Vitamin D metabolism in the Damara mole-rat is altered by exposure to sunlight yet mineral metabolism is unaffected

Vitamin D may be endogenously synthezised in the skin in the presence of sunlight or, alternatively, acquired from dietary sources. Cryptomys damarensis appear to have a naturally impoverished vitamin D status with low plasma concentrations of both 25-hydroxyvitamin D (25(OH)D; <5 ng/ml) and 1,25-dihydroxyvitamin D (1,25(OH)₂D; <20 pg/ml). We attribute this to their underground habitat and herbivorous habits. We questioned whether these subterranean mammals could utilize sunlight-mediated pathways and therefore compared vitamin D metabolism and function when animals were (a) housed naturally (control), (b) given an oral vitamin D₃ (D₃) supplement (1 IU/g dry matter food eaten per day) and (c) exposed to 10 h of sunlight. Control animals exhibited a highly efficient apparent fractional absorption of both calcium (Ca) and inorganic phosphorus (P(i)) (>90%), passive mode of intestinal mineral uptake, yet tightly regulated serum ionized calcium (Ca²⁺). The ratio of 25(OH)D-1α-hydroxylase (1-OHase) to 25 (OH)D-24R-hydroxylase (24-OHase) activity in the kidney, corresponded with a state of vitamin D deficiency. Cryptomys damarensis responded to both oral D₃ supplementation and sun exposure by an increase in plasma concentration of 1,25(OH)₂D with a commensurate decline (P<0.05) in 1-OHase activity, and a resulting decrease (P<0.05) in the ratio of 1-OHase:24-OHase activity. Despite these changes, the intestinal mode of Ca uptake and plasma total Ca, Ca²⁺ and P(i) remained unchanged with either treatment. Responses to sunlight were less pronounced than that of oral D₃ supplementation. These data confirm that naturally vitamin D-deficient mole-rats can convert vitamin D to the active hormone 1,25(OH)₂D, and indicate that mole-rats function optimally at the low concentrations of vitamin D metabolites found naturally. Furthermore, these animals exhibit a highly efficient vitamin D-independent mode of intestinal Ca absorption.