The p-nitrophenyl phosphate-strontium procedure for the localization of the phosphatase component of Na-K-activated adenosine triphosphatase was evaluated using rat renal cortex as a test tissue. The results obtained by light microscopy were unexpected in that reaction product was found only on the brush borders of proximal tubule cells; this reaction was ouabain-resistant, K-independent and partially Mg-dependent, but could be completely inhibited by l-tetramisole. Electron microscopy showed that a reaction was also present on the cytoplasmic surfaces of the lateral and basal plasma membranes of the proximal and distal tubule cells. That seen in the distal tubule was sensitive to ouabain but not to l-tetramisole, whereas that in the proximal tubule showed a mixture of ouabain-sensitive and l-tetramisole-sensitive components. It is concluded that the procedure as originally described is not specific, demonstrating alkaline phosphatase as well as Na-K-adenosine triphosphatase, but that this problem may be overcome by the use of an alkaline phosphatase inhibitor.
