VITAMIN-D METABOLISM IN THE DAMARA MOLE-RAT IS ALTERED BY EXPOSURE TOSUNLIGHT 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. Cryptomy s 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 att ribute this to their underground habitat and herbivorous habits. We qu estioned whether these subterranean mammals could utilize sunlight-med iated pathways and therefore compared vitamin D metabolism and functio n when animals were (a) housed naturally (control), (b) given an oral vitamin D-3 (D-3) supplement (1 IU/g dry matter food eaten per day) an d (c) exposed to 10 h of sunlight. Control animals exhibited a highly efficient apparent fractional absorption of both calcium (Ca) and inor ganic phosphorus (P-i) (> 90%), passive mode of intestinal mineral upt ake, yet tightly regulated serum ionized calcium (Ca2+). The ratio of 25(OH)D-1 alpha-hydroxylase (1-OHase) to 25(OH)D-24R-hydroxylase (24-O Hase) activity in the kidney, corresponded with a state of vitamin D d eficiency. Cryptomys damarensis responded to both oral D-3 supplementa tion and sun exposure by an increase in plasma concentration of 1,25(O H)(2)D 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 acti vity. Despite these changes, the intestinal mode of Ca uptake and plas ma total Ca, Ca2+ and P-i remained unchanged with either treatment. Re sponses to sunlight were less pronounced than that of oral D-3 supplem entation. These data confirm that naturally vitamin D-deficient mole-r ats can convert vitamin D to the active hormone 1,25(OH)(2)D, and indi cate that mole-rats function optimally at the low concentrations of vi tamin D metabolites found naturally. Furthermore, these animals exhibi t a highly efficient vitamin D-independent mode of intestinal Ca absor ption.