The nature of the neutral Na+−Cl−-coupled entry at the apical membrane of rabbit gallbladder epithelium: I. Na+/H+, Cl−/HCO 3 − double exchange and Na+−Cl− symport

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Abstract
Cl− influx at the luminal border of the epithelium of rabbit gallbladder was measured by 45-sec exposures to36Cl− and3H-sucrose (as extracellular marker). Its paracellular component was evaluated by the use of 25mm SCN− which immediately and completely inhibits Cl− entry into the cell. Cellular influx was equal to 16.7μeq cm−2 hr−1 and decreased to 8.5μeq cm−2 hr−1 upon removal of HCO 3 − from the bathing media and by bubbling 100% O2 for 45 min. When HCO 3 − was present, cellular influx was again about halved by the action of 10−4 m acetazolamide, 10−5 to 10−4 m furosemide, 10−5 to 10−4 m 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonate (SITS), 10−3 m amiloride. The effects of furosemide and SITS were tested at different concentrations of the inhibitor and with different exposure times: they were maximal at the concentrations reported above and nonadditive. In turn, the effects of amiloride and SITS were not additive. Acetazolamide reached its maximal action after an exposure of about 2 min. When exogenous HCO 3 − was absent, the residual cellular influx was insensitive to acetazolamide, furosemide and SITS. When exogenous HCO 3 − was present in the salines, Na+ removal from the mucosal side caused a slow decline of cellular Cl− influx; conversely, it immediately abolished cellular Cl− influx in the absence of HCO 3 − . In conclusion, about 50% of cellular influx is sensitive to HCO 3 − , inhibitable by SCN−, acetazolamide, furosemide, SITS and amiloride and furthermore slowly dependent on Na+. The residual cellular influx is insensitive to bicarbonate, inhibitable by SCN−, resistant to acetazolamide, furosemide, SITS and amiloride, and immediately dependent on Na+. Thus, about 50% of apical membrane NaCl influx appears to result from a Na+/H+ and Cl−/HCO 3 − exchange, whereas the residual influx seems to be due to Na+−Cl− contranport on a single carrier. Whether both components are simultaneously present or the latter represents a cellular homeostatic counterreaction to the inhibition of the former is not clear.