Sodium-pump density of cells from dog tracheal mucosa

Uptake of tritiated ouabain by cells isolated from dog tracheal epithelium showed two components: a saturable component with a Km of 5.1 X 10(-8) M and a maximal uptake of 8.3 X 10(5) molecules/cell and a nonsaturating component of uptake that was linear with concentration. Several criteria indicated that the saturable uptake component represented binding to the Na+-K+-ATPase. To estimate the average surface area per cell, a known number of cells were pelleted and weighed, and the average surface area was calculated, assuming the cells to be perfectly spherical. The validity of this assumption was confirmed by comparing the calculated surface areas of cells in isotonic and hypotonic media. From the values for maximal saturable uptake and average surface area, a pump density of approximately 2,400 sites/micron2 was calculated. Given that the apical membrane lacks Na pumps and accounts for only approximately 5% of the total surface area, this value corresponds to the pump density of the basolateral cell membrane. The pump densities of ciliated, goblet, and basal cells were compared by autoradiography. The three cell types had approximately the same density of pump sites.