The use of lead as an ionic tracer for investigating routes of passive fluid transfer.

Several inorganic ions, including lead, barium, silver, and thallium, have been tested as possible tracers for demonstrating fluid-accessible channels in functional epithelia at the ultrastructural level. The most useful of the ionic tracers examined was the lead (plumbous) ion, administered for short time intervals (less than 2 min) and "captured" with phosphate used as the buffer in the fixative. Passive fluid and ion-accessible channels of rat parotid salivary gland have been examined with this method. At short tracer infusion times (0.5-1.0 min), localization of the tracer was primarily extracellular, although intracellular deposits were observed in the following sites: smooth membrane-delimited endocytic vesicles of both epithelial and connective tissue cells, inner Golgi cisternae, and occasional cisternae of rough endoplastic reticulum. The lead tracer readily penetrated tight junctions between parotid acinar cells but rarely passed through the tight junctions between intercalated duct cells and did not penetrate junctions of striated duct cells. Fat cells observed in the stroma of this gland were the only cells that exhibited lead tracer in the cytosol, suggesting that the plasmalemma of this cell type is more permeable to exogenous ions than the plasmalemma of other cell types present in this gland.