Parathyroid hormone and the regulation of acid-base balance

Parathormone (PTH) excess limits renal bicarbonate reabsorption. This may aggravate the acidosis in patients with renal insufficiency and secondary hyperparathyroidism. Why parathormone, the primary action of which is thought to be stabilization of the inonized fraction of calcium, affects acid-base balance remains uncertain. Parathormone not only promotes the release of calcium from bone but also mobilizes salts, including bicarbonate and phosphate. Accumulation of these anions in the extracellular fluid would limit the ionization of calcium. Teleologically it is not unexpected to find that, coincident with evolution of a mechanism which permits rapid mobilization of calcium from bone, a system had to develop which removed the byproducts of bone dissolution. If this concept is valid, parathormone-induced depression of renal bicarbonate reabsorption in uremia represents an undesired side effect of an adaptive mechanism. This would extend Bricker's "trade-off" hypothesis which ascribes metabolic bone disease due to PTH-induced phosphate loss to include metabolic acidosis resulting from diminished renal bicarbonate regeneration. Parathyroidectomy or phosphate restriction have been proposed for correction of the side effects of secondary hyperparathyroidism. These therapeutic manipulations cannot be recommended for general use. A more rational apprach for prevention of secondary hyperparathyroidism is the combined use of phosphate restriction with a short-acting vitamin D derivative.