Functional colocalization of water channels and proton pumps in endosomes from kidney proximal tubule

The apical membrane of mammalian proximal tubule undergoes rapid membrane cycling by exocytosis and endocytosis. Osmotic water and ATP-driven proton transport were measured in endocytic vesicles from rabbit and rat proximal tubule apical membrane labeled in vivo with the fluid phase marker fluorescein-dextran. Osmotic water permeability (Pf) was determined from the time course of fluorescein-dextran fluorescence after exposure of endosomes to an inward osmotic gradient in a stopped-flow apparatus. Pf was 0.009 (rabbit) and 0.029 cm/s (rat) (23 degrees C) and independent of osmotic gradient size. Pf in rabbit endosomes was inhibited reversibly by HgCl2 (KI = 0.2 mM) and had an activation energy of 6.4 +/- 0.5 kcal/mol (15-35 degrees C). Endosomal proton ATPase activity was measured from the time course of internal pH, measured by fluorescein-dextran fluorescence, after the addition of external ATP. Endosomes contained an ATP-driven proton pump that was sensitive to N-ethylmaleimide and insensitive to vanadate and oligomycin. In response to saturating [ATP] the pump acidified the endosomal compartment at a rate of 0.17 (rat) and 0.029 pH unit/s (rabbit); at an external pH of 7.4, the steady-state pH was 6.4 (rat) and 6.5 (rabbit). To examine whether water channels and the proton ATPase were present in the same endosome, the time course of fluorescein-dextran fluorescence was measured in response to an osmotic gradient in the presence and absence of ATP. ATP did not alter endosome Pf, but decreased the amplitude of the fluorescence signal by 43 +/- 3% (rabbit) and 47 +/- 4% (rat).(ABSTRACT TRUNCATED AT 250 WORDS)