Metabolism and action of the hormone vitamin D. Its relation to diseases of calcium homeostasis

Extensive experimental evidence has established a significant role of calciferol in the maintenance of normal calcium homeostasis. Present knowledge indicates that vitamin D(3) must first be converted to 25-OH-D(3) and then to 1,25(OH)(2)D(3), the most active known form of the steroid. Many of the factors regulating the rate of production of this last steroid from its precurser have been evaluated, and the concept that vitamin D functions as a steroid hormone seems to be well established. Deranged action of calciferol, caused by impaired metabolism of the steroid or through altered sensitivity of target tissues, may be involved in the pathophysiology of several disease states with abnormal calcium metabolism. It is noted that liver disease, osteomalacia due to anticonvulsant therapy, chronic renal failure, hypophosphatemic rickets, hypoparathyroidism, hyperparathyroidism, sarcoidosis and idiopathic hypercalciuria have possible relation to alterations in metabolism or action of vitamin D. The future clinical availability of 1,25(OH)(2)D(3) and other analogs of this steroid may offer potential therapeutic benefit in the treatment of certain of the disease entities discussed.

Familial hypophosphatemic rickets: defective transport of inorganic phosphate by intestinal mucosa

Uptake of inorganic phosphate is impared in intestinal mucosa from hemizygous males and heterozygous females with X-linked familial hypophosphatemic rickets. Considerable intrafamilial and interfamilial variation in uptake of inorganic phosphate is observed in affected patients. Uptake by normal mucosa is concentrative and energy-dependent, and is mediated by at least two systems with widely different affinities. These results lend direct support to the thesis that the primary metabolic disturbance in this disease results from impaired transport of inorganic phosphate in kidney and gut.

Loss of a parathyroid hormone-sensitive component of phosphate transport in X-linked hypophosphatemia

Mutant hemizygotes with X-linked hypophosphatemia lack a parathyroid hormone-sensitive component of inorganic phosphate transport in kidney; female heterozygotes retain a variable proportion of this type of transport. The residual mechanism for reabsorption in affected males allows inorganic phosphate efflux from the kidney to urine so that net "secretion" is sometimes observed; the latter is directly proportional to the serum concentration of inorganic phosphate. Calcium acts on the kidney tubule to enhance net reabsorption by this component of inorganic phosphate transport.