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 exer cise. Each horse underwent an incremental exercise test in which it ra n 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 r ate 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 w as greater (P < 0.05) in the IN group (28.6 +/- 0.5 liters) compared w ith SP horses (22.7 +/- 2.2 liters). Exercise produced a significant i ncrease in Hct in both groups of horses; however, the increases were g reater (P < 0.05) at all exercise intensities in IN compared with SP h orses. Hct increased rapidly during the first step (4 m/s) in the exer cise test in the IN animals. After this, Hct increased slowly in IN ho rses, 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 calc ulate the relative change in PV. Exercise produced a significant decre ase 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 an d 7 m/s steps of the exercise test. These data suggest that the initia l increase in Hct in the horse is primarily due to contraction of the spleen and mobilization of red blood cells at the onset of exercise. H owever, percentage changes in plasma volume, calculated using Hct in t he splenectomized horses and the change in PV that was calculated usin g plasma total solids concentration calculated for both groups of hors es, suggest that part of this initial increase in Hct in the intact ho rses is also due to a 5-10% decrease in PV. Changes in Hct after accom modation of the red cell reserve appear to be due to decreases in PV.