The effects of cold exposure on tissue sensitivity and responsiveness to insulin were determined by the euglycemic insulin clamp technique. Insulin was infused at rates of 0.2, 0.5, 1.0, 6.0, and 30.0 mU x kg-1 x min-1 into five adult sheep in a warm environment and after cold exposure (0 degree C) from 7 to 23 days. Cold exposure increased basal plasma glucose concentration and basal glucose irreversible loss. Glucose metabolic clearance rate (MCR) was significantly increased by cold exposure at all rates of insulin infusion, with increases ranging from 44 to 72%. The insulin concentration causing half-maximal stimulation of glucose MCR was unchanged by environment (warm, 42 microU/ml; cold, 36 microU/ml). Combined alpha + beta-adrenergic blockade did not affect the increased response to insulin during cold exposure. Endogenous (hepatic) glucose production was inhibited by insulin to a similar extent in the two environments and was less sensitive to insulin than was glucose utilization or MCR. The results suggest that cold exposure increases the responsiveness to insulin of a postreceptor event in peripheral tissues.

Whole-body and mammary kinetics of blood glucose were measured in lactating goats exposed to thermoneutral, moderate hot, and severe hot environments for 4 d. Milk yields were reduced by 3 and 13% during moderate and severe heat exposure, but heat production was unchanged during the experiment. Concentrations of plasma glucose and free fatty acids did not change during heat exposure. Concentration of thyroxine tended to decrease and concentration of prolactin increased with increasing temperature. Whole-body turnover of blood glucose decreased significantly during both moderate and severe heat exposure. Blood glucose oxidation rate and contribution of blood glucose to total carbon dioxide production were not influenced by heat exposure. Mammary glucose uptake tended to decrease during heat exposure, and this reduction may account for the decreased whole-body turnover of blood glucose. The lactose concentration in milk was decreased on the 4th d of severe heat exposure. A relatively low production of milk lactose was apparently derived from blood glucose. These results suggest that the whole-body turnover of blood glucose decreases in step with a decrease in mammary glucose uptake during heat exposure in lactating goats.

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.

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The heat production of sheep exposed to OoC increased2.14 times compared with that at2O'C. At 0'C, the percentage of heat derived from oxidation of acetic acid decreased but that of free fatty acids increased remarkably. The substances which comprise 5OVa of total heat production remained unknown. Bergman (1963).The total heat production, 3.69 kcal/kgox'h at 20"C increased to 7.90 kcal/kgor'h at OoC.This meant the extent of oxidation of energy substances increased in total by a factor of2.74. The turnover rates of the substances examined at 20 and 0"C are shown in Table 1.The experiments were carried out under 20oC The turnover rate of acetic acid, which is the and OoC on the 4th day after exposure. The mainenergysourcefortheruminant,showedno amount of feed given was the same in both tem-difference between the two regimes but that of perature conditions. glucose increased significantly at 0"C. SinceThe isotope dilution method was used to es-propionic acid, which is the most important glutimate the metabolic parameters of energy-cogenic substance for the ruminant, was infused yielding substances. The substances used were into the rumen, the data in Table I are not the U-'oC-labelled glucose, acetic, propionic, glu-turnover rate but the production rate. Due to the tamic acids, alanine, glycerol and l-raC palmitic decrease of the production rate of propionic acid can. J. Anim. Sci.64 (Suppt.): [265][266] (Sept. 1984)