Influence of bicarbonate on parathyroid hormone-induced changes in fluid absorption by the proximal tubule.

Segments of the proximal tubule of the rabbit kidney were perfused in vitro in order to examine the influence of bicarbonate on the reduction in fluid absorption that occurs following the acute administration of parathyroid hormone (PTH). Studies were performed using either normal ultrafiltrate as perfusion fluid and normal rabbit serum as bath or low bicarbonate ultrafiltrate as perfusion fluid and low bicarbonate rabbit serum as bath. Low bicarbonate fluids were prepared by replacement of bicarbonate with chloride. In the presence of normal concentrations of bicarbonate, the addition of PTH to the bath (1 U/ml) resulted in a decrease in the fluid absorption rate (Jv) from 1.13 +/- 0.08 to 0.60 +/- 0.04 nl/mm X min (p is less than 0.001) in 23 convoluted segments and from 0.64 +/- 0.05 to 0.46 +/- 0.05 nl/mm - mm (P less than 0.01) in ten straight portions. Simultaneous with the PTH-induced reduction in Jv, the chloride concentration in the collected fluid changed from 119.0 +/- 2.0 to 113.4 +/- 1.1 mEq/liter (P less than 0.01) in the pars convoluta and from 117.7 +/- 0.6 to 114.0 +/- 1.9 mEq/liter (P less than 0.01) in the pars recta. However, there was no change in the net flux of chloride which averaged 42.58 +/- 5.00 pEq/mm - min during the control periods. Additional studies were performed in eight convoluted segments during perfusion on a randomized basis with low bicarbonate fluids as well as during perfusion with fluids having normal levels of bicarbonate. As before, in the presence of normal levels of bicarbonate, PTH reduced Jv from 1.16 +/- 0.15 to 0.68 +/- 0.07 nl/mm - min (P less than 0.001) and the chloride concentration in the collected fluid ([Cl]o) from 118.6 +/- 2.9 to 111.6 +/- 1.3 mEq/liter (P less than 0.005). Substitution of low bicarbonate fluids for normal bicarbonate fluids resulted in a decrease in Jv from 1.16 +/- 0.15 to 0.74 +/- 0.10 nl/mm - min (P less than 0.001). In the presence of low bicarbonate fluids, the addition of PTH resulted in no further decrease in Jv (0.74 +/- 0.10 vs. 0.72 +/- 0.10 nl/mm - min). These data indicate that in the proximal tubule the PTH-induced reduction in fluid absorption may be mediated by changes in bicarbonate absorption.