T. Pitcher et al., VITAMIN-D METABOLISM IN THE DAMARA MOLE-RAT IS ALTERED BY EXPOSURE TOSUNLIGHT YET MINERAL METABOLISM IS UNAFFECTED, Journal of Endocrinology, 143(2), 1994, pp. 367-374
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.