Distribution of Na⁺ pump sites in the frog gallbladder

Abstract

THE energy-dependent transport of fluid across epithelia has been best explained by the three-compartment conception of Curran and MacIntosh¹. This model couples the active ‘pumping’ of solute with the elaboration of an absorbate of specific tonicity by postulation of local osmosis within the epithelium. This reasoning has been extended in the ‘standing-gradient’ hypothesis², which accounts for isotonic or hypertonic fluid transport by means of epithelial geometry and the specific transport properties of the cell membrane. The hypothesis depends upon a quantitative assessment of several parameters (membrane hydraulic conductivity, solute-pump distribution and dimensions of epithelial intercellular spaces)³. For example, the model best explains isotonic reabsorption by the gallbladder when it is assumed that the solute-input region (the site of energy-dependent Na⁺ pumping) is confined to the apical portion of the lateral intercellular space². To test this important assumption, we studied the distribution of Na⁺ pumps in frog gallbladders with radioautography of ³H-ouabain binding sites⁵. This is a specific procedure for identifying the site of the enzyme, Na⁺-K⁺ ATPase^6,7, which in this tissue⁴, as well as others, is equated with the Na⁺ pump. We find that Na⁺ pumps are present only on the basal–lateral surface of the epithelial cells, consistent with their providing solute to the intercellular spaces as the site for local osmosis. However, since the pumps are not limited to the apical portion of the intercellular space, the results do not support the ‘standing-gradient’ view of isotonic fluid transport.