Transcellular bicarbonate transport in rabbit gallbladder epithelium: mechanisms and effects of cyclic AMP

Abstract

HCO₃ permeation through rabbit gallbladder epithelium has been investigated in vitro using voltage-clamp, pH-stat and microelectrode techniques. Mucosa-to-serosa flux of HCO₃ (≈4.9 μmol cm⁻²h⁻¹) was dependent on luminal Na and inhibited by amiloride (1 mmol/l, luminal bath), methazolamide (0.1 mmol/l, both sides), and ouabain (30 μmol/l, serosal bath). Maximal rates of serosa-to-mucosa flux of HCO₃ (≈2.8 μmol cm⁻²h⁻¹) required serosal Na and mucosal Cl. This flux was inhibited by ouabain, 4-acetamido-4′-isothiocyanato-stilbene-2,2′-disulfonic acid (1 mmol/l, serosal bath), and 5-nitro-2-(3-phenylpropylamino)-benzole acid (0.1 mmol/l, luminal bath). Ineffective were methazolamide (0.1 mmol/l, both sides) and amiloride (1 mmol/l, serosal bath). 8-Br-cAMP (1 mmol/l, serosal bath) largely inhibited the absorptive and moderately stimulated the secretory flux. In tissue conductance, short-circuit current, and transmural voltage prostaglandin E₁ (1 μmol/l, serosal bath) and 8-Br-cAMP caused moderate to negligible increases. No significant alterations of apical membrane potential (≈ −65 mV) and the apparent ratio of membrane resistances (R_a/R_b;≈1.9) were found. Cell membranes responded to luminal Cl removal mostly with a slow hyperpolarization that was mitigated by 8-Br-cAMP or, in some cases, converted into a small, transient depolarization. Our results are best explained by transcellular HCO₃ transport in both directions. In secretion, basolateral HCO₃ entry occurs by some form of co-transport with Na, and apical exit by Cl/HCO₃ exchange. cAMP opens no major electrodiffusive pathway for apical anion efflux. In absorption, HCO₃ import from the lumen into the cell is secondary to cAMP-sensitive Na/H exchange.