Intracellular pH and bicarbonate activities in rabbit colon

The short-circuited epithelium of rabbit colon is thought to actively absorb chloride ion by a mechanism in the mucosal cell membrane that exchanges chloride for bicarbonate ion. If this model is correct bicarbonate may be accumulated above electrochemical equilibrium across the mucosal cell membrane. To test this model intracellular pH was measured using a new hydrogen ion-selective liquid membrane microelectrode that is fast, highly selective, and easy to fabricate with a very small tip diameter. These measurements show that the average intracellular pH in this epithelium is 6.9 ± 0.1. The mucosal cell membrane electrical potential difference, measured by conventional open-tipped microelectrodes, averaged -52 ± 3 mV. Intracellular pH is above a value predicted for an equilibrium distribution of hydrogen ion across both cell membranes, implying that a mechanism exists for 'uphill' extrusion of this ion from the cell. Intracellular bicarbonate activity calculated from these measurements averaged 8 ± 1 mM. The electrochemical potential gradient for bicarbonate across the mucosal membrane averaged -28 ± 2 mV, demonstrating that intracellular bicarbonate is concentrated above an equilibrium distribution across the mucosal membrane. Thus energy in the 'downhill' electrochemical potential gradient for bicarbonate exit from the cell may drive the entrance of chloride into this epithelium and energize transepithelial chloride absorption.