Understanding how β adrenergic agonists influence the physiology of heat stress could lead to mitigation options. We sought to investigate body surface temperatures in feedlot wethers supplemented with ractopamine or zilpaterol and exposed to heat stress for 18 d. Corneal and skin temperatures were assessed via infrared thermography at 1- and 2-m distances. Rectal temperatures and circulating leukocytes, metabolites, and electrolytes were also measured. Heat stress increased (P < 0.05) rectal temperatures in unsupplemented and zilpaterol-supplemented lambs but not in ractopamine-supplemented lambs. Heat stress also increased (P < 0.05) surface temperatures of the cornea, nose, ear, and back, regardless of supplement. Observations were comparable between thermography performed at 1 and 2 m, and higher emissivity settings generally produced less variation. Heat stress tended to increase (P = 0.08) blood monocytes in unsupplemented but not ractopamine- or zilpaterol-supplemented lambs. Granulocytes were increased (P < 0.05) by heat stress in ractopamine-supplemented lambs but decreased (P < 0.05) in zilpaterol-supplemented lambs. Blood glucose, triglycerides, and cholesterol did not differ among groups, and blood lactate was reduced (P < 0.05) by heat stress in zilpaterol-supplemented lambs only. Blood Na+ was reduced (P < 0.05) and Ca2+ increased (P < 0.05) by heat stress, regardless of supplement. These findings indicate that β1- and β2-adrenergic agonists differentially relieve some but not all heat stress-induced changes in stress indicators. Moreover, corneal and skin surface temperatures measured by infrared thermography reasonably identified body temperature changes at a distance of 2 m.

中文翻译：

---

在补充莱克多巴胺或齐帕特罗的湿羊中，持续的热应激会升高角膜和体表温度并改变循环白细胞和代谢指标

了解 β 肾上腺素能激动剂如何影响热应激的生理学可能会导致缓解选择。我们试图研究补充莱克多巴胺或齐帕特罗并暴露于热应激 18 天的饲养场的体表温度。通过 1 米和 2 米距离的红外热成像评估角膜和皮肤温度。还测量了直肠温度和循环白细胞、代谢物和电解质。热应激在未补充和补充齐帕特罗的羔羊中增加（P < 0.05）直肠温度，但在补充莱克多巴胺的羔羊中没有增加。热应激也增加（P < 0.05）角膜、鼻子、耳朵和背部的表面温度，无论是否补充。在 1 米和 2 米处进行的热成像观察结果具有可比性，并且较高的发射率设置通常产生较小的变化。热应激倾向于增加（P = 0.08）未补充但不补充莱克多巴胺或齐帕特罗的羔羊的血液单核细胞。在补充莱克多巴胺的羔羊中，粒细胞因热应激而增加（P < 0.05），而在补充齐帕特罗的羔羊中粒细胞减少（P < 0.05）。各组之间的血糖、甘油三酯和胆固醇没有差异，并且仅在补充齐帕特罗的羔羊中，热应激降低了血乳酸（P < 0.05）。无论补充如何，热应激导致血液 Na+ 减少（P < 0.05）和 Ca2+ 增加（P < 0.05）。这些发现表明，β1- 和 β2- 肾上腺素能激动剂不同地缓解一些但不是所有热应激引起的应激指标变化。而且，