Effect of chronic metabolic acidosis on net electrolyte transport in rat colon

Rats fed NH4Cl (5 meq.100 g body wt-1.day-1) for one week developed chronic metabolic acidosis and had an arterial blood pH and plasma HCO3- concentration of 7.27 +2- 0.02 and 16.2 +/- 0.8 meq/l, respectively; control animals had values of 7.36 +/- 0.01 and 22.4 +/- 0.5 meq/l, respectively. Net electrolyte transport was measured in proximal and distal colonic segments by in situ perfusion. In proximal colon, chronic metabolic acidosis increased HCO3- absorption from 3.3 +/- 0.8 to 6.4 +/- 0.6 mu eq.min-1.g-1 but did not alter Na+ absorption. In distal colon, although Na+ transport was unaffected, chronic acidosis reduced HCO3- secretion from -6.9 +/- 0.8 to -4.4 +/- 0.7 mu eq.min-1.g-1 and increased voltage from -18.9 +/- 2.0 to -51.1 +/- 4.2 mV. To evaluate the dependence of these effects on altered arterial pH and HCO3- concentration, NaHCO3 was infused intravenously, raising pH and HCO3- concentration to 7.53 +/- 0.04 and 23.9 +/- 1.7 meq/l, respectively. Although acute correction of chronic metabolic acidosis reduced HCO3- absorption in proximal colon, it did not affect HCO3- secretion or voltage in the distal segment, suggesting that proximal and distal colon respond differently to chronic metabolic acidosis. These results also suggest that chronic metabolic acidosis alters the mechanisms of ion transport in distal colon.