Distribution of Na+-pump sites in transporting epithelia.

There are three techniques for the localization of intraepithelial Na+, K+-ATPase (usually equated with the Na+-pump) that offer reasonable specificity and resolution: the nitrophenylphosphatase assay of Ernst, the immunoferritin procedure of Kyte, and the radioautographic localization of tritiated ouabain as developed by Stirling. These have now been applied to a wide range of epithelia covering the four classes of interest here: isotonic and hypertonic absorbers and isotonic and hypertonic secretors. A review of published results reveals that in every case (except for the choroid plexus) the enzyme is preferentially located on the basolateral surface of the transporting epithelial cells so that a simple correlation of structure and function in terms of the Koefoed-Johnsen and Ussing hypothesis does not seem possible. With little dispute that this enzyme is, nonetheless, the probable site for conversion of metabolic energy to transport-related work, we summarized as well the more macroscopic structural characteristics of epithelia which serve to typify each of the four classes in terms of the direction and tonicity of transported fluid. The apparently systematic differences in cell shape and cell-cell junctions that are summarized here may well be an important consideration for the development of a useful holistic theory with which to explain the transepithelial transport of salt and water.