Natural high ambient temperature-induced respiratory hypocapnia without activation of the hypothalamic-pituitary-adrenal axis in lactating goats

Background and Aim: Activation of breathing, the hypothalamic-pituitary-adrenal (HPA) axis, and plasma antioxidant defense are adaptive mechanisms in lactating dairy goats fed during the summer season. However, an excess of these responses can interfere with the gas exchange. This study aimed to investigate the effect of natural high ambient temperature (HTa) on blood gas parameters and their relation to the HPA axis and antioxidant defense. Materials and Methods: Six mid-lactating goats were included in this study and were fed in individual pens for 2 weeks. The data on ambient conditions, physiological responses, and blood chemistry were measured for two sampling days (D7 and D14), 1 week apart during the late summer season. On this two-sampling day, the main physiological responses to HTa, including respiration rate (RR), rectal temperature (Tr), blood gas, and blood chemistry, were measured in the morning and afternoon. Results: Goats from both D7 and D14 increased RR and Tr significantly according to morning and afternoon periods. In addition, goats were at the hypocapnia stage during afternoon panting without a change in blood pH and bicarbonate levels. Interestingly, HTa-induced hypocapnia was not accompanied by an increase in plasma cortisol levels. Finally, ΔTa was negatively correlated with changes in glutathione peroxidase activity. Conclusion: The natural HTa (ΔTa; 5–8°C) in this study activated evaporative heat dissipation and was high enough to induce respiratory hypocapnia. Importantly, this ΔTa did not activate the HPA axis but was correlated with a change in antioxidant defense. Therefore, under natural HTa in tropical conditions, respiratory hypocapnia is the first line of physiological response in goats within a specific range of natural ΔTa (5–8°C).