Acute and repeated exposure for 8-13 consecutive days to exercise in humid heat was studied. Twelve fit subjects exercised at 150 W [45% of maximum O2 uptake (V.O2,max)] in ambient conditions of 35 degrees C and 87% relative humidity which resulted in exhaustion after 45 min. Average core temperature reached 39.9 +/- 0.1 degrees C, mean skin temperature (T-sk) was 37.9 +/- 0.1 degrees C and heart rate (HR) 152 +/- 6 beats min-1 at this stage. No effect of the increasing core temperature was seen on cardiac output and leg blood flow (LBF) during acute heat stress. LBF was 5.2 +/- 0.3 l min-1 at 10 min and 5.3 +/- 0.4 l min-1 at exhaustion (n = 6). After acclimation the subjects reached exhaustion after 52 min with a core temperature of 39.9 +/- 0.1 degrees C, T-sk 37.7 +/- 0.2 degrees C, HR 146 +/- 4 beats min-1. Acclimation induced physiological adaptations, as shown by an increased resting plasma volume (3918 +/- 168 to 4256 +/- 270 ml), the lower exercise heart rate at exhaustion, a 26% increase in sweating rate, lower sweat sodium concentration and a 6% reduction in exercise V.O2. Neither in acute exposure nor after acclimation did the rise of core temperature to near 40 degrees C affect metabolism and substrate utilization. The physiological adaptations were similar to those induced by dry heat acclimation. However, in humid heat the effect of acclimation on performance was small due to physical limitations for evaporative heat loss.
Eight men were studied during graded (47, 77, and 100% of maximal oxygen uptake) and prolonged (76%) exhaustive treadmill running. During graded exercise the glucagon concentration increased 35% from 81 plus or minus 7 pg/ml (mean and SE) at rest to 109 plus or minus 17 after the heaviest load. During prolonged exercise glucagon increased progressively to three times (226 plus or minus 40) the resting value. Norepinephrine increased from 0.40 plus or minus 0.06 ng/ml to 2.22 plus or minus 0.39, epinephrine from 0.07 plus or minus 0.01 to 0.42 plus or minus 0.13 during graded, and to 1.51 plus or minus 0.08 and 0.33 plus or minus 0.04, respectively, during prolonged exercise. Insulin concentrations were depressed during work except for the heaviest load. Fatty acids rose throughout prolonged exercise, whereas blood glucose significantly diminished 30 min afterward. Glucagon concentrations correlated significantly with norepinephrine and epinephrine concentrations during prolonged and with epinephrine during graded exercise. Although increments in catecholamines were similar, the glucagon secretion was larger during prolonged than during graded exercise. While increments in catecholamines might explain increased glucagon secretion during graded exercise, they cannot account completely for the rise of glucagon during prolonged exercise.
Key pointsr Weightlessness in space induces initially an increase in stroke volume and cardiac output, accompanied by unchanged or slightly reduced blood pressure.r It is unclear whether these changes persist throughout months of flight. r Here, we show that cardiac output and stroke volume increase by 35-41% between 3 and 6 months on the International Space Station, which is more than during shorter flights.r Twenty-four hour ambulatory brachial blood pressure is reduced by 8-10 mmHg by a decrease in systemic vascular resistance of 39%, which is not a result of the suppression of sympathetic nervous activity, and the nightly dip is maintained in space.r It remains a challenge to explore what causes the systemic vasodilatation leading to a reduction in blood pressure in space, and whether the unexpectedly high stroke volume and cardiac output can explain some vision acuity problems encountered by astronauts on the International Space Station.Abstract Acute weightlessness in space induces a fluid shift leading to central volume expansion. Simultaneously, blood pressure is either unchanged or decreased slightly. Whether these effects persist for months in space is unclear. Twenty-four hour ambulatory brachial arterial pressures were automatically recorded at 1-2 h intervals with portable equipment in eight male astronauts: once before launch, once between 85 and 192 days in space on the International Space Station and, finally, once at least 2 months after flight. During the same 24 h, cardiac output (rebreathing method) was measured two to five times (on the ground seated), and venous blood was sampled once (also seated on the ground) for determination of plasma catecholamine concentrations. The 24 h average systolic, diastolic and mean arterial pressures (mean ± SE) in space were reduced by 8 ± 2 mmHg (P = 0.01; ANOVA), 9 ± 2 mmHg (P < 0.001) and 10 ± 3 mmHg (P = 0.006), respectively. The nightly blood pressure dip of 8 ± 3 mmHg (P = 0.015) was maintained. Cardiac stroke volume and output increased by 35 ± 10% and 41 ± 9% (P < 0.001); heart rate and catecholamine concentrations were unchanged; and systemic vascular resistance was reduced by 39 ± 4% (P < 0.001). The increase in cardiac stroke volume and output is more than previously observed during short duration flights and might be a precipitator for some of the vision problems encountered by the astronauts. The spaceflight vasodilatation mechanism needs to be explored further.
The effect of resistant starch (RS) on postprandial plasma concentrations of glucose, lipids, and hormones, and on subjective satiety and palatability ratings was investigated in 10 healthy, normal-weight, young males. The test meals consisted of 50 g pregelatinized starch (0% RS) (S) or 50 g raw potato starch (54% RS) (R) together with 500 g artificially sweetened syrup. After the R meal postprandial plasma concentrations of glucose, lactate, insulin, gastric inhibitory polypeptide (GIP), glucagon-like peptide-1, and epinephrine were significantly lower compared with after the S meal. Moreover, subjective scores for satiety and fullness were significantly lower after the R meal than after the S meal. Differences in GIP, texture, and palatability may have been involved in these findings. In conclusion, the replacement of digestible starch with RS resulted in significant reductions in postprandial glycemia and insulinemia, and in the subjective sensations of satiety.
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