A method of isolating the thick luminal membrane from homogenates of bladder epithelium is described, which entails pretreatment of the epithelium with fluorescein mercuric acetate and centrifugation of the homogenate on sucrose density gradients. A hexagonal array of hexamers is illustrated by negative contrast staining in whole mounts of the isolated thick membrane. Subunits are also shown in tangential sections of this thick membrane, in fixed, embedded bladder epithelium. The significance of the subunits is discussed in the context of membrane structure and permeability.

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