Blood glucose kinetics in whole body and mammary gland of lactating goats exposed to heat.

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.