Effect of systemic acid-base disorders on ileal intracellular pH and ion transport

We previously reported that changes in ileal net Na absorption correlated with arterial pH, changes in net HCO3 secretion correlated with the plasma HCO3 concentration, and changes in net Cl absorption correlated with arterial CO2 partial pressure (PCO2) during the systemic acid-base disorders. To determine whether changes in intracellular pH (pHi) and HCO3 concentration [( HCO3]i) mediated these effects, we measured pHi and calculated [HCO3]i in the distal ileal mucosa of anesthetized, mechanically ventilated Sprague-Dawley rats using 5,5-[14C]dimethyloxazolidine-2,4,-dione and [3H]inulin. Rats were studied during normocapnia, acute respiratory acidosis, and alkalosis, and uncompensated and pH-compensated acute metabolic acidosis and alkalosis. When animals in all groups were considered, mucosal pHi was not altered, but there were strong correlations between mucosal [HCO3]i and both arterial PCO2 (r = 0.97) and [HCO3] (r = 0.61). When we considered the rates of ileal electrolyte transport that characterized these acid-base disorders [A. N. Charney and L.P. Haskell, Am. J. Physiol. 245 (Gastrointest. Liver Physiol. 8): G230-G235, 1983], we found strong correlations between mucosal [HCO3]i and both net Cl absorption (r = 0.88) and net HCO3 secretion (r = 0.82). These findings suggest that the systemic acid-base disorders do not affect ileal mucosal pHi but do alter mucosal [HCO3]i as a consequence of altered arterial PCO2 and [HCO3]. The effects of these disorders on ileal net Cl absorption and HCO3 secretion may be mediated by changes in [HCO3]i. Arterial pH does not appear to alter ileal Na absorption through changes in the mucosal acid-base milieu.