The distribution of (Na+ + K+) ATPase over the plasma membranes of distal convoluted tubules from canine kidney has been determined. This enzyme is responsible for the coupled active transport of Na+ and K+ across animal cell membranes. Ultrathin frozen sections were cut from fragments of renal cortex and specifically stained with antibodies, which recognize antigenic sites on the enzyme, and ferritin-conjugated goat antirabbit gamma-globulins. It is demonstrated that (Na+ + K+) ATPase is distributed uniformly and at high concentration over the plasma membranes which form the intercellular spaces of this epithelium. The enzyme is located on the luminal surface of the tubules as well but at a much lower concentration. These results, in combination with those of previous determinations of the cation fluxes across this epithelium, can be used to formulate a complete description of the cation movements through this tissue.

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