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.