TRANSPORT ADENOSINE TRIPHOSPHATASE CYTOCHEMISTRY II. CYTOCHEMICAL LOCALIZATION OF OUABAIN-SENSITIVE, POTASSIUM-DEPENDENT PHOSPHATASE ACTIVITY IN THE SECRETORY EPITHELIUM OF THE AVIAN SALT GLAND
A cytochemical procedure is described for the ultrastructural localization of K-dependent, ouabain-sensitive nitrophenyl phosphatase activity in avian salt gland. Cryostat sections (50 µ) of paraformaldehyde-fixed tissue were incubated in a kinetically defined medium containing: 5 mM p-nitrophenyl phosphate, 10 mM MgCl2, 10 mM KCl, 100 mM Tris-HCl buffer (pH 8.5 or 9.0) and 20 mM SrCl2 to precipitate hydrolyzed phosphate. After incubation at room temperature, the sections were treated with Pb(NO3)2 to convert SrPi to PbPi precipitates for visualization in the electron microscope. Reaction product was localized on the cytoplasmic side of the secretory cell lateral and basal plasma membranes. Little, if any, reaction product was associated with the apical surfaces of the secretory cells or with endothelial surfaces of capillaries. Appropriate control experiments indicated that deposition of reaction product was dependent on Mg and K and was sensitive to ouabain. Furthermore, nonenzymatic hydrolysis of nitrophenyl phosphate did not occur in the medium, and deposition of artifactually produced precipitates did not resemble deposition of enzymatically produced precipitates. The relationship of this localization to transport adenosine triphosphatase cytochemistry is discussed, and the physiologic implications of the localization for tracing the route of active Na transport in the salt gland are considered.