Effects of parathyroid hormone on renal tubular reabsorption of calcium, sodium, and phosphate.

AGUS, ZALMAN S., LAURENCE B. GARDNER, LAURENCE H. BECK, AND MARTIN GOLDBERG. Efects of parathyroid hormone on renal tubular reabsorption of calcium, sodium, and phosphate. Am. J. Physiol. 224(5) : 1143-l 148. 1973.-In order to study the relationships between sodium, calcium, and phosphate reabsorption in the proximal tubule and the effects of parathyroid hormone (PTH) on these relationships, recollection micropuncture and clearance studies were performed in dogs. Proximal tubular fluid/ plasma ultrafiltrate ratios for calcium ((TP/UF)cJ and phosphate ((TF/UF) ) p were 1.1 and 0.7 during the nondiuretic state, and reabsorption of both ions was directly proportional to sodium and fluid reabsorption. PTH resulted in a parallel inhibition of proximal tubular sodium and calcium reabsorption but caused a disproportionate inhibition of phosphate reabsorption ((TF/UF), rose to 0.9). The bulk of rejected phosphate was excreted in the urine. Sodium clearance rose slightly, whereas calcium clearance fell despite the proximal inhibition. Dibutyryl cyclic adenosine monophosphate (cyclic AMP) produced identical effects in the proximal tubule and similar urinary changes. Thus, the reabsorption of calcium and phosphate is closely related to sodium transport in the proximal tubule. Parathyroid hormone, probably via cyclic AMP, inhibits the proximal reabsorption of all three ions, but in a nonparallel manner. Phosphaturia results because the proximally rejected phosphate is not reabsorbed distally to any great extent. On the other hand, PTH reduces calcium excretion by stimulating distal calcium reabsorption out of proportion to that of sodium.