Figure 1.Sequential response of duodenal epithelial cells to luminal acid. In the left panel, steady-state pHi about 7.1 when no acid is present. In the succeeding panels to the right, luminal acid rapidly acidifies the epithelial cells. Low pHi increases the activity of the basolateral sodium-bicarbonate cotransporter (NBC), which increases cellular bicarbonate concentration, increasing cellular buffering power. When luminal pH returns to neutrality, acid diffuses out of the cell. The excess intracellular base produces a supernormal pH or overshoot. This in turn activates the bicarbonate secretory mechanism (CFTR), which produces bicarbonate secretion.