Influence of simulated microgravity on cardiac output and blood flow distribution during exercise.

Rats exposed to simulated conditions of microgravity by head-down suspension (HDS) exhibit reductions in aerobic capacity. This may be due 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. Measurements were obtained at rest and during light-intensity (15 m/min) and heavy-intensity (25 m/min; 10% grade) treadmill exercise. Cardiac output was similar in HDS and cage control rats at rest and light exercise but was significantly lower in HDS rats (-33%) during heavy exercise. Soleus muscle blood flow (ml/min) was lower at rest and during exercise in HDS rats; however, when expressed relative to muscle mass (ml.min-1.100 g-1), soleus blood flow was lower only during light exercise. Plantaris muscle blood flow was lower in HDS rats during heavy exercise. Blood flow to the ankle flexor, knee extensor, and knee flexor muscles was not altered by HDS. Blood flow to the spleen and kidney was significantly higher in HDS rats. It was concluded that the reduction in aerobic capacity associated with HDS is due in part to an impaired ability to augment cardiac output during exercise.