Vitamin D3 (D3) is produced endogenously from 7-dehydrocholesterol by irradiation and is an important secosteroid for the absorption of calcium and phosphate. Lithocholic acid (LCA) increases intestinal paracellular calcium absorption in a vitamin D receptor-dependent manner in vitamin D-deficient rats. The plateau zokor (Myospalax baileyi), a strictly subterranean species, and plateau pika are endemic to the Qinghai-Tibet Plateau. To verify whether the zokors were deficient in D3 and reveal the effects of hypoxia on D3 metabolism in the zokors and pikas, we measured the levels of 25(OH)D3, calcium, and LCA, and quantified the expression levels of D3 metabolism-related genes. The results showed an undetectable serum level of 25(OH)D3 and a significantly higher concentration of LCA in the serum of plateau zokor, but its calcium concentration was within the normal range compared with that of plateau pika and Sprague-Dawley rats. With increasing altitude, the serum 25(OH)D3 levels in plateau pika decreased significantly, and the mRNA and protein levels of CYP2R1 (in the liver) and CYP27B1 (in the kidney) in plateau pika decreased significantly. Our results indicate that plateau zokors were deficient in D3 and abundant in LCA, which might be a substitution of D3 in the zokor. Furthermore, hypoxia suppresses the metabolism of D3 by down-regulating the expression of CYP2R1 and CYP27B1 in plateau pika.
Keywords: Qinghai-Tibet Plateau; hypoxia; lithocholic acid; plateau pika (Ochotona curzoniae); plateau zokor (Myospalax baileyi); vitamin D3.