Sodium-proton exchange across the apical membrane of the alveolar type II cell of the fetal sheep.
In order to detect and characterise Na(+)-H+ countertransport in the fetal lung epithelium we have studied under a variety of conditions the effect of an outward facing H+ gradient on Na+ uptake into purified apical membrane vesicles prepared from alveolar type II cells. Kinetic analysis of the data reveals both a diffusional and a saturable component of total Na+ uptake. Evidence for the presence of a Na(+)-H+ exchanger is demonstrated by (1) stimulation of Na+ uptake by proton loading of vesicles both in the presence and absence of chemical voltage clamping; (2) saturation kinetics with respect to external Na+ with a Km of 16 mM and a Vmax of 2.1 nmol/mg protein per min; (3) amiloride inhibition of Na+ uptake driven by pH gradient. We conclude that although diffusion may be the major component of total Na+ uptake at physiological external Na+ concentration, Na(+)-H+ countertransport provides a possible mechanism for the acidification of fetal lung liquid in-vivo in addition to its established role in intracellular pH and volume regulation.
Shaw
A M
AM
Department of Child Health, The University, Dundee U.K.
Steele
L W
LW
Butcher
P A
PA
Ward
M R
MR
Olver
R E
RE
D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism*; D002455:Cell Division; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D005333:Fetus; D006859:Hydrogen / Q000378:metabolism; D007700:Kinetics; D011650:Pulmonary Alveoli / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D012756:Sheep; D017923:Sodium-Hydrogen Exchangers
D002352:Carrier Proteins; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D006859:Hydrogen