Seasonal variation in physiological responses to mild cold air in young and older men

Eight men aged 60–65 years and six men aged 20–25 years, wearing only swimming trunks, were exposed to an air temperature of 17° C and 45% R.H. in each of the four seasons. The increase in the rate of metabolic heat production for the older group in the cold test was significantly higher in summer and autumn than in winter and spring (P<0.05), but did not differ in the young group between seasons. Compared to the young group the was significantly greater for the older group (due to a marked increase in four individuals) in summer and autumn (P<0.04). At the end of the period of cold exposure, the decrements of rectal temperature (T _re), mean skin temperature ( ; due to a marked decrease in four individuals) and foot skin temperature (T _foot) were significantly greater for the older group compared to the young group at all times of the year (P<0.003). Seasonal variations in the two groups were similar, e.g., theT_re gradually became smaller from summer to winter (P<0.05) and then increased slightly in the spring (P=0.07).T _foot for both groups decreased from summer to autumn (P<0.01) and remained unchanged subsequently. No seasonal variations were observed for in either group. The increase in diastolic blood pressure (BP_d) during the test was significantly smaller in winter in both groups (P<0.05). BP_d became larger again during spring in the older group (P<0.01), but remained low in the young group. The BP_d was significantly greater for the older group than the young group in winter and spring (P<0.05). Compared to young men these results suggest that older men may lose the tolerance acquired by earlier cold acclimatization as seen by the BP_d responses, and have a somewhat lower thermoregulatory capability in coping with mild cold air in all seasons.     

Chronobiological effects on exercise performance and selected physiological responses

Previous studies investigating the impact of circadian rhythms on physiological variables during exercise have yielded conflicting results. The purpose of the present investigation was to examine maximal aerobic exercise performance, as well as the physiological and psychophysiological responses to exercise, at four different intervals (0800 hours, 1200 hours, 1600 hours, and 2000 hours) within the segment of the 24-h day in which strenuous physical activity is typically performed. Ten physically fit, but untrained, male university students served as subjects. The results revealed that exercise performance was unaffected by chronobiological effects. Similarly, oxygen uptake, minute ventilation and heart rate showed no time of day influences under pre-, submaximal, and maximal exercise conditions. Ratings of perceived exertion were unaffected by time of day effects during submaximal and maximal exercise. In contrast, rectal temperature exhibited a significant chronobiological rhythm under all three conditions. Under pre- and submaximal exercise conditions, significant time of day effects were noted for respiratory exchange ratio, while a significant rhythmicity of blood pressure was evident during maximal exercise. However, none of these physiological variables exhibited significant differential responses (percent change from pre-exercise values) to the exercise stimulus at any of the four time points selected for study. Conversely, resting plasma lactate levels and lactate responses to maximal exercise were found to be significantly sensitive to chronobiological influences. Absolute post-exercise plasma norepinephrine values, and norepinephrine responses to exercise (percent change from pre-exercise values), also fluctuated significantly among the time points studied. In summary, these data suggest that aerobic exercise performance does not vary during the time frame within which exercise is normally conducted, despite the fact that some important physiological responses to exercise do fluctuate within that time period. Accepted: 18 August 1997     

Comparative physiological responses of normotensive and essentially hypertensive men to exercise in the heat

Six essentially hypertensive men (average resting arterial pressure of 150/97 mm Hg) and eight normotensive controls (average resting arterial pressure of 115/73 mm Hg) were tested during 1 h of dynamic leg exercise in a warm environment. The groups were well matched for age, VO2 max, body surface area, weight, and body fat. Environmental conditions were 38 degrees C dry-bulb, 28 degrees C wet-bulb; exercise intensity was approximately 40% VO2 max (85-90 W). There were no significant intergroup differences in core or mean skin temperatures, calculated heat exchange variables, heart, or sweat rates. Blood pressure differences between the groups were maintained (P less than 0.01). The hypertensive group responded with a significantly lower stroke index (P less than 0.01) and cardiac index (P less than 0.01), and a decreased slope of the rise in forearm blood flow (P less than 0.01) due to an higher vascular resistance (P less than 0.01). The combined heat load (M + R + C) presented was not sufficient to override the hypertensives' higher cutaneous vasoconstrictor tone. However, on a practical basis, the hypertensives were able to tolerate exercise in the heat as well as their normotensive counterparts.     

Effect of lowered muscle temperature on the physiological response to exercise in men

The effect of low muscle temperature on the response to dynamic exercise was studied in six healthy men who performed 42 min of exercise on a cycle ergometer at an intensity of 70% of their maximal O2 uptake. Experiments were performed under control conditions, i.e. from rest at room temperature, and following 45 min standing with legs immersed in a water bath at 12 degrees C. The water bath reduced quadriceps muscle temperature (at 3 cm depth) from 36.4 (SD 0.5) degrees C to 30.5 (SD 1.7) degrees C. Following cooling, exercise heart rate was initially lower, the mean difference ranged from 13 (SD 4) beats.min-1 after 6 min of exercise, to 4 (SD 2) beats.min-1 after 24 min of exercise. Steady-state oxygen uptake was consistently higher (0.2 l.min-1). However, no difference could be discerned in the kinetics of oxygen uptake at the onset of exercise. During exercise after cooling a significantly higher peak value was found for the blood lactate concentration compared to that under control conditions. The peak values were both reached after approximately 9 min of exercise. After 42 min of exercise the blood lactate concentrations did not differ significantly, indicating a faster rate of removal during exercise after cooling. We interpreted these observations as reflecting a relatively higher level of muscle hypoxia at the onset of exercise as a consequence of a cold-induced vasoconstriction. The elevated steady-state oxygen uptake may in part have been accounted for by the energetic costs of removal of the extra lactate released into the blood consequent upon initial tissue hypoxia.     

Natural variation in germination responses of Arabidopsis to seasonal cues and their associated physiological mechanisms

Background and Aims

Despite the intense interest in phenological adaptation to environmental change, the fundamental character of natural variation in germination is almost entirely unknown. Specifically, it is not known whether different genotypes within a species are germination specialists to particular conditions, nor is it known what physiological mechanisms of germination regulation vary in natural populations and how they are associated with responses to particular environmental factors.

Methods

We used a set of recombinant inbred genotypes of Arabidopsis thaliana, in which linkage disequilibrium has been disrupted over seven generations, to test for genetic variation and covariation in germination responses to distinct environmental factors. We then examined physiological mechanisms associated with those responses, including seed-coat permeability and sensitivity to the phytohormones gibberellic acid (GA) and abscisic acid (ABA).

Key Results

Genetic variation for germination was environment-dependent, but no evidence for specialization of germination to different conditions was found. Hormonal sensitivities also exhibited significant genetic variation, but seed-coat properties did not. GA sensitivity was associated with germination responses to multiple environmental factors, but seed-coat permeability and ABA sensitivity were associated with specific germination responses, suggesting that an evolutionary change in GA sensitivity could affect germination in multiple environments, but that of ABA sensitivity may affect germination under more restricted conditions.

Conclusions

The physiological mechanisms of germination responses to specific environmental factors therefore can influence the ability to adapt to diverse seasonal environments encountered during colonization of new habitats or with future predicted climate change.     

Adaptive variation in physiological traits underpinning stem elongation responses among nodally-rooting stoloniferous herbs

For nodally-rooting stoloniferous herbs a close physiological link exists between the growth of nodal roots and development of shoot tissues of phytomers distal to them. Furthermore, in Trifolium, presence of nodal roots at phytomers proximal to the youngest nodal root on a stem influences the allocation of carbon along the stem when that stem encounters a patch preventing further formation of nodal roots. In this study six further phylogenetically and morphologically diverse stoloniferous species, selected so that they varied along the phalanx–guerrilla continuum (Acaena (Rosaceae), Leptinella (Asteraceae), Mentha (Lamiaceae), Vinca (Apocynaceae), Lamiastrum (Lamiaceae) and Glechoma (Lamiaceae)), were used to test the null hypothesis: that in all stoloniferous species the proportional allocation of carbon to the youngest nodal root decreases if proximally located roots are present when their stems extend into an environment preventing further nodal rooting. As Glechoma and Lamiastrum failed to change their proportional allocation to the youngest nodal root whilst the other five species varied their proportional allocation, with the species with shortest internodes (Acaena and Leptinella) changing the most, we reject this hypothesis. In addition, upon the transition to a dry environment, internode lengths in species with longer internodes (Glechoma and Lamiastrum) were markedly increased (doubled) whilst those in species with shorter internodes showed no response. We suggest this response is driven by apical bud tissues sensing changes in the humidity of their micro-environment and then modifying the length of internodes developing proximal to them. The extent of both these physiological responses correlated linearly with internode length of species (or position along the phalanx–guerrilla continuum), suggesting that these three traits were co-evolving, possibly in response to variation in grain of environmental heterogeneity, in such a way that the trade-off between the explorative and exploitive components of growth was underpinned and the performance of species in particular niches optimised as a result.     

Seasonal variation in work performance and heart rate response to exercise. A study of 1,835 middle-aged men

A near maximal bicycle exercise test in 1,835 presumably healthy Norwegian men indicated a seasonal variation in physical fitness. Thus, the total amount of work performed was significantly higher, and the work pulse on equivalent loads significantly lower during the summer than during the autumn. Although the differences were small, they may invalidate comparisons made between exercise tests in populations tested at different seasons of the year. In particular, there was a considerable and sudden change in the above mentioned parameters from June--August to September--October.     

Effects of delayed onset muscle soreness on selected physiological responses to submaximal running

This study examined the effects of delayed-onset muscle soreness (DOMS) on selected physiological responses to submaximal exercise. Seven male and four female subjects (Ss) aged 21-37 years completed two submaximal running sessions at an individualized pace corresponding to a blood lactate concentration (bLa) of approximately 2.5 mmol x L(-1). Following the first session (T1), Ss performed a series of lower extremity resistance exercises designed to induce DOMS. Subjects were then retested (T2) 24-30 hours later, during which time all Ss experienced DOMS. Oxygen uptake, heart rate (HR), respiratory exchange ratio, rating of perceived exertion (RPE), and bLa were measured every 6 minutes. Significant trial effects (p < 0.05) were observed for HR and RPE. HR was significantly higher during T1 at minutes 6 and 12 (p < 0.05), and RPE values were significantly higher at T2 during all recording periods (p < 0.05). Results from this study suggest that DOMS does not affect submaximal oxygen uptake. However, DOMS does appear to affect one's perception of effort.     

Overeating of palatable food is associated with blunted leptin and ghrelin responses

Palatable food is rich in fat and/or sucrose. In this study we examined the long-term effects of such diets on food intake, body weight, adiposity and circulating levels of the satiety peptide leptin and the hunger peptide ghrelin. The experiments involved rats and mice and lasted 5 weeks. In rats, we examined the effect of diets rich in fat and/or sucrose and in mice the effect of a high fat diet with or without sucrose in the drinking water. Animals fed with the palatable diets had a larger intake of calories, gained more weight and became more adipose than animals fed standard rat chow. Fasted animals are known to have low serum leptin and high serum ghrelin and to display elevated serum leptin and lowered serum ghrelin postprandially. With time, a sucrose-rich diet was found to raise the fasting level of leptin and to lower the fasting level of ghrelin in rats. A fat-rich diet suppressed serum ghrelin without affecting serum leptin; high sucrose and high fat in combination greatly reduced serum ghrelin and raised serum leptin in the fasted state. The mRNA expression of leptin in the rat stomach was up-regulated by sucrose-rich (but not by fat-rich) diets, whereas the expression of ghrelin seemed not to be affected by the palatable diets. Mice responded to sucrose in the drinking water with elevated serum leptin (fasted state) and to all palatable diets with low serum ghrelin. The expression of both leptin and ghrelin mRNA in the stomach was suppressed in fasted mice that had received a high fat diet for 5 weeks. We conclude that the expression of leptin mRNA in stomach and the concentration of leptin in serum were elevated in response to sucrose-rich rather than fat-rich diets, linking leptin with sucrose metabolism. In contrast, the expression of ghrelin and the serum ghrelin concentration were suppressed by all palatable diets, sucrose and fat alike. In view of the increased body weight and adiposity neither elevated leptin nor suppressed ghrelin were able to control/restrain the overeating that is associated with palatable diets.     

Time-motion and physiological assessments of ice hockey performance.

Ice hockey performance was studied during 10 contests to assess time-motion characteristics and associated physiological changes that occur for each position. Depending on the position played, the actual playing time per game for forwards and defensemen ranged between 20.7 and 28.0 min. The number of shifts ranged from 14 to 21 with an average playing time of 85.4 s/shift. Further analysis of each shift indicated that there was an average of 2.3 play stoppages which averaged 27.1 s, producing a continuous playing time of 39.7 s. Telemetered recordings of heart rate during each shift revealed sustained rates of between 170 and 174 beats/min over the three periods of the game. For both the forwards and defensemen, values for blood lactates were highest during the first and second periods (x = 78.0 and 66.1 mg/100 ml), then declined considerably during the third period (x = 44.5 mg/100 ml). Blood glucose showed a similar tendency (x = 139.3, 133.7, 114.1 mg/100 ml), while hematocrit and total protein showed little tendency to change.     

Hypoxemia and blunted hypoxic ventilatory responses in mice lacking heme oxygenase-2

Heme oxygenase (HO) catalyzes physiological heme degradation and consists of two structurally related isozymes, HO-1 and HO-2. Here we show that HO-2-deficient (HO-2(-/-)) mice exhibit hypoxemia and hypertrophy of the pulmonary venous myocardium associated with increased expression of HO-1. The hypertrophied venous myocardium may reflect adaptation to persistent hypoxemia. HO-2(-/-) mice also show attenuated ventilatory responses to hypoxia (10% O2) with normal responses to hypercapnia (10% CO2), suggesting the impaired oxygen sensing. Importantly, HO-2(-/-) mice exhibit normal breathing patterns with normal arterial CO2 tension and retain the intact alveolar architecture, thereby excluding hypoventilation and shunting as causes of hypoxemia. Instead, ventilation-perfusion mismatch is a likely cause of hypoxemia, which may be due to partial impairment of the lung chemoreception probably at pulmonary artery smooth muscle cells. We therefore propose that HO-2 is involved in oxygen sensing and responsible for the ventilation-perfusion matching that optimizes oxygenation of pulmonary blood.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

In six male subjects the sweating thresholds, heart rate (fc), as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO2max)], were compared at ambient temperatures (Ta) of 5 degrees C (LT) and 24 degrees C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO2) at the end of each rest period was higher in LT than MT (P less than 0.05). During 20-min exercise at 40% VO2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (Tre) but at lower mean skin (Tsk) and mean body temperatures (Tb) in LT than MT experiments (P less than 0.001). The exercise induced VO2 increase was greater only at the end of the light (40% VO2max) exercise in the cold in comparison with MT (P less than 0.001). Both fc and blood lactate concentration [1a]b were lower at the end of LT than MT for moderate (60% VO2max) and heavy (80% VO2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower Tb and Tsk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24 degrees C since the increases in Tre, fc and [1a]b were lower at the lower Ta.     

Pollinator responses to variation in floral display and flower size in dioecious Sagittaria latifolia (Alismataceae)

In animal-pollinated plants with unisexual flowers, sexual dimorphism in floral traits may be the consequence of pollinator-mediated selection. Experimental investigations of the effects of variation in flower size and floral display on pollinator visitation can provide insights into the evolution of floral dimorphism in dioecious plants. Here, we investigated pollinator responses to experimental arrays of dioecious Sagittaria latifolia in which we manipulated floral display and flower size. We also examined whether there were changes in pollinator visitation with increasing dimorphism in flower size. In S. latifolia, males have larger flowers and smaller floral displays than females. Visitation by pollinators, mainly flies and bees, was more frequent for male than for female inflorescences and increased with increasing flower size, regardless of sex. The number of insect visits per flower decreased with increasing floral display in males but remained constant in females. Greater sexual dimorphism in flower size increased visits to male inflorescences but had no influence on the number of visits to female inflorescences. These results suggest that larger flower sizes would be advantageous to both females and males, and no evidence was found that females suffer from increased flower-size dimorphism. Small daily floral displays may benefit males by allowing extended flowering periods and greater opportunities for effective pollen dispersal.     

Effect of electrical stimulation of leg muscles on physiological responses during arm-cranking exercise in healthy men

The purpose of this study was to investigate the hypothesis that changes in physiological responses during arm-cranking exercise using electrical stimulation of the leg muscles (ACE-ES) compared to arm-cranking exercise alone (ACE) in able-bodied subjects (ABS) are based on an increase in active muscle mass rather than the enhancing effect of the leg muscle pump. In ABS the sympathetic nervous system induced vasoconstriction and activity of the leg muscle pump are intact, therefore, a normal redistribution of blood takes place during exercise. Consequently, ES should have no additional effect on the redistribution of blood in these ABS during exercise and, thus, changes in physiological responses will be based on an increase in active muscle mass. A group of 11 ABS performed three maximal arm-cranking tests. In the first test peak power output (PO _peak) was determined. The other tests were both submaximal and maximal ACE, once with ACE-ES and once with ACE. The PO _peak was not significantly different between ACE-ES and ACE. Oxygen uptake (O₂) increased significantly during ACE-ES compared to ACE. Cardiac output (), stroke volume (SV), heart rate and ventilation were not significantly different during ACE-ES compared to ACE. Respiratory exchange ratios were significantly lower during ACE-ES compared to ACE at 60% PO _peak and at maximal exercise. In conclusion, ACE-ES caused significant increases in O₂ with a lack of elevation in and SV during submaximal and maximal exercise in ABS. The results of this study suggest that changes in physiological responses during ACE-ES are based on an increase in the active muscle mass rather than stimulation of the leg muscle pump. Accepted: 6 August 1996     

Computerized graphic display of physiological data collected during human stereotactic surgery.

An on-line computerized graphic display has been developed for use during stereotactic operations. This depicts in the form of figurine charts and alph-numeric symbols, appropriately oriented on saggital brain diagrams, the results of serial threshold stimulation of the brain. The display facilitates choice of target sites and the data can be stored in a tape library from which search-and-plot programs can be activated for any type or combination of types of response.     

Circadian variation in swim performance.

Previous findings of time-of-day differences in athletic performance could be confounded by diurnal fluctuations in environmental and behavioral "masking" factors (e.g., sleep, ambient temperature, and energy intake). The purpose of this study was to examine whether there is a circadian rhythm in swim performance that is independent of these masking factors. Experienced swimmers (n = 25) were assessed for 50-55 consecutive hours in the laboratory. The swimmers followed a 3-h "ultra-short" sleep-wake cycle, involving 1 h of sleep in darkness and 2 h of wakefulness in dim light, that was repeated throughout the observation. The protocol distributes behavioral and environmental masking factors equally across the 24-h period. Each swimmer was scheduled to perform six maximal-effort 200-m swim trials that were distributed equally across eight times of day (n = 147 trials). Each trial was separated by 9 h. A cosine fit of intra-aural temperature data established the time of the lowest body temperature (Tmin). Swim performances were z-transformed and compared across the eight times of day and across twelve 2-h intervals relative to Tmin. Analysis of covariance, controlling for trial number, revealed a significant (P < 0.001) pattern in swim performance relative to environmental and circadian times of day. Performance peaked 5-7 h before Tmin (approximately 2300) and was worst from 1 h before to 1 h after Tmin (approximately 0500). Mean swim performance was 169.5 s; circadian variation from peak to worst performance was 5.8 s. These data suggest a circadian rhythm in athletic performance independent of environmental and behavioral masking effects.