Cholecalciferol has no effect on calcium and inorganic phosphorus balance in a naturally cholecalciferol-deplete subterranean mammal, the naked mole rat (Heterocephalus glaber)

Naked mole rats, Heterocephalus glaber, have no obvious source of cholecalciferol (D₃) available to them, given their underground habitat and tubiferous diet. They have undetectable levels of 25-OH-D₃ and as such appear to be naturally deplete in D₃. The effect of an oral D₃ supplement on mineral balance and homeostasis was therefore investigated. This D₃ treatment did not affect circulating levels of Ca²⁺ and inorganic phosphorus (P(i)). Nor did D₃ treatment affect mineral intake and absorption. The Ca²⁺ and P(i) present in the food was efficiently extracted and absorbed, resulting in an apparent fractional absorption (AFA) efficiency exceeding 98%. Irrespective of D₃ treatment, the amount of Ca²⁺ and P(i) absorbed was positively correlated with the amount ingested, suggesting that intestinal uptake is by a passive D₃-independent process. After D₃ supplementation urinary Ca²⁺ secretion was unchanged; however, the amount of P(i) excreted in the urine increased (P ≤ 0.05). This resulted in a concomitant decline in P(i) AFR (P ≤ 0.02 from 99.95 ± 0.02% to 99.82 ± 0.03%). Almost all the Ca²⁺ and P(i) in the glomerular filtrate were reabsorbed, facilitating AFR efficiencies that approach physiological maxima (> 99%). Changes in AFR efficiency with D₃ supplementation are therefore of no biological significance. Net mineral flux of both elements, irrespective of D₃ treatment, was positive. It is speculated that the ever-growing incisors of these animals act as mineral dumps and assist in the tight regulation of plasma Ca²⁺ and P(i). These data suggest that naked mole rats utilize mechanisms independent of D₃ in regulating mineral homeostasis and are therefore well-adapted to an environment devoid of sunlight.