ROLE FOR GASTROINTESTINAL ENDOTOXINS IN ENHANCEMENT OF HEAT TOLERANCEBY PHYSICAL-FITNESS

To further elucidate mechanisms underlying the higher heat tolerance o f physically fit compared with sedentary people, we have investigated the possibility that endotoxins (of gastrointestinal origin) act, as i n the normal development of fever, to raise body temperature and there fore reduce heat tolerance. In an initial series of experiments, five physically fit and four sedentary sheep were exposed twice at rest to an environment of 42/35 degrees C (dry/wet bulb temperature). When ani mals were given normal saline iv, rectal temperature (T-re) rose at a significantly higher rate in sedentary than in fit animals, this confi rms that heat tolerance is improved by physical fitness. Treatment wit h iv indomethacin did not affect the rate of rise of T-re in fit anima ls. In sedentary animals, however, T-re was lowered to approximate tha t of fit animals. Because indomethacin blocks prostaglandin pathways i nvolved in endotoxin-induced fever, the indomethacin induced improveme nt of heat tolerance of sedentary but not fit animals supports the con tention that endotoxins play a role in determining that difference in heat tolerance. In a second series of experiments, quantitative cardio vascular measurements were made by using radioactive microspheres. Und er normothermic conditions, blood flows in the brain, ileum, and diaph ragm were higher in fit than in sedentary animals. During hyperthermia up to T-re of 42 degrees C (in a 42/39 degrees C environment), fit co mpared with sedentary animals exhibited I) a greater increase in cardi ac output, 2) an increase in blood flow through arteriovenous anastomo ses to higher and better maintained levels, 3)less reduction in blood flow to the ileum, and 4) greater increase in blood flows to the myoca rdium, turbinates, nasal mucosa, and respiratory muscles. Endotoxins a re likely to come from the gut lumen, because reduction of gut blood f low forms part of the normal response to heat stress. We suggest that improvement of heat tolerance by physical fitness is caused by a great er cardiovascular capacity that permits not only greater perfusion of heat-loss tissues but the maintenance of a better gastrointestinal tra ct blood supply, thereby better maintaining the normal barrier to move ment of endotoxins from gut lumen to plasma. Sedentary people, with th eir lower cardiovascular capacity, redistribute more blood flow away f rom the gut during environmentally induced hyperthermia, thus allowing endotoxin-induced fever to aggravate hyperthermia.