Aerobic training and cutaneous vasodilation in young and older men.
To determine the effect and underlying mechanisms of exercise training and the influence of age on the skin blood flow (SkBF) response to exercise in a hot environment, 22 young (Y; 18-30 yr) and 21 older (O; 61-78 yr) men were assigned to 16 wk of aerobic (A; YA, n = 8; OA, n = 11), resistance (R; YR, n = 7; OR, n = 3), or no training (C; YC, n = 7; OC, n = 7). Before and after treatment, subjects exercised at 60% of maximum oxygen consumption (VO2 max) on a cycle ergometer for 60 min at 36 degrees C. Cutaneous vascular conductance, defined as SkBF divided by mean arterial pressure, was monitored at control (vasoconstriction intact) and bretylium-treated (vasoconstriction blocked) sites on the forearm using laser-Doppler flowmetry. Forearm vascular conductance was calculated as forearm blood flow (venous occlusion plethysmography) divided by mean arterial pressure. Esophageal and skin temperatures were recorded. Only aerobic training (functionally defined a priori as a 5% or greater increase in VO2 max) produced a decrease in the mean body temperature threshold for increasing forearm vascular conductance (36.89 +/- 0.08 to 36.63 +/- 0.08 degrees C, P < 0.003) and cutaneous vascular conductance (36.91 +/- 0.08 to 36.65 +/- 0.08 degrees C, P < 0.004). Similar thresholds between control and bretylium-treated sites indicated that the decrease was mediated through the active vasodilator system. This shift was more pronounced in the older men who presented greater training-induced increases in VO2 max than did the young men (22 and 9%, respectively). In summary, older men improved their SkBF response to exercise-heat stress through the effect of aerobic training on the cutaneous vasodilator system. (+info)
Effect of heat stress on rabbit esophageal epithelium.
Hot beverages expose the esophageal epithelium to temperatures as high as 58 degrees C. To study the impact of such temperatures, rabbit esophageal epithelium was exposed to luminal heat or both luminal and serosal heat while mounted in Ussing chambers. Luminal heat, mimicking exposure to hot beverages, reduced potential difference (PD) and resistance (R) when applied at >/=49 degrees C and reduced short-circuit current (Isc) at >/=60 degrees C. At >/=60 degrees C, subepithelial blisters developed. Higher temperatures reduced R only moderately and reversibly. In contrast, the Isc declined sharply and irreversibly once threshold was reached. Luminal and serosal heat also reduced PD, Isc, and R, although the threshold for reduction in Isc was now similar to that for R. Additionally, luminal and serosal heat reduced Isc more than R for any given temperature and resulted in blisters at lower temperatures (50 degrees C) than luminal heat alone. The heat-induced decline in Isc was attributed in part to inactivation of Na-K-ATPase activity, although other transport systems could have been equally affected, and the decline in R to an increase in paracellular permeability. The latter effect on R also contributed to an increase in tissue sensitivity to luminal acid damage. Consumption of hot beverages exposes the esophagus to temperatures that can negatively impact epithelial structure and function. Impaired barrier function by heat increases the risk of esophageal damage by subsequent contact with (refluxed) gastric acid. These findings help explain in part the association between esophageal disease and consumption of hot beverages. (+info)
Can gender differences during exercise-heat stress be assessed by the physiological strain index?
A physiological strain index (PSI) based on rectal temperature (Tre) and heart rate (HR) was recently suggested to evaluate exercise-heat stress. The purpose of this study was to evaluate PSI for gender differences under various combinations of exercise intensity and climate. Two groups of eight men each were formed according to maximal rate of O2 consumption (VO2 max). The first group of men (M) was matched to a group of nine women (W) with similar (P > 0.001) VO2 max (46.1 +/- 2.0 and 43.6 +/- 2.9 ml. kg-1. min-1, respectively). The second group of men (MF) was significantly (P < 0. 001) more fit than M or W with VO2 max of 59.1 +/- 1.8 ml. kg-1. min-1. Subjects completed a matrix of nine experimental combinations consisting of three different exercise intensities for 60 min [low, moderate, and high (300, 500, and 650 W, respectively)] each at three climates (comfortable, hot wet, and hot dry [20 degrees C 50% relative humidity (RH), 35 degrees C 70% RH, and 40 degrees C 35% RH, respectively]). No significant differences (P > 0.05) were found between matched genders (M and W) at the same exposure for sweat rate, relative VO2 max (%VO2 max), and PSI. However, MF had significantly (P < 0.05) lower strain than M and W as reflected by %VO2 max and PSI. In summary, PSI applicability was extended for exercise-heat stress and gender. This index continues to show potential for wide acceptance and application. (+info)
Age and renal prostaglandin inhibition during exercise and heat stress.
Aging is associated with a number of physiological changes that may cause the kidney to rely to a greater extent on vasodilatory PGs for normal functioning. Acute exercise has been shown to cause renal vasoconstriction that may be partially buffered by vasodilatory PGs. To determine the relative importance of renal PGs during exercise in older adults, we compared the renal effects of the PG inhibitor ibuprofen (1.2 g/day for 3 days) vs. a placebo control in a cohort of eight younger (24 +/- 2 yr) and eight older (64 +/- 2 yr) women during treadmill exercise ( approximately 57% maximal oxygen consumption) in the heat (36 degrees C). This over-the-counter dose of ibuprofen reduced renal PG (i.e., PGE2) excretion by 47% (P < 0. 05). Acute exercise in the heat caused dramatic decreases in glomerular filtration rate, renal blood flow, and sodium excretion in both age groups. PG inhibition was associated with greater decreases in urine production and free water clearance (P < 0.05). There were no drug-related declines in glomerular filtration rate or renal blood flow. We conclude that PG inhibition has only modest effects on renal function during exercise. Also, the lack of hemodynamic changes with PG inhibition indicates that healthy well-hydrated older women are not in a renal PG-dependent state. (+info)
The free-convective anomaly.
Persons exposed to high temperature, or to equivalent environmental factors, have quantifiable reactions, such as reducing the resistance to both heat and moisture flow in skin tissues and clothing needed to maintain thermal equilibrium. The one-to-one relationship between this resistance in the walking person and temperature, with the other factors neutral, is the basis for the apparent temperature scale and the derived heat index. When this approach is taken to assess the thermal environment for a still person exposed to heat in still air, there is a zone of ambient conditions in which there are three solutions to the heat-balance equation. Extraordinary thermal stress occurs, depending slightly on other conditions, at ambient temperatures near 41 degrees C, especially at high humidity, because of the difficulty in carrying sweat vapor from the person when free convection is minimal. This anomaly is examined for a range of ambient vapor pressures and extra radiation. The rapid rise in heat stress when ambient temperature just exceeds body temperature in still conditions may explain the severity of some observed distress. (+info)
Low doses of melatonin and diurnal effects on thermoregulation and tolerance to uncompensable heat stress.
This study examined whether the reported hypothermic effect of melatonin ingestion increased tolerance to exercise at 40 degrees C, for trials conducted either in the morning or afternoon, while subjects were wearing protective clothing. Nine men performed four randomly ordered trials; two each in the morning (0930) and afternoon (1330) after the double-blind ingestion of either two placebo capsules or two 1-mg capsules of melatonin. Despite significant elevations in plasma melatonin to over 1,000 ng/ml 1 h after the ingestion of the first 1-mg dose, rectal temperature (T(re)) was unchanged before or during the heat-stress exposure. Also, all other indexes of temperature regulation and the heart rate response during the uncompensable heat stress were unaffected by the ingestion of melatonin. Initial T(re) was increased during the afternoon (37.1 +/- 0.2 degrees C), compared with the morning (36.8 +/- 0.2 degrees C) exposures, and these differences remained throughout the uncompensable heat stress, such that final T(re) was also increased for the afternoon (39.2 +/- 0.2 degrees C) vs. the morning (39.0 +/- 0.3 degrees C) trials. Tolerance times and heat storage were not different among the exposures at approximately 110 min and 16 kJ/kg, respectively. It was concluded that this low dose of melatonin had no impact on tolerance to uncompensable heat stress and that trials conducted in the early afternoon were associated with an increased T(re) tolerated at exhaustion that offset the circadian influence on resting T(re) and thus maintained tolerance times similar to those of trials conducted in the morning. (+info)
Role of radical oxygen species in rat testicular germ cell apoptosis induced by heat stress.
The present study was designed to clarify the role of radical oxygen species in testicular germ cell apoptosis induced by heat stress. Testicular cells isolated from immature rats were cultured with or without elevated temperature, and occurrence of apoptosis in these cells was defined by the appearance of DNA fragmentation following agarose gel electrophoresis and by flow cytometric quantification of apoptotic cells. At 32.5 degrees C, < 1% of cells showed signs of apoptosis throughout the culture period, whereas under heat stress, the proportion of apoptotic cells increased to 5% at 37 degrees C after 24 h of culture, or to 14% after 1-h exposure at 43 degrees C followed by 23-h culture at 32.5 degrees C. Similar to the effect of heat stress, exogenously supplied oxygen free radicals also induced apoptosis. In contrast, treatment with catalase significantly attenuated heat stress-induced apoptosis. Furthermore, heat stress of testicular cells was associated with an increased intracellular peroxide level as measured by a fluorescent probe, 2', 7'-dichlorofluorescin diacetate. In conclusion, our data indicate the involvement of radical oxygen species during testicular germ cell apoptosis induced by heat stress. This study provides a useful in vitro model for the study of testicular germ cell apoptosis. (+info)
Heat-related illnesses and deaths--Missouri, 1998, and United States, 1979-1996.
Although heat-related illness and death are readily preventable, exposure to extremely high temperatures caused an annual average of 381 deaths in the United States during 1979-1996. Basic behavioral and environmental precautions are essential to preventing adverse health outcomes associated with sustained periods of hot weather (daytime heat index of > or = 105 F [> or = 40.6 C] and a nighttime minimum temperature of 80 F [26.7 C] persisting for at least 48 hours). This report describes four heat-related deaths that occurred in Missouri during 1998, summarizes heat-related deaths in the United States during 1979-1996, describes risk factors associated with heat-related illness and death, especially in susceptible populations (young and elderly, chronically ill, and disabled persons), and recommends preventive measures. (+info)