Changes in Functional Activity of Platelets after Repeated Physical Exercise

The changes in platelet aggregation and retraction parameters were studied in 23 young men with high working capacities (PWC₁₇₀ > 130 W) after dosed repeated exercise with two equal loads of a moderate (N ₁ = 1.36 W/kg, t ₁ = 6.94 min) and a high (N ₂ = 2.87 W/kg, t ₂ = 3.09 min) intensity with a 1-h rest between them. Three types of response of ADP-dependent platelet aggregation to the exercise were revealed: an increase after each load and a decrease to the initial value at rest; an increase after the first load, persistence at rest, and a decrease after the second load; and a decrease after both loads and restoration at rest. The changes in platelet activity in the different types of response depended on the initial values and the course of the rest. The changes in most parameters of aggregation and retraction after loads and at rest correlated with their initial values. Muscular activity raised platelet aggregability when it was initially low and lowered it when it was initially high. The preceding muscle load amplified these correlations, and the repeated muscle load amplified them even more pronouncedly.__________Translated from Fiziologiya Cheloveka, Vol. 31, No. 4, 2005, pp. 102–107.Original Russian Text Copyright © 2005 by Golyshenkov, Mel’nikova, Tairova.     

Seasonal and individual characteristics of the response of the fibrinolytic system of blood to physical exercise

The response of the fibrinolytic system of the blood of 33 untrained subjects (16 males and 17 females 18 years old) to a single 20-min bicycle-ergometric large-capacity load (N _male = 2.66 W/kg, N _female = 2.3 W/kg) was studied for 1 year. It is known that the blood fibrinolytic activity at rest has seasonal characteristics: it is relatively high in autumn and spring and low in winter and summer. On average, physical activity in winter and summer stimulates the blood clot lysis, whereas in autumn and spring this stimulation is not observed. The direction and intensity of the response of the fibrinolytic system to exercise have individual features: in autumn and spring they depend on its initial state (increase when the blood FA is low and decrease when it is high) and fluctuate widely, whereas in winter and summer this dependence is not observed. According to the direction of response to the load and its persistence, three types of responses can be distinguished: (a) hyperfibrinolytic, which is characterized by an increase in FA during exercise throughout the year, with a constantly low initial level of fibrinolytic activity at rest; (b) hypofibrinolytic, which is characterized by a stable suppression of fibrinolysis after exercise regardless of the season and the initial level of fibrinolytic activity, with a high fibrinolytic activity at rest in autumn and spring and a low fibrinolytic activity in winter and summer; and (c) unstable, in which the direction of the response of the fibrinolytic system to exercise varies. It is concluded that the development of thromboembolic complications after exercise can be expected with the highest and lowest probability in the subjects with the hypofibrinolytic and hyperfibrinolytic response, respectively.     

Role of the Initial State in the Response of the Hemostasis System to Heavy Exercise

The authors studied changes in the hemostasis system while working on bicycle ergometer with and without manifest fatigue. The direction and value of the change in blood coagulation time and natural lysis of a blood clot under the influence of exercise correlated with the initial state of the system. Work mostly inhibited blood coagulation when its initial values high and accelerated it when they were low. When fibrinolytic activity of blood at rest was low, it was stimulated; when it was high, it was inhibited. A similar relation between the initial values and response to exercise characterized several indices of the plasma link of hemostasis, such as plasma coagulation time, fibrinogen concentration, activity of antithromboplastins and antithrombin III, and euglobulin clot lysis time. Fatigue led to more manifest individual changes in most of the indices of coagulant, anticoagulant, and fibrinolytic activity of blood. As a rule, the value of correlation between the initial state and changes in the indices increased. This suggests strengthening of the role of the initial state in the hemostasis system response to exercise.     

The use of critical power as a determinant for establishing the onset of blood lactate accumulation

Eight highly trained male kayakers were studied to determine the relationship between critical power (CP) and the onset of blood lactate accumulation (OBLA). Four exercise sessions of 90 s, 240 s, 600 s, and 1200 s were used to identify the CP of each kayaker. Each individual CP was obtained from the line of best fit (LBF_CP) obtained from the progressive work output/time relationships. The OBLA was identified by the 4 mmol·l^–1 blood lactate concentration and the work output at this level was determined using a lactate curve test. This consisted of paddling at 50 W for 5 min after which a 1-min rest was taken during which a 25-l blood sample was taken to analyse for lactate. Exercise was increased by 50 W every 5 min until exhaustion, with the blood sample being taken in the 1-min rest period. The exercise intensity at the OBLA for each subject was then calculated and this was compared to the exercise intensity at the LBF_CP. The intensity at LBF_CP was found to be significantly higher (t=2.115, P<0.05) than that at the OBLA of 4 mmol·1^–1. These results were further confirmed by significant differences being obtained in blood lactate concentration (t=8.063, P<0.05) and heart rate values (t=2.90, P<0.05) obtained from the exercise intensity at LBF_CP over a 20-min period and that of the anaerobic threshold (Th_an) parameters obtained from the lactate/heart rate curve. These differences suggest that CP and Th_an are different physiological events and that athletes have utilised either one or the other methods for monitoring training and its effects.     

Screening of a high fibrinolytic enzyme producing strain and characterization of the fibrinolytic enzyme produced from <Emphasis Type="Italic">Bacillus subtilis</Emphasis> LD-8547

A fibrinolytic enzyme producing strain Bacillus subtilis LD-8 was isolated from douchi, a traditional Chinese soybean-fermented food. After mutagenesis treatments by UV, NTG (N-methyl-N′-nitroso-N-nitroso-guanidine) and γ-radiation, a high fibrinolytic enzyme producing strain B. subtilis LD-8547 was obtained. Under optimum condition, LD-8547 was able to yield the average fibrinolytic activity of 4220 U/mL in 15 L fermenter. The strong fibrin-specific enzyme was purified from supernatant of B. subtilis LD-8547 culture broth using the combination of various steps. The optimal temperature and pH value of this fibrinolytic enzyme were 50 °C and 8.0, respectively. The molecular weight was about 30 kDa measured by SDS-PAGE. The amidolytic activity of this fibrinolytic enzyme was inhibited completely by 1 mmol/L phenylmethanesulfonyl fluoride (PMSF), but EDTA and EGTA did not affect the enzyme activity. The apparent K _m and V _max values were 0.521 mmol/L and 0.049 mmol/min, respectively. In vitro assays revealed that the enzyme could catalyze blood clot lysis effectively, indicating that this enzyme could be a useful thrombolytic agent.     

Effect of arm cranking exercise on skin blood flow of lower limb in people with injuries to the spinal cord

The purpose of this study was to examine whether arm cranking exercise induces changes in skin blood flow in the paralyzed lower limbs of people with injuries to the spinal cord (PISC). Ten PISC with lesions located between Th5 and L5 and six control subjects performed arm cranking exercise for 6 min at three intensities, 10, 30 and 50 W, at a room temperature of 25°C. Oxygen uptake (Vo₂) and heart rate (HR) were measured for the last 2 min of each exercise period. The skin blood flow at the anterior thigh (BF_sk,t) was continuously monitored using laser Doppler flowmetry for the whole 6-min period and for the first 10 min of recovery following exercise. During exercise, the PISC showed lower Vo₂ and greater HR than the control subjects. No increase in BF_sk,t was found in six of the PISC with lesions at or above Th12, irrespective of the exercise intensity. On the other hand, in PISC with lesions at L1 or below, BF_sk,t increased significantly (P < 0.05) with an increase in Vo₂ and HR, although the BF_sk,t at a given Vo₂ and HR was lower than that in the control subjects. These results would suggest that arm exercise can promote the blood circulation in the skin of the lower limbs if the injury level is below L1.     

Increases in factor VIII complex and fibrinolytic activity are dependent on exercise intensity

Components of the factor VIII complex increase and activation of the fibrinolytic system occur during exercise. The relation between the duration and intensity of exercise and the relative changes in the VIII complex and fibrinolytic system have not been previously examined. Five healthy male subjects were exercised with three protocols: a graded progressive exercise test to exhaustion on a cycle ergometer with 50-W increments every 4 min, steady-state exercise, 15 min at 5 and 125 W each, and an acute 30-s maximal exercise test on a cycle ergometer. Venous blood samples were drawn at base line, during the last 30 s of each power output in the graded exercise, at 5-min intervals for the steady-state exercise, and for up to 1 h after completion of exercise in all three protocols. At the maximum exercise intensities, increases in plasma lactate concentration ([La]), O2 uptake, and [H+] were observed. Components of the VIII complex [VIII procoagulant, VIII procoagulant antigen, VIII-related antigen (VIIIR:Ag), VIII ristocetin cofactor activity] abruptly rose at only the highest work intensities, whereas the whole blood clot lysis time began to gradually shorten much earlier at low work intensities. There were no qualitative changes in the factor VIIIR:Ag on crossed immunoelectrophoresis nor was there evidence of thrombin generation as determined by fibrinopeptide A generation. We conclude that during exercise the changes observed in the coagulation and fibrinolytic systems are related to the intensity of the exercise, which is reflected by increases in plasma [La] and [H+], and that the fibrinolytic system is activated before the changes in the VIII complex are observed.     

Control of the rate of phosphocreatine resynthesis after exercise in trained and untrained human quadriceps muscles

We examined the effect of differences in exercise intensity on the time constant (t _c) of phosphocreatine (PCr) resynthesis after exercise and the relationships betweent _c and maximal oxygen uptake (VO_2max) in endurance-trained runners (n = 5) and untrained controls (n = 7) (average VO_2max = 66.2 and 52.0 ml · min^–1 · kg^–1, respectively). To measure the metabolism of the quadriceps muscle using phosphorus nuclear magnetic resonance spectroscopy, we developed a device which allowed knee extension exercise inside a magnet. All the subjects performed four types of exercise: light, moderate, severe and exhausting. The end-exercise PCr: [PCr + inorganic phosphate (P_i)] ratio decreased significantly with the increase in the exercise intensity (P < 0.01). Although there was little difference in the end-exercise pH, adenosine diphosphate concentration ([ADP]) and the lowest intracellular pH during recovery between light and moderate exercise, significant changes were found at the two higher intensities (P < 0.01). These changes for runners were smaller than those for the controls (P < 0.05). The _c remained constant after light and moderate exercise and then lengthened in proportion to the increase in intensity (P < 0.05). The runners had a lowert _c at the same PCr and pH than the controls, particularly at the higher intensity (P < 0.05). There was a significant correlation betweent _c and [ADP] in light exercise and betweent _c and both end-exercise PCr and pH in severe and exhausting exercise (P < 0.05). The threshold of changes in pH andt _c was a PCr: (PCr + P_i) ratio of 0.5. There was a significant negative correlation between the VO_2max andt _c after all levels of exercise (P<0.05).However, in the controls a significant correlation was found in only light and moderate exercise (P < 0.05). These findings suggest the validity of the use oft _c at an end-exercise PCr:(PCr + P_i) ratio of more than 0.5 as a stable index of muscle oxidative capacity and the correlation between local and general aerobic capacity. Moreover, endurance-trained runners are characterized by the faster PCr resynthesis at the same PCr and intracellular pH.     

Heart rate variability in exercising humans: effect of water immersion

Power spectrum analysis of heart-rate variability was made in seven men [mean age 22 (SEM 1) years] in head-out water immersion (W) and in air (A, control) at rest and during steady-state cycling to maximal intensity (maximum oxygen uptake, V˙O_2max). At rest W resulted in a trebled increase in the total power (P < 0.05), coupled with minimal changes in the power (as a percentage of the total) of the high frequency peak (HF, centred at 0.26 Hz; 18% vs 28%) and of the low frequency peak (LF, 0.1 Hz; 24% vs 32%). A third peak at about 0.03 Hz (very low frequency, VLF) represented the remaining power both in W and A. These changes as a whole indicated that immersion caused a vagal dominance in cardiac autonomic interaction, due to the central pooling of blood and/or the pressure of water on the trunk. Exercise caused a decrease in the total power in W and A. The LF% did not change up to about 50% V˙O_2max, thereafter decreasing towards nil in both conditions. The HF% decreased in similar ways in W and A to about half at 55%–60% V˙O_2max and then increased to reach 1.5 times the resting values at V˙O_2max. The central frequency of HF increased linearly with oxygen uptake, showing a tendency to be higher in W than in A at medium to high intensities. The VLF% remained unchanged. The lack of differences in the LF peak between W and A during exercise would suggest that blood distribution had no effect on the readjustments in control mechanisms of arterial pressure. On the other hand, the findings of similar HF powers and the very similar values for ventilation in W and A confirmed the direct effect of the respiratory activity in heart rate modulation during exercise. Accepted: 25 August 1997     

Estimating tissue‐specific discrimination factors and turnover rates of stable isotopes of nitrogen and carbon in the smallnose fanskate Sympterygia bonapartii (Rajidae)

This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskate Sympterygia bonapartii by feeding six adult individuals, maintained in captivity, with a constant diet for 365 days. TDFs were estimated as the difference between δ¹³C or δ¹⁵N values of the food and the tissues of S. bonapartii after they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: C t95%_blood = 150 days, N t95%_blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ¹³C and Δ¹⁵N values in blood and muscle using all individuals were Δ¹³C_blood = 1·7‰, Δ¹³C_muscle = 1·3‰, Δ¹⁵N_blood = 2·5‰ and Δ¹⁵N_muscle = 1·5‰, but there was evidence of differences of c.0·4‰ in the Δ¹³C values between sexes. The present values for TDFs and turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed that S. bonapartii has relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ¹³C values in S. bonapartii blood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ¹³C = 1·5‰). For Δ¹⁵N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ¹⁵N c. 3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ¹⁵N values could lead to underestimates of actual trophic positions. Finally, the evidence of differences in TDFs between sexes reveals a need for more targeted research.     

Energy cost and energy sources in karate

Energy costs and energy sources in karate (wado style) were studied in eight male practitioners (age 23.8 years, mass. 72.3 kg, maximal oxygen consumption (VO_2max) 36.8 ml · min^–1 · kg^–1) performing six katas (formal, organized movement sequences) of increasing duration (from approximately. 10 s to approximately 80 s). Oxygen consumption (VO₂) was determined during pre-exercise rest, the exercise period and the first 270 s of recovery in five consecutive expired gas collections. A blood sample for lactate (la^–) analysis was taken 5 min after the end of exercise. The overall amount of O₂ consumed during the exercise and in the following recovery increased linearly with the duration of exercise (t) from approximately 1.51 (for t equal to 10.5 s (SD 1.6)) to approximately 5.81, for t equal to 81.5 s (SD 1.0). The energy release from la^– production (VO_21a ^–) calculated assuming that an increase of 1 mmol · l^–1 la^– corresponded to a VO₂ of 3 mlO₂ · kg^–1 was negligible for t equal to or less than 20 s and increased to 17.3 ml · kg^–1 (la^– = 5.8 mmol · l^–1 above resting values) for t equal approximately to 80 s. The overall energy requirement (VO_2eq) as given by the sum of VO₂ and VO_2la ^– was described by VO_2eq = 0.87 + 0.071 · t (n = 64; r ² = 0.91), where VO_2eq is in litres and t in seconds. This equation shows that the metabolic power (VO_2eq · t ^–1) for this karate style is very high: from approximately 9.51 · min^–1 for t equal to 10 s to approximately 4.91 · min^–1 for t equal to 80 s, i.e. from 3.5 to 1.8 times the subjects' VO_2max. The fraction of VO_2eq derived from the amount of O₂ consumed during the exercise increased from 11% for t equal to 10 s to 41 % for t equal to 80 s whereas VO_21a ^– was negligible far t equal to or less than 20 s and increased to 13 % o for t equal to 80 s. The remaining fraction (from 90% for t equal to 10 s to 46% for t equal to 80 s), corresponding to the amount of O₂ consumed in the recovery after exercise, is derived from anaerobic alactic sources, i.e. from net splitting of high energy phosphates during the exercise.     

Exercise prescription and thrombogenesis

Lifestyle habits, such as exercise, may significantly influence risk of major vascular thrombotic events. The risk of primary cardiac arrest has been shown to transiently increase during vigorous exercise, whereas regular moderate-intensity exercise is associated with an overall reduced risk of cardiovascular diseases. What are the mechanisms underlying these paradoxical effects of vigorous exercise versus exercise training on thrombotic modification? This review analyzes research regarding effects and their underlying mechanisms of acute exercise, endurance training, and deconditioning on platelets, coagulation, and fibrinolysis. Evidence suggests that (i) light, acute exercise ( < or = 49% VO(2 max)) does not affect platelet reactivity and coagulation and increases fibrinolytic activity; (ii) moderate, acute exercise (50 to approximately 74% VO(2 max)) suppresses platelet reactivity and enhances fibrinolysis, which remains unchanged in the coagulation system; and, (iii) strenuous, acute exercise ( > or = 75% VO(2 max)) enhances both platelet reactivity and coagulation, simultaneously promoting fibrinolytic activity. Therefore, moderate exercise is likely a safe and effective exercise dosage for minimizing risk of cardiovascular diseases by inducing beneficial anti-thrombotic changes. Moreover, moderate-intensity exercise training reduces platelet reactivity and enhances fibrinolysis at rest, also attenuating enhanced platelet reactivity and augmenting hyper-fibrinolytic activity during strenuous exercise. However, these favorable effects of exercise training on thrombotic modification return to a pre-training state after a period of deconditioning. These findings can aid in determining appropriate exercise regimes to prevent early thrombotic events and further hinder the cardiovascular disease progression.     

Pedogenesis, permafrost, and soil moisture as controlling factors for soil nitrogen and carbon contents across the Tibetan Plateau

We investigated the main parameters [e.g. mean annual air temperature , mean annual soil temperature, mean annual precipitation, soil moisture (SM), soil chemistry, and physics] influencing soil organic carbon (C_org), soil total nitrogen (N_t) as well as plant available nitrogen (N_min) at 47 sites along a 1200 km transect across the high‐altitude and low‐latitude permafrost region of the central‐eastern Tibetan Plateau. This large‐scale survey allows testing the hypothesis that beside commonly used ecological variables, diversity of pedogenesis is another major component for assessing carbon (C) and nitrogen (N) cycling. The aim of the presented research was to evaluate consequences of permafrost degradation for C and N stocks and hence nutrient supply for plants, as the transect covers all types of permafrost including heavily degraded areas and regions without permafrost. Our results show that SM is the dominant parameter explaining 64% of C_org and 60% of N variation. The extent of the effect of SM is determined by permafrost, current aeolian sedimentation occurring mostly on degraded sites, and pedogenesis. Thus, the explanatory power for C and N concentrations is significantly improved by adding CaCO₃ content (P=0.012 for C_org; P=0.006 for N_t) and soil texture (P=0.077 for C_org; P=0.015 for N_t) to the model. For soil temperature, no correlations were detected indicating that in high‐altitude grassland ecosystems influenced by permafrost, SM overrides soil temperature as the main driving parameter at landscape scale. It was concluded from the current study that degradation of permafrost and corresponding changes in soil hydrology combined with a shift from mature stages of pedogenesis to initial stages, have severe impact on soil C and plant available N. This may alter biodiversity patterns as well as the development and functioning of the ecosystems on the Tibetan Plateau.     

Fibrinolytic activity is not dependent upon exercise mode in post-myocardial infarction patients

In this study we investigated possible differences in fibrinolytic activity in cardiac patients while they performed treadmill and cycle ergometry. Thirteen post-myocardial infarction patients completed two maximal exercise tests on treadmill and cycle ergometers. Blood was collected before and after each exercise test and was analyzed for the fibrinolytic variables, tissue plasminogen activator (t-PA) and plasminogen activator inhibitor-1 (PAI-1) activity, and lactate. Maximal oxygen uptake, heart rate, and ventilation were greater (P < 0.05) on the treadmill than during cycle ergometry, however, blood lactate was similar between modes. t-PA activity significantly increased with exercise (P < 0.05) and there was a trend toward a reduction in PAI-1 activity with exercise, but this did not reach statistical significance. The fibrinolytic responses to maximal exercise did not differ between the two modes of exercise studied. Therefore, exercise intensity, but not the mode of exercise, appeared to be the primary determinant of the fibrinolytic response to acute exercise in these patients. Accepted: 29 January 1998     

The interaction of alcohol radicals with human hemoglobin

Aqueous deoxyhemoglobin solutions (2 mg/ml) were gamma-irradiated by a⁶⁰Co source in the presence of methanol, ethanol, 1-butanol andt-butanol under N₂O or argon. The effects of the interaction of the particular alcohol radical species with hemoglobin were determined according to the detected spectral alterations in the visible range. The amounts of stable final products in the form of methemoglobin (MetHb) and the sum of hemichromes and cholehemichromes (Hemichr) were estimated in irradiated preparations. For preparations irradiated under N₂O, the radiation yield for MetHb formation was threefold lower in the presence of ethanol and 1-butanol [G(MetHb)=0.33] compared with preparations irradiated in the presence oft-butanol or without alcohol [G(MetHb)=1.00]. The yield of hemichromes and cholehemichromes in preparations irradiated under N2O increased in the order: ethanol (G=0.38), 1-butanol (G=0.52),t-butanol (G=0.59), and in the absence of alcohol (G=0.72). The high effectivity oft-butanol radicals for iron oxidation and Hb destruction is apparently due to their oxidative properties, compared with the other radicals. It was also shown that ethanol radicals reduce MetHb 10 times more effectively [G(Fe(II)) = 2.5] compared witht-butanol radicals [G(Fe(II)) = 0.24]. For samples irradiated under argon all the observed changes were similar, regardless of the presence of alcohols. This effect can be attributed to reconstruction reactions of Hb molecules in the presence of both oxidizing (OH ort-but·) and reducing agents (e _aq ^/– ). The following sequence of effectivities of water radiolysis products and secondary alcohol radicals for hemoglobin destruction has been identified: meth·, eth·1-but·e _aq ^/– t-but··OH.This work was supported by State Committee for Scientific Research (Poland), grant no. 44509203     

Daily activity and body temperature

Body temperature varies between 36 and 39° C in states ranging from sleep to high levels of sustained exercise, but it is not known whether this continuum of body temperature is related to a continuum of activity. Calorimetric studies of sedentary days were undertaken with four levels of food intake, men doing mild sustained exercise, and men and women walking and cycling vigorously. Steady states of metabolism were followed by slow exponential changes to steady states of heat loss (Q), followed in turn by changes in rectal temperature (T _re). Regression analysis showed a continuous, curvilinear relationship between Q andT _re from the low end of the activity spectrum (50 W) to progressively higher levels of exercise (600 W). These related continua of activity and body temperature appear to be the result of heat regulation.     

Inhibitory Potency of Erythrina variegata Proteinase Inhibitors toward Serine Proteinases in the Blood Coagulation and Fibrinolytic Systems

The Erythrina variegata Kunitz family trypsin inhibitors, ETIa and ETIb, prolonged the activated partial thromboplastin time (APTT) and also the prothrombin time (PT) of human plasma, but the Kunitz family chymotrypsin inhibitor, ECI, and Bowman–Birk family inhibitor, EBI, from E. variegata hardly prolonged these times. Trypsin inhibitors ETIa and ETIb inhibited the amidolytic activity of factor Xa, and ETIb but not ETIa inhibited plasma kallikrein. Neither ETIa nor ETIb exhibited any inhibitory activity toward β-factor XIIa and thrombin. Furthermore, trypsin inhibitors ETIa and ETIb inhibited plasmin, a serine proteinase in the fibrinolytic system, whereas ECI and EBI did not. These results indicate that Erythrina Kunitz proteinase inhibitors possess different potency toward serine proteinases in the blood coagulation and fibrinolytic systems, in spite of their high similarity in amino acid sequence.     

Molecular-Dynamics Simulations for the Density Autocorrelation Function in a Supercooled Fluid Phase

Abstract

We have re-calculated the self part of the density autocorrelation function F_s(k, t) (incoherent scattering function) for the binary soft-sphere fluid with a much longer molecular-dynamics (MD) simulation than our previous MD calculations, and with a larger system size (N = 4000) to a longer time window as well as to study a system-size dependence, if it exists. The full density autocorrelation function F(k, t) was also computed. It is found that all F(k, t)'s that we have computed in this work can be fitted over a wide range of time steps (at least over three figures of the decay) by a Williams-Watts stretched exponential function F_s(k, t) = A exp [— (t/t ₀)^β], where A, β and t ₀ are adjustable parameters. Other significant dynamical behaviours were also presented in mean square displacements and non-Gaussian parameters for highly supercooled fluids with N = 4000. The present results are compatible to our previous computations with N = 500, but a significant size dependence is suggested.     

Physical dilatation of the nostrils lowers the thermal strain of exercising humans

Increased nasal air flow during exercise was examined as a possible heat loss avenue contributing to selective brain cooling in hyperthermic humans. On 2 separate days, eight subjects [mean (SE) age, 26.4 (1.2) years] exercised on a cycle ergometer in a warm room [28 (0.2)°C; 28 (5)% relative humidity] to induce a moderate level of hyperthermia. In one session the nostrils were physically dilatated [average dilatation 1.55 (0.17) times] and in the other they were not (control). Both sessions started with a 5-min resting period; then subjects pedaled at 60 W for 5 min, 100 W for 15 min, and 150 W for 20 min. During dilatation both tympanic temperature (T _ty) and forehead skin blood flow, estimated by laser doppler velocimetry, were significantly lower than during the control exercise of 150 W. Rates of increase of (T _ty) during the 100-W exercise were the same in both conditions; however, during the 150-W exercise with dilatated nostrils (T _ty) increased at a rate significantly lower than during control [1.1 (0.3)°C·h^–1 vs 1.5 (0.4)°C·h^–1]. The change in the rate of increase of T _ty between conditions was significantly correlated to the degree of nostril dilatation (r = –0.77, P = 0.02), suggesting that the lower (T _ty) observed was due to nostril dilatation. Facial skin temperature was not significantly different between sessions. The results suggest that the nasal cavity may act as a heat exchanger in selective brain cooling of exercising humans.