Alexander G, Hales J R, Stevens D, Donnelly J B
J Dev Physiol. 1987 Feb;9(1):1-15.
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.
本研究探讨了热应激绵羊的胎儿生长迟缓问题，并提供了有关热适应机制的信息。利用放射性微球测量了怀孕80 - 100天的清醒绵羊的局部毛细血管血流量，首先是在热中性环境中，接着是在暴露于40摄氏度、水汽压25 mmHg的热环境2.5 - 6小时后，然后是在每天在热环境中度过9小时后的7天以及15 - 20天后。首次热暴露使胎盘小叶中的母体血流量减少，并且在周期性加热期间一直处于较低水平。母体一侧胎盘血流量的减少可能是热应激绵羊胎儿发育迟缓机制的一部分。每天暴露于热环境的前2小时内直肠温度上升速率的降低表明对热产生了适应。与此同时，四肢皮肤和鼻粘膜中的血流量逐渐增加，这些组织在急性热应激期间有助于散热。然而，在同样与散热有关的呼吸肌中，则在对急性热的初始反应后血流量没有进一步增加，这表明热适应并非由于喘气能力增强所致。肠道、胰腺和脂肪组织等其他组织中的血流量逐渐减少。这些血流量的变化具有可能的适应性意义。
