The stoichiometry of K+/H+ antiport was measured fluorometrically by the static head method in highly purified vesicles from goblet cell apical membranes of larval lepidopteran midgut. The measured stoichiometry of 1 K+/2 H+ explains how the antiport results in electrophoretic exchange of extracellular H+ for intracellular K+, driven by the voltage component of the proton-motive force of an H+ translocating V-ATPase that is located in the same membrane. In turn, the exchange of K+ for H+ helps to explain how the midgut contents are alkalinized to a pH of 11.
