Binding of [3H]ouabain by the dog's tracheal epithelium shows a nonspecific component depending linearly on ouabain concentration, and a specific saturable component with a Km of 10(-7) M. Control experiments showed that the tracer taken up was not trapped within the extracellular space nor bound to tissue collagen. Inhibition of the saturable uptake by high K, metabolic inhibition, low Na, and low temperature indicated that binding was to Na/K ATPase. One-sided exposures of tissue sheets to tracer showed that the submucosal side took up 10 X as much tracer as the luminal. Autoradiography localized tracer uptake under all conditions to the cells' basolateral membranes
