Effects of acute and prolonged exposure to heat on regional blood flows in pregnant sheep.

This study deals with fetal growth retardation in heat-exposed sheep, and provides information on mechanisms of acclimation to heat. Radioactive microspheres were used to measure regional capillary blood flows in conscious sheep 80-100 days pregnant, at first in a thermoneutral environment, next after 2.5-6 h exposure to a hot environment of 40 degrees C, 25 mmHg water vapour pressure, and then 7 days and 15-20 days later, after spending 9 h daily in the heat. Maternal blood flow in the placental cotyledons was decreased by the first heat exposure, and remained depressed during the period of periodic heating. Reduction in placental blood flow on the maternal side could be part of the mechanism by which fetal development is retarded in heat-stressed sheep. Acclimation to heat was indicated by a decrease in the rate of rise in rectal temperature during the first 2 h of daily exposure to heat. Concomitantly, there was a progressive increase in the blood flow in extremity skin and nasal mucosa, which are tissues concerned with facilitation of heat loss during acute heat stress. However, in respiratory muscles which are also concerned with heat-loss, there was no further increase in flow after the initial response to to acute heat, indicating that heat acclimation is not due to an increased ability to pant. Blood flow in other tissues such as gut, pancreas and adipose tissue progressively decreased. These changes in blood flow have possible adaptive significance.