Dose-dependent effects of hypergravity on body mass in mature rats.

INTRODUCTION: Previous reports have shown that exposure to hypergravity decreases rat body mass during the initial phase, with this decrease and level of gravity showing a dose-response relationship. The present study examined whether rate of body mass gain after the initial phase of exposure is attenuated by hypergravity in a dose-dependent manner and sought to identify any threshold. METHODS: Male 10-wk-old rats (n = 64) were used, with 16 rats serving as 1.0-G controls, and 48 rats exposed to hypergravity for 14 d in 4 groups (1.5, 2.0, 2.5, and 3.5 G; n = 12 each). Body mass gain was evaluated according to slope of change in body mass from day 7 of exposure to hypergravity, as both absolute and relative values. RESULTS: Slopes of body mass gain did not differ between the 1.0- and 1.5-G groups (6.09 and 5.75 g x d(-1), respectively), but were significantly less for the 2.0-, 2.5-, and 3.5-G groups (4.91, 3.03 and 1.99 g x d(-1), respectively) than for the 1.0- and 1.5-G groups. Body mass gain as a relative value did not differ between the 1.0-, 1.5-, and 2.0-G groups (1.5 +/- 0.2, 1.6 +/- 0.6 and 1.4 +/- 0.3 g x d(-1) x 100 g(-1) body mass, respectively), but was significantly less for the 2.5- and 3.5-G groups (1.1 +/- 0.6 and 0.8 +/- 0.3 g x d(-1) x 100 g(-1) body mass, respectively) than for the 1.0-, 1.5-, and 2.0-G groups. Absolute values and rate of body mass gain were reduced with increases in gravity. CONCLUSION: Exposure to hypergravity attenuates body mass gain in a dose-dependent manner, with a threshold possibly existing between 1.5- and 2.5-G for 10-wk-old male rats.

Morphometric changes in vagal nerves of fourth generation mice passage-bred in a 2-G environment.

INTRODUCTION: Previous studies have shown that microgravity induces both functional and structural adaptations in the autonomic nerves. Functional adaptation to hypergravity has also been reported, but structural change has not yet been isolated. The purpose of this study was to evaluate structural adaptation to hypergravity in the parasympathetic nerve. METHOD: We selected fourth generation mice which were passage-bred in a 2-G environment by cycles of coupling, delivery, and growth. Complete left cervical vagal nerves of these mice were studied in transverse sections by electron microscopy. The number of small (diameter < 5 microm, thin and light-stained myelin sheath) and large (diameter > 5 microm, thick and dark-stained myelin sheath) myelinated fibers was counted. RESULTS: The total number of all myelinated fibers (2 G: 795 +/- 103, 1 G: 644 +/- 60) and the number of small myelinated fibers (2 G: 657 +/- 95, 1 G: 522 +/- 66) were significantly greater in the 2-G mice than those in the 1-G mice (p < 0.05). The number of large myelinated fibers in the 2-G mice was greater than that in the 1-G mice, although it was not statistically significant (2 G: 138 +/- 15, 1-G: 122 +/- 16; p = 0.091). DISCUSSION: The results show that the autonomic nerves can adapt structurally to hypergravity. We contend that the present results are due to the fact that the mice were passage-bred. As far as we know, this is the first report to show an increase in myelinated fibers in autonomic nerves under prolonged exposure to an increased G environment.

Changes in bone turnover markers during 14-day 6 degrees head-down bed rest.

Osteoporosis caused by exposure to microgravity represents a serious clinical concern, but the mechanisms have yet to be fully elucidated. The present research aimed to elucidate the effects of microgravity environments on bone turnover, with a specific focus on changes in bone resorption markers such as type I collagen cross-linked N-telopeptides (NTx) and deoxypyridinoline (Dpyr), for which scant data are available regarding detailed time course. Methods using 6 degrees head-down bed rest were utilized to simulate a microgravity environment. Eleven adult male volunteers underwent 6 degrees head-down bed rest for 14 days; measurements were made of serum and urine Ca concentrations, in addition to osteocalcin (OC), bone alkaline phosphatase (ALP), NTx, and Dpyr as bone turnover markers. By the end of bed rest, concentrations of bone ALP had significantly increased, but OC displayed a tendency toward decrease. Concentrations of Dpyr significantly increased from day 6, remaining elevated until the end of bed rest. Concentrations of NTx significantly increased on day 13 and at the end of bed rest. Serum and urinary concentrations of Ca increased significantly at the end of bed rest. Bone ALP represents a relatively early marker of osteoblast differentiation at the matrix maturation phase and OC is a late marker in osteoblast differentiation at the calcification phase. The present results therefore suggest an absolute increase in bone resorption and normal or reduced bone formation, together causing prominent uncoupling and rapid bone loss after simulated microgravity. Moreover, the present results suggest that bone resorption is enhanced at an early stage of exposure to microgravity environments.