Active transport of potassium by the cecropia midgut. VI. Microelectrode potential profile.

Potential differences can only appear across barriers with high electrical resistance. Therefore it should be possible to locate the potential produced by an electrogenic pump across such high resistance barriers by using intracellular microelectrodes. An opportunity to locate such a pump is afforded by the electrogenic active transport of K across the midgut of Hyalophora cecropia (Harvey, Haskell & Nedergaard, 1968). The structure of the midgut encourages the attempt to locate the electrogenic pump. Two types of cell, columnar cells (65 /i tall) and goblet cells (35 fi tall) are arranged in a single layer one cell thick to make up the epithelium. Its basal surface is covered by a basement lamina (10 ft thick), surrounded by an inner sheet of circular muscles and outer bundles of longitudinal muscles, and supplied with tracheoles (Anderson & Harvey, 1966). Presumably the midgut potential is generated by the epithelial cells because they form the only continuous cellular layer in the midgut. The first step in locating the potassium pump is to determine where within the epithelial cells the electrogenic potential appears.