Changes in blood volume and potassium concentration during exercise in rats

We measured the changes in circulating blood volume (BV), plasma sodium ([Na+]), and plasma potassium ([K+]) concentrations continuously, during and after exercise in euhydrated and dehydrated rats. Rats ran on a treadmill for 15 min at 600, 900, and 1,200 m/h, and the recovery period lasted 45 min. The BV decreased immediately after onset of exercise, and reached a plateau value in about 5 min of exercise. With 1,200 m/h exercise, a further decrease in blood volume was observed in both groups. The decline in BV was reversely related to the exercise intensity in both groups although the decrease was less in the dehydration group. During and after exercise, there were no significant changes in plasma [Na+] at any speed in either group. Plasma [K+] increased sharply at the onset of exercise and the magnitude of the increase was directly related to the exercise intensity. However, at all three exercise intensities the change in plasma [K+] in the dehydrated group was only about 60% of that measured in the euhydrated control group. These findings suggest that dehydration reduces the wash-out of K+ from the interstitial space of the exercising muscle.