title: Na+/H+ exchange modulates acidification of early rat liver endocytic vesicles.
abstract: Endocytic vesicles are acidified by an electrogenic vacuolar H(+)-ATPase. These studies examined whether rat liver endosomes also exhibit Na+/H+ exchange and whether this transporter alters acidification. Extravesicular Na+ caused saturable proton efflux from acidified endosomes with a Michaelis constant for Na+ of 7.6 mM, whereas an in-to-out Na+ gradient caused endosome acidification without MgATP and accelerated acidification with MgATP. Na(+)-driven proton fluxes were little altered by valinomycin or carbonyl cyanide m-chlorophenylhydrazone. Na+/H+ exchange was inhibited by Li+ but was not affected by K+, Cl-, amiloride (1 mM), or 5-(N,N-dimethyl) amiloride (0.1 mM). Na+/H+ exchange was detected in "early" but not in "late" liver endosomes or in lysosomes. These data suggest that early rat liver endosomes exhibit Na+/H+ exchange that, immediately after endosome formation, may accelerate vesicular acidification. Because of its insensitivity to amiloride, this exchanger may be a pharmacologically altered form of Na+/H+ exchanger-1 or a new isoform.
mesh_terms: D000143:Acids / Q000378:metabolism; D000818:Animals; D004705:Endocytosis; D011992:Endosomes / Q000378:metabolism; D007700:Kinetics; D008094:Lithium / Q000494:pharmacology; D008099:Liver / Q000378:metabolism*; D008297:Male; D011188:Potassium / Q000494:pharmacology; D011309:Preservation, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D017923:Sodium-Hydrogen Exchangers / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D000143:Acids; D017923:Sodium-Hydrogen Exchangers; D008094:Lithium; D011188:Potassium