Immunolocalization of aquaporins 1, 3, and 5 in the nasal respiratory mucosa of a panting species, the sheep (Ovis aries)

• Sheep, a panting species, were exposed ambient temperatures (Ta) of 21 °C or 38 °C.
• Aquaporin (AQP) 1, 3 and 5 were localized in respiratory mucosa using histochemistry.
• AQP localization suggests a plausible route for rapid transepithelial water movement.
• AQP localization in respiratory mucosa is unaffected by increased Ta and panting rate.
• AQP distribution indicates adaptations in respiratory mucosa specific to panting species.

The nasal respiratory mucosa is the primary site for evaporative water loss in panting species, necessitating the movement of water across the nasal epithelium. Aquaporins (AQP) are protein channels that facilitate water movement in various fluid transporting tissues of non-panting species. Whether the requirement for enhanced capacity for transepithelial water movement in the nasal respiratory mucosa of panting species has led to differences in AQP localization is unknown. Using immunohistochemistry, we report the localization of AQP1, 3, and 5 in the nasal respiratory mucosa of sheep being exposed to ambient temperatures of ~21 °C or ~38 °C for 4.5 h before death (n=3/treatment). Exposure to either treatment resulted in panting. While exposure to ~38 °C resulted in a higher respiratory frequency (mean difference: 82 breaths min−1; P<0.001) than exposure to ~21 °C, there was no difference in the localization of AQPs. Connective tissue and vascular endothelial cells expressed AQP1. Glandular acini expressed AQP1 and apically localized AQP5, which was also present in glandular duct cells. Ciliated columnar epithelial cells expressed AQP5 apically and AQP3 basolaterally. Basal cells expressed AQP3. The distribution and co-localization of AQPs in the ovine nasal respiratory mucosa is different to that reported in non-panting species and may reflect the physiological demands associated with enhanced respiratory evaporation. We propose that AQP1, 3, and 5 may constitute a transepithelial water pathway via glandular secretions and across the surface epithelium, which provides a possible means for rapid and controllable water movement in the nasal respiratory mucosa of a panting species.