The risk of renal stone formation during and after long duration space flight.

BACKGROUND: The formation of a renal stone during space flight may have serious negative effects on the health of the crewmember and the success of the mission. Urinary biochemical factors and the influence of dietary factors associated with renal stone development were assessed during long duration Mir Space Station missions. METHODS: Twenty-four-hour urine samples were collected prior to, during and following long duration space flight. The relative urinary supersaturation of calcium oxalate, calcium phosphate (brushite), sodium urate, struvite and uric acid were determined. RESULTS: Changes in the urinary biochemistry of crewmembers during long duration spaceflight demonstrated increases in the supersaturation of the stone-forming salts. In-flight hypercalciuria was evident in a number of individual crewmembers and 24-hour dietary fluid intake and urine volume were significantly lower. During flight, there was a significant increase in brushite supersaturation. CONCLUSIONS: These data suggest acute effects of space flight and postflight changes in the urinary biochemistry favoring increased crystallization in the urine. The effects of dietary intake, especially fluid intake, may have a significant impact on the potential for renal stone formation. Efforts are now underway to assess the efficacy of a countermeasure to mitigate the increased risk.

Urine volume and its effects on renal stone risk in astronauts.

BACKGROUND: Urine composition in astronauts during and immediately after spaceflight changes in ways that increase the renal stone-forming potential for calcium oxalate, calcium phosphate, and uric acid saturation. We examined the effect of urine volume on the risk of renal stone formation in 356 astronauts. METHODS: Renal stone-forming risk was evaluated from 24-h urine samples collected from astronauts before and after 4- to 17-d Space Shuttle flights. Urinary chemistries were performed and the relative supersaturations of calcium oxalate, brushite, sodium urate, struvite, and uric acid saturation were calculated from the biochemical results. RESULTS: Urinary supersaturation levels of stone-forming salts were inversely related to urinary output both before and after spaceflight. Urine volume > 2 L x d(-1) reduced the risk of renal-stone development without affecting urinary citrate concentrations as compared with the increased risk observed in those astronauts who excreted urine volumes < L x d(-1). CONCLUSION: Results from this study indicate that increasing daily urinary output alone is an effective countermeasure to reduce the renal stone-forming risk immediately after spaceflight. However, increasing urinary output during flight may not be entirely effective in minimizing the potential risk of renal stone formation due to the changes in the urine chemistry in astronauts exposed to microgravity. KEYWORDS: urine volume, spaceflight, renal calculi.

Effects of microgravity on urinary osteopontin.

Increased risk of renal stone formation during space flight has been linked primarily to increased calcium excretion from bone demineralization induced by space flight. Other factors contributing to increased risk include increased urinary calcium oxalate supersaturation, while urinary citrate, magnesium and volume are all decreased. The aim of this study was to increase the predictive value of stone risk profiles for crew members during space flight by evaluating the excretion of urinary protein inhibitors of calcium crystallization so that more comprehensive stone risk profiles could relate mineral saturation to the concentrations of inhibitor proteins. Levels of urinary osteopontin (uropontin) are reported in a series of 14 astronauts studied before, during, and after space flights. During space flight, a compensatory increase in uropontin excretion was not observed. However, the uropontin excretion of a majority of astronauts was increased during the period after space flight and was maximal at 2 wk after landing. The downward shift in the molecular size of uropontin observed in samples obtained during space flight was shown to result from storage at ambient temperature during flight, rather than an effect of microgravity on uropontin synthesis.

Mathematical model to estimate risk of calcium-containing renal stones.

BACKGROUND/AIMS: Astronauts exposed to microgravity during the course of spaceflight undergo physiologic changes that alter the urinary environment so as to increase the risk of renal stone formation. This study was undertaken to identify a simple method with which to evaluate the potential risk of renal stone development during spaceflight. METHOD: We used a large database of urinary risk factors obtained from 323 astronauts before and after spaceflight to generate a mathematical model with which to predict the urinary supersaturation of calcium stone forming salts. RESULT: This model, which involves the fewest possible analytical variables (urinary calcium, citrate, oxalate, phosphorus, and total volume), reliably and accurately predicted the urinary supersaturation of the calcium stone forming salts when compared to results obtained from a group of 6 astronauts who collected urine during flight. CONCLUSIONS: The use of this model will simplify both routine medical monitoring during spaceflight as well as the evaluation of countermeasures designed to minimize renal stone development. This model also can be used for Earth-based applications in which access to analytical resources is limited.

Space flight and the risk of renal stones.

Exposure to the microgravity environment results in many metabolic and physiological changes to humans. Body fluid volumes, electrolyte levels, and bone and muscle undergo changes as the human body adapts to the weightless environment. This investigation examined the role of these physiologic changes to the potentially serious consequences of renal stone formation. The influence of dietary factors on the urinary biochemistry were assessed. Data collected immediately after Space Shuttle flights indicated changes in the urine chemistry favoring an increased risk of calcium oxalate and uric acid stone formation (Whitson et al., 1993). During short term Shuttle space flights, in-flight changes observed included increased urinary calcium and decreased urine volume, pH and citrate resulting in a greater risk for calcium oxalate and calcium phosphate stone formation (Whitson et al, 1997). Results from long duration Shuttle-Mir missions followed a similar trend and demonstrated decreased fluid intake and urine volume resulting in a urinary environment saturated with the calcium stone-forming salts. The increased risk occurs rapidly upon exposure to microgravity, continues throughout the space flight and following landing.

Renal stone risk assessment during Space Shuttle flights.

PURPOSE: The metabolic and environmental factors influencing renal stone formation before, during, and after Space Shuttle flights were assessed. We established the contributing roles of dietary factors in relationship to the urinary risk factors associated with renal stone formation. MATERIALS AND METHODS: 24-hr. urine samples were collected prior to, during space flight, and following landing. Urinary and dietary factors associated with renal stone formation were analyzed and the relative urinary supersaturation of calcium oxalate, calcium phosphate (brushite), sodium urate, struvite and uric acid were calculated. RESULTS: Urinary composition changed during flight to favor the crystallization of calcium-forming salts. Factors that contributed to increased potential for stone formation during space flight were significant reductions in urinary pH and increases in urinary calcium. Urinary output and citrate, a potent inhibitor of calcium-containing stones, were slightly reduced during space flight. Dietary intakes were significantly reduced for a number of variables, including fluid, energy, protein, potassium, phosphorus and magnesium. CONCLUSIONS: This is the first in-flight characterization of the renal stone forming potential in astronauts. With the examination of urinary components and nutritional factors, it was possible to determine the factors that contributed to increased risk or protected from risk. In spite of the protective components, the negative contributions to renal stone risk predominated and resulted in a urinary environment that favored the supersaturation of stone-forming salts. Dietary and pharmacologic therapies need to be assessed to minimize the potential for renal stone formation in astronauts during/after space flight.

Quantification of urinary uric acid in the presence of thymol and thimerosal by high-performance liquid chromatography.

A high-performance liquid chromatographic method was developed as an alternative to automated enzymatic analysis of uric acid in human urine preserved with thymol and/or thimerosal. Uric acid (tR = 10 min) and creatinine (tR = 5 min) were separated and quantified during isocratic elution (0.025 M acetate buffer, pH 4.5) from a mu Bondapak C18 column. The uric-acid peak was identified chemically by incubating urine samples with uricase. The thymol/thimerosal peak appeared at 31 min during the washing step and did not interfere with the analysis. We validated the high-performance liquid chromatographic method for linearity, precision and accuracy, and the results were found to be excellent.

Assessing total body and extracellular water from bioelectrical response spectroscopy.

We developed and validated assessments for total body water (TBW) and extracellular water (ECW) by using two resistance values of a new electric circuit model (CM) (two resistors; a capacitor and an inductor) with or without body mass. Fluid shifts occurring after 40 min of supine rest did not increase the validity of either estimate. CM estimates were valid; r = 0.941 to 0.969, low SE of estimates of 1.15-2.28 kg, nonsignificant mean differences (CM - dilution; %delta = -0.4 to 1.3%) that were close to the expected measurement errors for TBW (+/- 1%) and ECW (+/- 5%), and Bland-Altman pairwise comparisons that showed equivalence between methods. The CM estimates of TBW and ECW had marginally better validity than the previously published bioimpedance models. The advantage of the CM model is its assessments of multiple fluid spaces and that it does not require gender-specific equations. We conclude that CM estimate of TBW is acceptable, whereas further validation is needed before the ECW estimate should be used in a clinical or research setting.

Acute effects of head-down tilt and hypoxia on modulators of fluid homeostasis.

In an effort to understand the interaction between acute postural fluid shifts and hypoxia on hormonal regulation of fluid homeostasis, the authors measured the responses to head-down tilt with and without acute exposure to normobaric hypoxia. Plasma atrial natriuretic peptide (ANP), cyclic guanosine monophosphate (cGMP), cyclic adenosine monophosphate (cAMP), plasma aldosterone (ALD), and plasma renin activity (PRA) were measured in six healthy male volunteers who were exposed to a head-down tilt protocol during normoxia and hypoxia. The tilt protocol consisted of a 17 degrees head-up phase (30 minutes), a 28 degrees head-down phase (1 hour), and a 17 degrees head-up recovery period (2 hours, with the last hour normoxic in both experiments). Altitude equivalent to 14,828 ft was simulated by having the subjects breathe an inspired gas mixture with 13.9% oxygen. The results indicate that the postural fluid redistribution associated with a 60-minute head-down tilt induces the release of ANP and cGMP during both hypoxia and normoxia. Hypoxia increased cGMP, cAMP, ALD, and PRA throughout the protocol and significantly potentiated the increase in cGMP during head-down tilt. Hypoxia had no overall effect on the release of ANP, but appeared to attenuate the increase with head-down tilt. This study describes the acute effects of hypoxia on the endocrine response during fluid redistribution and suggests that the magnitude, but not the direction, of these changes with posture is affected by hypoxia.

Human cardiovascular response to sympathomimetic agents during head-down bed rest: the effect of dietary sodium.

Changes in sympathoadrenal function and cardiovascular deconditioning have long been recognized as a feature of the physiological adaptation to microgravity. The deconditioning process, coupled with altered hydration status, is thought to significantly contribute to orthostatic intolerance upon return to Earth gravity. The cardiovascular response to stimulation by sympathomimetic agents before, during, and after exposure to simulated microgravity was determined in healthy volunteers equilibrated on normal or high sodium diets in order to further the understanding of the deconditioning process.

Rat tail suspension causes a decline in insulin receptors.

Decreased muscular activity results in weakness and muscular atrophy. Coincident with this protein catabolic state is glucose intolerance and hyperinsulinemia. Rats were tail suspended for 7 to 14 days to accomplish unloading of the hindlimbs. Insulin resistance was documented in these animals by a 14 day tail suspension-related 26% increase in serum glucose in spite of a 253% increase in serum insulin concentration. Microsomal membranes were prepared from hindlimb muscles and specific binding of insulin and insulin-like growth factor I (IGF-I) were determined in these membranes. Insulin binding was decreased by 27% at 7 days and by 21% at 14 days. In contrast, IGF-I binding was unchanged at 7 days and was increased by 24% at 14 days. Liver membrane insulin receptors also had declined by 14 days of suspension, suggesting that the change in insulin receptors was a generalized, humorally-mediated phenomenon. These data suggest that tail suspension in rats results in insulin resistance, hyperinsulinemia, a decline in insulin receptors in liver and muscle, and a relative increase in muscle membrane IGF-I receptors. These data are consistent with the hypothesis that resistance to insulin's effects on protein metabolism in skeletal muscle may contribute to the protein catabolism associated with decreased muscular activity.

Alterations in renal stone risk factors after space flight.

Exposure to the microgravity environment of space produces a number of physiological changes of metabolic and environmental origin that could increase the potential for renal stone formation. Metabolic, environmental and physicochemical factors that influence renal stone risk potential were examined in 24-hour urine samples from astronauts 10 days before launch and on landing day to provide an immediate postflight assessment of these factors. In addition, comparisons were made between male and female crewmembers, and between crewmembers on missions of less than 6 days and those on 6 to 10-day missions. Results suggest that immediately after space flight the risk of calcium oxalate and uric acid stone formation is increased as a result of metabolic (hypercalciuria, hypocitraturia, pH) and environmental (lower urine volume) derangements, some of which could reflect residual effects of having been exposed to microgravity.

Autophosphorylation of cultured skin fibroblast insulin receptors from patients with severe insulin resistance and acanthosis nigricans.

The severe insulin resistance with acanthosis nigricans seen in young women without insulin-receptor autoantibodies is characterized by hyperinsulinemia and decreased in vivo responsiveness to insulin. We evaluated the potential cellular defects in insulin-receptor binding and autophosphorylation in 12 subjects with this syndrome. When evaluated as a group, insulin binding to freshly isolated monocytes was 55% that of controls. Specific binding of insulin to skin fibroblasts in monolayer culture was 49% that of controls. Maximal insulin-stimulated receptor autophosphorylation was only 27% that of controls. Individual data demonstrated that the diminished autophosphorylation activity was out of proportion to the diminished fibroblast insulin binding in cell lines from most subjects and was less than 50% of the predicted activity in 6 of the 12 studied cell lines. These data are consistent with genetically determined defects leading to diminished numbers of cell surface insulin receptors with intact tyrosine kinase autophosphorylation in many of our cell lines. However, in at least half, there appeared to be an additional defect beyond insulin binding, resulting in a disproportionate decrease in insulin-sensitive phosphorylation of the insulin-receptor beta-subunit.

High affinity antibody from hen's eggs directed against the human insulin receptor and the human IGF-I receptor.

Large quantities of high affinity antibodies directed against the human insulin receptor and the human insulin-like growth factor-I (IGF-I) receptor were obtained from hen's eggs. Hens were immunized with human placental membranes and human liver membranes by intramuscular injections. Specific antibodies to the receptors were demonstrated in serum and egg yolks at 5 weeks and these antibodies presisted for at least 6 months. Antibodies from egg yolks were purified by the polyethylene glycol precipitation technique of Polson et al. The eggs provided the equivalent of about 450 ml of immunized serum per month per bird. The purified antibodies were approximately equally reactive with receptors for insulin or IGF-I. Antibodies immunoprecipitated affinity-labeled receptors, inhibited binding of each ligand, and were capable of stimulating 2-deoxyglucose uptake in rat adipocytes and thymidine incorporation in cultured fibroblasts. The presence of antibodies directed against the IGF-I receptor in those hens immunized with human liver membranes was unexpected, since liver membranes possess little or no IGF-I binding activity. We conclude that antibodies against human antigens may be relatively easily obtained by immunization of hens and purification of those antibodies from eggs.

Photochemical cross-linking of 125I-hydroxyphenylisopropyl adenosine to the A1 adenosine receptor of rat adipocytes.

Rat adipocyte plasma membranes were incubated with the A1 adenosine receptor agonist, 125I-hydroxyphenylisopropyl adenosine (1 nM) and then treated with the photoactive cross-linking agent, ANB-NOS. The membranes were solubilized and analyzed by SDS-PAGE and autoradiography. A single protein, Mr approx. 38,000, was specifically labeled. Reduction with 2-mercaptoethanol did not affect the apparent Mr of the labeled protein. Labeling was inhibited by the adenosine receptor agonists, HPIA, PIA and NECA, and by the antagonist, theophylline, but was unaffected by inosine. We conclude that the A1 adenosine receptor is a single protein of Mr approx. 38,000.