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Muscle damage, hydration, electrolyte balance and vasopressin concentrations in successful and exhausted endurance horses.

Arginine vasopressin (AVP) is released in response to depletion of the effective circulating blood volume. Endurance horses might develop exhaustion because loss of water and electrolytes in sweat with hypovolaemia and electrolytes abnormalities. Therefore, AVP should act in the control of volemia in these animals. This research evaluates the differences in AVP, hydration, electrolyte status and serum muscle enzymes in successful endurance horses in comparison with those eliminated from the competition because of exhaustion. Eighteen endurance horses, divided into two groups, successful (n = 13) and eliminated at the vet-gates because of exhaustion, dehydration and/or lack of recovery of heart rate (n = 5), were studied during a competition of 76.2 km. Jugular venous blood samples were collected before the event (BF), and at the vet-gates, at 30 km (PH1), 53.6 km (PH2) and at the end (PH3). Endurance exercise in successful horses induced significant increases from BF in Na at PH1, in Mg, CK, LDH at PH2 and microhaematocrit (MHT), total serum proteins (TSP), albumin (ALB), creatinine (CREAT) and lactate (LA) at PH3, together with a decreased in Cl at PH2 and Ca at PH3. Exhausted horses had higher MHT, Na, Ca, TSP, CREAT, LA, and AVP than successful at PH2 and PH3, whereas Cl was lower in exhausted horses. Velocity during the ride was higher in the exhausted group. These results indicate that exhausted endurance horses have laboratorial findings corresponding with a deeper dehydration and increased release of AVP. Therefore, this parameter could be used as a biomarker of early exhaustion and hypovolaemia. Additionally, AVP does not appear to be affected by exercise velocity or covered distance in successful horses.