Influence of simulated microgravity on sympathetic nervous system activity and the blood flow response to acute exercise.

Abstract

Rats exposed to head down suspension (HDS) exhibit reductions in aerobic capacity characterized by lower measures of maximal oxygen uptake (VO₂ max). This deconditioning process may be due in part to an impaired ability to augment cardiac output and to redistribute blood flow during exercise. The purpose of this investigation was to measure cardiac output and blood flow distribution in rats that were exposed to 14 days of HDS or cage control conditions. In addition, the role of the sympathetic nervous system in these responses was determined. The results from the blood flow experiments indicated that cardiac output was similar in HDS and control rats at rest and during light intensity exercise. In contrast, cardiac output was significantly lower in HDS rats (-33%) during heavy intensity exercise. Soleus and plantaris muscle blood flow was lower during exercise in HDS rats whereas blood flow to the ankle flexor, knee extensor, and knee flexor muscles was not altered. Blood flow to the spleen and kidneys was significantly higher during exercise in HDS rats. Estimated sympathetic nerve traffic, as determined by the rate of norepinephrine depletion, was significantly lower in the heart (-82%) and spleen (-42%), and higher in the soleus muscle (47%). Sympathectomized rats had significantly lower measures of VO₂ max than sham controls (20%), and did not exhibit a reduction in aerobic capacity when exposed to HDS. In contrast, adrenal demedullated rats had lower VO₂ max values than sham controls (20%) and did exhibit a reduction in aerobic capacity when suspended. From these results, it was concluded that the reduction in aerobic capacity in HDS rats was due primarily to an impaired ability to augment cardiac output and to redistribute blood flow during exercise. In addition, it was concluded that the impaired ability to make these cardiovascular adjustments was mediated by an altered sympathetic response to exercise.