AJP - Renal Physiology, Vol 267, Issue 1 106-F113, Copyright © 1994 by American Physiological Society
Acidification of vasopressin-induced endosomes in toad urinary bladder
F. Emma, H. W. Harris and K. Strange
Division of Nephrology, Children's Hospital, Harvard Medical School, Boston, Massachusetts 02115.
It is well established that water channels (WC) are removed from the apical
membrane of vasopressin-sensitive epithelia by endocytosis. The processing
and the ultimate fate of endocytosed WC is, however, incompletely
understood. In many cells, endosome acidification plays an important role
in the processing and sorting of endocytosed proteins. Endosome
acidification in the toad urinary bladder was therefore examined in vivo by
fluorescence ratio video microscopy after induction of endocytosis by
vasopressin removal and transepithelial water flow in the presence of the
pH-sensitive fluid phase marker
2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein-dextran. Fifteen minutes
after induction of endocytosis, the majority of endosomes had a neutral or
slightly acidic pH. The number of acidic endosomes increased progressively
with time. Two hours after endocytosis began, 98% of the endosomes had a pH
< 6.0. Bafilomycin completely blocked endosome acidification, indicating
that H+ transport is mediated by a vacuolar H(+)-adenosinetriphosphatase.
Bafilomycin had no effect on transepithelial water flow in bladders
repetitively stimulated by vasopressin. These findings, as well as the work
of other investigators, suggest that if WC recycling occurs, it is not
dependent on acidification of the endosomal compartment. Acidification of
vasopressin-induced endosomes most likely represents a terminal event in
the endocytic pathway.
