Influence of heat stress on exercise-induced changes in regional blood flow in sheep.

Radioactive microspheres were used to measure cardiac output and blood flow to most major tissues in sheep at rest and during treadmill exercise (3- to 6-fold increase in metabolic rate for 30 min) in thermoneutral (TN) [dry bulb temperature (Tdb) = 16 degrees C, wet bulb temperature (Twb) = 12 degrees C] and mildly hot (MH) (Tdb = 40 degrees C, Twb = 23 degrees C) environments. During exercise, rectal temperature increased more under MH than under TN conditions; exercise-induced changes in the major central cardiovascular parameters were unaffected by MH. Exercise in TN caused mild hypocapnia, and in MH, severe respiratory alkalosis. Skin blood flow in the torso decreased during exercise in TN and MH. Extremity skin blood flow was increased by heat but not exercise. Exercise-induced increases in flows to respiratory muscles and upper respiratory tract tissues were greatly enhanced in MH. Exercise caused large increases in blood flow to fore- and hindlimb muscles, which were less in MH than in TN. Effects of MH on exercise-induced changes in flow to these and other tissues (e.g., abdominal viscera and adipose tissue) are discussed in terms of the conflicting requirements of energy expenditure and body temperature regulation during exercise in sheep and other species, particularly humans.