Isoproterenol inhibits fluid-phase endocytosis from early to late endosomes

We have shown recently that isoproterenol affects both the cellular locatio n and the morphology of late endosomes in a pa-dependent manner [Marjomaki et al,, Eur. J. Cell Biol, 65, 1-13 (1994)]. In this study, using fluoresce nce and quantitative electron microscopy, we wanted to examine further what is the fate of internalized markers during their translocation from early to late endosomes under isoproterenol treatment. Fluorescein dextran intern alized for 30 min (10-min pulse followed by a 20-min chase) showed accumula tion in the cellular periphery during isoproterenol treatment in contrast t o the control cells, which accumulated dextran in the perinuclear region. Q uantitative electron microscopy showed that the markers accumulated in the early endosomes and putative carrier vesicles, In addition, different parti culate markers that were internalized sequentially accumulated in similar s tructures due to the isoproterenol treatment, altogether suggesting that is oproterenol retards the translocation of markers to the later structures. P relabelling of the late endosomes with fluorescent dextran or BSA-coated go ld particles showed that isoproterenol causes a reduction of the mean size of the prelabelled late endosomes as well as a shift of these vesicles to t he cellular periphery. Isoproterenol had no apparent effect on the morpholo gy nor on the location of lysosomes, Percoll fractionation shelved that the changes in late endosomal location and morphology did not change their cha racteristic density. Furthermore, electron microscopy showed that, in the c ellular periphery, these late endosomal elements did not fuse with early en dosomal structures, which is in agreement with the results of biochemical i n vitro cell-free assays carried out by others. In conclusion, the results show that isoproterenol inhibits transport from early to late endosomes in a manner that may be pH- and/or Ca2+-dependent. Simultaneously, isoproteren ol causes fragmentation of the late endosomal compartment and the shift of these fragments to the cellular periphery, where they have a restricted abi lity to fuse with earlier endosomal structures.