Iodination (¹²⁵I) of the apical plasma membrane of toad bladder epithelium: Electron-microscopic autoradiography and physiological effects

Summary

The apical plasma membrane of toad bladder epithelial cells has been enzymatically iodinated, using lactoperoxidase, H₂O₂ (generated by a glucose-glucose oxidase system) and NaI. The site of labeling was demonstrated by electron-microscopic autoradiography; the silver grains (¹²⁵I) were found exclusively overlying the luminal plasma membranes of the epithelium. The iodination reaction reached completion in less than 5 min. The dependence of the degree of iodination on NaI concentrations (range=6.3×10⁻⁸ to 6.3×10⁻²m) in the mucosal medium was determined. The results suggest that three classes of sites are iodinated within this concentration range. At concentrations of NaI of 6.3×10⁻⁶m or less, iodination of the apical membrane had no significant effect on either the fine structure of the epithelium or on electrophysiological properties. The baseline short-circuit current (SCC) remained steady and the response to vasopressin was unimpaired. At concentrations of 6.3×10⁻⁵m NaI and greater, the baseline SCC was depressed and the response to vasopressin was partially inhibited. The results indicate that¹²⁵I may serve as a covalent marker (specific for tyrosine and histidine residues) of the apical plasma membrane of epithelia.