Witten M L, Balagtas M P, Keller R L, Hays A M, Braun E J, Vargas J, Padilla M T, Hall J N
Department of Pediatrics and Steele Memorial Children's Research Center, University of Arizona College of Medicine, Tucson, USA.
Aviat Space Environ Med. 1999 Jan;70(1):11-4.
Exposure to microgravity or simulated microgravity causes significant shifts in body fluids which may initiate physiological adaptations to the microgravity stressor. It is imperative to understand the physiological adaptations to microgravity in order to develop appropriate countermeasures to the deleterious aspects (i.e., muscle and bone wasting) of long-term spaceflights.
The significant shifts in body fluids by 45 degrees head-down tilt can be measured by changes in organ weight/body weight (OW/BW) ratios and non-invasively by spiral computed tomography.
In a previous study (14), rats were weighed and exposed to either 45 degrees head-down tilt (45HDT) or a prone control position for one of the following experimental times: 0.5 h, 1 h, 2 h, 4 h, 8 h, or 24 h. A radioactive tracer was injected intramuscularly immediately prior to the start of the experimental time periods. At the end of the experiment, the major organs were harvested, weighed, and measured for gamma radiation levels. We used the organ weights from this previous study to calculate OW/BW ratios for the present study. Additionally, in the present study, rats in the 14-d experimental groups were weighed, lightly anesthetized to facilitate placement in the 45HDT position, and placed in a specially designed 45HDT cage (45HDT group) or left unrestrained in the cages (control group). At the end of the 14-d experimental time period, the rats were anesthetized and their lung densities measured with spiral computed tomography.
The OW/BW ratios for the liver, kidneys, and spleen of 24 h 45HDT rats were significantly lower (p<0.05) than control values while at 1 h the 45HDT rats had a higher kidney OW/BW ratio. Lung density from the 14-d 45HDT rats was 24.4% greater than control rats' values.
The physiological change due to the 45HDT position to simulate microgravity begins as early as 1 h, and the kidney appears to be the first organ affected. Spiral computed tomography may offer a viable method of non-invasively measuring organ densities in the 45HDT model. The OW/BW data generated in the present study does not correlate with the changes in radioactive tracer distribution data from our previous study.
暴露于微重力或模拟微重力环境会导致体液显著变化，这可能引发对微重力应激源的生理适应。为了制定应对长期太空飞行有害方面（即肌肉和骨骼萎缩）的适当对策，了解对微重力的生理适应至关重要。
通过45度头低位倾斜引起的体液显著变化可以通过器官重量/体重（OW/BW）比值的变化来测量，也可以通过螺旋计算机断层扫描进行无创测量。
在先前的一项研究（14）中，对大鼠进行称重，并将其暴露于45度头低位倾斜（45HDT）或俯卧对照体位，持续以下实验时间之一：0.5小时、1小时、2小时、4小时、8小时或24小时。在实验时间段开始前立即肌肉注射放射性示踪剂。实验结束时，采集主要器官，称重并测量其γ辐射水平。我们使用先前研究中的器官重量来计算本研究中的OW/BW比值。此外，在本研究中，对14天实验分组中的大鼠进行称重，轻度麻醉以便于放置在45HDT体位，然后将其置于专门设计的45HDT笼中（45HDT组）或在笼中自由活动（对照组）。在14天实验时间段结束时，对大鼠进行麻醉，并用螺旋计算机断层扫描测量其肺部密度。
24小时45HDT大鼠肝脏、肾脏和脾脏的OW/BW比值显著低于对照值（p<0.05），而在1小时时，45HDT大鼠的肾脏OW/BW比值较高。14天45HDT大鼠的肺部密度比对照大鼠的值高24.4%。
由于45HDT体位模拟微重力引起的生理变化早在1小时就开始了，肾脏似乎是第一个受影响的器官。螺旋计算机断层扫描可能提供一种在45HDT模型中无创测量器官密度的可行方法。本研究中生成的OW/BW数据与我们先前研究中放射性示踪剂分布数据的变化不相关。
