INFLUENCE OF SIMULATED MICROGRAVITY ON CARDIAC-OUTPUT AND BLOOD-FLOW DISTRIBUTION DURING EXERCISE

Rats exposed to simulated conditions of microgravity by head-down susp ension (HDS) exhibit reductions in aerobic capacity. This may be due t o an impaired ability to augment cardiac output and to redistribute bl ood flow during exercise. The purpose of this investigation was to mea sure cardiac output and blood flow distribution in rats that were expo sed to 14 days of HDS or cage control conditions. Measurements were ob tained at rest and during Light-intensity (15 m/min) and heavy-intensi ty (25 m/min; 10% grade) treadmill exercise. Cardiac output was simila r in HDS and cage control rats at rest and light exercise but was sign ificantly lower in HDS rats (-33%) during heavy exercise. Soleus muscl e blood flow (ml/min) was lower at rest and during exercise in HDS rat s; however, when expressed relative to muscle mass (ml . min(-1). 100 g(-1)), soleus blood flow was lower only during light exercise. Planta ris muscle blood flow was lower in HDS rats during heavy exercise. Blo od flow to the ankle lower, knee extensor, and knee flexor muscles was not altered by HDS. Blood flow to the spleen and kidney was significa ntly higher in HDS rats. It was concluded that the reduction in aerobi c capacity associated with HDS is due in part to an impaired ability t o augment cardiac output during exercise.