Turnover and oxidation rates of blood glucose and heat production in sheep exposed to heat.

Five shorn sheep were used to study effects of heat exposure (30 degrees C, relative humidity 70%) on metabolism of blood glucose and heat production. The isotope dilution method was applied for determination of metabolism of blood glucose in a thermoneutral environment (20 degrees C, relative humidity 70%) and on the 4th and 10th days of heat exposure. Concentrations of free fatty acids and thyroxine of plasma decreased during heat exposure with no reduction in feed intake. At 20 degrees C, glucose turnover rate, percentage of glucose oxidized, and contribution of exhaled carbon dioxide derived from glucose to the total carbon dioxide production were 6.2 mg/kg.75 per min, 34%, and 11%. Glucose turnover rate decreased significantly during heat exposure, but the proportion of glucose oxidized was unchanged. Heat production decreased almost parallel to the glucose turnover rate during heat exposure, resulting in an unchanged contribution of carbon dioxide derived from glucose to the total production of carbon dioxide. Blood glucose metabolism and heat production of sheep decrease, but the contribution of blood glucose to heat production remains unchanged during exposure to 30 degrees C.