Effects of (CO2 + HCO3-) on electrical conductance in cortical thick ascending limbs.

These experiments in isolated mouse cortical thick ascending limbs (cTALH) provide information about: the relative contributions of cellular and paracellular pathways to the transepithelial electrical conductance Ge (mS cm-2); the effects of (CO2 + HCO3-) on Ge; the ratio of net K+ secretion (JKnet) to net Cl- absorption (JClnet); and the K+ requirement for apical membrane furosemide-sensitive NaCl entry. The combination of luminal Ba++, zero K+ reduced Ge; at 5 mM luminal Ba++, the residual conductance was about 75% of the control Ge. The Ba++-insensitive Ge of 73.1 +/- 6.4 mS cm-2 was only slightly greater than the shunt conductance of 57 mS cm-2 computed from the sum of the dissipative bath to lumen fluxes of 22Na+ and 36Cl-. Moreover, luminal 5 mM Ba++, zero K+ had no effect on the Na+/Cl- permselectivity ratio of the paracellular pathway. Thus, Ba++ blockaded transcellular conductance by blocking apical membrane K+ channels. The combination of (CO2 + HCO3-) in external solutions increased Ge solely by augmenting the Ba++-sensitive, that is, transcellular, component of Ge. Finally, in paired experiments, the ratio JKnet/JClnet was 0.27 +/- 0.05; and both Ve, the spontaneous transepithelial voltage (mV), and the equivalent short circuit current Je (pEq sec-1 cm-2) were reduced dramatically by luminal K+ omission. Thus, apical membranes of the cTALH appear to contain a pathway for net K+ secretion, and apical membrane NaCl entry may involve co-transport with K+. Transcellular conductance contributes at least 25% to the total Ge, and the combination (CO2 + HCO3-) augments transcellular conductance.