Which of the following is TRUE regarding bone mineralisation?A. The actions of Vitamin D are exerted largely through the liver
Normal regulation of bone mineralisation occurs through maintenance of serum calcium and phosphorus levels and is achieved through the actions of vitamin D and parathyroid hormone (PTH). The actions of both hormones are exerted largely through the kidney. Vitamin D3, which has minimal biological activity, requires two hydroxylations – first occurs in the liver and the second within the tubular cell, through the action of 25-hydroxylase to form 25-hydroxycholecalciferol (calcidiol). The calcidiol is then transported to the kidney, where it is filtered and reabsorbed by renal tubular cells. These cells contain both 1α-hydroxylase and 24α-hydroxylase, and produce either the active 1,25-dihydroxycholecalciferol (calcitriol) or the inactive 24,25-dihydroxycholecalciferol. Parathormone increases the active calcium reabsorption at the level of the distal tubule. Secondly, it decreases phosphate reabsorption in the proximal convoluted tubule (and the distal tubule, to a lesser degree) and thirdly, it stimulates calcitriol production by increasing 1α-hydoxylase levels and decreasing 24α-hydroxylase levels.