Vitamin D and the kidney

The kidney has a unique function in mineral homeostasis and plays a vital role in the control of plasma calcium and phosphorus. The kidney plays a vital role in the conservation of calcium and phosphorus. Besides being the site of synthesis of 1,25(OH)₂D₃, the kidney responds to the hormone by increasing the efficiency of calcium and phosphorus reabsorption. 25-Hydroxyvitamin D₃-1α-hydroxylase is a multicomponent, cytochrome P-450-containing enzyme in the mitochondria of renal proximal tubular cells. Elements of the calcium transport systems including calbindin D_28K, calbindin D_9K, the epithelial calcium channel, and the plasma membrane calcium pump all localize to the distal portion of the nephron and are regulated directly or indirectly by 1,25(OH)₂D₃. The reabsorption of calcium in the kidney is controlled by several factors such as the filtered load of sodium, urine flow, and the activity of several hormones, most notably, parathyroid hormone (PTH), 1,25-dihydroxyvitamin D (1,25(OH)₂D) and calcitonin, in addition to others. The kidney is the major site of synthesis of 1,25 (OH)₂D, the active, hormonal form of vitamin D. Finally, the kidney has an equally important role in the control of plasma phosphate, and the filtration and reabsorption of phosphate.