In 40 experiments on seven goats head and trunk temperatures were altered independently of each other and the effects on exercise performance on a treadmill (speed: 3 km/h, slope: 16%-20%) were observed. Brain temperature between 38.5 degrees C and 42.0 degrees C and trunk temperature between 39 degrees C and 43.5 degrees C did not reduce exercise performance or running time. Blood lactate concentration increased with rising brain and trunk temperatures, but did not exceed 13.1 mmol/l-1. Blood pressure and heart rate did not show any dependence on brain or trunk temperatures. Brain temperature between 42.0 degrees C and 42.9 degrees C shortened running time in 3 out of 12 experiments and reduced performance during shortlasting upward deviations of temperature. This suggests that in this species, the thermal safety limit to exercise is very close to that range of temperature which is likely to induce heat stroke.

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In three conscious goats, head and trunk temperatures were altered independently of each other by means of extracorporeal carotid heat exchangers and intravascular heat exchangers in the trunk veins. In 35 experiments heat production and heat loss were measured while head temperature was varied between 35.4 and 42.2 degrees C and trunk temperature between 34.5 and 42.4 degrees C. The largest temperature difference between head and trunk amounted to 6.6 degrees C. Head and trunk generated approximately equal fractions of the total core temperature input to the controller. The distribution of combinations of head and trunk temperatures resulting in constant levels of heat production and heat loss was consistent with the hypothesis that the total core temperature input to the controller equaled the sum of two identical inputs, both rising exponentially with temperature. The hypothesis implies that the input generated by core sensors of temperature in head and trunk is a continuum and conforms with the temperature-response curve of warm receptors.

A method has been developed to test the hypothesis that the deep tissues of the legs, e.g., skeletal muscle and/or periosteum, contain thermosensitive elements feeding signals into the temperature-regulating system. Stainless steel thermodes of 10 to 12-mm diameter and 100 to 150-mm length were chronically implanted into the marrow spaces of both humeri and femora, all of which have wide cavities and thin walls. Perfusing the thermodes with water of 0 degree C altered the temperature of the deep muscle layers by several degrees. The animals were further equipped with intravascular heat exchangers, which served to keep general body temperature constant during periods of leg cooling. Eighty experiments were performed in a hot and dry environment. During the middle period of each experiment the legs were cooled by perfusing the thermodes with water of 0 degree C. This caused respiratory evaporative heat loss to decrease by 0.15-0.20 W/kg. The small but significant response occurred at constant general body temperature and is therefore indicative of a local effect of the cooling on deep thermosensitive elements in the legs themselves and a neural afferent transmission of temperature signals into the temperature-regulating system.

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Thirty experiments were performed in two goats at an air temperature of +35 degrees C and a relative humidity of 33%. By means of heat exchangers, body core temperature (Tpaor) was adjusted to 39, 40.5, or 42 degrees C and maintained at these levels for 120 min. During the last 60 min the animals worked at a rate of 1.2 W/kg (treadmill, 3 km/h, 15%). Blood gases (arteriovenous O2 difference, Po2, Pco2), hemoglobin (Hb), blood lactate (LA), cardiac output (CO), blood pressure (MAP), heart rate (HR), metabolic rate (M), and respiratory evaporative heat loss (REHL) were determined. M, CO, HR, and Hb increased with exercise and were independent of Tpaor. At rest and exercise, REHL increased and Pco2 decreased at higher levels of Tpaor resulting in a respiratory alkalosis. During exercise this was accompanied by an increase in LA. At all instants, the concentrations of LA were higher at higher Tpaor. It is concluded that in a virtually nonsweating species like the goat the overall stress on the circulatory system caused by hyperthermia during exercise is relatively small while the behavior of blood LA is indicative of a temperature-dependent accumulation of LA also in the exercising muscle.

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