Absorption of short-chain fatty acids,sodium and water from the forestomach of camels

In camelids the ventral parts of compartments 1 and 2 (C1/C2) and the total surface of compartment 3 of the forestomach are lined with tubular glands, whereas in ruminants the surface of the forestomach is composed entirely of stratified, squamous epithelium. Thus, differences in absorption rates between these foregut fermenters can be expected. In five camels C1/C2 was temporarily isolated, washed and filled with buffer solutions. Absorption of short-chain fatty acids (SCFA) and net absorption of sodium and water were estimated relative to Cr-ethylenediaminetetraacetic acid as a fluid marker. SCFA were extensively absorbed in the forestomach; clearance rates of SCFA with different chain lengths were equal. After lowering the pH of solutions SCFA absorption rates increased, but much less than the increase of the non-ionized fraction. Absorption of propionate was lower when acetate had been added. Findings suggest that most of the SCFA in camels are transported in the ionized form, most likely via an anion exchange mechanism. Net water absorption is closely related to net sodium absorption. Apparently water absorption results from an iso-osmotic process. Differences between absorption mechanisms of SCFA from the forestomach of camelids and ruminants are discussed.     

Effects of oral, intraperitoneal and intrajugular rehydrations on water retention, rumen volume, kidney function and thirst satiation in goats.

1. In order to test the hypothesis that peripheral receptors are involved in the control of fluid re-distribution following acute dehydration and rapid rehydration, peripheral rehydrations (oral or intraperitoneal) were compared with central (intrajugular) rehydration. 2. The experiments were carried out with four goats dehydrated to about 20% of their initial mass. 3. Following peripheral rehydration, a higher proportion of water was retained in comparison with central rehydration, and this was related to a more effective kidney retention mechanism, i.e. lower GFR and higher tubular reabsorption. 4. Higher proportions of water were retained in the rumen in the peripheral rehydrations in comparison with the central one apparently due to increased saliva secretion. 5. Thirst saturation was more effective with the peripheral rehydration in comparison with the central one and was related to the amount retained in the rumen and to peripheral blood expansion (or dilution).     

Effects of water deprivation on urea metabolism in camels, desert sheep and desert goats fed dry desert grass

1. The effects of water restriction and complete water deprivation on nitrogen retention and urea recycling were examined in camels, desert sheep and desert goats fed only dry desert grass of 3.2% crude protein content. 2. All three species were in negative nitrogen balance when water was available ad libitum. 3. In camels urea recycling was consistently high (94-97%) and nitrogen balance did not change with treatment. 4. In sheep and goats urea recycling increased from 75% to 94% (sheep) and from 79% to 95% (goats) during water deprivation, and nitrogen balance improved to positive values. 5. In all species water deprivation depressed dry matter intake and increased apparent digestibility of dry matter and nitrogen. 6. The effects of water restriction were generally intermediate. 7. These results suggest that nitrogen metabolism in adult domestic livestock raised by nomadic pastoralists in Sudan is not harmed, and may in fact be improved by short periods of water deprivation.     

Prolactin and its involvement in fluid regulation in the bovine

Plasma prolactin (PRL) concentrations, osmolality, water consumption, feed intake, urine excretion, and fecal water output were determined in twelve steers of 3 breeds exposed to 5 feed and water regimes. Breed differences were found in water intake and plasma PRL concentrations when feed and water were ad lib., however, during any of the other 4 treatments, responses were similar between breeds. During dehydration and feed restriction, water intake, urine, fecal water, and plasma prolactin decreased; however, during hydration and refeeding such changes were not as clearly related. No consistent relationships between plasma prolactin and osmolality were found. Data suggests that PRL's role in fluid regulation in the bovine is most likely associated with alterations in renal hemodynamics rather than by changes in plasma osmolality.     

Some effects of water deprivation on dorcas gazelle (Gazella dorcas) in the Sudan

1. The effects of dehydration for 10 days and subsequent rehydration for 2 days on some physiological and biochemical parameters were studied in the Dorcas gazelle. 2. At the end of the 10 days of dehydration, feed intake and body weight were decreased by 42 and 29% of the control values, respectively. Rehydration restored most of the loss in feed intake and body weight. 3. Dehydration decreased the water content in the faeces of the gazelles by 41%. Rehydration restored 36% of the lost faecal water. Urine volume in dehydrated animals decreased by 66%, an effect which was readily reversed by rehydration. 4. Blood haemoglobin of dehydrated gazelles decreased by 22% but this effect was not reversed by rehydration for 2 days. 5. Dehydration for 10 days decreased the concentration of serum glucose by 34% and increased that of urea and albumin by 34 and 18%, respectively. Dehydration for 2 days reversed these effects completely. 6. The serum concentrations of sodium, potassium and chloride were increased by dehydration by 23, 44 and 18% respectively. After 2 days of rehydration the values returned to normal. 7. No change in the heart rate, pulse or temperature was found during the dehydration period. 8. The gazelles survived the 10 days of dehydration, although they tended to be drowsy, weak and emaciated during the last 2 days of dehydration. The animals appeared normal after rehydration.     

Food intake, conversion efficiency, and feeding behaviour of tobacco hornworm caterpillars given artificial diet of varying nutrient and water content

ABSTRACT Fifth stadium tobacco hornworm caterpillars, Manduca sexta (L.), given artificial diet diluted to varying extents with either cellulose or water compensated for the food's reduced nutrient content by eating more of it. This compensation was, however, in most cases not sufficient to maintain normal growth rates. When the water content of the diet was reduced, the insects ate less than the usual fresh weight of food but maintained their intake of nutrients. Nevertheless, growth rate was impaired. The insects were better able to compensate for dilution of their food with water than with cellulose. The efficiency of conversion of ingested food (ECI) was decreased when the diet was adulterated with cellulose. At moderate dilution (50% nutrient) this was due mostly to decreased approximate digestibility (AD), but at greater dilution (25% and 10% nutrient content) the efficiency of conversion of digested food (ECD) was decreased. ECI was maintained when the water content of the diet was increased to give 50% nutrient concentration, but was decreased when water content was changed more radically (200%, 25% and 10% nutrient diets). This was due mostly to increased metabolic costs (decreased ECD) in all cases. The retention time of food in the gut was progressively decreased (i.e. speed of passage was increased) as nutrients were replaced by cellulose. By contrast, dilution of the diet with water resulted in only slight changes in retention time, except at extreme dilution (10% nutrient content) when retention time was reduced. Compensation of food intake was achieved by spending more (or less) time eating. Video analysis of feeding behaviour showed that there were significant changes in the length of feeding bouts and of interfeed gaps when caterpillars fed on diets of altered composition. For diets diluted with cellulose, changes in bout length and bout frequency contributed substantially to the increased time spent feeding on the adulterated food. For diets diluted with water, however, almost all of the compensatory change in behaviour was due to increased bout length, with bout frequency affected only slightly. This suggests that volumetric feedback contributes principally to the termination of feeding bouts in caterpillars, while nutrient flow may affect both the initiation and termination of feeding.     

Distributions of antibovine-pepsinogen-positive cells in glandular regions of forestomach in the bactrian camel, Camelus bactrianus.

The present paper describes mucous cells in the forestomach glandular region which immunoreacted to antibovine pepsinogen serum in adult and young bactrian camels, Camelus bactrianus (female, 25 years old; female, 6 months old; from Hilar Inner Mongolia, People's Republic of China, in August 1988). These immunoreactive cells in forestomach glandular regions also reacted to Alcian blue (pH 1.0) and/or periodic acid-Schiff. Cross-reactivity of antibovine pepsinogen serum against extracts from the forestomach glandular region was analyzed by Western blotting, supporting that this serum could detect pepsin and/or pepsinogen in the bactrian camel. It was suggested that mucous cells in forestomach glandular regions produce pepsinogen molecules.     

Effects of water deprivation and rehydration on c-Fos and FosB staining in the rat supraoptic nucleus and lamina terminalis region

We studied cFos and FosB staining in the supraoptic nucleus (SON) the organum vasculosum of the lamina terminalis (OVLT) and the median preoptic nucleus (MnPO) in adult male rats after water deprivation (24 h, n = 11; 48 h, n = 12) and water deprivation with rehydration (22 h + water, n = 11; 46 h + water, n = 10). Control rats (n = 15) had water available ad libitum. Separate sets of serial sections from each brain were processed for immunocytochemistry using primary antibodies against either c-Fos or FosB protein. Plasma osmolality, vasopressin, hematocrit, and plasma proteins were measured in separate groups (n = 6-7). The number of c-Fos-positive cells in the SON was significantly increased after 24 and 48 h of water deprivation. In contrast, rehydrated groups were not different from control. Water deprivation significantly increased c-Fos staining in both the OVLT and the MnPO, but c-Fos staining was not altered by rehydration. FosB staining in the SON was significantly increased only by 48-h water deprivation, and this effect was significantly decreased by rehydration. In the MnPO and OVLT, FosB staining was significantly increased by water deprivation, and, like c-Fos staining, these increases were not affected by rehydration. Water deprivation significantly increased osmolality and hematocrit, as well as plasma protein and vasopressin concentrations. Plasma measurements from rehydrated rats were not different from control. We conclude that water deprivation and rehydration differentially affect c-Fos and FosB staining in a region-dependent manner.     

Effects of dehydration,rehydration, and hyperhydration in the lactating and non-lactating black Moroccan goat

The effects of water deprivation, rehydration and hyperhydration were investigated in the black Moroccan goat (Capra hircus). Mean daily water intake was 46 ± 5 ml/kg in lactating and 36 ± 4 ml/kg in non-lactating black Moroccan goats, and milk production 21 ± 1 ml/kg. Mean urine excretion was 8 ± 2 ml/kg body weight in both groups, and the daily water losses via evaporation and feces were estimated at 23 ± 3 ml/kg during lactation and 28 ± 4 ml/kg during non-lactation. Forty-eight hours of water deprivation caused a body weight loss of 9% and 6% in lactating and non-lactating goats, respectively, and a drop of 28% in milk production with only a slight decrease in food intake. After rehydration, the elevated plasma osmolality as well as Na and total protein concentrations returned to basal values within 2–3 hr, indicating a rapid absorption of the ingested water, but urine excretion did not increase. After hyperhydration (10% of body weight), 46% of the load was excreted by the kidneys within 6 hr. In conclusion, black Moroccan goats have a low water turnover, and they can retain water upon rehydration but not store excess water after hyperhydration.     

Solute and particle retention in the digestive tract of the Phillip's dikdik (Madoqua saltiana phillipsi), a very small browsing ruminant: biological and methodological implications

Morphological characteristics of the forestomach, as well as reports of a natural diet that mostly excludes monocots, suggest that dikdiks (Madoqua spp.), among smallest extant ruminants, should have a 'moose-type' forestomach physiology characterised by a low degree of selective particle retention. We tested this assumption in a series of feeding experiments with 12 adult Phillip's dikdiks (Madoqua saltiana phillipsi) on three different intake levels per animal, using cobalt-EDTA as a solute marker and a 'conventional' chromium-mordanted fibre (<2 mm; mean particle size 0.63 mm) marker for the particle phase. Body mass had no influence on retention measurements, whereas food intake level clearly had. Drinking water intake was not related to the retention of the solute marker. In contrast to our expectations, the particle marker was retained distinctively longer than the solute marker. Comparisons with results in larger ruminants and with faecal particle sizes measured in dikdiks suggested that in these small animals, the chosen particle marker was above the critical size threshold, above which particle delay in the forestomach is not only due to selective particle retention (as compared to fluids), but additionally due to the ruminal particle sorting mechanism that retains particles above this threshold longer than particles below this threshold. A second study with a similar marker of a lower mean particle size (0.17 mm, which is below the faecal particle size reported for dikdiks) resulted in particle and fluid retention patterns similar to those documented in other 'moose-type' ruminants. Nevertheless, even this smaller particle marker yielded retention times that were longer than those predicted by allometric equations based on quarter-power scaling, providing further support for observations that small ruminants generally achieve longer retention times and higher digestive efficiencies than expected based on their body size.     

Renal Na excretion in dehydrated and rehydrated adrenalectomized sheep maintained with aldosterone

The effect of water deprivation for 19 h on renal Na excretion of conscious adrenalectomized (ADX) sheep maintained on a constant intravenous infusion of aldosterone and cortisol (ADX-constant steroid sheep) was investigated. Both ADX and normal sheep showed large increases in renal Na excretion when they were deprived of water. ADX-constant steroid sheep also exhibited a normal postprandial natriuresis 3-6 h after feeding, whether or not water was available to drink. In another experiment, sheep deprived of water for 41 h were then allowed to drink water. Both normal and ADX-constant steroid sheep exhibited a large reduction of renal Na excretion in the 6 h after rehydration. Changes in plasma Na and K concentration and osmolality were similar in normal and ADX-constant steroid sheep during periods of dehydration and rehydration. These results show that change in aldosterone secretion is not a major factor in causing either dehydration-induced or postprandial natriuresis. Neither is it a major cause of rehydration-induced renal Na retention.     

Gestational and early postnatal dietary NaCl levels affect NaCl intake, but not stimulated water intake, by adult rats

We examined body fluid regulation by weanling (21-25 days) and adult (>60 days) male rats that were offspring of dams fed chow containing either 0.1, 1, or 3% NaCl throughout gestation and lactation. Weanling rats were maintained on the test diets until postnatal day 30 and on standard 1% NaCl chow thereafter. Ad libitum water intake by weanlings was highest in those fed 3% NaCl and lowest in those fed 0.1% NaCl. Adult rats maintained on standard NaCl chow consumed similar amounts of water after overnight water deprivation or intravenous hypertonic NaCl (HS) infusion regardless of early NaCl condition. Moreover, baseline and HS-stimulated plasma Na(+) concentrations also were similar for the three groups. Nonetheless, adult rats in the early 3% NaCl group consumed more of 0.5 M NaCl after 10 days of dietary Na(+) deprivation than did rats in either the 1% or 0.1% NaCl group. Interestingly, whether NaCl was consumed in a concentrated solution in short-term, two-bottle tests after dietary Na(+) deprivation or in chow during ad libitum feeding, adult rats in the 3% NaCl group drank less water for each unit of NaCl consumed, whereas rats in the 0.1% NaCl group drank more water for each unit of NaCl consumed. Thus gestational and early postnatal dietary NaCl levels do not affect stimulated water intake or long-term body fluid regulation. Together with our previous studies, these results suggest that persistent changes in NaCl intake and in water intake associated with NaCl ingestion reflect short-term behavioral effects that may be attributable to differences in NaCl taste processing.     

Water intake induced by water deprivation in the quail,Coturnix coturnix japonica

Mechanisms inducing drinking after water deprivation, and mechanisms terminating drinking after rehydration, were investigated in the quail, Coturnix coturnix japonica. 1. Water intake was induced after 4 h of water deprivation, and the amount of water drunk increased in proportion to the period of water deprivation. Drinking occurred immediately after deprivation. Drinking occurred immediately after deprived birds were given access to water, and continued for periods proportional to the period of water deprivation. 2. Plasma angiotensin II concentration increased, as did plasma osmolality and Na+ concentration, and blood volume decreased after water deprivation. The increase in plasma angiotensin II concentration and decrease in blood volume occurred soon after the start of water deprivation, whereas plasma osmolality and Na+ concentration did not increase until at least 4 h after the start of water deprivation. 3. These results indicate that extracellular dehydration and angiotensin II are responsible for the significant drinking that follows 4 h of water deprivation, and that cellular dehydration is also involved in the stimulation of drinking that occurs after longer periods of water deprivation. 4. Plasma osmolality and Na+ concentration in birds deprived of water for 48 h quickly returned to normal levels after the birds were allowed access to water. Plasma angiotensin II levels and blood volume also approached the values measured prior to water deprivation. However, the rate and degree of restoration of normal values were reduced, and normal values were not restored even after 1.5 h or rehydration when drinking terminated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chitinolytic bacteria in the minke whale forestomach

Minke whales consume large amounts of pelagic crustaceans. Digestion of the prey is initiated by indigenous bacteria in a rumen-like forestomach system. A major structural component of the crustacean exoskeleton is chitin, the beta-1,4-linked polymer of N-acetyl-D-glucosamine. The exoskeletons appear to dissolve completely in the non-glandular forestomach. Bacteria in the forestomach fluid of six krill-eating minke whales were enumerated and isolated using an anaerobic habitat-simulating culture medium. Median viable population densities ranged between 6.0 x 10(6) and 9.9 x 10(9) bacterial cells per mL forestomach fluid. Bacterial isolates (n = 44) cultured from the forestomach fluid of one minke whale mainly resembled strains of Eubacterium (25%), Streptococcus (18%), Clostridium (14%), and Bacteroides (11%). As much as 12% of the bacterial isolates were chitinolytic, while beta-N-acetylglucosaminidase activity was demonstrated in 54% of the isolates, and utilisation of N-acetyl-D-glucosamine was observed in 73%. The chitinolytic isolates resembled strains of Bacteroides, Bacteroidaceae, Clostridium, and Streptococcus. Scanning and transmission electron microscopy of partly digested krill from the minke whale forestomach revealed bacteria close to and inside the chitinous exoskeleton. The bacterial chitinase may act on the chitinous crustacean exoskeletons, thereby allowing other bacteria access to the nutritious soft inner tissues of the prey, and thus initiating its degradation and fermentation.     

Effect of bolus fluid intake on energy expenditure values as determined by the doubly labeled water method.

The doubly labeled water (DLW, 2H(2)18O) method is a highly accurate method for measuring energy expenditure (EE). A possible source of error is bolus fluid intake before body water sampling. If there is bolus fluid intake immediately before body water sampling, the saliva may reflect the ingested water disproportionately, because the ingested water may not have had time to mix fully with the body water pool. To ascertain the magnitude of this problem, EE was measured over a 5-day period by the DLW method. Six subjects were dosed with 2H2(18)O. After the reference salivas for the two-point determination were obtained, subjects drank water (700-1,000 ml), and serial saliva samples were collected for the next 3 h. Expressing the postbolus saliva enrichments as a percentage of the prebolus value, we found 1) a minimum in the saliva isotopic enrichments were reached at approximately 30 min with the minimum for 2H (95.48 +/- 0.43%) being significantly lower than the minimum for 18O (97.55 +/- 0.44, P less than 0.05) and 2) EE values calculated using the postbolus isotopic enrichments are appreciably higher (19.9 +/- 7.5%) than the prebolus reference values. In conclusion, it is not advisable to collect saliva samples for DLW measurements within approximately 1 h of bolus fluid intake.     

Intake, ingesta retention, particle size distribution and digestibility in the hippopotamidae

Although several aspects of the digestive physiology of the hippopotamidae-non-ruminating foregut fermenters-have been described, ingesta kinetics and passage characteristics of these species are not well understood. The most outstanding feature of the hippo digestive physiology reported so far is the very long mean ingesta retention times (MRTs) measured by Foose [Foose, T., 1982. Trophic strategies of ruminant versus nonruminant ungulates. PhD dissertation, University of Chicago, Chicago.]. Since those data had been investigated with animals without water access, we intended to measure MRT in hippos which were allowed to enter water pools during the night. MRT parameters as well as dry matter (DM) digestibility were determined in four common (Hippopotamus amphibius) and four pygmy hippos (Hexaprotodon liberiensis) on two different diets each using cobalt ethylendiamintetraacetate (Co-EDTA) as a fluid, chromium (Cr)-mordanted fibre (<2 mm) as a particle and acid detergent lignin (ADL) as an internal digestibility marker. Four of the animals additionally received cerium (Ce)-mordanted fibres (2-10 mm) as particle markers. Total MRTs for fluids and particles ranged between 20-35 and 48-106 h in the common and between 13-39 and 32-107 h in the pygmy hippos. The difference between fluid and particle retention was greater than usually reported in ruminants. Excretion patterns of the markers differed from those usually observed in ruminants but resembled those reported for macropods (kangaroos), indicating a plug-flow reactor-like physiology in the hippo forestomach (FRST). This finding complements other described similarities between the macropod and the hippo forestomach. The measurements of larger particle retention profiles suggest that in the hippo, larger particles might be excreted either faster or at the same rate as smaller particles, indicating a general difference between ruminants and hippos with respect to differential particle retention. The digestive physiology of hippos is characterised by a generally low food intake, long ingesta retention times and dry matter digestibilities lower than reported in ruminants. Moderate digestibilities in spite of long retention times might be the result of the generally high average ingesta particle size in hippos. The comparatively easy management of pygmy hippos, together with the significant correlations between food intake, MRT and digestibility in the pygmy hippos of this study, recommends this species for further studies on the interplay of these parameters in herbivore digestive physiology.     

Maintained serum sodium in male ultra-marathoners--the role of fluid intake, vasopressin, and aldosterone in fluid and electrolyte regulation

Exercise-associated hyponatremia (EAH) is a well know electrolyte disorder in endurance athletes. Although fluid overload is the most like etiology, recent studies, however, argued whether EAH is a disorder of vasopressin secretion. The aims of the present study were to investigate (i) the prevalence of EAH in male ultra-marathoners and (ii) whether fluid intake, aldosterone or vasopressin, as measured by copeptin, were associated with post-race serum sodium concentration ([Na+]). In 50 male ultra-marathoners in a 100?km ultra-marathon, serum [Na+], aldosterone, copeptin, serum and urine osmolality, and body mass were measured pre- and post-race. Fluid intake, renal function parameters and urine excretion were measured. No athlete developed EAH. Copeptin and aldosterone increased; a significant correlation was found between the change in copeptin and the change in serum [Na+], no correlation was found between aldosterone and serum [Na+]. Serum [Na+] increased by 1.6%; body mass decreased by 1.9?kg. The change in serum [Na+] and body mass correlated significantly and negatively. The fluid intake of ~?0.58?l/h was positively related to the change in body mass and negatively to both post-race serum [Na+] and the change in serum [Na+]. We conclude that serum [Na+] was maintained by both the mechanisms of fluid intake and the hormonal regulation of vasopressin.     

Mice deficient in oxytocin manifest increased saline consumption following overnight fluid deprivation

Male mice (9-13 mo of age) in which the gene for oxytocin (OT) had been deleted (OT -/-) were administered 0.5 M sodium chloride (NaCl) solution or tap water as a two-bottle choice test following overnight fluid deprivation (1600 to 1000 the following day). Compared with wild-type cohorts (OT +/+), OT-deficient mice ingested sevenfold greater amounts of saline in the first hour following reintroduction of fluids, P < 0.001, and fourfold greater amounts at the end of 6 h, P < 0.02. No significant difference in total water ingested was noted between the two genotypes at the end of either 1 or 6 h. If food deprivation accompanied the overnight fluid deprivation and food was reintroduced 1 h after the reintroduction of both water and saline, OT -/- mice still ingested greater amounts of saline, but not water, than OT +/+ mice at both 1 h, P < 0.001, and 6 h, P < 0.02. No differences were noted between genotypes in the daily intake of 0.5 M NaCl solution or water during a 3-day observation period before the overnight fluid deprivation. The volume of saline consumed in each 24-h observation period represented about one-tenth of the total fluids ingested in each genotype. We conclude that OT -/- mice display an enhanced salt appetite compared with OT +/+ mice when fluid deprived overnight. The salt appetite was only apparent in the presence of a perturbation such as fluid deprivation, which predisposes the animal to moderate hypovolemia. The observations support an inhibitory role for OT in the control of sodium appetite in mice.     

Paradoxes of body fluid volume regulation in health and disease. A unifying hypothesis.

The body''s normal homeostasis is maintained by the integrity of the excretory capacity of the kidneys. In advanced cardiac failure, however, the avidity of the renal sodium and water retention contributes to the occurrence of pulmonary congestion and peripheral edema. In patients with advanced cirrhosis, the kidneys again fail to excrete the amounts of sodium and water ingested, thus leading to ascites and peripheral edema. The signals for this renal retention of sodium and water in a patient with cirrhosis must be extrarenal because when the same kidneys are transplanted into persons with normal liver function, renal sodium and water retention no longer occurs; rather, the kidneys maintain normal fluid and electrolyte balance. Excessive sodium and water retention by the kidneys also occurs during pregnancy despite a 30% to 50% increase in plasma volume, cardiac output, and glomerular filtration rate. What are the afferent and efferent signals whereby normal kidneys retain sodium and water so that total extracellular, interstitial, and intravascular volumes expand far beyond those limits observed in normal subjects? These dilemmas are the subject of this review, in which a "unifying hypothesis of body fluid volume regulation" is presented.     

Early fluid retention and severe acute mountain sickness.

Field studies of acute mountain sickness (AMS) usually include variations in exercise, diet, and environmental conditions over days and development of clinically apparent edemas. The purpose of this study was to clarify fluid status in persons developing AMS vs. those remaining without symptoms during simulated altitude with controlled fluid intake, diet, temperature, and without exercise. Ninety-nine exposures of 51 men and women to reduced barometric pressure (426 mmHg = 16,000 ft. = 4,880 m) were carried out for 8-12 h. AMS was evaluated by Lake Louise (LL) and AMS-C scores near the end of exposure. Serial measurements included fluid balance, electrolyte excretions, and plasma concentrations, regulating hormones, and free water clearance. Comparison between 16 subjects with the lowest AMS scores near the end of exposure ("non-AMS": mean LL = 1.0, range = 0-2.5) and 16 others with the highest AMS scores ("AMS": mean LL = 7.4, range = 5-11) demonstrated significant fluid retention in AMS beginning within the first 3 h, resulting from reduced urine flow. Plasma Na+ decreased significantly after 6 h, indicating dilution throughout the total body water. Excretion of Na+ and K+ trended downward with time in both groups, being lower in AMS after 6 h, and the urine Na+-to-K+ ratio was significantly higher for AMS after 6 h. Renal compensation for respiratory alkalosis, plasma renin activity, aldosterone, and atrial natriuretic peptide were not different between groups, with the latter tending to rise and aldosterone falling with time of exposure. Antidiuretic hormone fell in non-AMS and rose in AMS within 90 min of exposure and continued to rise in AMS, closely associated with severity of symptoms and fluid retention.