined for fish acclimated to 28 degrees C. H. littorale was able to maintain a constant VO2 down to a PcO2 of 50 mm Hg, below which fish became dependent on the environmental O2 even with significant increases in fR. The fRA was kept constant around 1 breath h(-1) above 50 mm Hg and increased significantly below 40 mm Hg, reaching maximum values (about 4.5 breaths h(-1)) at 10 mm Hg. The lethality to sulfide concentrations under normoxic and hypoxic conditions were also determined along with the fRA. For the normoxic fish the sulfide lethal limit was about 70 microM, while in the hypoxic ones this limit increased to 87 muM. The high sulfide tolerance of H. littorale may be attributed to the air-breathing capability, which is stimulated by this compound.     

Effects of acute temperature changes on aerial and aquatic gas exchange, pulmonary ventilation and blood gas status in the South American lungfish, Lepidosiren paradoxa

Lungfish (Dipnoi) are probably sister group relative to all land vertebrates (Tetrapoda). The South American lungfish, Lepidosiren paradoxa, depends markedly on pulmonary gas exchange. In this context, we report on temperature effects on aquatic and pulmonary respiration, ventilation and blood gases at 15, 25 and 35 degrees C. Lung ventilation increased from 0.5 (15 degrees C) to 8.1 ml BTPS kg(-1) min(-1) (35 degrees C), while pulmonary O(2)-uptake increased from 0.06 (15 degrees C) to 0.73 ml STPD kg(-1) min(-1) (35 degrees C). Meanwhile aquatic O(2)-uptake remained about the same ( approximately 0.01 ml STPD kg(-1) min(-1)) at all temperatures. Concomitantly, the pulmonary gas exchange ratio (R(E)) rose from 0.11 (15 degrees C) to 0.62 (35 degrees C), because a larger fraction of total CO(2) output became eliminated by the lung. Accordingly, PaCO(2) rose from 13 (15 degrees C) to 37 mm Hg (35 degrees C), leading to a significant decrease of pHa at higher temperature (pHa=7.58-15 degrees C; 7.33-35 degrees C). The acid-base status of L. paradoxa was characterized by a generally low pH (7.4-7.5), high bicarbonate level (20-25 mM) and PaO(2) ( approximately 80 mm Hg). The increased dependence on the lung at higher temperature parallels data for amphibians. Further, the effects of bimodal gas exchange on temperature-dependent acid-base regulation closely resemble those of anuran amphibians.     

Heart rate and rate of oxygen consumption during flight of the barnacle goose, Branta leucopsis

We report the first measurements of heart rate (f(H)) and the rate of oxygen consumption (V(O(2))) during flights from a species of bird larger than 500 g. V(O(2))was obtained from nine forward flapping flights of 8.9 min mean duration at a mean speed of 13.2 m s(-1) performed by three barnacle geese of mean mass 1.68 kg. Mean V(O(2))was 332 ml min(-1)or 201 ml min(-1) kg(-1). Sixteen flights were obtained from two of these birds equipped with heart rate data loggers, both when they were wearing a V(O(2)) mask and when they were not. During flights with the mask (mean duration 7.4 min), mean f(H) was 472 beats per min and during flights without the mask (mean duration 8.0 min) it was 391 beats per min. Heart rate was also measured in another goose flying without a respiratory mask and mean f(H) for all the three birds (mean mass 1.7 kg) flying without a mask for an average of 7.9 min at 13 m s(-1) was 378 beats per min. Resting f(H) for these three birds was 79 beats per min. The values of f(H) during flight are greater than those obtained from the same species during their autumn migration from Spitsbergen to southern Scotland. The possible reasons for this are discussed.     

Hydrogen peroxide production by alveolar macrophages is increased and its concentration is elevated in the breath of rats exposed to hypoxia: relationship to lung lipid peroxidation

Hypoxic exposure triggers a generation of reactive oxygen species that initiate free radical damage to the lung. Hydrogen peroxide is the product of alveolar macrophages detectable in the expired breath. We evaluated the significance of breath H(2)O(2) concentration for the assessment of lung damage after hypoxic exposure and during posthypoxic period. Adult male rats were exposed to normobaric hypoxia (10 % O(2)) for 3 hours or 5 days. Immediately after the hypoxic exposure and then after 7 days or 14 days of air breathing, H(2)O(2) was determined in the breath condensate and in isolated lung macrophages. Lipid peroxidation was measured in lung homogenates. Three-hour hypoxia did not cause immediate increase in the breath H(2)O(2); 5-day hypoxia increased breath H(2)O(2) level to 458 %. After 7 days of subsequent air breathing H2O2 was elevated in both groups exposed to hypoxia. Increased production of H(2)O(2) by macrophages was observed after 5 days of hypoxia and during the 7 days of subsequent air breathing. Lipid peroxidation increased in the periods of enhanced H(2)O(2) generation by macrophages. As the major increase (1040 %) in the breath H(2)O(2) concentration found 7 days after 3 hours of hypoxia was not accompanied by lipid peroxidation, it can be concluded that the breath H(2)O(2) is not a reliable indicator of lung oxidative damage.     

Oxidative metabolism and anaerobic glycolysis during repeated exercise

The purpose of the present study was to examine aerobic and muscle anaerobic energy production during supramaximal repeated exercise. Eight subjects performed three 2-min bouts of cycling (EX1-EX3) at an intensity corresponding to about 125 % of VO2 max separated by 15 min of rest. Ventilatory variables were measured breath by breath during the exercise and a muscle biopsy was taken before and after each exercise bout. Blood samples were collected before and after each cycling period and during the recovery periods. Total work in the first 2 min bout of cycling, EX1, [46.3 +/- 2.1 KJ] was greater than in the second, EX2, (p < 0.01) and in the third, EX3, (p < 0.05). The ATP utilization [4.0 +/- 1.4 mmol x (kg dry weight)(-1), EX1] during the three exercise bouts was the same. The decrement in muscle phosphocreatine (PCr) [46.8 +/- 8.5 mmol x (kg dry weight)(-1), EX1] was also similar for the three exercise bouts. Muscle lactate accumulation was greater (p < 0.05) during EX1 compared to EX2 and EX3. The total oxygen consumption was the same for the three exercise bouts, but when it is corrected for the total work performed, oxygen uptake during EX2 (153 +/- 9 ml x KJ(-1)) and EX3 (150 +/- 9 ml x KJ(-1)) was higher (p < 0.01 and p < 0.05, respectively) than during EX1 (139 +/- 8 ml x KJ(-1)). The present data suggest that oxidative metabolism does not compensate for the reduction of anaerobic glycolysis during repeated fatiguing exercise.     

Hypercapnia does not affect functional residual capacity enlargement induced by chronic hypoxia

To determine whether changes in partial pressure of CO2 participate in mechanism enlarging the lung functional residual capacity (FRC) during chronic hypoxia, we measured FRC and ventilation in rats exposed either to poikilocapnic (group H, F(I)O2 0.1, F(I)CO2 <0.01) or hypercapnic (group H+CO2, F(I)O2 0.1, F(I)CO2 0.04-0.05) hypoxia for the three weeks and in the controls (group C) breathing air. At the end of exposure a body plethysmograph was used to measure ventilatory parameters (V'(E), f(R), V(T)) and FRC during air breathing and acute hypoxia (10 % O2 in N2). The exposure to hypoxia for three weeks increased FRC measured during air breathing in both experimental groups (H: 3.0+/-0.1 ml, H+CO2: 3.1+/-0.2 ml, C: 1.8+/-0.2 ml). During the following acute hypoxia, we observed a significant increase of FRC in the controls (3.2+/-0.2 ml) and in both experimental groups (H: 3.5+/-0.2 ml, H+CO2: 3.6+/-0.2 ml). Because chronic hypoxia combined with chronic hypercapnia and chronic poikilocapnic hypoxia induced the same increase of FRC, we conclude that hypercapnia did not participate in the FRC enlargement during chronic hypoxia.     

A comparison of adrenergic stress responses in three tropical teleosts exposed to acute hypoxia

Experiments were performed to assess the afferent and efferent limbs of the hypoxia-mediated humoral adrenergic stress response in selected hypoxia-tolerant tropical fishes that routinely experience environmental O(2) depletion. Plasma catecholamine (Cat) levels and blood respiratory status were measured during acute aquatic hypoxia [water Po(2) (Pw(O(2))) = 10-60 mmHg] in three teleost species, the obligate water breathers Hoplias malabaricus (traira) and Piaractus mesopotamicus (pacu) and the facultative air breather Hoplerythrinus unitaeniatus (jeju). Traira displayed a significant increase in plasma Cat levels (from 1.3 +/- 0.4 to 23.3 +/- 15.1 nmol/l) at Pw(O(2)) levels below 20 mmHg, whereas circulating Cat levels were unaltered in pacu at all levels of hypoxia. In jeju denied access to air, plasma Cat levels were increased markedly to a maximum mean value of 53.6 +/- 19.1 nmol/l as Pw(O(2)) was lowered below 40 mmHg. In traira and jeju, Cat release into the circulation occurred at abrupt thresholds corresponding to arterial Po(2) (Pa(O(2))) values of approximately 8.5-12.5 mmHg. A comparison of in vivo blood O(2) equilibration curves revealed low and similar P(50) values (i.e., Pa(O(2)) at 50% Hb-O(2) saturation) among the three species (7.7-11.3 mmHg). Thus Cat release in traira and jeju occurred as blood O(2) concentration was reduced to approximately 50-60% of the normoxic value. Intravascular injections of nicotine (600 nmol/kg) elicited pronounced increases in plasma Cat levels in traira and jeju but not in pacu. Thus the lack of Cat release during hypoxia in pacu may reflect an inoperative or absent humoral adrenergic stress response in this species. When allowed access to air, jeju did not release Cats into the circulation at any level of aquatic hypoxia. The likeliest explanation for the absence of Cat release in these fish was that air breathing, initiated by aquatic hypoxia, prevented Pa(O(2)) values from falling to the critical threshold required for Cat secretion. The ventilatory responses to hypoxia in each species were similar, consisting generally of increases in both frequency and amplitude. These responses were not synchronized with or influenced by plasma Cat levels. Thus the acute humoral adrenergic stress response does not appear to stimulate ventilation during acute hypoxia in these tropical species.     

Effects of intermittent hypoxic training on amino and fatty acid oxidative combustion in human permeabilized muscle fibers.

The effects of concurrent hypoxic/endurance training on mitochondrial respiration in permeabilized fibers in trained athletes were investigated. Eighteen endurance athletes were divided into two training groups: normoxic (Nor, n = 8) and hypoxic (H, n = 10). Three weeks (W1-W3) of endurance training (5 sessions of 1 h to 1 h and 30 min per week) were completed. All training sessions were performed under normoxic [160 Torr inspired Po(2) (Pi(O(2)))] or hypoxic conditions ( approximately 100 Torr Pi(O(2)), approximately 3,000 m) for Nor and H group, respectively, at the same relative intensity. Before and after the training period, an incremental test to exhaustion in normoxia was performed, muscle biopsy samples were taken from the vastus lateralis, and mitochondrial respiration in permeabilized fibers was measured. Peak power output (PPO) increased by 7.2% and 6.6% (P < 0.05) for Nor and H, respectively, whereas maximal O(2) uptake (Vo(2 max)) remained unchanged: 58.1 +/- 0.8 vs. 61.0 +/- 1.2 ml.kg(-1).min(-1) and 58.5 +/- 0.7 vs. 58.3 +/- 0.6 ml.kg(-1).min(-1) for Nor and H, respectively, between pretraining (W0) and posttraining (W4). Maximal ADP-stimulated mitochondrial respiration significantly increased for glutamate + malate (6.27 +/- 0.37 vs. 8.51 +/- 0.33 mumol O(2).min(-1).g dry weight(-1)) and significantly decreased for palmitate + malate (3.88 +/- 0.23 vs. 2.77 +/- 0.08 mumol O(2).min(-1).g dry weight(-1)) in the H group. In contrast, no significant differences were found for the Nor group. The findings demonstrate that 1) a 3-wk training period increased the PPO at sea level without any changes in Vo(2 max), and 2) a 3-wk hypoxic exercise training seems to alter the intrinsic properties of mitochondrial function, i.e., substrate preference.     

Longitudinal distribution of chlorine absorption in human airways: a comparison to ozone absorption.

The bolus inhalation method was used to measure the fraction of inhaled chlorine (Cl(2)) and ozone (O(3)) absorbed during a single breath as a function of longitudinal position in the respiratory system of 10 healthy nonsmokers during oral and nasal breathing at respired flows of 150, 250, and 1,000 ml/s. At all experimental conditions, <5% of inspired Cl(2) penetrated beyond the upper airways and none reached the respiratory air spaces. On the other hand, larger penetrations of O(3) beyond the upper airways occurred as flow increased and during nasal than during oral breathing. In the extreme case of oral breathing at 1,000 ml/s, 35% of inhaled O(3) penetrated beyond the upper airways and approximately 10% reached the respiratory air spaces. Mass transfer theory indicated that the diffusion resistance of the tissue phase was negligible for Cl(2) but important for O(3). The gas phase resistances were the same for Cl(2) and O(3) and were directly correlated with the volume of the nose and mouth during nasal and oral breathing, respectively.     

Metabolism and thermoregulation during fasting in king penguins, Aptenodytes patagonicus, in air and water

We measured oxygen consumption rate (Vo(2)) and body temperatures in 10 king penguins in air and water. Vo(2) was measured during rest and at submaximal and maximal exercise before (fed) and after (fasted) an average fasting duration of 14.4 +/- 2.3 days (mean +/- 1 SD, range 10-19 days) in air and water. Concurrently, we measured subcutaneous temperature and temperature of the upper (heart and liver), middle (stomach) and lower (intestine) abdomen. The mean body mass (M(b)) was 13.8 +/- 1.2 kg in fed and 11.0 +/- 0.6 kg in fasted birds. After fasting, resting Vo(2) was 93% higher in water than in air (air: 86.9 +/- 8.8 ml/min; water: 167.3 +/- 36.7 ml/min, P < 0.01), while there was no difference in resting Vo(2) between air and water in fed animals (air: 117.1 +/- 20.0 ml O(2)/min; water: 114.8 +/- 32.7 ml O(2)/min, P > 0.6). In air, Vo(2) decreased with M(b), while it increased with M(b) in water. Body temperature did not change with fasting in air, whereas in water, there were complex changes in the peripheral body temperatures. These latter changes may, therefore, be indicative of a loss in body insulation and of variations in peripheral perfusion. Four animals were given a single meal after fasting and the temperature changes were partly reversed 24 h after refeeding in all body regions except the subcutaneous, indicating a rapid reversal to a prefasting state where body heat loss is minimal. The data emphasize the importance in considering nutritional status when studying king penguins and that the fasting-related physiological changes diverge in air and water.     

Measurement of cardiac output during exercise by open-circuit acetylene uptake.

Noninvasive measurement of cardiac output (QT) is problematic during heavy exercise. We report a new approach that avoids unpleasant rebreathing and resultant changes in alveolar PO(2) or PCO(2) by measuring short-term acetylene (C(2)H(2)) uptake by an open-circuit technique, with application of mass balance for the calculation of QT. The method assumes that alveolar and arterial C(2)H(2) pressures are the same, and we account for C(2)H(2) recirculation by extrapolating end-tidal C(2)H(2) back to breath 1 of the maneuver. We correct for incomplete gas mixing by using He in the inspired mixture. The maneuver involves switching the subject to air containing trace amounts of C(2)H(2) and He; ventilation and pressures of He, C(2)H(2), and CO(2) are measured continuously (the latter by mass spectrometer) for 20-25 breaths. Data from three subjects for whom multiple Fick O(2) measurements of QT were available showed that measurement of QT by the Fick method and by the C(2)H(2) technique was statistically similar from rest to 90% of maximal O(2) consumption (VO(2 max)). Data from 12 active women and 12 elite male athletes at rest and 90% of VO(2 max) fell on a single linear relationship, with O(2) consumption (VO(2)) predicting QT values of 9.13, 15.9, 22.6, and 29.4 l/min at VO(2) of 1, 2, 3, and 4 l/min. Mixed venous PO(2) predicted from C(2)H(2)-determined QT, measured VO(2), and arterial O(2) concentration was approximately 20-25 Torr at 90% of VO(2 max) during air breathing and 10-15 Torr during 13% O(2) breathing. This modification of previous gas uptake methods, to avoid rebreathing, produces reasonable data from rest to heavy exercise in normal subjects.     

Metabolic equivalent: one size does not fit all.

The metabolic equivalent (MET) is a widely used physiological concept that represents a simple procedure for expressing energy cost of physical activities as multiples of resting metabolic rate (RMR). The value equating 1 MET (3.5 ml O2 x kg(-1) x min(-1) or 1 kcal x kg(-1) x h(-1)) was first derived from the resting O2 consumption (VO2) of one person, a 70-kg, 40-yr-old man. Given the extensive use of MET levels to quantify physical activity level or work output, we investigated the adequacy of this scientific convention. Subjects consisted of 642 women and 127 men, 18-74 yr of age, 35-186 kg in weight, who were weight stable and healthy, albeit obese in some cases. RMR was measured by indirect calorimetry using a ventilated hood system, and the energy cost of walking on a treadmill at 5.6 km/h was measured in a subsample of 49 men and 49 women (26-45 kg/m2; 29-47 yr). Average VO2 and energy cost corresponding with rest (2.6 +/- 0.4 ml O2 x kg(-1) x min(-1) and 0.84 +/- 0.16 kcal x kg(-1) x h(-1), respectively) were significantly lower than the commonly accepted 1-MET values of 3.5 ml O2 x kg(-1) x min(-1) and 1 kcal x kg(-1) x h(-1), respectively. Body composition (fat mass and fat-free mass) accounted for 62% of the variance in resting VO2 compared with age, which accounted for only 14%. For a large heterogeneous sample, the 1-MET value of 3.5 ml O2 x kg(-1) x min(-1) overestimates the actual resting VO2 value on average by 35%, and the 1-MET of 1 kcal/h overestimates resting energy expenditure by 20%. Using measured or predicted RMR (ml O2 x kg(-1) x min(-1) or kcal x kg(-1) x h(-1)) as a correction factor can appropriately adjust for individual differences when estimating the energy cost of moderate intensity walking (5.6 km/h).     

Heterogeneity of local myocardial flow and oxidative metabolism

In mammalian hearts, local myocardial flow (LMF) varies between 20 and 200% of the mean. It is not clear whether oxidative metabolism has a similar degree of heterogeneity. Therefore, we investigated the relation between LMF and local oxidative metabolism in isolated rabbit hearts. Buffer oxygenation with (18)O(2) resulted in labeled myocardial oxidation water (H(2)(18)O). In four hearts, myocardial oxygen consumption (MVO(2)) was calculated from the H(2)(18)O production and compared with that calculated according to Fick. In eight additional hearts, LMF was measured using microspheres. Coronary venous H(2)(18)O kinetics and local H(2)(18)O residues were determined and analyzed by mathematical modeling. MVO(2) recovery from H(2)(18)O was >93% compared with that according to Fick. LMF ranged from 1.91 to 11.24 ml. min(-1). g(-1), and local H(2)(18)O residue ranged from 0.41 to 1.04 micromol/g. Both variables correlated (r = 0.62, n = 64, P < 0.001). Measurements in nine hearts were fitted by modeling using capillary permeability-surface area products (PS(c)) from 2 to 10 ml. min(-1). g(-1). With flow-proportional PS(c), a 3.33-fold difference in LMF was associated with a 6.45-fold difference in local MVO(2). Both LMF and local oxidative metabolism are spatially heterogeneous, and they correlate to one another.     

Factorial scopes of cardio-metabolic variables remain constant with changes in body temperature in the varanid lizard, Varanus rosenbergi

The majority of information concerning the cardio-metabolic performance of varanids during exercise is limited to a few species at their preferred body temperature (T(b)) even though, being ectotherms, varanids naturally experience rather large changes in T(b). Although it is well established that absolute aerobic scope declines with decreasing T(b), it is not known whether changes in cardiac output (V(b)) and/or tissue oxygen extraction, (Ca(O2) - Cv(O2)), are in proportion to the rate of oxygen consumption (Vo(2)). To test this, we studied six Rosenberg's goannas (Varanus rosenbergi) while at rest and while maximally exercising on a treadmill both at 25 and 36 degrees C. During maximum exercise both at 25 and 36 degrees C, mass-specific rate of oxygen consumption (Vo(2kg)) increased with an absolute scope of 8.5 ml min(-1) kg(-1) and 15.7 ml min(-1) kg(-1), respectively. Interestingly, the factorial aerobic scope was temperature-independent and remained at 7.0 which, at each T(b), was primarily the result of an increase in V(bkg), governed by approximate twofold increases both in heart rate (f(H)) and cardiac stroke volume (V(Skg)). Both at 25 degrees C and 36 degrees C, the increase in V(bkg) alone was not sufficient to provide all of the additional oxygen required to attain maximal Vo(2kg), as indicated by a decrease in the blood convection requirement V(bkg)/Vo(2kg); hence, there was a compensatory twofold increase in (Ca(O2) - Cv(O2)). Although associated with an increase in hemoglobin-oxygen affinity, a decrease in T(b) did not impair unloading of oxygen at the tissues and act to reduce (Ca(O2) - Cv(O2)); both Ca(O2)) and Cv(O2)) were maintained across T(b). The change in Vo(2kg) with T(b), therefore, is solely reliant on the thermal dependence of V(bkg). Maintaining a high factorial aerobic scope across a range of T(b) confers an advantage in that cooler animals can achieve higher absolute aerobic scopes and presumably improved aerobic performance than would otherwise be achievable.     

Isolation and characterization of microsatellite markers from the marine isopods Serolis paradoxa and Septemserolis septemcarinata (Crustacea: Peracarida)

This study reports the successful isolation of highly informative microsatellite marker sets for two marine serolid isopod species. For Serolis paradoxa (Fabricius, 1775), 13, and for Septemserolis septemcarinata (Miers, 1875), eight polymorphic microsatellite markers were isolated using the reporter genome enrichment protocol. The number of alleles per locus (N(A) ) and the observed heterozygosity (H(O) ) encompass a wide range of variation within S. paradoxa (N(A) 3-31, H(O) 6-89%) and S. septemcarinata (N(A) 2-18, H(O) 9-94%). The suitability of the newly isolated markers for population genetic studies is evaluated.     

13CO2 washout dynamics during intermittent exercise in children and adults.

To test the hypothesis that children store less CO2 than adults during exercise, we measured breath 13CO2 washout dynamics after oral bolus of [13C]bicarbonate in nine children [8 +/- 1 (SD) yr, 4 boys] and nine (28 +/- 6 yr, 5 males) adults. Gas exchange [O2 uptake and CO2 production (Vco2)] was measured breath by breath during rest and during light (80% of the anaerobic threshold) intermittent exercise. Breath samples were obtained for subsequent analysis of 13CO2 by isotope ratio mass spectrometry. The tracer estimate of Vco2 was highly correlated to Vco2 measured by gas exchange (r = 0.97, P < 0.0001). The mean residence time was shorter in children (50 +/- 5 min) compared with adults (69 +/- 7 min, P < 0.0001) at rest and during exercise (children, 35 +/- 7 min; adults, 50 +/- 11 min, P < 0.001). The estimate of stored CO2 (using mean Vco2 measured by gas exchange and mean residence time derived from tracer washout) was not statistically different at rest between children (254 +/- 36 ml/kg) and adults (232 +/- 37 ml/kg). During exercise, CO2 stores in the adults (304 +/- 46 ml/kg) were significantly increased over rest (P < 0.001), but there was no increase in children (mean exercise value, 254 +/- 38 ml/kg). These data support the hypothesis that CO2 distribution in response to exercise changes during the growth period.     

Cardiorespiratory responses of the facultative air-breathing fish jeju, Hoplerythrinus unitaeniatus (Teleostei, Erythrinidae), exposed to graded ambient hypoxia

The jeju, Hoplerythrinus unitaeniatus, is equipped with a modified part of the swim bladder that allows aerial respiration. On this background, we have evaluated its respiratory and cardiovascular responses to aquatic hypoxia. Its aquatic O2 uptake (V(O2)) was maintained constant down to a critical P(O2) (P(cO2)) of 40 mm Hg, below which V(O2) declined linearly with further reductions of P(iO2). Just below P(cO2), the ventilatory tidal volume (V(T)) increased significantly along with gill ventilation (V(G)), while respiratory frequency changed little. Consequently, water convection requirement (V(G)/V(O2)) increased steeply. The same threshold applied to cardiovascular responses that included reflex bradycardia and elevated arterial blood pressure (P(a)). Aerial respiration was initiated at water P(O2) of 44 mm Hg and breathing episodes and time at the surface increased linearly with more severe hypoxia. At the lowest water P(O2) (20 mm Hg), the time spent at the surface accounted for 50% of total time. This response has a character of a temporary emergency behavior that may allow the animal to escape hypoxia.     

L-arginine enhances aerobic exercise capacity in association with augmented nitric oxide production.

We tested whether supplementation with L-arginine can augment aerobic capacity, particularly in conditions where endothelium-derived nitric oxide (EDNO) activity is reduced. Eight-week-old wild-type (E(+)) and apolipoprotein E-deficient mice (E(-)) were divided into six groups; two groups (LE(+) and LE(-)) were given L-arginine (6% in drinking water), two were given D-arginine (DE(+) and DE(-)), and two control groups (NE(+) and NE(-)) received no arginine supplementation. At 12-16 wk of age, the mice were treadmill tested, and urine was collected after exercise for determination of EDNO production. NE(-) mice demonstrated a reduced aerobic capacity compared with NE(+) controls [maximal oxygen uptake (VO(2 max)) of NE(-) = 110 +/- 2 (SE) vs. NE(+) = 122 +/- 3 ml O(2). min(-1). kg(-1), P < 0.001]. This decline in aerobic capacity was associated with a diminished postexercise urinary nitrate excretion. Mice given L-arginine demonstrated an increase in postexercise urinary nitrate excretion and aerobic capacity in both groups (VO(2 max) of LE(-) = 120 +/- 1 ml O(2). min(-1). kg(-1), P < 0.05 vs. NE(-); VO(2 max) of LE(+) = 133 +/- 4 ml O(2). min(-1). kg(-1), P < 0.01 vs. NE(+)). Mice administered D-arginine demonstrated an intermediate increase in aerobic capacity in both groups. We conclude that administration of L-arginine restores exercise-induced EDNO synthesis and normalizes aerobic capacity in hypercholesterolemic mice. In normal mice, L-arginine enhances exercise-induced EDNO synthesis and aerobic capacity.     

Kinetics of intramuscular triglyceride fatty acids in exercising humans.

A pulse ([(14)C]palmitate)-chase ([(3)H]palmitate) approach was used to study intramuscular triglyceride (imTG) fatty acid and plasma free fatty acid (FFA) kinetics during exercise at approximately 45% peak O(2) consumption in 12 adults. Vastus lateralis muscle was biopsied before and after 90 min of bicycle exercise; (3)H(2)O production, breath (14)CO(2) excretion and lipid oxidation (indirect calorimetry) rates were measured during exercise. Results: during exercise, 8.2+/-1.2 and 8.4+/-0.7 micromol x kg(-1) x min(-1) of imTG fatty acids and plasma FFA, respectively, were oxidized according to isotopic measurements. The sum of these two values was not different (P = 0.6) from lipid oxidation by indirect calorimetry (15.4 +/-1.6 micromol x kg(-1) x min(-1)); the isotopic and indirect calorimetry values were correlated (r = 0.79, P<0.005). During exercise, imTG turnover rate was 0.32+/-0.07%/min (6.0+/-2.0 micromol of imTG x kg wet muscle(-1) x min(-1)) and plasma FFA were incorporated into imTG at a rate of 0.7+/-0.1 micromol x kg wet muscle(-1) x min(-1). The imTG pool size did not change during exercise. This pulse-chase, dual tracer appears to be a reasonable approach to measure oxidation and synthesis kinetics of imTG.     

Renomedullary interstitial cells in culture; the osmolality and oxygen tension influence the extracellular amounts of hyaluronan and cellular expression of CD44.

Our previous studies have suggested a role for renomedullary interstitial cells (RMICs) and renal medullary hyaluronan (HA) in water homeostasis. In the present study, cultured rat RMICs were used to examine the relationship of osmolality and oxygen tension on the extracellular amount of HA in the culture and to the cellular immunoreactivity to CD44, a HA binding protein. Under isotonic (330 mOsm(.)kg(-1) H(2)O), normoxic (20% O(2)) conditions, supernatant from sub-confluent RMICs contained 120+/-37 pg 10(4) cells(-1) 24 h(-1) of HA. Under hyperosmotic conditions (630 mOsm kg(-1) H(2)O), HA in the supernatant was decreased by 42% and under hypoosmotic conditions (230 mOsm kg(-1) H(2)O) it was doubled. Under hypoxic, iso-osmolar conditions (5% and 1% O(2), 330 mOsm kg(-1) H(2)O) this HA content was decreased by 56 and 48%, respectively, compared with normoxic, iso-osmolal conditions. Expression of CD44 on sub-confluent cells increased with increasing osmolality, as shown by immunostaining and flow cytometric analysis. The increases in CD44 from 330 to 630, 930 and 1230 mOsm kg(-1) H(2)O amounted to 5, 142 and 212%, respectively. Low oxygen tension (5% O(2)) decreased the intensity of CD44 immunofluorescence by 31%. Cell viability was similar at all conditions studied. In summary, these data indicate that cultured RMICs produce HA and are immunoreactive to CD44. In the supernatant of RMICs, the HA content decreases under hyperosmotic, hypoxic conditions. Conversely, CD44 immunoreactivity increases under hyperosmotic conditions. These results may explain our previous in vivo findings of a decreased renal papillary HA content during anti-diuresis and an increased content during water diuresis. The results support the concept that RMICs play an important role in renal water handling.