Phosphate Reabsorption in the Distal Convoluted Tubule

Abstract

Phosphate excretion by the mammalian kidney is subject to regulation by a number of factors including, but not limited to, dietary phosphorus intake, parathyroid hormone (PTH), and extracellular volume expansion. The primary quantitative importance of the proximal tubule in this regulation is well known, but the role of more distal nephron segments has been less clearly defined. It has long been recognized that the fraction of filtered phosphate delivered to distal convolutions accessible to micropuncture on the surface of the rat kidney may exceed fractional excretion, especially in phosphate retaining states, suggesting that further phosphate reabsorption occurs in the distal tubule and/or collecting duct (1). Early microinjection studies, however, failed to reveal significant distal phosphate reabsorption (2,3). Since the composition of the final urine reflects the pooled contributions of both superficial and deep nephrons, nephron heterogeneity, with more avid reabsorption in proximal tubules of juxtamedullary nephrons, was suggested as an alternative to distal reabsorption to explain the low urinary phosphate excretion (4). These two postulated mechanisms, distal reabsorption and nephron heterogeneity, need not of course be mutually exclusive.