Effect of arterial carbon dioxide tension on amiloride-sensitive sodium absorption in the colon

To examine the nature of the electroneutral sodium chloride absorptive process affected by arterial carbon dioxide tension (PCO2), we measured the effects of amiloride on colonic sodium absorption at concentrations (0.75 mM) known to inhibit cell membrane sodium-hydrogen ion exchange. During sequential in situ perfusions of distal colon with amiloride-free and amiloride-containing solutions, water and electrolyte transport was measured in anesthetized, mechanically ventilated rats during normocapnia, respiratory alkalosis, or respiratory acidosis. During amiloride-free perfusions, alkalosis decreased and acidosis increased net water, sodium, and chloride absorption without changing the transmural potential difference. Perfusion of amiloride (0.75 mM) caused a similar fractional decrease in net sodium absorption in alkalotic (-53.3 +/- 10.2%), normocapnic (-46.3 +/- 6.5%), and acidotic rats (-57.2 +/- 5.2%). Net water (-43%) and chloride absorption also exhibited equivalent fractional reductions in the three acid-base states during amiloride perfusion, although net chloride absorption was reduced only about 20%. These results suggest that the specific colonic sodium absorptive process affected by arterial PCO2 is an amiloride-sensitive, sodium-hydrogen ion exchange process. Arterial PCO2 probably also affects a mucosal chloride-bicarbonate exchange process that results in its overall effect on electroneutral sodium chloride absorption by the distal colon.