Direct demonstration of the cellular location of sodium pumping constitutes a key problem in the solution of intestinal sodium absorption. Utilizing silicone-impregnated epoxy sections of freeze-dried, osmium-fixed tissue, ouabain-3H and inulin-3H light microscope radioautographs have been produced which show that: lateral but not brush border membranes of rabbit small intestine bind ouabain-3H (high specific activity) with an affinity so great that a subsequent washing in ouabain-free medium has little effect on binding; lateral membrane binding is not apparent with low specific activity ouabain-3H, and inulin-3H and ouabain-3H (low specific activity) in the cores of the villi do not equilibrate with the intercellular spaces. Preliminary tracer measurements of ouabain-3H and inulin-14C spaces also agree with these findings As ouabain is a specific inhibitor of active sodium transport, these observations provide direct support for the view that lateral membrane pumping of sodium into the intercellular spaces causes, through osmotic forces on water, a flow of fluid out of these spaces into the interstitium.

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