Differential covalent labeling of apical and basal-lateral membranes of the epithelium of the toad bladder.

The apical (luminal) plasma membrane of toad bladder epithelial cells has been labeled with (125I) diazo-diiodo sulfanilic acid (125I-DDISA) as demonstrated by electron-microscopic autoradiography. The silver grains (125I) were localized exclusively to the apical surface. At concentrations of DDISA of 10(-3) M or less, binding to the apical membrane had no significant effect on the fine structure of the epithelium. At concentrations of DDISA of 10(-6) M or less, the baseline short-circuit current (SCC), and the response to cyclic 3',5'-adenosine monophosphate (cAMP) plus theophylline were unimpaired. At 10(-5) M, baseline SCC was unchanged and the response to cyclic AMP plus theophylline was enhanced. At concentrations of 10(-4) M and greater baseline SCC was depressed and the response to the nucleotide inhibited. The basal-lateral epithelial plasma membranes were labeled by exposing the serosal side to pyridoxal phosphate and reducing the resultant Schiff base with sodium borotritide (3H-NaBH1). In electron-microscopic autoradiographs, the silver grains (3H) were found over the basal and lateral surfaces of the epithelium. At concentrations of pyridoxal phosphate of 10(-4) M and 3H-NaBH1 of 10(-3) M, there were no significant changes in the fine structure of the epithelium. Addition of pyridoxal phosphate (10(-4) M) and NaBH4 (10(-3) M) to the serosal side decreased the baseline SCC significantly but not the response to vasopressin. Covalent attachment of the 125I and the 3H was indicated by resistance to elution in the preparation of the sections for electron-microscopy and the reagent requirements for binding.