RATE AND COMPOSITION OF SWEAT FLUID LOSSES ARE UNALTERED BY HYPOHYDRATION DURING PROLONGED EXERCISE IN HORSES

Rate and ionic composition of sweat fluid losses and partitioning of e vaporative heat loss into respiratory and cutaneous components were de termined in six horses during three 15-km phases of exercise at simila r to 40% of maximal O-2 uptake. Pattern of change in sweat rate (SR) a nd composition was similar during each phase. SR increased rapidly for the first 20 min of exercise but remained at similar to 24-28 ml.m(-2 ).min(-1) during the remainder of each phase. Similarly, the concentra tions of Na and Cl in sweat increased until 30 min of exercise but wer e 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 similar to 21% in plasma V olume and similar to 11% in total body water. The similar to 6% hypohy dration was not associated with an alteration in SR, sweat composition , or heat storage. Respiratory and cutaneous evaporative heat loss rep resented similar to 23 and 70%, respectively, of the total heat dissip ated, and the partitioning of heat loss was similar in each exercise p hase. We conclude that SR and the relative proportions of respiratory and cutaneous evaporative heat loss are unchanged in horses during pro longed low-intensity exercise despite moderate hypohydration.