Micropuncture study of handling of phosphate by proximal and distal nephron in normal and parathyroidectomized rat. Evidence for distal reabsorption.

Abstract

The handling of phosphate by the nephron of the rat was investigated by proximal and distal tubular micropunctures in three experimental conditions: 1. in normal animals, 2. several days after parathyroidectomy (“chronic”) and 3. immediately after parathyroidectomy (“acute”). In proximal tubules of normal animals phosphate reabsorption appears to be gradient limited. The concentration gradient is already reached in the early proximal. Consequently, proximal phosphate reabsorption is almost completed in this segment. Moreover, reabsorption is higher in early proximal in normal than in acute although the reverse is observed at the late accessible proximal. This indicates that the initial part of the proximal nephron in normal has an increased capacity to reabsorb phosphate. Comparison of phosphate delivery at early distal and late proximal tubule suggests that, in normal and chronic, phosphate reabsorption either stops in the pars recta or is compensated by a net addition. Phosphate delivery is higher in superficial distal tubules than in ureteral urine, which is interpreted as a phosphate reabsorption by the terminal nephron since the heterogeneity of the nephrons population cannot explain the results. There are several arguments suggesting that this reabsorption is an active process. Moreover it appears to be inhibited by PTH.