Hormonal Control of Calcium & Phosphate Metabolism

Vitamin D (Calcitriol)
vitamin D or calcitriol is a steroid hormone . Cholecalciferol D3 is generated in the skin of animals when light energy is absorbed by a precursor molecule 7-dehydrocholesterol. Vitamin D3, does not have significant biological activity. Rather, it must be metabolized within the body to the hormonally-active form known as 1,25-dihydroxycholecalciferol. This transformation occurs in two steps,
• Within the liver, cholecalciferal is hydroxylated to 25-hydroxycholecalciferol by the enzyme 25-hydroxylase.
• Within the kidney, 1-alpha-hydroxylase act on 25-hydroxycholecalciferol, yielding 1,25-dihydroxycholecalciferol, the biologically active form.
1-alpha-hydroxylase in the kidney is tightly regulated and serves as the major control point in production of the active hormone. The major inducer of 1-alpha-hydroxylase is parathyroid hormone.
Physiological Effects of Vitamin D
Vitamin D is well known as a hormone involved in mineral metabolism and bone growth.
• Its most dramatic effect is to facilitate intestinal absorption of calcium. Vitamin D stimulates the expression of a number of proteins involved in transporting calcium from the lumen of the intestine, across the epithelial cells and into blood (calcium transporters - calbindin).
• The crutial effect of vitamin D on bone is to provide the proper balance of calcium and phosphorus to support mineralization.
• It stimulates tubular reabsorption of calcium and phosphate
Disease States
Vitamin D deficiency: The classical manifestations of vitamin D deficiency is rickets, which is seen in children and results in bony deformaties including bowed long bones. Deficiency in adults leads to the disease osteomalacia. Both rickets and osteomalacia reflect impaired mineralization of newly synthesized bone matrix, and usually result from a combination of inadequate exposure to sunlight and decreased dietary intake of vitamin D.
Osteoporosis is characterized by demineraIization of bone resulting in the progressive loss of bone mass . This is reflected in demineralization After the age of 60, osteoporosis is seen . There is reduced bone strength and an increased risk of fractures