Response to 45 degrees head-down tilt as measured by organ weight body weight ratios and spiral computed tomography

Background: Exposure to microgravity or simulated microgravity causes signi ficant shifts in body fluids which may initiate physiological adaptations t o the microgravity stressor. It is imperative to understand the physiologic al adaptations to microgravity in order to develop appropriate countermeasu res to the deleterious aspects (i.e., muscle and bone wasting) of long-term spaceflights. Hypothesis: The significant shifts in body fluids by 45 degr ees head-down tilt can be measured by changes in organ weight/body weight ( OW/BW) ratios and non-invasively by spiral computed tomography. Methods: 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 trac er was injected intramuscularly immediately prior to the start of the exper imental time periods. At the end of the experiment, the major organs were h arvested, weighed, and measured for gamma radiation levels. We used the org an weights from this previous study to calculate OW/BW ratios for the prese nt study. Additionally, in the present study, rats in the 14-d experimental groups were weighed, lightly anesthetized to facilitate placement in the 4 5HDT 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 e xperimental time period, the rats were anesthetized and their lung densitie s measured with spiral computed tomography. Results: The OW/BW ratios for t he 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 kidn ey OW/BW ratio. Lung density from the 14-d 45HDT rats was 24.4% greater tha n control rats' values. Conclusions: The physiological change due to the 45 HDT position to simulate microgravity begins as early as 1 h, and the kidne y appears to be the first organ affected. Spiral computed tomography may of fer a viable method of non-invasively measuring organ densities in the 45HD T model. The OW/BW data generated in the present study does not correlate w ith the changes in radioactive tracer distribution data from our previous s tudy.