Vitamins D : Function

The overall function of calcitriol is to maintain adequate serum levels of Ca²⁺. It performs this function by 1) increasing uptake of Ca²⁺ by the intestine, 2) minimizing loss of Ca²⁺ by the kidney by increasing reabsorption, and 3) stimulating resorption (demineralization) of bone when blood Ca²⁺ is low (see Fig. 1).

Figure 1:  Metabolism and actions of vitamin D. [Note: Calcitonin, a thyroid hormone, decreases blood calcium (Ca²⁺) by inhibiting mobilization from bone, absorption from the intestine, and reabsorption by the kidney. It opposes the actions of PTH.] mRNA = messenger RNA; 25-OH-D3 = 25-hydroxycholecalciferol; 1,25-diOH-D3 = 1,25-dihydroxycholecalciferol.
1. Effect on the intestine: Calcitriol stimulates intestinal absorption of Ca²⁺ by first entering the intestinal cell and binding to a cytosolic receptor. The 1,25-diOH-D3–receptor complex then moves to the nucleus where it selectively interacts with response elements on the DNA. As a result,

Ca2+ uptake is enhanced by increased expression of the calcium-binding protein calbindin. Thus, the mechanism of action of 1,25-diOH-D3 is typical of steroid hormones .
2. Effect on bone: Bone is composed of collagen and crystals of Ca5(PO₄)₃OH (hydroxylapatite). When blood Ca2+ is low, 1,25-diOH-D3 stimulates bone resorption by a process that is enhanced by PTH. The result is an increase in serum Ca2+. Therefore, bone is an important reservoir of Ca²⁺ that can be mobilized to maintain serum levels. [Note: PTH and calcitriol also work together to prevent renal loss of Ca²⁺.]