Rate and composition of sweat fluid losses are unaltered by hypohydration during prolonged exercise in horses

Abstract

Rate and ionic composition of sweat fluid losses and partitioning of evaporative heat loss into respiratory and cutaneous components were determined in six horses during three 15-km phases of exercise at approximately 40% of maximal O2 uptake. Pattern of change in sweat rate (SR) and composition was similar during each phase. SR increased rapidly for the first 20 min of exercise but remained at approximately 24-28 ml . m-2 . min-1 during the remainder of each phase. Similarly, the concentrations of Na and Cl in sweat increased until 30 min of exercise but were unchanged thereafter. Sweat osmolality and concentrations of Na and Cl were positively correlated with SR. Sweat K concentration decreased during exercise but was not correlated with SR. Fluid losses were 33.8 +/- 1.5 liters, resulting in decreases of approximately 21% in plasma volume and approximately 11% in total body water. The approximately 6% hypohydration was not associated with an alteration in SR, sweat composition, or heat storage. Respiratory and cutaneous evaporative heat loss represented approximately 23 and 70%, respectively, of the total heat dissipated, and the partitioning of heat loss was similar in each exercise phase. We conclude that SR and the relative proportions of respiratory and cutaneous evaporative heat loss are unchanged in horses during prolonged low-intensity exercise despite moderate hypohydration.

Equine sweating and anhidrosis Part 1 – equine sweating

“…The osmolality of sweat samples collected during exercise was 303 mOsm in cool dry and 339 mOsm in hot dry conditions 62 . Kingston et al 24 . found a sweat osmolality of 290–320 mOsm and that osmolality and concentrations of Na + and Cl − were positively correlated with sweat rate during exercise.…”

“…Nonetheless, during exercise, c . 90% of the metabolic heat load of Equidae is dissipated, of which 70% is lost via sweat evaporation and 23% via respiratory tract evaporation 24 . This proportion is not surprising as the horse demonstrates probably the highest sweat rate in the animal kingdom 25 .…”

“…Sweat rates are not effectively altered by dehydration induced either by water deprivation or by furosemide treatment, indicating that neither pre‐existing hypohydration nor progressive dehydration during exercise in a cool environment alters sweating rate 24,27 . However, Geor and McCutcheon 59 found a decline in sweating during the last 30 min of exercise in horses given no fluid compared with hyperhydrated horses, and suggested that at high thermal demands, preservation of euhydration is necessary for maintenance of a high sweating rate.…”

Porcentagem De Perdas De Peso Em Cavalos Submetidos Ao Esforço De Enduro Na Regão Centro Oeste Do Brasil.

“…Perdas de fluidos levam a um aumento da osmolalidade, causando estímulo primário a sede. Frequentemente, esta taxa de ingestão de água é associada com a ingestão de comida e pode compensar perdas hidroeletrolíticas e, alguns animais podem inclusive, terminar a prova mais pesado que na largada (KINGSTON et al 1997;SCHOTT II et al 1997). …”

“…Como em atletas da espécie humana, a avaliação da perda de peso corporal de cavalos durante o exercício prolongado tem sido considerada uma forma acurada de se estimar a perda de fluidos através do suor (Kingston et al, 1997). Assim sendo, o objetivo deste trabalho foi monitorar as perdas de peso sofridas por cavalos submetidos à provas de enduro de 20, 40, 80 e 160 km de distância.…”