Seasonal dynamics of vitamin D metabolism in the oviduct of the Chinese Brown frog (Rana dybowskii)

The R. dybowskii oviductal tissue undergoes significant expansion during pre-brumation rather than the pre-spawning period, reflecting a unique physiological adaptation for brumation. While vitamin D is known to support growth and reproductive functions in various organisms, its role in seasonal oviductal hypertrophy remains unclear. In this study, the vitamin D metabolic pathway's role in regulating this process was investigated. Morphometric measurements revealed a significant increase in oviductal weight and luminal diameter during pre-brumation compared to pre-spawning, while histological examination showed marked glandular cell hypertrophy in the ampullary region. Correspondingly, 1,25-(OH)₂D₃ was significantly elevated in the oviduct during pre-brumation. Immunohistochemistry revealed that vitamin D metabolic enzymes Cyp2r1, Cyp27b1, Cyp24a1, the vitamin D receptor (Vdr), and the reductase Dhcr7 were localized in both epithelial cells and glandular cells during pre-spawning, but became predominantly restricted to epithelial cells during pre-brumation. qRT-PCR and western blot analyses confirmed higher expression of Cyp2r1, Cyp27b1, Cyp24a1, and Vdr, but reduced expression of Dhcr7 during pre-brumation. To further evaluate potential functional associations, 25-(OH)D₃ and 1,25-(OH)₂D₃ levels were found to be positively correlated with the mRNA expression of steroidogenic ( star , cyp11a1 ) and proliferative ( mki67 , and pcna ) markers across the two periods. Transcriptome analyses further supported these findings, identifying enrichment of differentially expressed genes within the vitamin and steroid hormone biosynthesis pathways. Collectively, these results suggest vitamin D metabolism is locally upregulated in the oviduct during pre-brumation, potentially via autocrine/paracrine mechanisms, and may contribute to seasonal reproductive adaptation in R. dybowskii .