SYNCHRONIZED ENDOCYTOSIS AND INTRACELLULAR SORTING IN ALVEOLAR MACROPHAGES - THE EARLY SORTING ENDOSOME IS A TRANSIENT ORGANELLE

Incubation of alveolar macrophages in hypoosmotic K+-containing buffer s results in persistent cell swelling and an inability to undergo regu latory volume decrease, We demonstrate that cells incubated in hypo-K show an inhibition of endocytosis without any observed alteration in recycling, The inhibition of endocytosis affected al forms of membrane internalization, receptor and fluid phase, Both increased cell volume and the inhibition of endocytosis could be released upon return of ce lls to iso-Na+ buffers. The ability to synchronize the endocytic appar atus allowed us to examine hypotheses regarding the origin and maturat ion of endocytic vesicles. Incubation in hypo-K+ buffers had no effect on the delivery of ligands to degradative compartments or on the retu rn of previously internalized receptors to the cell surface. Thus, mem brane recycling and movement of internalized components to lysosomes o ccurred in the absence of continued membrane influx. We also demonstra te that fluorescent lipids, that had been incorporated into early endo somes, returned to the cell surface upon exposure of cells to hypo-Kbuffers. These results indicate that the early sorting endosome is a t ransient structure, whose existence depends upon continued membrane in ternalization. Our data supports the hypothesis that the transfer of m aterial to lysosomes can best be explained by the continuous maturatio n of endosomes.