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 (pHi) 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 pHi 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 pHi 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 microM) 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.