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J Appl Physiol 92: 3-12, 2002;
8750-7587/02 $5.00
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Vol. 92, Issue 1, 3-12, January 2002

Calcium metabolism and cardiovascular function after spaceflight

Daniel C. Hatton1,2, Qi Yue1, Jacqueline Dierickx2, Chantal Roullet1, Keiichi Otsuka1, Mitsuaki Watanabe1, Sarah Coste2, Jean Baptiste Roullet1, Thongchan Phanouvang1, Eric Orwoll3, Shiela Orwoll3, and David A. McCarron1

Divisions of 1 Nephrology, Hypertension, and Clinical Pharmacology and 3 Endocrinology, Metabolism, and Clinical Nutrition, Department of Medicine, and 2 Department of Behavioral Neuroscience, Oregon Health Sciences University, Portland, Oregon 97201


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

To determine the influence of dietary calcium on spaceflight-induced alterations in calcium metabolism and blood pressure (BP), 9-wk-old spontaneously hypertensive rats, fed either high- (2%) or low-calcium (0.02%) diets, were flown on an 18-day shuttle flight. On landing, flight animals had increased ionized calcium (P < 0.001), elevated parathyroid hormone levels (P < 0.001), reduced calcitonin levels (P < 0.05), unchanged 1,25(OH)2D3 levels, and elevated skull (P < 0.01) and reduced femur bone mineral density. Basal and thrombin-stimulated platelet free calcium (intracellular calcium concentration) were also reduced (P < 0.05). There was a tendency for indirect systolic BP to be reduced in conscious flight animals (P = 0.057). However, mean arterial pressure was elevated (P < 0.001) after anesthesia. Dietary calcium altered all aspects of calcium metabolism (P < 0.001), as well as BP (P < 0.001), but the only interaction with flight was a relatively greater increase in ionized calcium in flight animals fed low- compared with high-calcium diets (P < 0.05). The results indicate that 1) flight-induced disruptions of calcium metabolism are relatively impervious to dietary calcium in the short term, 2) increased ionized calcium did not normalize low-calcium-induced elevations of BP, and 3) parathyroid hormone was paradoxically increased in the high-calcium-fed flight animals after landing.

spontaneously hypertensive rat; microgravity; dietary calcium; platelet intracellular calcium; parathyroid hormone


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

DURING SPACEFLIGHT, THE MUSCULOSKELETAL system is beset by a progressive loss of bone calcium (see Refs. 28, 38). The loss of calcium from bone results in hypercalcemia, which resets calcium regulation such that there is reduced gut absorption of calcium. Over the long term, the diminished absorption of calcium contributes to the progressive negative calcium balance that develops. Whereas the obvious concern with loss of calcium is bone health, there are likely to be ramifications for other physiological systems of equal importance to health. The cardiovascular system is particularly sensitive to calcium status. Research in hypertension has provided abundant evidence that variations in calcium intake and metabolism are associated with significant differences in blood pressure within populations (15, 20). There is a strong correlation between negative calcium balance and elevated blood pressure in all human populations. Continued losses of calcium in space can reasonably be expected to result in a similar outcome, if not during spaceflight, then in the prolonged period of time required to reestablish calcium balance after flight.

The purpose of the present experiment was twofold. The first goal was to establish the impact of variations in dietary calcium on microgravity-induced alterations in calcium metabolism, bone density, and blood pressure. The possibility that dietary calcium might ameliorate some of the deficits in calcium metabolism caused by spaceflight has not been examined, except in studies of simulated weightlessness (11). Of particular interest was the interaction of diet and spaceflight on cellular calcium metabolism. Whereas mobilization of calcium from bone stores and attendant alterations in systemic calcium metabolism have long been recognized as a consequence of spaceflight, there have been few investigations of intracellular calcium regulation relative to spaceflight, despite the critical importance of calcium as an intracellular messenger. Vascular smooth muscle cell (VSMC) intracellular calcium concentration ([Ca2+]i) levels are positively related to vascular contraction, vessel tone, and blood pressure (9) and may represent one avenue through which spaceflight could influence vascular function (8), thereby altering blood pressure regulation.

The second goal was to examine the mechanisms responsible for dietary calcium-induced alterations in blood pressure. Spaceflight provides a unique opportunity to investigate the influence of dietary calcium on blood pressure because of the possibility of dissociating calcium content of the diet from calcium-regulating hormones. Under conditions on Earth, low-calcium diets reduce ionized calcium levels, causing an elevation of parathyroid hormone (PTH) and 1,25(OH)2D3 to correct the deficit. However, in microgravity, bone resorption may result in sufficiently elevated levels of circulating ionized calcium in animals fed low-calcium diets to normalize calcium-regulating hormones, resulting in an animal on a low-calcium diet with normal calcium-regulating hormone levels. These special circumstances may provide insight into the diet-induced alterations in calcium metabolism responsible for maintaining alterations in blood pressure.

These issues were examined using spontaneously hypertensive rats (SHR) fed either high- or low-calcium diets. The SHR is an animal model of human essential hypertension with calcium-sensitive blood pressure. High-calcium diets lower blood pressure, whereas low-calcium diets increase blood pressure in this strain (16). It was hypothesized that, for SHR fed low-calcium diets, short-term exposure to microgravity would mobilize bone calcium, resulting in decreased bone mineral density (BMD), increased circulating ionized calcium, normalized calcium-regulating hormones, reduced platelet [Ca2+]i levels, and lower blood pressures. It was anticipated that the increase in calcium availability would be relatively greater in the low-calcium diet group than in the high-calcium diet group, and, consequently, it was expected that blood pressure would be comparable between animals fed high- and low-calcium diets after the relatively short duration of a shuttle flight.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The protocols used in these experiments were approved by the Institutional Animal Care and Use Committees at Oregon Health Sciences University, Ames Research Center, Johnson Space Center, and Kennedy Space Center.

Flight Experiment

Animals. Fourteen male SHR were obtained from Taconic Farms (Germantown, NY) at 21 days of age. On arrival at Kennedy Space Center, the animals were placed on either a high- (2.0%) or low-calcium (0.2%) diet (Teklad, Madison, WI) for the duration of the experiment. Calcium content was the only difference between the present diet and the typical space bar diet consumed by rodents on shuttle flights. The animals were assigned to flight and vivarium control groups based on receipt date from the vendor. Shipments were offset by 3 days to allow for testing of all animals at the same age.

Procedure. When the animals were 7 wk of age, two animal enclosure modules (AEM) containing seven animals each were flown on the space shuttle Columbia (STS-80). One AEM contained high-calcium diet and the other low-calcium diet animals. The flight lasted 18 days. Three hours after landing, the animals were available for experimentation.

The tail-cuff method (NARCO Biosystems, Houston, TX) was used to measure indirect systolic blood pressure (SBP) in all animals within the first 1.5 h of receipt. After a warm-up period of 20 min during which the animal's core temperature was increased by 1°C through exposure to a circulating water bath set in the floor of the restrainers, a series of three measurements of blood pressure were taken. The average value of the three measurements was considered SBP for the animal. Immediately after tail-cuff blood pressure was measured on the first animal, it was transferred to another room and anesthetized with halothane (2% in O2), and a catheter was inserted into the carotid artery for measurement of direct arterial blood pressure and collection of blood. Mean arterial pressure (MAP) was measured for 5 min using a Statham P23-ID pressure transducer in line with a Grass model 7P1 direct-current preamplifier (Grass Instrument, Quincy, MA). Data were recorded on a chart recorder before blood samples were collected for determination of blood calcium, calcium-regulating hormones, and platelet [Ca2+]i. Thereafter, one rat was killed every 0.5 h for the first eight rats, followed by a break for 2.5 h. The final six rats were also killed one every 0.5 h. From beginning to end, it took a total of 14 h to finish testing. Experiments on the vivarium control animals were conducted on exactly the same schedule as that for the flight animals but 3 days later.

Platelet calcium measurements. Platelet [Ca2+]i was measured using fura 2-AM (Sigma Chemical, St. Louis, MO). Washed platelets were incubated for 30 min at room temperature with 3 µM/l fura 2-AM diluted from a 2 mM stock solution in dimethyl sulphoxide (Sigma Chemical). Platelets were then centrifuged at 400 g for 10 min with 2 µM/l PGE1 to remove extracellular fura 2-AM. The platelet pellet was resuspended in HEPES buffer solution and incubated with 1 mM/l CaCl2 for 3 min at 37°C.

For measurement, 500-µl aliquots of platelet suspension were continually stirred by a magnetic stir bar in a microquartz cuvette maintained at 37°C in a dual-excitation wavelength fluorescence spectrophotometer (F2000, Hitachi, Tokyo, Japan). The cells were alternately excited with ultraviolet light at 340 and 380 nm, and emission at 510 nm was detected.

Thereafter, platelets were stimulated by either 1 U/ml rat thrombin (Sigma Chemical) or 5 µM/l ionomycin (Sigma Chemical). The ionomycin dose was sufficient to obtain a maximal response of calcium discharge from intracellular stores. These measurements were performed in duplicate. At the end of each experiment, the platelets were lysed with 50 µM/l digitonin to obtain the fluorescent signal at minimal calcium saturation in the presence of 10 mM/l EGTA (pH 8.3) and maximal calcium saturation in the presence of 1 mM/l calcium. All values were corrected for autofluorescence by subtraction of fluorescence of unloaded platelets and test reagents. Platelet calcium was calculated as described by Grynkiewiczx et al. (13).

Systemic calcium assays. Ionized calcium was measured from whole blood within 1 min of withdrawal from the animal using a calcium-specific electrode (AVL 9140, AVL Instruments, Roswell, GA). Total calcium was measured from serum using a COBAS Bioanalyzer (Roche, Somerville, NJ). Serum 1,25(OH)2D3 was measured with a radioreceptor assay kit, and calcitonin was measured with a RIA kit from Diasorin (Stillwater, MN). PTH was measured with an immunoradiometric assay kit that measures intact and N-terminal forms of PTH (Nichols Institute, San Juan Capistrano, CA).

BMD measurements. On return to Oregon Health Sciences University, animal carcasses were scanned for bone area, mineral content, and density measurements by dual-energy X-ray absorptiometry (DEXA) using a Hologic 4500A (Hologic, Waltham, MA) equipped with rat whole body scan software (version 8.11a). The scan field size was 30 × 18 cm with a resolution of 279 × 201 pixels. Analysis was performed for whole body and two subregions of interest: the skull and femur. Subsequently, the right femur was excised and scanned with high-resolution software (version 8.17h) with a scan field of 6 × 5 cm and a resolution of 159 × 185 pixels. For scanning, femurs were placed in a thin-walled plastic container filled with 2.5 cmH2O. Because the autoanalysis was unable to detect all of the bone in the animals on low-calcium diets, bone profiles for all animals were done manually. Great care was taken to keep the profile 1-2 pixels inside the bone edge.

AEM Control Experiment

After the flight experiment, an AEM control (AEMC) experiment was conducted at Oregon Health Sciences University. The experiment was designed to determine which of the parameters measured in the flight experiment were sensitive to AEM housing conditions. Consequently, the experiment simulated as closely as possible the flight experiment in all respects, except launch, landing, and spaceflight. The source of animals, the procedure, the timing of experiments, and collection of data were the same as described for the flight experiment. Animals assigned to the AEM condition remained in the AEM for a period identical to that experienced by the flight animals. Temperature and lighting conditions simulated shuttle flight conditions. AEMC animals were treated in the same way as control animals in the flight experiment.

Data Analysis

ANOVA, correlations, and stepwise linear regression analysis were conducted using SPSS for Windows 6.0. For ANOVA, repeated-measures designs were used where appropriate. Contrasts were made using Tukey tests. Pearson's r was used for correlation analysis. Stepwise linear regression was done with a criterion of P < 0.05 to enter and P > 0.10 to remove. Trend analysis, using time of testing as the independent variable, was used to determine whether there were any linear or higher order trends over time in blood pressure responses as a consequence of adaptation to normal gravity. A probability of 0.05 was used to establish statistical significance.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Flight Results

Body weights for the flight rats and vivarium control rats were comparable for the two diet groups at testing (201 ± 7 vs. 198 ± 14 g for low calcium and 219 ± 12 vs. 220 ± 13 g for high calcium for flight vs. vivarium control). However, the flight rats were heavier than the control rats preflight (133 ± 8 g for flight and 113 ± 9 g for control). Thus there was significantly less weight gain in the flight rats than in the controls (P < 0.001) between preflight and postflight. The high-calcium diet animals had significantly greater body weight than the low-calcium animals (P < 0.001).

Whole blood ionized calcium and total serum calcium (Fig. 1) were significantly higher in the high-calcium diet groups both before and after flight (P < 0.001). After flight, there was a significant elevation of ionized and total calcium in both diet groups (P < 0.001). The increase in ionized calcium was greatest in the low-calcium diet group, resulting in a significant diet × flight interaction (P < 0.05).


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  		Fig. 1.   Blood ionized (A) and total calcium (B) in flight and control animals on high- and low-calcium diets. Values are means ± SE. There were significant effects for diet (P < 0.001) and flight (P < 0.001) and a significant interaction between diet and flight for ionized calcium (P < 0.05) due to a greater increase in the low-calcium flight animals. Significant difference between diet groups: bbb P < 0.001. Significant difference between flight groups: a P < 0.05.

PTH, 1,25(OH)2D3, and calcitonin results are presented in Fig. 2. After landing, the flight groups had higher PTH levels than the vivarium controls, regardless of the diet condition (P < 0.001). There was a significant effect of diet on PTH, with the high-calcium diet groups having lower levels of PTH, regardless of the flight condition (P < 0.001). The 1,25(OH)2D3 was highest in the low-calcium diet (P < 0.001) and did not change after flight. In contrast, calcitonin values (P < 0.005) were significantly reduced after flight, whereas animals fed low-calcium diets had significantly higher levels of calcitonin (P < 0.005) than those in the high-calcium group. Thus diet and flight influenced calcium-regulating hormones independently.


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  		Fig. 2.   Calcium-regulating hormones in flight and control animals on high- and low-calcium diets. A: parathyroid hormone; B: 1,25(OH)2D3; C: calcitonin. Values are means ± SE. Significant difference between diet groups: bbb P < 0.001. Significant difference between flight groups: aa P < 0.01, aaa P < 0.001.

Data on BMD are shown in Fig. 3. BMD was markedly different between diet groups for all bone measurements. Measurement of the whole carcass indicated that total BMD was significantly greater in the flight animals (P < 0.001). Data from the same scan showed that BMD in the skull was significantly increased in the flight animals (P < 0.01), whereas the femur was not different between flight and vivarium control animals (P > 0.05). However, analysis of the femur after it was excised indicated that BMD was significantly lower in the flight animals (P < 0.05) than in vivarium control animals.


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  		Fig. 3.   Bone mineral density (BMD) in total body (A), skull (B), and femur [in vivo (C) and ex vivo (D)] for flight and control animals on high- and low-calcium diets. Values are means ± SE. Significant difference between diet groups: bbb P < 0.001. Significant difference between flight groups: a P < 0.05, aa P < 0.01, aaa P < 0.001.

Platelet [Ca2+]i levels under basal and stimulated conditions are presented in Fig. 4. Basal and thrombin-stimulated levels of [Ca2+]i were significantly lower after flight than in the control animals (P < 0.05). Platelet [Ca2+]i ionomycin was comparable between flight and control animals. These flight-induced changes applied to both diet groups. Diet had an independent effect on intracellular calcium. High-calcium diets resulted in lower thrombin- and ionomycin-stimulated [Ca2+]i than did low-calcium diets (P < 0.05). There was no difference between diet groups for basal intracellular calcium levels (P > 0.05).


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  		Fig. 4.   Platelet intracellular calcium concentration ([Ca2+]i) in flight and control animals on high- and low-calcium diets. A: basal platelet; B: thrombin-stimulated platelet; C: ionomycin-releasable platelet. Values are means ± SE. Significant difference between diet groups: b P < 0.05. Significant difference between flight groups: a P < 0.05.

Blood pressure after flight is shown in Fig. 5. Animals on low-calcium diets had higher blood pressure than did animals on high-calcium diets (P < 0.001), regardless of the method of measuring blood pressure. SBP was somewhat lower (P = 0.057) in the flight animals than in the control animals after flight. In contrast, MAP measured while the animals were anesthetized was significantly higher in the flight animals than in the control animals (P < 0.001). Trend analysis indicated that there were no apparent order effects in the blood pressure data that would be indicative of a change in blood pressure over time due to readaptation to Earth's gravity.


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  		Fig. 5.   Systolic blood pressure (SBP; A) and mean arterial pressure (MAP; B) in flight and control animals on high- and low-calcium diets. SBP was taken by using a tail-cuff method in conscious animals. MAP was measured while the animals were anesthetized with halothane. Values are means ± SE. Significant difference between diet groups: bbb P < 0.001. Significant difference between flight groups: aaa P < 0.001.

Linear regression was used to develop a model for predicting MAP and SBP in the flight and control groups based on measures of calcium metabolism. Because the number of variables that can be included in linear regression models is limited by the number of subjects in the analysis, a combination of variables representing systemic calcium (ionized calcium), calcium-regulating hormones (PTH), intracellular calcium (ionomycin-stimulated [Ca2+]i), and BMD (excised femoral BMD) was selected. For the flight animals, PTH emerged as the best predictor of MAP (r = 0.77, beta  = 0.093, t = 4.275, P = 0.001), whereas femoral BMD was most predictive of SBP (r = 0.720, beta  = -418, t = -3.589, P < 0.005). In the vivarium controls, ionomycin-releasable calcium stores predicted both MAP (r = 0.801, beta  = 0.221, t = 4.224, P < 0.005) and SBP (r = 0.795, beta  = 0.276, t = 4.143, P < 0.005).

In both the high- and low-calcium diet flight groups, there was an inverse relationship between basal levels of platelet [Ca2+]i and MAP, as shown in Fig. 6, that was not present in the vivarium control group. The correlation between basal-free intracellular calcium and MAP was r = 0.82 (P < 0.05) in the low-calcium fed animals and r = 0.87 in the high-calcium fed animals (P < 0.05).


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  		Fig. 6.   Correlation between MAP and basal platelet [Ca2+]i in flight animals on high (Hi)- and low (Lo)-calcium diets. r = 0.82 in the low-calcium group and 0.87 in the high-calcium group. Regr, regression.

AEMC experiment. Data from the AEM experiment are presented in Table 1. Overall, there were few significant differences between animals housed in AEMs and their control animals. However, there were significant interactions between diet and AEM condition for body weight (P < 0.05), 1,25(OH)2D3 (P < 0.001), and BMD of the femur (P < 0.05), suggesting that housing conditions did influence calcium metabolism. The interactions occurred because the AEM animals fed low-calcium diets had greater weight, higher 1,25(OH)2D3 levels, and lower BMD in the femur than did their counterparts in the AEMC condition, whereas the AEM animals fed high-calcium diets showed the opposite profile relative to the AEMC animals fed high-calcium diets.

                              
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  		Table 1.   AEM experiment data


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Calcium Metabolism

On landing, whole blood ionized calcium was significantly elevated in the flight animals. As expected, there was a significant interaction between diet and spaceflight for ionized calcium. Indeed, ionized calcium levels in the low-calcium diet animals after flight were comparable to those of the high-calcium diet animals before flight. Yet, despite the pronounced increase in ionized calcium in the low-calcium-fed animals after spaceflight, there were no other significant interactions between diet and flight among the variables measured.

Calcium-regulating hormones on landing vs. during spaceflight. The significant elevation in whole blood ionized calcium failed to reduce 1,25(OH)2D3 in the animals fed low-calcium diets. This outcome is commensurate with reports of a transient fall in 1,25(OH)2D3 levels followed by a return to control levels during simulated weightlessness in rats (7, 14, 37) but leaves open the question of whether or not the 1,25(OH)2D3 levels obtained after landing are representative of levels that prevailed during flight. The only data on that issue come from Smith et al. (33), who measured ionized calcium and calcium-regulating hormones in astronauts before, during, and 3 h after flight. Their data suggest that values for ionized calcium and 1,25(OH)2D3 did not change much immediately after flight, but PTH, on the other hand, increased rapidly after landing. Thus ionized calcium and 1,25(OH)2D3 levels may be close to the values that prevailed during flight, whereas it seems likely that the elevation in PTH observed in the present study reflects a postflight phenomena.

The PTH data from Smith et al. (33) dovetail with other reports that PTH is either normal or reduced while exposed to microgravity but increased after return to normal gravity (4, 40). Similarly, Arnaud et al. (1) found slightly elevated levels of PTH after landing in rats exposed to microgravity. One possible explanation for the elevated PTH could be that sustained elevations of ionized calcium during spaceflight may have reset the set point for calcium regulation by, for example, altering the expression of calcium-sensing receptors in the parathyroid gland (6). If so, a slight reduction in ionized calcium on landing would trigger a large increase in PTH because of the steep ionized calcium-PTH curve in young rats (36).

Blood Pressure

Diet-induced blood pressure differences were sustained, despite the marked increases in blood ionized calcium levels in the low-calcium diet group in the flight condition. Assuming that ionized calcium increased shortly after entering microgravity (33), these results suggest that normalizing circulating ionized calcium levels, at least for a period of 18 days, is not sufficient to lower blood pressure in the low-calcium animals. Failure of increased ionized calcium to lower blood pressure in the low-calcium-fed flight animals suggests that some other aspect of calcium metabolism may have been responsible for the sustained differences in blood pressure between diet groups.

Blood pressure and calcium-regulating hormones. Diet-induced differences in calcium-regulating hormones were maintained after spaceflight and may have contributed to the diet-induced variations in blood pressure. In the present study, both PTH and 1,25(OH)2D3 were elevated in the low-calcium-diet groups. There is evidence that both PTH and 1,25(OH)2D3 may modulate blood pressure (31). PTH is an acute vasodilator, but prolonged exposure, either through diet or infusion, has been reported to attenuate the vasodilatory effect (26, 34), whereas subacute infusion in humans has been shown to elevate blood pressure (10). The chronic effects of PTH may explain the positive relationship between blood pressure and PTH levels that have been observed in humans (25). 1,25(OH)2D3, on the other hand, has been shown to increase vascular contractility in the SHR (2, 3), which may contribute to increased vascular tone. Calcitonin has not been considered a primary variable in blood pressure regulation, as it has little impact on cardiovascular function, at least acutely (30).

Based on linear regression, PTH was the best predictor of MAP in the flight animals when the data were collapsed across diet conditions. However, within each diet condition for the flight animals, basal platelet [Ca2+]i had the highest correlation with MAP. Platelet [Ca2+]i levels, both basal and stimulated, have been found to have a tight, positive correlation with blood pressure in humans (9) and experimental models of hypertension (27). The significant inverse relationship between basal calcium levels and MAP in both the high- and low-calcium flight groups in the present study suggests that there was a unique and powerful factor influencing both MAP and basal platelet [Ca2+]i levels after flight.

Nitric oxide and flight-induced alterations of blood pressure. A recent report of elevated nitric oxide synthase activity in the vasculature of animals subjected to simulated weightlessness (32) suggests the possibility that nitric oxide may have been that factor. Nitric oxide lowers blood pressure through a dose-dependent fall in sympathetic nervous system activity (39). However, halothane also acts like a nitric oxide synthase inhibitor (17, 29) that elevates blood pressure. Thus, whereas the principal effect of halothane is a reduction of blood pressure through a dose-dependent fall in sympathetic nervous system outflow (39), the disruption of nitric oxide activity represents an opposing force that drives blood pressure upward. If the flight animals had higher levels of nitric oxide activity, halothane-induced inhibition of nitric oxide activity may have resulted in higher blood pressure in the flight groups. At the same time, nitric oxide may have acted to reduce basal and thrombin-stimulated [Ca2+]i levels in the flight animals. Nitric oxide lowers intracellular calcium in platelets and VSMCs (19) and, if nitric oxide levels were elevated in the vasculature of animals exposed to spaceflight, a reduction in platelet intracellular calcium might be expected.

Relationship between blood pressure and intracellular calcium. In contrast to the flight animals, the linear regression model that best predicted both MAP and SBP in the vivarium control animals was ionomycin-stimulated [Ca2+]i levels. This outcome suggests the possibility that diet-induced alterations in intracellular calcium stores may be the proximal cause of diet-induced alterations in blood pressure. Many investigators use platelet [Ca2+]i levels as a surrogate for VSMC [Ca2+]i levels, and it has been suggested that the higher levels of calcium stores may represent a greater calcium discharge capacity on stimulation in VSMC and, therefore, a greater potential to increase vascular resistance and elevate blood pressure (28).

Where MAP was closely associated with intracellular calcium levels, SBP in the flight animals was best predicted by BMD. A growing number of studies in humans (5, 22), as well as rats (21), have reported a significant inverse relationship between blood pressure and BMD. That relationship most likely reflects bone mineralization as a cumulative marker of whole body calcium metabolism. As such, it reflects the long-term relationship between calcium metabolism and blood pressure. The inverse relationship between SBP and femoral BMD indicates that spaceflight did not disrupt the normal relationship between bone mineralization and blood pressure.

Bone

The increase in BMD of the skull and decline in the femur after spaceflight are commensurate with results from other studies (18). These outcomes provide further evidence that alterations in bone mineral content during spaceflight are region specific (23). In contrast, the finding of increased whole skeleton BMD in the flight animals in the present study appears to be anomalous. However, because most studies of the effects of spaceflight on bone in rats have been done on selected regions of the skeleton (for a review, see Ref. 35) rather than the whole skeleton, there are no comparison data.

Housing effects vs. DEXA software. It is possible that the spaceflight environment may have promoted bone mineralization in this strain. The AEM represents a complex environment, both in terms of the numbers of cage mates and in cage surface area for climbing, which may facilitate activity. Wronski et al. (38) reported that, under conditions of group housing in AEMs, spaceflight has minimal effects on bone mass and bone formation in rapidly growing rats. Subsequently, Morey-Holton et al. (23) found that group housing reduced the response of bone to spaceflight by as much as 80%. Data from the present AEM study also indicate an influence of housing condition on bone mineralization. Animals on high-calcium diets developed greater BMD when housed in the AEM. These outcomes raise the possibility that the differential housing conditions between spaceflight and vivarium control animals in the present study may have resulted in greater bone mineralization in the flight animals.

On the other hand, there is reason to suspect that the measures of whole body BMD in the present study may have been flawed. The disparity in outcomes between ex vivo and in vivo estimates of BMD in the femur suggests that the DEXA software had difficulty resolving small differences between groups in vivo. The DEXA methodology has been shown to have good reliability in estimating BMD in rats (12), with coefficients of variability ranging from 0.52 to 2.2%. Nevertheless, the correlation between DEXA and ash BMC has been reported to be greater for in vitro samples (0.99) than in vivo samples (0.89) (12). In the present study, BMD in the femur was overestimated in both the low- and high-calcium flight groups and considerably underestimated in the high-calcium control group based on in vivo compared with ex vivo measurement. Inaccurate estimations of BMD in the flight animals may have contributed to the finding of increased whole body BMD in the flight animals relative to the vivarium control animals in the present study.

Lack of interaction between diet and flight. Dietary calcium did not modify the effects of spaceflight on bone mineralization. This finding is commensurate with that of Globus et al. (11) regarding rats exposed to hindlimb suspension. Of greater interest will be the impact of dietary calcium on long-term recovery from spaceflight. Lafage-Proust et al. (18) noted that the effects of spaceflight on bone were more pronounced 4 wk after recovery than they were immediately after recovery, suggesting that the harmful effects of spaceflight were delayed. At that time, any interactive effect between diet and flight may be more apparent.

Summary

In summary, the present study shows that spaceflight alters both systemic and intracellular calcium metabolism. In the short term, the effect of spaceflight on calcium metabolism appears to be impervious to variations in dietary calcium. The hypothesis that spaceflight would eliminate the diet-induced difference in blood pressure was not supported, even though ionized calcium was increased to a significantly greater degree in the low-calcium-diet flight animals. This outcome suggests that something other than ionized calcium is responsible for maintaining the diet-induced difference in blood pressure. The observation that PTH was unexpectedly elevated on landing in the high-calcium flight group, even though ionized calcium was significantly higher than in vivarium control animals, suggests that there may have been resetting of the set point for PTH release during spaceflight.


    ACKNOWLEDGEMENTS

This work was supported by National Aeronautics and Space Administration Grant no. NCC-2-934.


    FOOTNOTES

Address for reprint requests and other correspondence: D. C. Hatton, Dept. of Behavioral Neuroscience, Mail Code L470, Oregon Health Sciences Univ., 3181 SW Sam Jackson Park Road, Portland, OR 97201 (E-mail: hattond{at}ohsu.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 8 May 2000; accepted in final form 26 September 2001.


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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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D. C. Hatton, Q. Yue, J. Chapman, H. Xue, J. Dierickx, C. Roullet, S. Coste, J. B. Roullet, and D. A. McCarron
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