ACIDIFICATION OF 3 TYPES OF LIVER ENDOCYTIC VESICLES - SIMILARITIES AND DIFFERENCES

Endocytosed ligands move through a series of progressively more acidic vesicles. These differences in pH (pH(i)) could reflect differences i n ion transport mechanisms. Vesicles representing three stages of endo cytosis, compartment for uncoupling of receptor and ligand (CURL), mul tivesicular bodies (MVB), and receptor recycling compartment (RRC), we re studied, and all exhibited ATP-dependent electrogenic acidification that was a saturable function of medium chloride. Initial rates of ac idification differed (RRC > CURL > MVB), and proton influx was similar for CURL and RRC but slower for MVB. Steady-state ATP-dependent pH(i) in the three vesicles was more similar. Vesicle membrane potential wa s substantial (+41 to +69 mV) in low-chloride medium and greatest for RRC but was low (-6 to +6 mV) in 140 mM KCI. These vesicles also exhib ited -22 to -37 mV Donnan potentials. Steady-state pump-generated prot on electrochemical gradients (Delta ($) over tilde mu(H+)) ranged from 114 to 175 mV and were greater for CURL and RRC than for MVB; however , Delta ($) over tilde mu(H+) changed little over a 140-fold differenc e in chloride concentration. Proton leak rates were faster in CURL and RRC than in MVB, but proton efflux was similar. Finally, proton fluxe s and permeabilities, calculated with regard to surface area, differed in the opposite direction (MVB > CURL > RRC). Thus, for the endocytic vesicles studied, intrinsic differences in proton flux and in vesicle geometry could be demonstrated that contributed to differences in pre -steady-state vesicle pH(i).