CO2-stimulated NaCl absorption in the mouse renal cortical thick ascending limb of Henle. Evidence for synchronous Na +/H+ and Cl-/HCO3- exchange in apical plasma membranes.

These experiments evaluated salt transport processes in isolated cortical thick limbs of Henle (cTALH) obtained from mouse kidney. When the external solutions consisted of Krebs-Ringer bicarbonate (KRB), pH 7.4, and a 95% O2-5% CO2 gas phase, the spontaneous transepithelial voltage (Ve, mV, lumen-to-bath) was approximately mV; the net rate of Cl- absorption (JnetCl) was approximately 3,600 pmols s-1 cm-2; the net rate of osmotic solute absorption Jnetosm was twice JnetCl; and the net rate of total CO2 transport (JnetCO2) was indistinguishable from zero. Thus, net Cl- absorption was accompanied by the net absorption of a monovalent cation, presumably Na+, and net HCO3- absorption was negligible. This salt transport process was stimulated by (CO2 + HCO3-): omission of CO2 from the gas phase and HCO3- from external solutions reduced JnetCl, Jnetosm, and Ve by 50%. Furthermore, 10(-4) M luminal furosemide abolished JnetCl and Ve entirely. The lipophilic carbonic anhydrase inhibitor ethoxzolamide (10(-4) M, either luminal or peritubular) inhibited (CO2 + HCO3-)-stimulated JnetCl, Jnetosm, and Ve by approximately 50%; however, when the combination (CO2 + HCO3-) was absent, ethoxzolamide had no detectable effect on salt transport. Ve was reduced or abolished entirely by omission of either Na+ or Cl- from external solutions, by peritubular K+ removal, by 10(-3) M peritubular ouabain, and by 10(-4) M luminal SITS. However, Ve was unaffected by 10(-3) M peritubular SITS, or by the hydrophilic carbonic anhydrase inhibitor acetazolamide (2.2 x 10(-4) M, lumen plus bath). We interpret these data to indicate that (CO2 + HCO3-)-stimulated NaCl absorption in the cTALH involved two synchronous apical membrane antiport processes: one exchanging luminal Na+ for cellular H+; and the other exchanging luminal Cl- for cellular HCO3- or OH-, operating in parallel with a (CO2+ HCO3-)-independent apical membrane NaCl cotransport mechanism.