Polarization and density of Na-pumps in the inner ear of the chick embryo during early stages of development.

The otic vesicle consists of a pseudostratified epithelium with some features of transporting epithelia. The present work questions whether Na-pumps are polarized in this epithelium and what is the relation between the location or density of pumps and development. This was done by measuring the binding of [3H]ouabain to isolated otic vesicles in developmental stages 16 to 22. The results show the presence of specific ouabain-binding sites located in the inward-facing membrane of the otic vesicle epithelium. Binding was saturable at increasing concentrations of ouabain and was highly sensitive to the external K+ concentration with half-maximal inhibition below 0.5 mM, indicating that the binding sites and Na-pump sites are identical. A transient stage-dependent increase in the density of Na-pumps during the period that precedes growth was observed. Evidence is given against this being directly related to an increased net fluid transport rate despite the fact that pump sites were always polarized throughout these stages. The conclusions are that (1) the otic vesicle epithelium is a polarized structure with the possibility of vectorial transport of solutes and water and (2) the increased number of pumps may operate as a regulatory mechanism during normal proliferative growth.