3 UNRELATED PERTURBATIONS SIMILARLY UNCOUPLE FLUID, BULK-MEMBRANE, AND RECEPTOR ENDOSOMAL FLOW IN RAT FETAL FIBROBLASTS

We have compared the effects of three perturbations (treatment with 2 mu M monensin, potassium depletion, and incubation in 0.35 M NaCl) on recycling of internalized fluid-phase, bulk-membrane, and receptor-med iated tracers in rat fetal fibroblasts. Monensin accelerated 2-fold th e regurgitation of the fluid-phase tracer horseradish peroxidase (HRP) , as previously described in these cells after potassium depletion or upon incubation in hypertonic medium (1), and all treatments severely inhibited transfer of HRP from endosomes to lysosomes. In comparison, recycling of -(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoyl) glucos ylsphingosine (C6-NBD-GlcCer), a fluorescent lipid used as a bulk-memb rane probe, was not significantly affected while transferrin recycling was slowed down a-fold. The striking similarities of these unrelated perturbations in their distinct effects on fluid, bulk-membrane, and r eceptor addressing point to common targets regulating these mechanisms . (C) 1997 Academic Press.