Vasopressin and galanin expression in the hypothalamus of two African rodents, Taterillus gracilis and Steatomys caurinus, subjected to water-restriction.

The expression of arginine-vasopressin (AVP) and galanin (GAL) was studied by immunohistochemistry and in situ hybridization in the hypothalamus of two species of African rodents. In the wild, these animals experience successive arid and wet seasons that alternately stimulate their antidiuretic and diuretic systems. In this study, animals were subjected to both standardized laboratory conditions and to eight days of water-restriction. Under both sets of conditions, AVP and GAL were detected in the supraoptic nucleus (SON), paraventricular nucleus (PVN), and median eminence (ME). AVP and GAL responses to water-restriction differed in the two species, as did behavioral adaptations to the hot-dry season. In Taterillus gracilis, AVP- and GAL-LI (like immunoreactivity) peptide and mRNA levels increased in the SON. AVP-LI peptide and mRNA levels increased in the PVN, whereas only AVP-LI peptide levels increased in the ME. Pituitary gland AVP pools were unchanged by water deprivation, whereas urinary AVP levels and osmolality increased. The AVP response is typical of that of desert rodents, favoring survival under conditions of water-restriction. In Steatomys caurinus, which estivates, AVP and GAL-LI peptide levels decreased in the hypothalamus, as they did in the laboratory rat. In the SON, AVP, and GAL mRNA levels increased, whereas, in the PVN, only AVP mRNA levels increased. Pituitary gland AVP levels decreased, whereas urinary AVP levels and osmolality increased. In both species, the changes in the amount of GAL-LI peptide appeared to be closely linked to changes in AVP levels, suggesting that this peptide is involved in the osmoregulatory response to water-restriction.

Comparison of passive heat or exercise-induced dehydration on renal water and electrolyte excretion: the hormonal involvement.

The effects of hydromineral hormones and catecholamines on renal water and electrolyte excretion were examined during and after dehydration induced by either passive heat or exercise. Eight healthy young Caucasian subjects participated in three separate trials, each including three consecutive phases. Phases 1 and 3 involved a 90-min period at rest in a thermoneutral environment, while phase 2 involved a 120-min period designed to provide: (1) euhydration (control trial), (2) passive heat-induced dehydration of 2.8% body mass, or (3) exercise-induced dehydration of 2.8% body mass. During the two dehydration procedures, the decreases in urine flow and sodium excretion were more marked during exercise (P < 0.05). An increase in plasma catecholamines occurred only during exercise, together with a reduction in creatinine clearance and more marked increases in plasma renin and aldosterone than during passive heat exposure (P < 0.05). Although plasma vasopressin was elevated during the two dehydration procedures, urine osmolality did not change and, moreover, free water clearance increased during exercise (P < 0.05). Plasma levels of atrial natriuretic peptide increased markedly only during exercise compared to the other trials (P < 0.05). After the dehydration procedures, urine flow decreased again and urine osmolality increased markedly (P < 0.05), while plasma vasopressin remained elevated. These results suggest that sympathoadrenal activation during exercise plays a major role in the more marked reduction in diuresis and natriuresis than during passive heat exposure. Despite high plasma vasopressin concentrations during the two dehydrating events, the observed antidiuresis was not due to an increased renal concentrating ability, and the vasopressin was more effective after the dehydration procedures.

Influence of head-down bed rest on the circadian rhythms of hormones and electrolytes involved in hydroelectrolytic regulation.

We investigated in six men the impact of a 17-day head-down bed rest (HDBR) on the circadian rhythms of the hormones and electrolytes involved in hydroelectrolytic regulation. This HDBR study was designed to mimic an actual spaceflight. Urine samples were collected at each voiding before, during and after HDBR. Urinary excretion of aldosterone, arginine vasopressin (AVP), cyclic guanosine monophosphate (cGMP), cortisol, electrolytes (Na+ and K+) and creatinine were determined. HDBR resulted in a significant reduction of body mass (P < 0.01) and of caloric intake [mean (SEM) 2,778 (37) kcal.24 h(-1) to 2,450 (36) kcal.24 h(-1), where 1 kcal.h(-1) = 1.163 J.s(-1); P< 0.01]. There was a significant increase in diastolic blood pressure [71.8 (0.7) mmHg vs 75.6 (0.91) mmHg], with no significant changes in either systolic blood pressure or heart rate. The nocturnal hormonal decrease of aldosterone was clearly evident only before and after HDBR, but the day/night difference did not appear during HDBR. The rhythm of K+ excretion was unchanged during HDBR, whereas for Na+ excretion, a large decrease was shown during the night as compared to the day. The circadian rhythm of cortisol persisted. These data suggest that exposure to a 17-day HDBR could induce an exaggeration of the amplitude of the Na+ rhythm and abolition of the aldosterone rhythm.

Endocrine responses to 7 days of head-down bed rest and orthostatic tests in men and women.

The objective of this study was to investigate plasma volume (PV), total body water, hormones and hydroelectrolyte responses in eight males (25-40 years) and eight females (25-31 years) during 7 days of exposure to simulated microgravity (-6 degrees head-down bed rest, HDBR). Bed rest is a model that has commonly been used to simulate spaceflight. Heart rate (HR), blood pressure (BP) and vasoactive hormone responses were studied before and after HDBR during a 10-min stand test. No change in total body water and body mass was noted in either sex. The decrease in PV was similar in both men (9.1 +/- 1.4%) and women (9.4 +/- 0.8%). Urinary normetanephrine (NMN) was decreased during HDBR in both sexes. Urinary metanephrine (MN) and plasma catecholamines were unchanged. Daily urinary excretion of urea, an indirect index of protein breakdown, was increased only in the female subjects during HDBR. Plasma active renin (AR) and aldosterone were increased in both sexes, but urinary atrial natriuretic peptide (ANP) and arginine vasopressin (AVP) were unchanged throughout the study. Also, the hormonal responses to 7 days of HDBR were comparable between men and women. Moreover, the results show similar cardiovascular and endocrine responses to standing after HDBR. However, the orthostatic intolerance following HDBR was associated with a blunted increase in noradrenaline (NA) only in the women during the stand test. It is concluded that: (i) 7 days of physical inactivity achieved during HDBR resulted in a reduced sympathetic activity in both sexes and alterations in protein metabolism in women and (ii) standing after HDBR resulted in an attenuated release of noradrenaline in women.

Effects of 17 days of head-down bed rest on hydro-electrolytic regulation in men.

Prolonged periods of head-down bed rest (HDBR) are commonly used to mimic the effects of microgravity. HDBR has been shown to produce, as in space, a cephalad redistribution of circulating blood volume with an increase in central blood volume which induces the early adaptations in blood volume regulating hormones. Changes in atrial natriuretic peptide (ANP), arginine vasopressin (AVP), renin activity and aldosterone have been observed. Many reports describe these endocrine adaptations but few investigations of rhythms are in the literature. We proposed to evaluate the circadian rhythms of the hormones and electrolytes involved in the hydro-electrolytic regulation during a HDBR study which was designed to simulate a 17-day spaceflight (Life and Microgravity Spacelab experiment, LMS, NASA).

Cardiac natriuretic peptide response to water restriction in the hormonal adaptation of two semidesert rodents from West Africa (Steatomys caurinus, Taterillus gracilis).

Two African rodents, Taterillus gracilis and Steatomys caurinus, native to regions of alternate dry and wet seasons, were studied under laboratory conditions. These species differ in estivation behavior, one undergoing pseudoestivation and the other strong estivation. One group of animals of each species was provided with unlimited access to seed and vegetables rich in water, mimicking the food availability of the wet season (control group). A second group of animals of each species was subjected to water restriction for 8 days, mimicking the natural drought that occurs during the dry-hot season. The effects of water restriction on osmoregulation and body water content were assessed from hematocrit, and plasma and urinary osmolalities (PO, UO). Whether the natriuretic peptide system was modified by the osmoregulator adaptation to aridity of these semidesert rodents was examined from measurements of atrial natriuretic peptide (ANP) levels in plasma, atria, and ventricles, in parallel with morphological studies. In both species, UO was increased by water restriction. In water-deprived T. gracilis, ANP levels were about twice (right atria: 1.08 +/- 0.16 microg/mg protein vs control: 0.40 +/- 0.06 microg/mg protein) and plasma concentrations half (0.28 +/- 0.06 ng/ml vs control: 0.64 +/- 0.07 ng/ml) those in control animals. In S. caurinus these variables were not affected by water availability (right atria water restricted: 2. 20 +/- 0.15 microg/mg protein vs control: 2.86 +/- 0.37 microg/mg protein; plasma ANP water restricted: 0.80 +/- 0.12 ng/ml vs control: 0.90 +/- 0.16 ng/ml). Consistent with these quantitative results, immunohistochemical and ultrastructural observations showed an increase in immunostaining for both the N- and the C-terminal ANP and a larger number of granules in the atria of T. gracilis following water restriction, whereas there was no visible change in S. caurinus. Thus, water restriction induced a decrease in ANP secretion in T. gracilis, increasing cardiac storage alongside a reduced urine production. In contrast, in S. caurinus, the natriuretic system was not affected by an 8-day period of water restriction.

Adaptations to a 7-day head-down bed rest with thigh cuffs.

PURPOSE: Thigh cuffs were two elastic strips fixed at the upper part of each thigh, which limits the shift of fluid from the legs into the cardio-thoracic region. The purpose of this study was to examine the effects of thigh cuffs on hormonal and plasma volume responses and orthostatic tolerance during a 7-day head-down bed rest (HDBR). METHODS: Orthostatic tolerance, plasma volume, total body water, blood volume-regulating hormones, and hydro-electrolyte responses were measured in eight healthy men (age range, 25-40 yr), using thigh cuffs 10 h daily during 7 d of -6 degrees HDBR. RESULTS: Thigh cuffs worn during HDBR attenuated the decrease in plasma volume observed after HDBR (thigh cuffs: -5.85 +/- 0.95% vs control: -9.09 +/- 0.82%, P < or = 0.05). During this experiment, there was no significant change in total body water. Thus, the hypovolemia did not result from a loss of water but from a fluid shift from the blood compartment into the interstitial and/or intracellular compartment. Hormonal responses during HDBR and stand test were not modified by the thigh cuffs. Thigh cuffs had no significant effect on the clinical symptoms of orthostatic intolerance after HDBR. CONCLUSIONS: Thigh cuffs worn during HDBR blunted the decrease in plasma volume but did not reduce orthostatic intolerance; thus, they are not a completely effective countermeasure. Furthermore, hypovolemia seems to be necessary but not sufficient to induce orthostatic intolerance after HDBR.

Better preservation of endothelial function and decreased activation of the fetal renin-angiotensin pathway with the use of pulsatile flow during experimental fetal bypass.

OBJECTIVE: Pulsatile flow was shown to overcome the progressive rise in peripheral and placental vascular resistances observed during steady-flow bypass, this rise being counteracted by inhibition of nitric oxide synthase. This study quantifies the release of endothelial vasoactive substances during a 60-minute in utero model of fetal bypass. METHODS: Fetuses were randomly allocated into 1 of 2 groups (steady flow, n = 8, or pulsatile flow, n = 13) and subjected to bypass through central cannulation and perfusion with either a centrifugal or pulsatile (125 beats x min(-1)) blood pump. RESULTS: Lactate concentration was high, starting at fetal exteriorization and increasing during fetal preparation in the 2 groups. Once bypass was established, the rise was significant only in the steady-flow group. Plasma nitric oxide metabolites, similar before bypass, reached higher levels during pulsatile flow at the end of bypass (99+/-9 vs. 82+/-23 micromol x L(-1); P =.037). Levels of urinary nitric oxide metabolites were significantly higher in the pulsatile-flow than in the steady-flow group (764+/-143 vs. 508+/-240 micromol x L(-1); P =.005). Plasma cyclic guanosine monophosphate levels increased after 30 minutes of bypass in the pulsatile-flow group (25+/-18 vs. 12+/-8 pmol x mL(-1); P =.004), and urinary cyclic guanosine monophosphate excretion was higher in the pulsatile-flow group (517+/-450 vs. 118+/-78 pmol x mL(-1); P =.024). Plasma endothelin-1 levels increased in the 2 groups and were higher in the steady-flow group at 30 minutes (27+/-5 vs. 23+/-2 pg x mL(-1); P =.04) and 60 minutes of bypass (39+/-7 vs 32 +/- 6 pg x mL(-1); P =.04). Plasma renin concentration increased significantly during bypass only in the steady-flow group (26+/-10 vs. 57+/-42 in ng A1 x mL(-1) x h(-1); P =.04). CONCLUSIONS: Improved placental and peripheral perfusion during fetal pulsatile-flow bypass may be mediated by preservation of fetal/maternal endothelial nitric oxide biosynthetic mechanisms and/or decreased activation of the fetal renin-angiotensin pathway.

Plasma volume changes during and after acute variations of body hydration level in humans.

This study examined plasma volume changes (deltaPV) in humans during periods with or without changes in body hydration: exercise-induced dehydration, heat-induced dehydration and glycerol hyperhydration. Repeated measurements of plasma volume were made after two injections of Evans blue. Results were compared to deltaPV calculated from haematocrit (Hct) and blood haemoglobin concentration ([Hb]). Eight well-trained men completed four trials in randomized order: euhydration (control test C), 2.8% dehydration of body mass by passive controlled hyperthermia (D) and by treadmill exercise (60% of their maximal oxygen uptake, VO2max) (E), and hyperhydration (H) by glycerol ingestion. The Hct, [Hb], plasma protein concentrations and plasma osmolality were measured before, during and after the changes in body hydration. Different Hct and [Hb] reference values were obtained to allow for posture-induced variations between and during trials. The deltaPV values calculated after two Evans blue injections were in good agreement with deltaPV calculated from Hct and [Hb]. Compared to the control test, mean plasma volume declined markedly during heat-induced dehydration [-11.4 (SEM 1.7)%] and slightly during exercise-induced dehydration [-4.2 (SEM 0.9)%] (P < 0.001 compared to D), although hyperosmolality was similar in these two trials. Conversely, glycerol hyperhydration induced an increase in plasma volume [+7.5 (SEM 1.0)%]. These results would indicate that, for a given level of dehydration, plasma volume is dramatically decreased during and after heat exposure, while it is better maintained during and after exercise.

Changes in the sympathetic nervous system induced by 42 days of head-down bed rest.

Changes in autonomic nervous system activity could be linked to the orthostatic intolerance (OI) that individuals suffer after a spaceflight or head-down bed rest (HDBR). We examined this possibility by assessing the sympathetic nervous system activity during 42 days of HDBR in seven healthy men. Heart rate variability was studied with the use of power spectral analysis, which provided indicators of the sympathetic (SNSi) and parasympathetic (PNSi) nervous system influences on the heart. Urinary catecholamines and the spontaneous baroreflex sensitivity were measured. Urinary catecholamines decreased by 21.3%, showing a decrease in SNSi. Heart rate variability was greatly reduced during 42 days of HDBR with a drop in PNSi but with no significant changes in SNSi. The baroreflex sensitivity was greatly reduced (30.7%) on day 42 of HDBR. These results suggest a dissociation between the catecholamine response and the SNSi of the heart rate. This dissociation could be the consequence of an increase in beta-adrenergic receptor density and/or activity induced by a decrease in catecholamines during HDBR. The subjects who suffered from OI also had a greater sympathetic response and much lower baroreflex sensitivity when supine than those who finished the stand test. However, the mean response of all subjects indicated that the sympathetic activity (catecholamine excretion) was probably slightly inhibited during HDBR and could contribute to OI.

Presence of atrial natriuretic peptide in two desert rodents: comparison with rat.

Atrial natriuretic peptide (ANP) was characterized and assayed in plasmas, hearts, and brains of two Algerian desert rodents, Psammomys obesus and Meriones libycus along with vasopressin, which was assayed in hypophyses and hypothalami. Using reverse-phase high-performance liquid chromatography and radioimmunoassay, we showed, in plasmas and hearts of both species of desert rats, the presence of peptides similar to rat N- and C-terminal ANP but in lower amounts than in Wistar rats. Conversely, C-terminal ANP was abundantly detected in hypophyses from Meriones libycus rats. As these peptides, through their diuretic and natriuretic activities, are involved in body fluid regulation and electrolyte balance, the reduction of ANP stores in both plasmas and hearts suggests that diuresis and natriuresis are lowered in both species of mammals adapted to arid environments. This could occur because of the vasopressin-mediated adaptation, but also in response to the low ANP involvement in hydro-osmotic regulations, even in Psammomys, which has a dietary salt loading. On the other hand, the higher C-terminal ANP contents in the hypophysis of Meriones than in Psammomys and Wistar rats remain to be understood.

Effects of a 60-day confinement on the blood pressure, hormonal responses and body fluids of a mixed crew.

During the EXEMSI experiment, an international crew of 4 subjects (1 woman and 3 men) was confined for 60 days in a normobaric diving chamber (with 1060 mbar atmospheric pressure) to simulate life in a space station and to assess the effects of confinement on psychological and physiological factors. Blood pressure and blood volume regulating hormones (atrial natriuretic peptide, renin, aldosterone) and urine data (24-h urine outputs, ionogram) were measured before (BDC: baseline data collection), during (D: day) and after (R: recovery) confinement. We also measured energy expenditure and total body water, 14 days before, and after 27 days of confinement, by the double-labeled water method. We found a marked increase in 24-h urine output during most of the confinement in the men and the woman. Body weight (-1.8 +/- 0.9 kg) and energy expenditure (-1064 +/- 143 kcal/d, p<0.01) decreased in the 3 men. The total body water (TBW) decreased by 1.5 +/- 1.2 l in the men. Stress was not indicated by plasma and urine catecholamines but plasma growth hormone was elevated on D2 (p<0.01 vs. BDC) in the men. This study shows that confinement conditions can modify body fluid (increases in 24-h urine outputs and TBW changes) and energetic metabolisms.

Cardiovascular and hormonal changes induced by a simulation of a lunar mission.

BACKGROUND: This is the first simulation of a 14-d lunar mission including 6 d on the Moon. HYPOTHESIS: We hypothesized that a lunar gravity simulation in the middle of a head-down tilt (HDT) might result in some reversal of body fluid/hormonal responses, and influence cardiovascular deconditioning. METHODS: Six men (28 +/- 2.5 yr) were placed in bed rest (BR): in (HDT) (-6 degrees) to simulate microgravity during the travel (two 4-d periods), and in head-up tilt (HUT) (+10 degrees) (6-d period) to simulate lunar gravity (1/6 g). Muscular exercise was performed during the HUT period to simulate 6 h of lunar EVA. Heart rate variability (HRV) and hormonal responses were studied. RESULTS: An orthostatic arterial hypotension was observed after the BR (tilt test) in 4 of the 6 subjects. Plasma volume measured at D14 decreased by -11.1% (vs. D-3, sitting position). A decrease in atrial natriuretic peptide (26 +/- 3.5 pg.ml-1 (D14) vs. 37.9 +/- 3.5 pg.ml-1 (D-3, sitting) and an increase in plasma renin activity (198 +/- 9.2 mg.L-1.min-1 (D14) vs. 71 +/- 9.2 mg.L-1.min-1 (D-3, sitting) were observed during the BR, more pronounced in HUT at 7:00 p.m. Sympathetic-parasympathetic balance (HRV) at rest showed a decrease in parasympathetic indicator and an increase in sympathetic indicator in BR (p < 0.05), without differences within HDT and HUT periods. CONCLUSION: These changes were mostly similar to those reported in spaceflights, and HDT. Although the exposure to 1/6 g with exercise modified some hormonal and body fluid responses, this partial gravity simulation was not sufficient to prevent the decrease in orthostatic tolerance observed here as well as after Apollo lunar missions.

Haematocrit, plasma volume and noradrenaline in humans during simulated weightlessness for 42 days.

Previous results from our laboratory demonstrate that changes in haematocrit (Hct) and haemoglobin concentration (Hb) underestimate the relative (%) change in plasma volume (PV) in seated subjects during simulation of weightlessness by water immersion. Therefore, we examined whether changes in Hct and Hb would accurately reflect the changes in PV in seven subjects during simulation of weightlessness by another model, 6 degrees head-down tilted bed rest (HDBR), for 42 days. Since we have previously observed unexpectedly high plasma levels of noradrenaline (NA) in astronauts during space flight, we also took the opportunity to measure this variable. The measurements were compared with those of the supine horizontal position before and after HDBR. During HDBR, PV measured by the Evans blue dye dilution technique decreased by 6.1 +/- 2.8% (P < 0.05) on day 2 and 9.6 +/- 2.2% (P < 0.05) on the 42nd day compared with that of the supine, horizontal position. Based on changes in Hct and Hb, PV decreased similarly by 8.3 +/- 2.8 and 10.2 +/- 3.2% (P < 0.05) respectively. There were no differences comparing the results of the two methods (P > 0.05). Forearm venous plasma NA was unchanged during the whole course of HDBR compared with that of the pre-HDBR supine position. It is concluded that changes in Hct and Hb reliably reflect the changes in PV comparing prolonged HDBR with the pre- and post-HDBR horizontal, supine position. Thus, changes in Hct and Hb might accurately reflect the change in PV during weightlessness in humans provided that the horizontal supine position is used as the ground-based reference. Furthermore, the results of this study, as well as of previous studies from space, confirm that NA release is unchanged or even increased during weightlessness.

Effects of hydration state on hormonal and renal responses during moderate exercise in the heat.

The effects of hydromineral hormones and catecholamines on renal concentrating ability at different hydration states were examined in five male volunteers while they performed three trials. Each of these trials comprised a 60-min exercise bout on a treadmill (at 50% of maximal oxygen uptake) in a warm environment (dry bulb temperature, 35 degrees C; relative humidity, 20-30%). In one session, subjects were euhydrated before exercise (C). In the two other sessions, after thermal dehydration (loss of 3% body mass) which markedly reduced plasma volume (PV) and increased plasma osmolality (osm[pl]), the subjects exercised either not rehydrated (Dh) or rehydrated (Rh) by drinking 600 ml of mineral water before and 40 min after the onset of exercise. During exercise in the Dh compared to C state, plasma renin, aldosterone, arginine vasopressin (AVP), noradrenaline and adrenaline concentrations were increased (P < 0.05). A reduction in creatinine clearance and urine flow was also observed (P < 0.05) together with a decrease in urine osmolality, osmolar clearance and sodium excretion, while free water clearance increased (P < 0.05). However, compared to Dh, Rh partially restored PV and osm(pl) and induced a marked reduction in the time courses of both the plasma AVP and catecholamine responses (P < 0.05). Values for renal water and electrolyte excretion were intermediate between those of Dh and C. Plasma atrial natriuretic peptide presented similar changes whatever the hydration state. These results demonstrate that during moderate exercise in the heat, renal concentrating ability is paradoxically reduced by prior dehydration in spite of high plasma AVP levels, and might be the result of marked activation of the sympatho-adrenal system. Rehydration, by reducing this activation, could partially restore the renal concentrating ability despite the lowered plasma AVP.

Adaptation of plasma volume in humans to prolonged head-down bed rest.

NASA: Changes in plasma volume were studied in subjects who underwent 42 days of head-down bed rest or a one hour change in posture between upright and head-down tilt. Changes in hematocrit and heomoglobin concentration were also measured. Results are presented and discussed in terms of physiological adaptation to postural changes.^ieng

Restraint vs. hindlimb suspension on fluid and electrolyte balance in rats.

To determine the effect of hindlimb suspension on body fluid volume, salt and water balance, and relevant hormones, two series of experiments were performed in an experimental protocol including periods of isolation (7 days), horizontal attachment (7 days), and suspension (14 days). 1) During the first experiment, water and electrolyte balance, arginine vasopressin (AVP), and guanosine 3',5'- cyclic monophosphate (cGMP) were determined in urine, atrial natriuretic peptide in plasma and atria, and renin concentration and AVP in plasma in 30 rats. 2) During the second experiment, blood volume and extracellular fluid volume were measured by a dilution technique (Evans blue and sodium thiocyanate) in another 30 rats. We observed a pronounced and early effect of horizontal attachment on the renal variables. After 48 h, diuresis (49%), natriuresis (44%), kaliuresis (36%), osmotic load (39%), creatinine (28%), and AVP excretion (155%) were significantly increased in attached rats (P < 0.05). There was no short-term (24-h) effect of suspension on urine flow and Na+, K+, creatinine, and AVP excretion, but the urine cGMP decreased significantly (45%; P < 0.05). Significant decreases in natriuresis, kaliuresis, urine creatinine, and osmotic load occurred in the suspension group 7 days after suspension. After the 14-day tail suspension, plasma volume and extracellular fluid volume measured in suspended rats were not different from isolated rat values, whereas plasma volume increased by 15% (P < 0.05) in the attached rats. Plasma immunoreactive plasma atrial natriuretic levels of suspended rats were significantly reduced by 35% vs. isolated rats (P < 0.001) and by 18% vs. attached rats (P < 0.05). By using this experimental protocol, the physiological alterations revealed that suspension produced some acute and long-term effects, but the fixation to the suspension device, restraint, and confinement have their own influence on fluid distribution and renal function.

Cardiovascular and hormonal response during a 4-week head-down tilt with and without exercise and LBNP countermeasures.

To determine whether exercise and Lower Body Negative Pressure (LBNP) during 28 days of -6 degrees head-down tilt (HDT) would modify orthostatic tolerance and blood volume regulating hormones, twelve healthy men were assigned to either a no- countermeasure (No-CM, n=6), or a countermeasure (CM, n=6) group. LBNP sessions consisted of 15 minutes exposure to -30 mm Hg, on days 16, 18, 20 and 22-28 of HDT. Muscular exercise began on day 8 and consisted of combined graded dynamic and isometric resistance bilateral leg exercise on a specially designed supine ergometer, in two sessions of 15-20 min. each, every day, 6 days per week. A tilt test was performed before and at the end of HDT. Changes in resting plasma volume from control day (D-5) to HDT day 24 were -11.2% for No-CM and -2.2% for CM. After HDT three among the 6 subjects of the No-CM group presented presyncopal or syncopal symptoms, no tilt test was interrupted in CM group. Atrial Natriuretic Peptide (ANP) decreased at day 7 for the two groups and remained low during all the HDT period for No-CM group only. Plasma Renin Activity and Aldosterone increased at day 7 and remained elevated for the two groups. Norepinephrine and epinephrine were unchanged. Elevated diuresis and natriuresis were evident during the first day of HDT. However, renal excretory patterns were different between the two groups: indeed, a decrease of Na+, ANP and cGMP was observed only in No-CM at Day 13 during HDT. Our data showed that the subjects of the No-CM group experienced a greater increase in heart rate and a decrease in systolic blood pressure during tilt tests after HDT; nevertheless, after HDT, blood pressure was better maintained in CM group during the tilt test. The plasma volume decrease measured at the end of HDT was significantly lower in CM group, in contrast, these countermeasures were ineffective in preventing at least certain changes in blood volume regulating hormones.

Water balance during and after marathon running.

To describe the time course of plasma volume alterations and the changes in the plasma concentrations of hormones regulating water balance in relation to a marathon race, six experienced marathon runners (five men, one women) aged 28 (SD 6) years were studied during and for the 3 days following a treadmill marathon run at 68 (SD 5) percent of maximal oxygen consumption. Haematocrit, haemoglobin, plasma protein (Prot) and electrolyte (Na+, K+) concentration, osmolality (osm), plasma concentrations of renin (Ren), aldosterone (Ald) and atrial natriuretic peptide (ANP) were determined at rest in a sitting position (T(-30)), and then after 30 min in an upright posture (R(0)), while running a marathon at 10 km (R(10)), 30 km (R(30)) and 42.2 km (R end), and after the marathon at 30 min (T(30)), 60 min (T(60)), 120 min (T(120)) and 24 h (TD(+1)), 48 h (TD(+2)) and 72 h (TD(+3)). The changes in plasma volume (PV), Prot, osm and Na+ observed during the race were nonsignificant. Significant increases in plasma concentration of K+ [4.8 (SD 0.6) vs 5.5 (SD 0.6) mmol*1(-1); P <0.01], Ren [38(SD 57) vs 197 (SD 145) pmol*1(-1); P <0.02] and Ald [175 (SD 142) vs 1632 (SD 490) pmol*1(-1); P <0.01] were observed at R(end). A significant increase of ANP (P <0.05) was only found after R(10). Body mass significantly decreased by 2.0 kg (P <0.01) during the race in spite of the ingestion of 1.46 (SD 0.34)1 of a 5 percent glucose solution. Urinary volume and Na+ excretion dropped significantly after the completion of the marathon in comparison with the day before [2600 vs 1452 ml*day(-1)(P <0.02) and 161.3 vs 97.1 mmol*1(-1) (P <0.05)]. At TD(+1) and TD(+2) a significant increase in PV was noted, compared to T(-30). The lack of a decrease in PV during the marathon may have been due to the production of 402 g of metabolic water and by the release of 1280 g of water stored in glycogen complexes in muscle and liver. Thus, the hormone response during the marathon may have been due to the effects of the exercise itself and not to the effects of dehydration. The postmarathon PV expansion may be explained by a protein shift to the intravascular space and by renal sodium retention.

Comparison of a 4-day confinement and head-down tilt on endocrine response and cardiovascular variability in humans.

The aim of this study was to determine the effects of a 4-day head-down tilt (HDT; -6 degrees) and 4-day confinement on several indicators that might reflect a state of cardiovascular deconditioning on eight male subjects. Measurements were made of endocrine responses, heart rate variability and spontaneous baroreflex response (SBR) slope before, during and after each intervention. Plasma volume decreased by 10 percent after the 4-day HDT. The concentration of active renin was increased and that of urinary atrial natriuretic peptide decreased during the 4-day experiment in both groups. Plasma arginine vasopressin concentration decreased significantly only after 4-day confinement. After the 4-day HDT, one of the spectrum analysis parameters was statistically changed: the parasympathetic indicator decreased significantly (P <0.05) whereas the sympathetic indicator and the total power spectrum were unaltered. After 4-day confinement spectrum analysis parameters were not statistically altered. A significant decrease of SBR (P <0.05) was noticed only after the 4-day HDT. These data would suggest that exposure to a 4-day HDT was sufficient to induce a cardiovascular deconditioning which may have been induced by confinement and inactivity.