Receptor-mediated endocytosis of rat preputial a-glucuronidase and the glycoconjugate mannose-BSA by rat alveolar macrophages is inhibited by chloroquine and ammonium chloride . We have previously reported that these drugs cause a loss of cell surface binding activity and that they do not inhibit internalization of receptor ligand complexes when incubated with cells at 37°C. In this report we more clearly delineate the intracellular site of weak base inhibition of receptor recycling and the mechanism of that inhibition . From our analysis of the kinetics of ligand transport we conclude that there are two functionally distinct intracellular pools of receptor . One of these, the cycling pool, is not sensitive to the presence of weak bases, and receptor-ligand complexes return from this pool to the cell surface intact . The second pool is responsible for the time-dependent intracellular delivery of ligand to acid vesicles, which is inhibited by weak bases. Chloroquine and ammonium chloride appear to inhibit the dissociation of receptor-ligand complexed in this second pool and thereby the production of free receptors for the continuation of receptor-mediated endocytosis. We examine the internalization and binding of ligand in normal and paraformaldehyde-treated cells and find that these are strongly affected by pH . In particular, the dissociation rate of receptor ligand complexes is enhanced >7 .5 fold by lowering the medium pH from 7 to 6. From these results we propose that weak bases raise the pH of acid intracellular compartments, slowing the rate of receptor-ligand dissociation and thereby reducing the cellular pool of free receptors available for further uptake of ligand . In addition, we demonstrate that receptorligand complexes cannot return to the cell surface from the amine-sensitive (acid) intracellular pool that led us to call this the nonreleasable pool . This final observation indicates that receptor movements through these two pools are functionally distinct processes.A number of macromolecules enter cells by receptor-mediated endocytosis; all involve binding to a specific receptor, and some have been shown to associate with coated pits on the plasma membrane and to be internalized in coated vesicles . Frequently, the ligand is deposited in lysosomes and degraded (for review, see reference 1) .This report describes some work done with the system that recognizes mannose-terminated glycoproteins. The mannosereceptor has been found on a number of cells of the reticuloendothelial system of the rat including Kupffer cells of the
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