EFFECT OF PARATHYROID-HORMONE ON ACID-BASE TRANSPORT IN RABBIT RENAL PROXIMAL TUBULE S3 SEGMENT

The effect of parathyroid hormone (PTH) on acid/base transport in isolated rabbit renal proximal tubule S3 segment was investigated with double-barreled and conventional microelectrodes. PTH (10 nM) induced a small depolarization and enhanced the initial rates of cell pH (pH(i)) increase and cell Cl- ([Cl-]i) decrease in response to bath Cl- removal by 28.0 +/- 2.1 % and 31.0 +/- 6.4 % respectively. The calculated initial HCO3- influx to bath Cl- removal was also enhanced by 28 %. On the other hand, PTH reduced the initial rate of pH(i) decrease to luminal Na+ removal in the absence of HCO3-/CO2 by 20.4 +/- 3.9 %. The PTH-induced depolarization was not accompanied with changes in steady-state pH(i) or [Cl-]i levels, but was greatly attenuated in the presence of ouabain (0.1 mM). Either dibutyryl-cAMP (0.1 mM) plus theophylline (1 mM) or forskolin (10 muM) alone could reproduce all the effects of PTH. These results indicate that (a) PTH inhibits the luminal Na+/H+ exchanger but stimulates the basolateral Cl-/HCO3- exchanger in the S3 segment; (b) the PTH-induced depolarization largely results from inhibition of Na+/K+-ATPase and (c) all these effects are at least partly mediated by a cAMP-dependent mechanism.