Resistance to the Phosphaturic Effect of Parathyroid Hormone during Phosphate Conservation is due to Enhanced Phosphate Reabsorption by the Pars Recta

Abstract

Studies of the renal adaptation to changes in the dietary intake of phosphate have provided important insights into the mechanisms of regulation of phosphate transport by the renal tubule. Tröhler, Bonjour and Fleisch (1) and Steele and DeLuca (2) showed that phosphate deprivation resulted in marked increases in phosphate reabsorption which were independent of plasma phosphate and parathyroid hormone (PTH). Studies utilizing several different approaches support the concept that adaptive changes occur in the proximal tubule. Studies utilizing microperfusion, micropuncture, brush border membrane vesicles and isolated tubules all illustrate enhanced phosphate transport by proximal tubules in animals fed a low phosphate diet (3–6). Further, rats fed a low phosphate diet provide a model of resistance to the phosphaturic responses to hormones (7). In the case of PTH, where the expected increases in cAMP excretion and hypocalciuria occur, there is little or no increase in phosphate excretion.