Plasma von Willebrand factor in thoroughbreds in response to high-intensity treadmill exercise.

OBJECTIVE: To determine whether plasma von Willebrand factor (vWf) concentration changes in horses during and after treadmill exercise. ANIMALS: 5 mature, fit Thoroughbreds. PROCEDURE: A blood sampling catheter was placed in the right jugular vein. A warm-up period was followed by a 3-minute rest period. Horses were galloped at racing pace until fatigued (about 2 minutes). Blood samples were collected prior to warm-up, during the postwarm-up rest period, 1 minute into the run, at cessation of the run, and 5 to 120 minutes after cessation of the run. vWf activity was measured by ELISA and corrected for plasma volume changes (measured by changes in plasma albumin concentration). Platelet-poor plasma from 10 clinically normal, resting horses was pooled, assigned a value of 100 U/dl, and served as a control for all assays.
RESULTS: vWf activity began increasing 1 minute after horses reached full speed. At 5 minutes after cessation of exercise, vWf values had increased by mean of 92% (P < 0.05) from baseline. vWf activity returned baseline by 15 minutes after exercise, and remained there until 90 minutes after exercise, when it began to increase. CONCLUSION AND CLINICAL RELEVANCE: The spontaneous decrease in vWf values after completion of exercise was unexpected because vWf has a long half-life in circulation. This unexpected finding is compatible with increased vWf consumption and suggests that microvascular trauma may occur in horses during strenuous exercise.