Seasonal Expression of Glucose Transporter 5 (GLUT-5) Protein in the Testes of Roundleaf Bats in Thailand

Bats have unique reproductive strategies that are closely related to testicular metabolic adaptations, such as prolonged sperm storage. This study examined the expression of glucose transporter 5 (GLUT-5), a fructose-specific member of the facilitative glucose transporter family, in the testes of roundleaf bats (<i>Hipposideros</i> spp.) collected from various locations in Thailand during their active reproductive season (July to September) and explored its association with biometric traits. To assess GLUT-5's localization and expression levels, testicular tissues from 50 adult males representing <i>Hipposideros larvatus</i>, <i>Hipposideros armiger</i>, and <i>Hipposideros lekaguli</i> species were analyzed using immunohistochemistry and Western blotting. Strong GLUT-5 immunoreactivity was observed in the cytoplasm of spermatocytes, spermatids, and spermatozoa, while weak staining was seen in spermatogonia. No GLUT-5 expression was detected in Leydig or Sertoli cells. Staining intensity varied significantly by month, with the highest levels observed in August (<i>p</i> < 0.05), exceeding those in July and September. Western blotting identified two GLUT-5 isoforms (55 and 100 kDa), with relative intensities that changed across the reproductive timeline. In parallel, morphometric analysis revealed that the height of the germinal epithelium and the diameter of the seminiferous tubules were significantly greater in July than in August and September, reflecting peak spermatogenic activity. These findings suggest that the seasonal regulation of fructose transport, along with changes in testicular architecture, may support testicular function and sperm maturation. The differential expression of GLUT-5 isoforms may reflect their distinct roles in body growth, reproductive maturation, and seasonal testicular activity in Hipposiderid bats.