Intracellular chloride activity of rabbit proximal straight tubule perfused in vitro

To examine the cellular mechanism of Cl- transport in the proximal tubule, cell Cl- activity (aiCl) was measured with double-barreled Cl(-)-selective microelectrodes in the rabbit proximal straight tubules (PST) perfused in vitro. When tubules were perfused and bathed with ultrafiltrate-like solution, aiCl (corrected for the interference from undetermined intracellular anions, 4.2 mM) was 17.8 +/- 0.5 mM (n = 90), and this value was 1.3 times higher than that predicted from passive distribution (basolateral membrane potential, Vbl = -51.9 +/- 0.8 mV). Reducing either luminal or bath Cl- indicated that the basolateral membrane plays a more important role as a determinant of aiCl. aiCl reduction rates induced by luminal Cl- removal was inhibited by 75% by 1 mM SITS added to the lumen but was not inhibited by furosemide (0.1 mM). Application of SITS in the lumen in the control condition, however, did not change aiCl appreciably. Reducing the luminal HCO3- from 25 to 5 mM did not significantly change aiCl or Vbl. Replacing luminal Na+ with choline decreased aiCl but these effects were still present when luminal Cl- was absent. We conclude that in the rabbit PST: 1) Cl- is taken up into the cell against the electrochemical gradient, 2) the basolateral membrane plays a more important role in the regulation of aiCl, and 3) luminal Cl- transport is SITS sensitive and Na+ independent.