Isoproterenol inhibits fluid-phase endocytosis from early to late endosomes

We have shown recently that isoproterenol affects both the cellular location and the morphology of late endosomes in a pH-dependent manner [Marjomäki et al., Eur. J. Cell Biol. 65, 1-13 (1994)]. In this study, using fluorescence 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 internalized for 30 min (10-min pulse followed by a 20-min chase) showed accumulation in the cellular periphery during isoproterenol treatment in contrast to the control cells, which accumulated dextran in the perinuclear region. Quantitative electron microscopy showed that the markers accumulated in the early endosomes and putative carrier vesicles. In addition, different particulate markers that were internalized sequentially accumulated in similar structures due to the isoproterenol treatment, altogether suggesting that isoproterenol retards the translocation of markers to the later structures. Prelabelling of the late endosomes with fluorescent dextran or BSA-coated gold 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 the cellular periphery. Isoproterenol had no apparent effect on the morphology nor on the location of lysosomes. Percoll fractionation showed that the changes in late endosomal location and morphology did not change their characteristic density. Furthermore, electron microscopy showed that, in the cellular periphery, these late endosomal elements did not fuse with early endosomal structures, which is in agreement with the results of biochemical in 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 Ca(2+)-dependent. Simultaneously, isoproterenol causes fragmentation of the late endosomal compartment and the shift of these fragments to the cellular periphery, where they have a restricted ability to fuse with earlier endosomal structures.