Sodium and chloride transport across the isolated porcine gallbladder

Porcine gallbladder, stripped of serosal muscle, mounted in Ussing chambers, and bathed in plasma-like Ringer solution generates a serosal positive transepithelial potential of 4-7 mV and a short-circuit current (I(sc)) of 50-120 μA/cm². Substitution of Cl with gluconate or HCO₃ with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) results in a 50% decrease in I(sc). Treatment with 1 mM amiloride (mucosal side) or 0.1 mM acetazolamide (both sides) causes 25-27% inhibition of the I(sc). Mucosal addition of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid inhibits the I(sc) by 17%. Serosal addition of 0.1 mM bumetanide inhibits the I(sc) by 28%. Amiloride (1 mM) inhibits the net transepithelial fluxes of Na and Cl by 55 and 41%, respectively. Substitution of Cl with gluconate inhibits the net Na flux by 50%, whereas substitution of HCO₃ with HEPES inhibits 85-90% of the net Na flux and changes Cl absorption to net secretion. Based on these results, its is hypothesized that Na and Cl transport across the apical membrane is mediated by two pathways, Na-H/Cl-HCO₃ exchange and Na-HCO₃ cotransport. Partial recycling of Cl and HCO₃ presumably occurs through a Cl conductive pathway and Cl-HCO₃ exchange, respectively, in the apical membrane. This results in net Na absorption, which accounts for most of the I(sc) observed under basal conditions. The effect of bumetanide on the basolateral membrane and the fact that Cl secretion occurs when HCO₃ is absent suggests that Cl secretion involves a basolateral NaCl or Na-K-Cl cotransport system arranged in series with a Cl conductive pathway in the apical membrane.