Identification of active cell in potassium transporting epithelium.

Abstract

Epithelial ion transport regulates the environment of cells and modulates the environment inside them, making possible the evolution of eukaryotic organisms. Analysis of the transport process requires that the route taken by ions as they are actively transported across epithelia be known. The route can be determined by locating ion pumps electrically and by measuring transport pool sizes kinetically provided that the epithelial structure is simple enough to enable the results to be interpreted in cellular terms. The route by which potassium is actively transported across the lepidopteran midgut is demonstrated here by a combination of electrical and kinetic analyses. Two sorts of cells, goblet and columnar cells, are distinguished by their electrical properties. The goblet cell is identified as the site of active potassium transport, the pump being localized in its apical membrane. The goblet cells are electrically coupled with columnar cells only when midguts that have been isolated from diet-reared larvae are short circuited. A corollary to this result is that the size of the pool of potassium which is involved in transport should be small when the goblet cells are not coupled with columnar cells and should become large when they are coupled. This corollary has been confirmed by direct measurement of the transport pools using isotope tracer kinetic analysis. This is the first time that the cell type responsible for active ion transport in a polymorphic tissue has been identified directly by physiological means.