A Heavy Metal Tracer Method for Demonstrating Fluid and Ion-Accessible Channels in Epithelia

Abstract

The use of enzymes such as horseradish peroxidase or of metal-containing macromolecules such as ferritin and iron dextran have been extremely valuable in tracer studies designed to determine the fate of exogenous macromolecules during endocytosis and in exocytosis/endocytosis coupling during secretion. An important unanswered question in the cell biology of secretion is: How do small molecules and inorganic ions gain access to the lumina of secretory epithelium during secretion? Macro molecular tracers cannot be used to answer this question since they are larger than the pore sizes involved, and most heavy-metal-containing small ionic tracers (such as lanthanum and ruthenium red) have been used only in the presence of fixatives to determine the existence of channels that remain patent in fixed cells and tissues. These tracers have only rarely been used to examine functioning fluid-permeable channels because of their toxicity to living cells (see Hayat, 1975). It is clear, however, that the equilibration of very small ions and molecules across a permeable barrier (e.g., fenestrated capillaries) occurs very rapidly, in a matter of perhaps 2-4 seconds.