9. Summary
Basolateral ammonium produces an inhibition of Cl− secretion the magnitude of which is dependent on the NH4 + to K+ concentration ratio. Inhibition is maximal at a mole fraction ratio of 0.25 K+ to NH4 +. This anomalous mole fraction effect is due to effects on the basolateral K+ channel as well as Na+-K+-2Cl− cotransporter. However, only Cl− loading, not K+ loading, appears affected in an anomalous mole fraction manner. Transepithelial current is only slightly inhibited relative to equilmolar K+ by NH4 +. As in other systems, both Na+-K+-ATPase and Na+-K+-2Cl− can act in Na+-NH4 +-ATPase and Na+-NH4 +-2Cl− transport modes. NH4 + conducts through most K+ channels and thus likely through the apical K+ channel present in native crypt cells. This suggests that, similar to the kidney, colonic secretory cells have the capacity to secrete NH4 + when in a K+-secreting mode with elevated basolateral NH4 + levels.