Role of decreased plasma volume in hematocrit alterations during incremental treadmill exercise in horses.

Six intact (IN) and six splenectomized (SP) mares were subjected to an incremental exercise test to examine the role of decreases in plasma volume (PV) in the changes in hematocrit (Hct) normally seen with exercise. Each horse underwent an incremental exercise test in which it ran on a treadmill up a fixed incline of 6 degrees. The test started at a speed of 4 m/s, and speed was increased 1 m/s each min until heart rate reached a plateau. Blood samples were obtained at rest and at the end of the 4, 5, 6, and 7 m/s steps of the exercise test. Resting PV was greater (P < 0.05) in the IN group (28.6 +/- 0.5 liters) compared with SP horses (22.7 +/- 2.2 liters). Exercise produced a significant increase in Hct in both groups of horses; however, the increases were greater (P < 0.05) at all exercise intensities in IN compared with SP horses. Hct increased rapidly during the first step (4 m/s) in the exercise test in the IN animals. After this, Hct increased slowly in IN horses, and values for the 5, 6, and 7 m/s steps paralleled the changes observed in the SP horses. Corrected Hct values were then used to calculate the relative change in PV. Exercise produced a significant decrease in PV in both groups of horses; however, there were no differences (P > 0.05) between the groups when IN horses (-13 +/- 3 and -18 +/- 2%) were compared with SP horses (-14 +/- 4 and -17 +/- 4%) at the 6 and 7 m/s steps of the exercise test.(ABSTRACT TRUNCATED AT 250 WORDS)