Insulin-induced alterations in the lactoperoxidase-catalyzed radioiodination of membrane proteins of the toad bladder epithelium

Insulin-stimulated sodium transport in the toad urinary bladder consists of two components, a brief element of rapid onset that is independent of protein synthesis, and a sustained increase, slower in onset, that is dependent uoon RNA and protein synthesis. The mucosal epithelium of the toad bladder was labeled bv lactoperoxidase-catalyzed radioiodination (¹²⁵I) following 15 min and 3 h exposure to insulin. The membranes of Ȝmitochondriarichȝ and Ȝgranularȝ mucosal cells from these tissues were analyzed by electrophoresis in SDS-urea. Compared to untreated tissues, membranes of Ȝgranularȝ mucosal cells from tissues exposed to insulin for 15 min contained a band (M = 15, 000) with significantly increased labeling. Bladders exposed to insulin for 3 h showed no consistent increase in labeling. These data suggest that there are differences in the conformation of apical membrane proteins during the two phases of hormone-induced sodium transport. The technique may also offer an opportunity to identify Ȝeffectorȝ proteins mediating this and other insulin responses.