Inter‐relationship of sodium, chloride, bicarbonate and acetate transport by the colon of the pig.

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Abstract
Net transport of Na+, Cl-, HCO3- and acetate was examined in the temporarily isolated colon of conscious pigs weighing 46 .+-. 8 kg. The entire colon absorbs 4.1 ml.cntdot.H2O, 0.8 m-equiv Na+, 1.3 meq acetate and secretes 0.5 meq HCO3-/min with a solution comparable to the normal contents. The absorptive capacity of the proximal and distal halves of the colon was comparable per unit dry weight of mucosa when each segment was presented with the same solution. A series of studies using ion replacement solutions showed that net Na+ absorption and net HCO3- accumulation in the lumen solution were both increased in the presence of acetate. Cl- absorption was independent of Na+ absorption and was accompanied by an equivalent net secretion of HCO3- in the absence of Na+. When NaCl in the perfusion solution was replaced with Na2SO4, Na+ and HCO3- were absorbed at equal rates. Final P[partial pressure]CO2 values observed in NaCl and Na2SO4 solutions were greater than those observed in plasma while the PCO2 of the Na acetate solution after perfusion was reduced to values below plasma concentrations. Hydration of CO2 in the lumen solution or mucosal cell provides a continuous source of H+ for absorption of the more permeable undissociated acid. An additional source of H ions may be provided by a Na-H exchange process located in one of the limiting cell membranes.